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<title>Warning Options (Using the GNU Compiler Collection (GCC))</title>
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<a name="Warning-Options"></a>
<div class="header">
<p>
Next: <a href="Debugging-Options.html#Debugging-Options" accesskey="n" rel="next">Debugging Options</a>, Previous: <a href="Diagnostic-Message-Formatting-Options.html#Diagnostic-Message-Formatting-Options" accesskey="p" rel="prev">Diagnostic Message Formatting Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Options-to-Request-or-Suppress-Warnings"></a>
<h3 class="section">3.8 Options to Request or Suppress Warnings</h3>
<a name="index-options-to-control-warnings"></a>
<a name="index-warning-messages"></a>
<a name="index-messages_002c-warning"></a>
<a name="index-suppressing-warnings"></a>
<p>Warnings are diagnostic messages that report constructions that
are not inherently erroneous but that are risky or suggest there
may have been an error.
</p>
<p>The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
</p>
<dl compact="compact">
<dd><a name="index-syntax-checking"></a>
</dd>
<dt><code>-fsyntax-only</code></dt>
<dd><a name="index-fsyntax_002donly"></a>
<p>Check the code for syntax errors, but don&rsquo;t do anything beyond that.
</p>
</dd>
<dt><code>-fmax-errors=<var>n</var></code></dt>
<dd><a name="index-fmax_002derrors"></a>
<p>Limits the maximum number of error messages to <var>n</var>, at which point
GCC bails out rather than attempting to continue processing the source
code. If <var>n</var> is 0 (the default), there is no limit on the number
of error messages produced. If <samp>-Wfatal-errors</samp> is also
specified, then <samp>-Wfatal-errors</samp> takes precedence over this
option.
</p>
</dd>
<dt><code>-w</code></dt>
<dd><a name="index-w"></a>
<p>Inhibit all warning messages.
</p>
</dd>
<dt><code>-Werror</code></dt>
<dd><a name="index-Werror"></a>
<a name="index-Wno_002derror"></a>
<p>Make all warnings into errors.
</p>
</dd>
<dt><code>-Werror=</code></dt>
<dd><a name="index-Werror_003d"></a>
<a name="index-Wno_002derror_003d"></a>
<p>Make the specified warning into an error. The specifier for a warning
is appended; for example <samp>-Werror=switch</samp> turns the warnings
controlled by <samp>-Wswitch</samp> into errors. This switch takes a
negative form, to be used to negate <samp>-Werror</samp> for specific
warnings; for example <samp>-Wno-error=switch</samp> makes
<samp>-Wswitch</samp> warnings not be errors, even when <samp>-Werror</samp>
is in effect.
</p>
<p>The warning message for each controllable warning includes the
option that controls the warning. That option can then be used with
<samp>-Werror=</samp> and <samp>-Wno-error=</samp> as described above.
(Printing of the option in the warning message can be disabled using the
<samp>-fno-diagnostics-show-option</samp> flag.)
</p>
<p>Note that specifying <samp>-Werror=</samp><var>foo</var> automatically implies
<samp>-W</samp><var>foo</var>. However, <samp>-Wno-error=</samp><var>foo</var> does not
imply anything.
</p>
</dd>
<dt><code>-Wfatal-errors</code></dt>
<dd><a name="index-Wfatal_002derrors"></a>
<a name="index-Wno_002dfatal_002derrors"></a>
<p>This option causes the compiler to abort compilation on the first error
occurred rather than trying to keep going and printing further error
messages.
</p>
</dd>
</dl>
<p>You can request many specific warnings with options beginning with
&lsquo;<samp>-W</samp>&rsquo;, for example <samp>-Wimplicit</samp> to request warnings on
implicit declarations. Each of these specific warning options also
has a negative form beginning &lsquo;<samp>-Wno-</samp>&rsquo; to turn off warnings; for
example, <samp>-Wno-implicit</samp>. This manual lists only one of the
two forms, whichever is not the default. For further
language-specific options also refer to <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options">C++ Dialect Options</a> and
<a href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options">Objective-C and Objective-C++ Dialect Options</a>.
</p>
<p>Some options, such as <samp>-Wall</samp> and <samp>-Wextra</samp>, turn on other
options, such as <samp>-Wunused</samp>, which may turn on further options,
such as <samp>-Wunused-value</samp>. The combined effect of positive and
negative forms is that more specific options have priority over less
specific ones, independently of their position in the command-line. For
options of the same specificity, the last one takes effect. Options
enabled or disabled via pragmas (see <a href="Diagnostic-Pragmas.html#Diagnostic-Pragmas">Diagnostic Pragmas</a>) take effect
as if they appeared at the end of the command-line.
</p>
<p>When an unrecognized warning option is requested (e.g.,
<samp>-Wunknown-warning</samp>), GCC emits a diagnostic stating
that the option is not recognized. However, if the <samp>-Wno-</samp> form
is used, the behavior is slightly different: no diagnostic is
produced for <samp>-Wno-unknown-warning</samp> unless other diagnostics
are being produced. This allows the use of new <samp>-Wno-</samp> options
with old compilers, but if something goes wrong, the compiler
warns that an unrecognized option is present.
</p>
<dl compact="compact">
<dt><code>-Wpedantic</code></dt>
<dt><code>-pedantic</code></dt>
<dd><a name="index-pedantic-1"></a>
<a name="index-Wpedantic"></a>
<p>Issue all the warnings demanded by strict ISO C and ISO C++;
reject all programs that use forbidden extensions, and some other
programs that do not follow ISO C and ISO C++. For ISO C, follows the
version of the ISO C standard specified by any <samp>-std</samp> option used.
</p>
<p>Valid ISO C and ISO C++ programs should compile properly with or without
this option (though a rare few require <samp>-ansi</samp> or a
<samp>-std</samp> option specifying the required version of ISO C). However,
without this option, certain GNU extensions and traditional C and C++
features are supported as well. With this option, they are rejected.
</p>
<p><samp>-Wpedantic</samp> does not cause warning messages for use of the
alternate keywords whose names begin and end with &lsquo;<samp>__</samp>&rsquo;. Pedantic
warnings are also disabled in the expression that follows
<code>__extension__</code>. However, only system header files should use
these escape routes; application programs should avoid them.
See <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>.
</p>
<p>Some users try to use <samp>-Wpedantic</samp> to check programs for strict ISO
C conformance. They soon find that it does not do quite what they want:
it finds some non-ISO practices, but not all&mdash;only those for which
ISO C <em>requires</em> a diagnostic, and some others for which
diagnostics have been added.
</p>
<p>A feature to report any failure to conform to ISO C might be useful in
some instances, but would require considerable additional work and would
be quite different from <samp>-Wpedantic</samp>. We don&rsquo;t have plans to
support such a feature in the near future.
</p>
<p>Where the standard specified with <samp>-std</samp> represents a GNU
extended dialect of C, such as &lsquo;<samp>gnu90</samp>&rsquo; or &lsquo;<samp>gnu99</samp>&rsquo;, there is a
corresponding <em>base standard</em>, the version of ISO C on which the GNU
extended dialect is based. Warnings from <samp>-Wpedantic</samp> are given
where they are required by the base standard. (It does not make sense
for such warnings to be given only for features not in the specified GNU
C dialect, since by definition the GNU dialects of C include all
features the compiler supports with the given option, and there would be
nothing to warn about.)
</p>
</dd>
<dt><code>-pedantic-errors</code></dt>
<dd><a name="index-pedantic_002derrors-1"></a>
<p>Give an error whenever the <em>base standard</em> (see <samp>-Wpedantic</samp>)
requires a diagnostic, in some cases where there is undefined behavior
at compile-time and in some other cases that do not prevent compilation
of programs that are valid according to the standard. This is not
equivalent to <samp>-Werror=pedantic</samp>, since there are errors enabled
by this option and not enabled by the latter and vice versa.
</p>
</dd>
<dt><code>-Wall</code></dt>
<dd><a name="index-Wall"></a>
<a name="index-Wno_002dall"></a>
<p>This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also
enables some language-specific warnings described in <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options">C++ Dialect Options</a> and <a href="Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options.html#Objective_002dC-and-Objective_002dC_002b_002b-Dialect-Options">Objective-C and Objective-C++ Dialect Options</a>.
</p>
<p><samp>-Wall</samp> turns on the following warning flags:
</p>
<div class="smallexample">
<pre class="smallexample">-Waddress
-Warray-bounds=1 <span class="roman">(only with</span> <samp>-O2</samp><span class="roman">)</span>
-Wbool-compare
-Wbool-operation
-Wc++11-compat -Wc++14-compat
-Wcatch-value <span class="roman">(C++ and Objective-C++ only)</span>
-Wchar-subscripts
-Wcomment
-Wduplicate-decl-specifier <span class="roman">(C and Objective-C only)</span>
-Wenum-compare <span class="roman">(in C/ObjC; this is on by default in C++)</span>
-Wformat
-Wint-in-bool-context
-Wimplicit <span class="roman">(C and Objective-C only)</span>
-Wimplicit-int <span class="roman">(C and Objective-C only)</span>
-Wimplicit-function-declaration <span class="roman">(C and Objective-C only)</span>
-Winit-self <span class="roman">(only for C++)</span>
-Wlogical-not-parentheses
-Wmain <span class="roman">(only for C/ObjC and unless</span> <samp>-ffreestanding</samp><span class="roman">)</span>
-Wmaybe-uninitialized
-Wmemset-elt-size
-Wmemset-transposed-args
-Wmisleading-indentation <span class="roman">(only for C/C++)</span>
-Wmissing-attributes
-Wmissing-braces <span class="roman">(only for C/ObjC)</span>
-Wmultistatement-macros
-Wnarrowing <span class="roman">(only for C++)</span>
-Wnonnull
-Wnonnull-compare
-Wopenmp-simd
-Wparentheses
-Wpointer-sign
-Wreorder
-Wrestrict
-Wreturn-type
-Wsequence-point
-Wsign-compare <span class="roman">(only in C++)</span>
-Wsizeof-pointer-div
-Wsizeof-pointer-memaccess
-Wstrict-aliasing
-Wstrict-overflow=1
-Wstringop-truncation
-Wswitch
-Wtautological-compare
-Wtrigraphs
-Wuninitialized
-Wunknown-pragmas
-Wunused-function
-Wunused-label
-Wunused-value
-Wunused-variable
-Wvolatile-register-var
</pre></div>
<p>Note that some warning flags are not implied by <samp>-Wall</samp>. Some of
them warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid in
some cases, and there is no simple way to modify the code to suppress
the warning. Some of them are enabled by <samp>-Wextra</samp> but many of
them must be enabled individually.
</p>
</dd>
<dt><code>-Wextra</code></dt>
<dd><a name="index-W"></a>
<a name="index-Wextra"></a>
<a name="index-Wno_002dextra"></a>
<p>This enables some extra warning flags that are not enabled by
<samp>-Wall</samp>. (This option used to be called <samp>-W</samp>. The older
name is still supported, but the newer name is more descriptive.)
</p>
<div class="smallexample">
<pre class="smallexample">-Wclobbered
-Wcast-function-type
-Wempty-body
-Wignored-qualifiers
-Wimplicit-fallthrough=3
-Wmissing-field-initializers
-Wmissing-parameter-type <span class="roman">(C only)</span>
-Wold-style-declaration <span class="roman">(C only)</span>
-Woverride-init
-Wsign-compare <span class="roman">(C only)</span>
-Wtype-limits
-Wuninitialized
-Wshift-negative-value <span class="roman">(in C++03 and in C99 and newer)</span>
-Wunused-parameter <span class="roman">(only with</span> <samp>-Wunused</samp> <span class="roman">or</span> <samp>-Wall</samp><span class="roman">)</span>
-Wunused-but-set-parameter <span class="roman">(only with</span> <samp>-Wunused</samp> <span class="roman">or</span> <samp>-Wall</samp><span class="roman">)</span>
</pre></div>
<p>The option <samp>-Wextra</samp> also prints warning messages for the
following cases:
</p>
<ul>
<li> A pointer is compared against integer zero with <code>&lt;</code>, <code>&lt;=</code>,
<code>&gt;</code>, or <code>&gt;=</code>.
</li><li> (C++ only) An enumerator and a non-enumerator both appear in a
conditional expression.
</li><li> (C++ only) Ambiguous virtual bases.
</li><li> (C++ only) Subscripting an array that has been declared <code>register</code>.
</li><li> (C++ only) Taking the address of a variable that has been declared
<code>register</code>.
</li><li> (C++ only) A base class is not initialized in the copy constructor
of a derived class.
</li></ul>
</dd>
<dt><code>-Wchar-subscripts</code></dt>
<dd><a name="index-Wchar_002dsubscripts"></a>
<a name="index-Wno_002dchar_002dsubscripts"></a>
<p>Warn if an array subscript has type <code>char</code>. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wchkp</code></dt>
<dd><a name="index-Wchkp"></a>
<p>Warn about an invalid memory access that is found by Pointer Bounds Checker
(<samp>-fcheck-pointer-bounds</samp>).
</p>
</dd>
<dt><code>-Wno-coverage-mismatch</code></dt>
<dd><a name="index-Wno_002dcoverage_002dmismatch"></a>
<p>Warn if feedback profiles do not match when using the
<samp>-fprofile-use</samp> option.
If a source file is changed between compiling with <samp>-fprofile-gen</samp> and
with <samp>-fprofile-use</samp>, the files with the profile feedback can fail
to match the source file and GCC cannot use the profile feedback
information. By default, this warning is enabled and is treated as an
error. <samp>-Wno-coverage-mismatch</samp> can be used to disable the
warning or <samp>-Wno-error=coverage-mismatch</samp> can be used to
disable the error. Disabling the error for this warning can result in
poorly optimized code and is useful only in the
case of very minor changes such as bug fixes to an existing code-base.
Completely disabling the warning is not recommended.
</p>
</dd>
<dt><code>-Wno-cpp</code></dt>
<dd><p><span class="roman">(C, Objective-C, C++, Objective-C++ and Fortran only)</span>
</p>
<p>Suppress warning messages emitted by <code>#warning</code> directives.
</p>
</dd>
<dt><code>-Wdouble-promotion <span class="roman">(C, C++, Objective-C and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wdouble_002dpromotion"></a>
<a name="index-Wno_002ddouble_002dpromotion"></a>
<p>Give a warning when a value of type <code>float</code> is implicitly
promoted to <code>double</code>. CPUs with a 32-bit &ldquo;single-precision&rdquo;
floating-point unit implement <code>float</code> in hardware, but emulate
<code>double</code> in software. On such a machine, doing computations
using <code>double</code> values is much more expensive because of the
overhead required for software emulation.
</p>
<p>It is easy to accidentally do computations with <code>double</code> because
floating-point literals are implicitly of type <code>double</code>. For
example, in:
</p><div class="smallexample">
<pre class="smallexample">float area(float radius)
{
return 3.14159 * radius * radius;
}
</pre></div>
<p>the compiler performs the entire computation with <code>double</code>
because the floating-point literal is a <code>double</code>.
</p>
</dd>
<dt><code>-Wduplicate-decl-specifier <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wduplicate_002ddecl_002dspecifier"></a>
<a name="index-Wno_002dduplicate_002ddecl_002dspecifier"></a>
<p>Warn if a declaration has duplicate <code>const</code>, <code>volatile</code>,
<code>restrict</code> or <code>_Atomic</code> specifier. This warning is enabled by
<samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wformat</code></dt>
<dt><code>-Wformat=<var>n</var></code></dt>
<dd><a name="index-Wformat"></a>
<a name="index-Wno_002dformat"></a>
<a name="index-ffreestanding-2"></a>
<a name="index-fno_002dbuiltin-1"></a>
<a name="index-Wformat_003d"></a>
<p>Check calls to <code>printf</code> and <code>scanf</code>, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string make
sense. This includes standard functions, and others specified by format
attributes (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>), in the <code>printf</code>,
<code>scanf</code>, <code>strftime</code> and <code>strfmon</code> (an X/Open extension,
not in the C standard) families (or other target-specific families).
Which functions are checked without format attributes having been
specified depends on the standard version selected, and such checks of
functions without the attribute specified are disabled by
<samp>-ffreestanding</samp> or <samp>-fno-builtin</samp>.
</p>
<p>The formats are checked against the format features supported by GNU
libc version 2.2. These include all ISO C90 and C99 features, as well
as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library&rsquo;s limitations. However, if <samp>-Wpedantic</samp> is used
with <samp>-Wformat</samp>, warnings are given about format features not
in the selected standard version (but not for <code>strfmon</code> formats,
since those are not in any version of the C standard). See <a href="C-Dialect-Options.html#C-Dialect-Options">Options Controlling C Dialect</a>.
</p>
<dl compact="compact">
<dt><code>-Wformat=1</code></dt>
<dt><code>-Wformat</code></dt>
<dd><a name="index-Wformat-1"></a>
<a name="index-Wformat_003d1"></a>
<p>Option <samp>-Wformat</samp> is equivalent to <samp>-Wformat=1</samp>, and
<samp>-Wno-format</samp> is equivalent to <samp>-Wformat=0</samp>. Since
<samp>-Wformat</samp> also checks for null format arguments for several
functions, <samp>-Wformat</samp> also implies <samp>-Wnonnull</samp>. Some
aspects of this level of format checking can be disabled by the
options: <samp>-Wno-format-contains-nul</samp>,
<samp>-Wno-format-extra-args</samp>, and <samp>-Wno-format-zero-length</samp>.
<samp>-Wformat</samp> is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wno-format-contains-nul</code></dt>
<dd><a name="index-Wno_002dformat_002dcontains_002dnul"></a>
<a name="index-Wformat_002dcontains_002dnul"></a>
<p>If <samp>-Wformat</samp> is specified, do not warn about format strings that
contain NUL bytes.
</p>
</dd>
<dt><code>-Wno-format-extra-args</code></dt>
<dd><a name="index-Wno_002dformat_002dextra_002dargs"></a>
<a name="index-Wformat_002dextra_002dargs"></a>
<p>If <samp>-Wformat</samp> is specified, do not warn about excess arguments to a
<code>printf</code> or <code>scanf</code> format function. The C standard specifies
that such arguments are ignored.
</p>
<p>Where the unused arguments lie between used arguments that are
specified with &lsquo;<samp>$</samp>&rsquo; operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to <code>va_arg</code> to skip the unused arguments. However,
in the case of <code>scanf</code> formats, this option suppresses the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
</p>
</dd>
<dt><code>-Wformat-overflow</code></dt>
<dt><code>-Wformat-overflow=<var>level</var></code></dt>
<dd><a name="index-Wformat_002doverflow"></a>
<a name="index-Wno_002dformat_002doverflow"></a>
<p>Warn about calls to formatted input/output functions such as <code>sprintf</code>
and <code>vsprintf</code> that might overflow the destination buffer. When the
exact number of bytes written by a format directive cannot be determined
at compile-time it is estimated based on heuristics that depend on the
<var>level</var> argument and on optimization. While enabling optimization
will in most cases improve the accuracy of the warning, it may also
result in false positives.
</p>
<dl compact="compact">
<dt><code>-Wformat-overflow</code></dt>
<dt><code>-Wformat-overflow=1</code></dt>
<dd><a name="index-Wformat_002doverflow-1"></a>
<a name="index-Wno_002dformat_002doverflow-1"></a>
<p>Level <var>1</var> of <samp>-Wformat-overflow</samp> enabled by <samp>-Wformat</samp>
employs a conservative approach that warns only about calls that most
likely overflow the buffer. At this level, numeric arguments to format
directives with unknown values are assumed to have the value of one, and
strings of unknown length to be empty. Numeric arguments that are known
to be bounded to a subrange of their type, or string arguments whose output
is bounded either by their directive&rsquo;s precision or by a finite set of
string literals, are assumed to take on the value within the range that
results in the most bytes on output. For example, the call to <code>sprintf</code>
below is diagnosed because even with both <var>a</var> and <var>b</var> equal to zero,
the terminating NUL character (<code>'\0'</code>) appended by the function
to the destination buffer will be written past its end. Increasing
the size of the buffer by a single byte is sufficient to avoid the
warning, though it may not be sufficient to avoid the overflow.
</p>
<div class="smallexample">
<pre class="smallexample">void f (int a, int b)
{
char buf [13];
sprintf (buf, &quot;a = %i, b = %i\n&quot;, a, b);
}
</pre></div>
</dd>
<dt><code>-Wformat-overflow=2</code></dt>
<dd><p>Level <var>2</var> warns also about calls that might overflow the destination
buffer given an argument of sufficient length or magnitude. At level
<var>2</var>, unknown numeric arguments are assumed to have the minimum
representable value for signed types with a precision greater than 1, and
the maximum representable value otherwise. Unknown string arguments whose
length cannot be assumed to be bounded either by the directive&rsquo;s precision,
or by a finite set of string literals they may evaluate to, or the character
array they may point to, are assumed to be 1 character long.
</p>
<p>At level <var>2</var>, the call in the example above is again diagnosed, but
this time because with <var>a</var> equal to a 32-bit <code>INT_MIN</code> the first
<code>%i</code> directive will write some of its digits beyond the end of
the destination buffer. To make the call safe regardless of the values
of the two variables, the size of the destination buffer must be increased
to at least 34 bytes. GCC includes the minimum size of the buffer in
an informational note following the warning.
</p>
<p>An alternative to increasing the size of the destination buffer is to
constrain the range of formatted values. The maximum length of string
arguments can be bounded by specifying the precision in the format
directive. When numeric arguments of format directives can be assumed
to be bounded by less than the precision of their type, choosing
an appropriate length modifier to the format specifier will reduce
the required buffer size. For example, if <var>a</var> and <var>b</var> in the
example above can be assumed to be within the precision of
the <code>short int</code> type then using either the <code>%hi</code> format
directive or casting the argument to <code>short</code> reduces the maximum
required size of the buffer to 24 bytes.
</p>
<div class="smallexample">
<pre class="smallexample">void f (int a, int b)
{
char buf [23];
sprintf (buf, &quot;a = %hi, b = %i\n&quot;, a, (short)b);
}
</pre></div>
</dd>
</dl>
</dd>
<dt><code>-Wno-format-zero-length</code></dt>
<dd><a name="index-Wno_002dformat_002dzero_002dlength"></a>
<a name="index-Wformat_002dzero_002dlength"></a>
<p>If <samp>-Wformat</samp> is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.
</p>
</dd>
<dt><code>-Wformat=2</code></dt>
<dd><a name="index-Wformat_003d2"></a>
<p>Enable <samp>-Wformat</samp> plus additional format checks. Currently
equivalent to <samp>-Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k</samp>.
</p>
</dd>
<dt><code>-Wformat-nonliteral</code></dt>
<dd><a name="index-Wformat_002dnonliteral"></a>
<a name="index-Wno_002dformat_002dnonliteral"></a>
<p>If <samp>-Wformat</samp> is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a <code>va_list</code>.
</p>
</dd>
<dt><code>-Wformat-security</code></dt>
<dd><a name="index-Wformat_002dsecurity"></a>
<a name="index-Wno_002dformat_002dsecurity"></a>
<p>If <samp>-Wformat</samp> is specified, also warn about uses of format
functions that represent possible security problems. At present, this
warns about calls to <code>printf</code> and <code>scanf</code> functions where the
format string is not a string literal and there are no format arguments,
as in <code>printf (foo);</code>. This may be a security hole if the format
string came from untrusted input and contains &lsquo;<samp>%n</samp>&rsquo;. (This is
currently a subset of what <samp>-Wformat-nonliteral</samp> warns about, but
in future warnings may be added to <samp>-Wformat-security</samp> that are not
included in <samp>-Wformat-nonliteral</samp>.)
</p>
</dd>
<dt><code>-Wformat-signedness</code></dt>
<dd><a name="index-Wformat_002dsignedness"></a>
<a name="index-Wno_002dformat_002dsignedness"></a>
<p>If <samp>-Wformat</samp> is specified, also warn if the format string
requires an unsigned argument and the argument is signed and vice versa.
</p>
</dd>
<dt><code>-Wformat-truncation</code></dt>
<dt><code>-Wformat-truncation=<var>level</var></code></dt>
<dd><a name="index-Wformat_002dtruncation"></a>
<a name="index-Wno_002dformat_002dtruncation"></a>
<p>Warn about calls to formatted input/output functions such as <code>snprintf</code>
and <code>vsnprintf</code> that might result in output truncation. When the exact
number of bytes written by a format directive cannot be determined at
compile-time it is estimated based on heuristics that depend on
the <var>level</var> argument and on optimization. While enabling optimization
will in most cases improve the accuracy of the warning, it may also result
in false positives. Except as noted otherwise, the option uses the same
logic <samp>-Wformat-overflow</samp>.
</p>
<dl compact="compact">
<dt><code>-Wformat-truncation</code></dt>
<dt><code>-Wformat-truncation=1</code></dt>
<dd><a name="index-Wformat_002dtruncation-1"></a>
<a name="index-Wno_002dformat_002doverflow-2"></a>
<p>Level <var>1</var> of <samp>-Wformat-truncation</samp> enabled by <samp>-Wformat</samp>
employs a conservative approach that warns only about calls to bounded
functions whose return value is unused and that will most likely result
in output truncation.
</p>
</dd>
<dt><code>-Wformat-truncation=2</code></dt>
<dd><p>Level <var>2</var> warns also about calls to bounded functions whose return
value is used and that might result in truncation given an argument of
sufficient length or magnitude.
</p></dd>
</dl>
</dd>
<dt><code>-Wformat-y2k</code></dt>
<dd><a name="index-Wformat_002dy2k"></a>
<a name="index-Wno_002dformat_002dy2k"></a>
<p>If <samp>-Wformat</samp> is specified, also warn about <code>strftime</code>
formats that may yield only a two-digit year.
</p></dd>
</dl>
</dd>
<dt><code>-Wnonnull</code></dt>
<dd><a name="index-Wnonnull"></a>
<a name="index-Wno_002dnonnull"></a>
<p>Warn about passing a null pointer for arguments marked as
requiring a non-null value by the <code>nonnull</code> function attribute.
</p>
<p><samp>-Wnonnull</samp> is included in <samp>-Wall</samp> and <samp>-Wformat</samp>. It
can be disabled with the <samp>-Wno-nonnull</samp> option.
</p>
</dd>
<dt><code>-Wnonnull-compare</code></dt>
<dd><a name="index-Wnonnull_002dcompare"></a>
<a name="index-Wno_002dnonnull_002dcompare"></a>
<p>Warn when comparing an argument marked with the <code>nonnull</code>
function attribute against null inside the function.
</p>
<p><samp>-Wnonnull-compare</samp> is included in <samp>-Wall</samp>. It
can be disabled with the <samp>-Wno-nonnull-compare</samp> option.
</p>
</dd>
<dt><code>-Wnull-dereference</code></dt>
<dd><a name="index-Wnull_002ddereference"></a>
<a name="index-Wno_002dnull_002ddereference"></a>
<p>Warn if the compiler detects paths that trigger erroneous or
undefined behavior due to dereferencing a null pointer. This option
is only active when <samp>-fdelete-null-pointer-checks</samp> is active,
which is enabled by optimizations in most targets. The precision of
the warnings depends on the optimization options used.
</p>
</dd>
<dt><code>-Winit-self <span class="roman">(C, C++, Objective-C and Objective-C++ only)</span></code></dt>
<dd><a name="index-Winit_002dself"></a>
<a name="index-Wno_002dinit_002dself"></a>
<p>Warn about uninitialized variables that are initialized with themselves.
Note this option can only be used with the <samp>-Wuninitialized</samp> option.
</p>
<p>For example, GCC warns about <code>i</code> being uninitialized in the
following snippet only when <samp>-Winit-self</samp> has been specified:
</p><div class="smallexample">
<pre class="smallexample">int f()
{
int i = i;
return i;
}
</pre></div>
<p>This warning is enabled by <samp>-Wall</samp> in C++.
</p>
</dd>
<dt><code>-Wimplicit-int <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wimplicit_002dint"></a>
<a name="index-Wno_002dimplicit_002dint"></a>
<p>Warn when a declaration does not specify a type.
This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wimplicit-function-declaration <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wimplicit_002dfunction_002ddeclaration"></a>
<a name="index-Wno_002dimplicit_002dfunction_002ddeclaration"></a>
<p>Give a warning whenever a function is used before being declared. In
C99 mode (<samp>-std=c99</samp> or <samp>-std=gnu99</samp>), this warning is
enabled by default and it is made into an error by
<samp>-pedantic-errors</samp>. This warning is also enabled by
<samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wimplicit <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wimplicit"></a>
<a name="index-Wno_002dimplicit"></a>
<p>Same as <samp>-Wimplicit-int</samp> and <samp>-Wimplicit-function-declaration</samp>.
This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wimplicit-fallthrough</code></dt>
<dd><a name="index-Wimplicit_002dfallthrough"></a>
<a name="index-Wno_002dimplicit_002dfallthrough"></a>
<p><samp>-Wimplicit-fallthrough</samp> is the same as <samp>-Wimplicit-fallthrough=3</samp>
and <samp>-Wno-implicit-fallthrough</samp> is the same as
<samp>-Wimplicit-fallthrough=0</samp>.
</p>
</dd>
<dt><code>-Wimplicit-fallthrough=<var>n</var></code></dt>
<dd><a name="index-Wimplicit_002dfallthrough_003d"></a>
<p>Warn when a switch case falls through. For example:
</p>
<div class="smallexample">
<pre class="smallexample">switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}
</pre></div>
<p>This warning does not warn when the last statement of a case cannot
fall through, e.g. when there is a return statement or a call to function
declared with the noreturn attribute. <samp>-Wimplicit-fallthrough=</samp>
also takes into account control flow statements, such as ifs, and only
warns when appropriate. E.g.
</p>
<div class="smallexample">
<pre class="smallexample">switch (cond)
{
case 1:
if (i &gt; 3) {
bar (5);
break;
} else if (i &lt; 1) {
bar (0);
} else
return;
default:
&hellip;
}
</pre></div>
<p>Since there are occasions where a switch case fall through is desirable,
GCC provides an attribute, <code>__attribute__ ((fallthrough))</code>, that is
to be used along with a null statement to suppress this warning that
would normally occur:
</p>
<div class="smallexample">
<pre class="smallexample">switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
&hellip;
}
</pre></div>
<p>C++17 provides a standard way to suppress the <samp>-Wimplicit-fallthrough</samp>
warning using <code>[[fallthrough]];</code> instead of the GNU attribute. In C++11
or C++14 users can use <code>[[gnu::fallthrough]];</code>, which is a GNU extension.
Instead of these attributes, it is also possible to add a fallthrough comment
to silence the warning. The whole body of the C or C++ style comment should
match the given regular expressions listed below. The option argument <var>n</var>
specifies what kind of comments are accepted:
</p>
<ul>
<li> <samp>-Wimplicit-fallthrough=0</samp> disables the warning altogether.
</li><li> <samp>-Wimplicit-fallthrough=1</samp> matches <code>.*</code> regular
expression, any comment is used as fallthrough comment.
</li><li> <samp>-Wimplicit-fallthrough=2</samp> case insensitively matches
<code>.*falls?[ \t-]*thr(ough|u).*</code> regular expression.
</li><li> <samp>-Wimplicit-fallthrough=3</samp> case sensitively matches one of the
following regular expressions:
<ul>
<li> <code>-fallthrough</code>
</li><li> <code>@fallthrough@</code>
</li><li> <code>lint -fallthrough[ \t]*</code>
</li><li> <code>[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?<br>FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?</code>
</li><li> <code>[ \t.!]*(Else,? |Intentional(ly)? )?<br>Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?</code>
</li><li> <code>[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?<br>fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?</code>
</li></ul>
</li><li> <samp>-Wimplicit-fallthrough=4</samp> case sensitively matches one of the
following regular expressions:
<ul>
<li> <code>-fallthrough</code>
</li><li> <code>@fallthrough@</code>
</li><li> <code>lint -fallthrough[ \t]*</code>
</li><li> <code>[ \t]*FALLTHR(OUGH|U)[ \t]*</code>
</li></ul>
</li><li> <samp>-Wimplicit-fallthrough=5</samp> doesn&rsquo;t recognize any comments as
fallthrough comments, only attributes disable the warning.
</li></ul>
<p>The comment needs to be followed after optional whitespace and other comments
by <code>case</code> or <code>default</code> keywords or by a user label that precedes some
<code>case</code> or <code>default</code> label.
</p>
<div class="smallexample">
<pre class="smallexample">switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
&hellip;
}
</pre></div>
<p>The <samp>-Wimplicit-fallthrough=3</samp> warning is enabled by <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wif-not-aligned <span class="roman">(C, C++, Objective-C and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wif_002dnot_002daligned"></a>
<a name="index-Wno_002dif_002dnot_002daligned"></a>
<p>Control if warning triggered by the <code>warn_if_not_aligned</code> attribute
should be issued. This is enabled by default.
Use <samp>-Wno-if-not-aligned</samp> to disable it.
</p>
</dd>
<dt><code>-Wignored-qualifiers <span class="roman">(C and C++ only)</span></code></dt>
<dd><a name="index-Wignored_002dqualifiers"></a>
<a name="index-Wno_002dignored_002dqualifiers"></a>
<p>Warn if the return type of a function has a type qualifier
such as <code>const</code>. For ISO C such a type qualifier has no effect,
since the value returned by a function is not an lvalue.
For C++, the warning is only emitted for scalar types or <code>void</code>.
ISO C prohibits qualified <code>void</code> return types on function
definitions, so such return types always receive a warning
even without this option.
</p>
<p>This warning is also enabled by <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wignored-attributes <span class="roman">(C and C++ only)</span></code></dt>
<dd><a name="index-Wignored_002dattributes"></a>
<a name="index-Wno_002dignored_002dattributes"></a>
<p>Warn when an attribute is ignored. This is different from the
<samp>-Wattributes</samp> option in that it warns whenever the compiler decides
to drop an attribute, not that the attribute is either unknown, used in a
wrong place, etc. This warning is enabled by default.
</p>
</dd>
<dt><code>-Wmain</code></dt>
<dd><a name="index-Wmain"></a>
<a name="index-Wno_002dmain"></a>
<p>Warn if the type of <code>main</code> is suspicious. <code>main</code> should be
a function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types. This warning
is enabled by default in C++ and is enabled by either <samp>-Wall</samp>
or <samp>-Wpedantic</samp>.
</p>
</dd>
<dt><code>-Wmisleading-indentation <span class="roman">(C and C++ only)</span></code></dt>
<dd><a name="index-Wmisleading_002dindentation"></a>
<a name="index-Wno_002dmisleading_002dindentation"></a>
<p>Warn when the indentation of the code does not reflect the block structure.
Specifically, a warning is issued for <code>if</code>, <code>else</code>, <code>while</code>, and
<code>for</code> clauses with a guarded statement that does not use braces,
followed by an unguarded statement with the same indentation.
</p>
<p>In the following example, the call to &ldquo;bar&rdquo; is misleadingly indented as
if it were guarded by the &ldquo;if&rdquo; conditional.
</p>
<div class="smallexample">
<pre class="smallexample"> if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the &quot;if&quot;. */
</pre></div>
<p>In the case of mixed tabs and spaces, the warning uses the
<samp>-ftabstop=</samp> option to determine if the statements line up
(defaulting to 8).
</p>
<p>The warning is not issued for code involving multiline preprocessor logic
such as the following example.
</p>
<div class="smallexample">
<pre class="smallexample"> if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);
</pre></div>
<p>The warning is not issued after a <code>#line</code> directive, since this
typically indicates autogenerated code, and no assumptions can be made
about the layout of the file that the directive references.
</p>
<p>This warning is enabled by <samp>-Wall</samp> in C and C++.
</p>
</dd>
<dt><code>-Wmissing-attributes</code></dt>
<dd><a name="index-Wmissing_002dattributes"></a>
<a name="index-Wno_002dmissing_002dattributes"></a>
<p>Warn when a declaration of a function is missing one or more attributes
that a related function is declared with and whose absence may adversely
affect the correctness or efficiency of generated code. For example, in
C++, the warning is issued when an explicit specialization of a primary
template declared with attribute <code>alloc_align</code>, <code>alloc_size</code>,
<code>assume_aligned</code>, <code>format</code>, <code>format_arg</code>, <code>malloc</code>,
or <code>nonnull</code> is declared without it. Attributes <code>deprecated</code>,
<code>error</code>, and <code>warning</code> suppress the warning.
(see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>).
</p>
<p><samp>-Wmissing-attributes</samp> is enabled by <samp>-Wall</samp>.
</p>
<p>For example, since the declaration of the primary function template
below makes use of both attribute <code>malloc</code> and <code>alloc_size</code>
the declaration of the explicit specialization of the template is
diagnosed because it is missing one of the attributes.
</p>
<div class="smallexample">
<pre class="smallexample">template &lt;class T&gt;
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template &lt;&gt;
void* __attribute__ ((malloc)) // missing alloc_size
allocate&lt;void&gt; (size_t);
</pre></div>
</dd>
<dt><code>-Wmissing-braces</code></dt>
<dd><a name="index-Wmissing_002dbraces"></a>
<a name="index-Wno_002dmissing_002dbraces"></a>
<p>Warn if an aggregate or union initializer is not fully bracketed. In
the following example, the initializer for <code>a</code> is not fully
bracketed, but that for <code>b</code> is fully bracketed. This warning is
enabled by <samp>-Wall</samp> in C.
</p>
<div class="smallexample">
<pre class="smallexample">int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
</pre></div>
<p>This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wmissing-include-dirs <span class="roman">(C, C++, Objective-C and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wmissing_002dinclude_002ddirs"></a>
<a name="index-Wno_002dmissing_002dinclude_002ddirs"></a>
<p>Warn if a user-supplied include directory does not exist.
</p>
</dd>
<dt><code>-Wmultistatement-macros</code></dt>
<dd><a name="index-Wmultistatement_002dmacros"></a>
<a name="index-Wno_002dmultistatement_002dmacros"></a>
<p>Warn about unsafe multiple statement macros that appear to be guarded
by a clause such as <code>if</code>, <code>else</code>, <code>for</code>, <code>switch</code>, or
<code>while</code>, in which only the first statement is actually guarded after
the macro is expanded.
</p>
<p>For example:
</p>
<div class="smallexample">
<pre class="smallexample">#define DOIT x++; y++
if (c)
DOIT;
</pre></div>
<p>will increment <code>y</code> unconditionally, not just when <code>c</code> holds.
The can usually be fixed by wrapping the macro in a do-while loop:
</p><div class="smallexample">
<pre class="smallexample">#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;
</pre></div>
<p>This warning is enabled by <samp>-Wall</samp> in C and C++.
</p>
</dd>
<dt><code>-Wparentheses</code></dt>
<dd><a name="index-Wparentheses"></a>
<a name="index-Wno_002dparentheses"></a>
<p>Warn if parentheses are omitted in certain contexts, such
as when there is an assignment in a context where a truth value
is expected, or when operators are nested whose precedence people
often get confused about.
</p>
<p>Also warn if a comparison like <code>x&lt;=y&lt;=z</code> appears; this is
equivalent to <code>(x&lt;=y ? 1 : 0) &lt;= z</code>, which is a different
interpretation from that of ordinary mathematical notation.
</p>
<p>Also warn for dangerous uses of the GNU extension to
<code>?:</code> with omitted middle operand. When the condition
in the <code>?</code>: operator is a boolean expression, the omitted value is
always 1. Often programmers expect it to be a value computed
inside the conditional expression instead.
</p>
<p>For C++ this also warns for some cases of unnecessary parentheses in
declarations, which can indicate an attempt at a function call instead
of a declaration:
</p><div class="smallexample">
<pre class="smallexample">{
// Declares a local variable called mymutex.
std::unique_lock&lt;std::mutex&gt; (mymutex);
// User meant std::unique_lock&lt;std::mutex&gt; lock (mymutex);
}
</pre></div>
<p>This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wsequence-point</code></dt>
<dd><a name="index-Wsequence_002dpoint"></a>
<a name="index-Wno_002dsequence_002dpoint"></a>
<p>Warn about code that may have undefined semantics because of violations
of sequence point rules in the C and C++ standards.
</p>
<p>The C and C++ standards define the order in which expressions in a C/C++
program are evaluated in terms of <em>sequence points</em>, which represent
a partial ordering between the execution of parts of the program: those
executed before the sequence point, and those executed after it. These
occur after the evaluation of a full expression (one which is not part
of a larger expression), after the evaluation of the first operand of a
<code>&amp;&amp;</code>, <code>||</code>, <code>? :</code> or <code>,</code> (comma) operator, before a
function is called (but after the evaluation of its arguments and the
expression denoting the called function), and in certain other places.
Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order,
since, for example, if two functions are called within one expression
with no sequence point between them, the order in which the functions
are called is not specified. However, the standards committee have
ruled that function calls do not overlap.
</p>
<p>It is not specified when between sequence points modifications to the
values of objects take effect. Programs whose behavior depends on this
have undefined behavior; the C and C++ standards specify that &ldquo;Between
the previous and next sequence point an object shall have its stored
value modified at most once by the evaluation of an expression.
Furthermore, the prior value shall be read only to determine the value
to be stored.&rdquo;. If a program breaks these rules, the results on any
particular implementation are entirely unpredictable.
</p>
<p>Examples of code with undefined behavior are <code>a = a++;</code>, <code>a[n]
= b[n++]</code> and <code>a[i++] = i;</code>. Some more complicated cases are not
diagnosed by this option, and it may give an occasional false positive
result, but in general it has been found fairly effective at detecting
this sort of problem in programs.
</p>
<p>The C++17 standard will define the order of evaluation of operands in
more cases: in particular it requires that the right-hand side of an
assignment be evaluated before the left-hand side, so the above
examples are no longer undefined. But this warning will still warn
about them, to help people avoid writing code that is undefined in C
and earlier revisions of C++.
</p>
<p>The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle cases.
Links to discussions of the problem, including proposed formal
definitions, may be found on the GCC readings page, at
<a href="http://gcc.gnu.org/readings.html">http://gcc.gnu.org/readings.html</a>.
</p>
<p>This warning is enabled by <samp>-Wall</samp> for C and C++.
</p>
</dd>
<dt><code>-Wno-return-local-addr</code></dt>
<dd><a name="index-Wno_002dreturn_002dlocal_002daddr"></a>
<a name="index-Wreturn_002dlocal_002daddr"></a>
<p>Do not warn about returning a pointer (or in C++, a reference) to a
variable that goes out of scope after the function returns.
</p>
</dd>
<dt><code>-Wreturn-type</code></dt>
<dd><a name="index-Wreturn_002dtype"></a>
<a name="index-Wno_002dreturn_002dtype"></a>
<p>Warn whenever a function is defined with a return type that defaults
to <code>int</code>. Also warn about any <code>return</code> statement with no
return value in a function whose return type is not <code>void</code>
(falling off the end of the function body is considered returning
without a value).
</p>
<p>For C only, warn about a <code>return</code> statement with an expression in a
function whose return type is <code>void</code>, unless the expression type is
also <code>void</code>. As a GNU extension, the latter case is accepted
without a warning unless <samp>-Wpedantic</samp> is used.
</p>
<p>For C++, a function without return type always produces a diagnostic
message, even when <samp>-Wno-return-type</samp> is specified. The only
exceptions are <code>main</code> and functions defined in system headers.
</p>
<p>This warning is enabled by default for C++ and is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wshift-count-negative</code></dt>
<dd><a name="index-Wshift_002dcount_002dnegative"></a>
<a name="index-Wno_002dshift_002dcount_002dnegative"></a>
<p>Warn if shift count is negative. This warning is enabled by default.
</p>
</dd>
<dt><code>-Wshift-count-overflow</code></dt>
<dd><a name="index-Wshift_002dcount_002doverflow"></a>
<a name="index-Wno_002dshift_002dcount_002doverflow"></a>
<p>Warn if shift count &gt;= width of type. This warning is enabled by default.
</p>
</dd>
<dt><code>-Wshift-negative-value</code></dt>
<dd><a name="index-Wshift_002dnegative_002dvalue"></a>
<a name="index-Wno_002dshift_002dnegative_002dvalue"></a>
<p>Warn if left shifting a negative value. This warning is enabled by
<samp>-Wextra</samp> in C99 and C++11 modes (and newer).
</p>
</dd>
<dt><code>-Wshift-overflow</code></dt>
<dt><code>-Wshift-overflow=<var>n</var></code></dt>
<dd><a name="index-Wshift_002doverflow"></a>
<a name="index-Wno_002dshift_002doverflow"></a>
<p>Warn about left shift overflows. This warning is enabled by
default in C99 and C++11 modes (and newer).
</p>
<dl compact="compact">
<dt><code>-Wshift-overflow=1</code></dt>
<dd><p>This is the warning level of <samp>-Wshift-overflow</samp> and is enabled
by default in C99 and C++11 modes (and newer). This warning level does
not warn about left-shifting 1 into the sign bit. (However, in C, such
an overflow is still rejected in contexts where an integer constant expression
is required.)
</p>
</dd>
<dt><code>-Wshift-overflow=2</code></dt>
<dd><p>This warning level also warns about left-shifting 1 into the sign bit,
unless C++14 mode is active.
</p></dd>
</dl>
</dd>
<dt><code>-Wswitch</code></dt>
<dd><a name="index-Wswitch"></a>
<a name="index-Wno_002dswitch"></a>
<p>Warn whenever a <code>switch</code> statement has an index of enumerated type
and lacks a <code>case</code> for one or more of the named codes of that
enumeration. (The presence of a <code>default</code> label prevents this
warning.) <code>case</code> labels outside the enumeration range also
provoke warnings when this option is used (even if there is a
<code>default</code> label).
This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wswitch-default</code></dt>
<dd><a name="index-Wswitch_002ddefault"></a>
<a name="index-Wno_002dswitch_002ddefault"></a>
<p>Warn whenever a <code>switch</code> statement does not have a <code>default</code>
case.
</p>
</dd>
<dt><code>-Wswitch-enum</code></dt>
<dd><a name="index-Wswitch_002denum"></a>
<a name="index-Wno_002dswitch_002denum"></a>
<p>Warn whenever a <code>switch</code> statement has an index of enumerated type
and lacks a <code>case</code> for one or more of the named codes of that
enumeration. <code>case</code> labels outside the enumeration range also
provoke warnings when this option is used. The only difference
between <samp>-Wswitch</samp> and this option is that this option gives a
warning about an omitted enumeration code even if there is a
<code>default</code> label.
</p>
</dd>
<dt><code>-Wswitch-bool</code></dt>
<dd><a name="index-Wswitch_002dbool"></a>
<a name="index-Wno_002dswitch_002dbool"></a>
<p>Warn whenever a <code>switch</code> statement has an index of boolean type
and the case values are outside the range of a boolean type.
It is possible to suppress this warning by casting the controlling
expression to a type other than <code>bool</code>. For example:
</p><div class="smallexample">
<pre class="smallexample">switch ((int) (a == 4))
{
&hellip;
}
</pre></div>
<p>This warning is enabled by default for C and C++ programs.
</p>
</dd>
<dt><code>-Wswitch-unreachable</code></dt>
<dd><a name="index-Wswitch_002dunreachable"></a>
<a name="index-Wno_002dswitch_002dunreachable"></a>
<p>Warn whenever a <code>switch</code> statement contains statements between the
controlling expression and the first case label, which will never be
executed. For example:
</p><div class="smallexample">
<pre class="smallexample">switch (cond)
{
i = 15;
&hellip;
case 5:
&hellip;
}
</pre></div>
<p><samp>-Wswitch-unreachable</samp> does not warn if the statement between the
controlling expression and the first case label is just a declaration:
</p><div class="smallexample">
<pre class="smallexample">switch (cond)
{
int i;
&hellip;
case 5:
i = 5;
&hellip;
}
</pre></div>
<p>This warning is enabled by default for C and C++ programs.
</p>
</dd>
<dt><code>-Wsync-nand <span class="roman">(C and C++ only)</span></code></dt>
<dd><a name="index-Wsync_002dnand"></a>
<a name="index-Wno_002dsync_002dnand"></a>
<p>Warn when <code>__sync_fetch_and_nand</code> and <code>__sync_nand_and_fetch</code>
built-in functions are used. These functions changed semantics in GCC 4.4.
</p>
</dd>
<dt><code>-Wunused-but-set-parameter</code></dt>
<dd><a name="index-Wunused_002dbut_002dset_002dparameter"></a>
<a name="index-Wno_002dunused_002dbut_002dset_002dparameter"></a>
<p>Warn whenever a function parameter is assigned to, but otherwise unused
(aside from its declaration).
</p>
<p>To suppress this warning use the <code>unused</code> attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
</p>
<p>This warning is also enabled by <samp>-Wunused</samp> together with
<samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wunused-but-set-variable</code></dt>
<dd><a name="index-Wunused_002dbut_002dset_002dvariable"></a>
<a name="index-Wno_002dunused_002dbut_002dset_002dvariable"></a>
<p>Warn whenever a local variable is assigned to, but otherwise unused
(aside from its declaration).
This warning is enabled by <samp>-Wall</samp>.
</p>
<p>To suppress this warning use the <code>unused</code> attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
</p>
<p>This warning is also enabled by <samp>-Wunused</samp>, which is enabled
by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wunused-function</code></dt>
<dd><a name="index-Wunused_002dfunction"></a>
<a name="index-Wno_002dunused_002dfunction"></a>
<p>Warn whenever a static function is declared but not defined or a
non-inline static function is unused.
This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wunused-label</code></dt>
<dd><a name="index-Wunused_002dlabel"></a>
<a name="index-Wno_002dunused_002dlabel"></a>
<p>Warn whenever a label is declared but not used.
This warning is enabled by <samp>-Wall</samp>.
</p>
<p>To suppress this warning use the <code>unused</code> attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
</p>
</dd>
<dt><code>-Wunused-local-typedefs <span class="roman">(C, Objective-C, C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wunused_002dlocal_002dtypedefs"></a>
<p>Warn when a typedef locally defined in a function is not used.
This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wunused-parameter</code></dt>
<dd><a name="index-Wunused_002dparameter"></a>
<a name="index-Wno_002dunused_002dparameter"></a>
<p>Warn whenever a function parameter is unused aside from its declaration.
</p>
<p>To suppress this warning use the <code>unused</code> attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
</p>
</dd>
<dt><code>-Wno-unused-result</code></dt>
<dd><a name="index-Wunused_002dresult"></a>
<a name="index-Wno_002dunused_002dresult"></a>
<p>Do not warn if a caller of a function marked with attribute
<code>warn_unused_result</code> (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>) does not use
its return value. The default is <samp>-Wunused-result</samp>.
</p>
</dd>
<dt><code>-Wunused-variable</code></dt>
<dd><a name="index-Wunused_002dvariable"></a>
<a name="index-Wno_002dunused_002dvariable"></a>
<p>Warn whenever a local or static variable is unused aside from its
declaration. This option implies <samp>-Wunused-const-variable=1</samp> for C,
but not for C++. This warning is enabled by <samp>-Wall</samp>.
</p>
<p>To suppress this warning use the <code>unused</code> attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
</p>
</dd>
<dt><code>-Wunused-const-variable</code></dt>
<dt><code>-Wunused-const-variable=<var>n</var></code></dt>
<dd><a name="index-Wunused_002dconst_002dvariable"></a>
<a name="index-Wno_002dunused_002dconst_002dvariable"></a>
<p>Warn whenever a constant static variable is unused aside from its declaration.
<samp>-Wunused-const-variable=1</samp> is enabled by <samp>-Wunused-variable</samp>
for C, but not for C++. In C this declares variable storage, but in C++ this
is not an error since const variables take the place of <code>#define</code>s.
</p>
<p>To suppress this warning use the <code>unused</code> attribute
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>).
</p>
<dl compact="compact">
<dt><code>-Wunused-const-variable=1</code></dt>
<dd><p>This is the warning level that is enabled by <samp>-Wunused-variable</samp> for
C. It warns only about unused static const variables defined in the main
compilation unit, but not about static const variables declared in any
header included.
</p>
</dd>
<dt><code>-Wunused-const-variable=2</code></dt>
<dd><p>This warning level also warns for unused constant static variables in
headers (excluding system headers). This is the warning level of
<samp>-Wunused-const-variable</samp> and must be explicitly requested since
in C++ this isn&rsquo;t an error and in C it might be harder to clean up all
headers included.
</p></dd>
</dl>
</dd>
<dt><code>-Wunused-value</code></dt>
<dd><a name="index-Wunused_002dvalue"></a>
<a name="index-Wno_002dunused_002dvalue"></a>
<p>Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to
<code>void</code>. This includes an expression-statement or the left-hand
side of a comma expression that contains no side effects. For example,
an expression such as <code>x[i,j]</code> causes a warning, while
<code>x[(void)i,j]</code> does not.
</p>
<p>This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wunused</code></dt>
<dd><a name="index-Wunused"></a>
<a name="index-Wno_002dunused"></a>
<p>All the above <samp>-Wunused</samp> options combined.
</p>
<p>In order to get a warning about an unused function parameter, you must
either specify <samp>-Wextra -Wunused</samp> (note that <samp>-Wall</samp> implies
<samp>-Wunused</samp>), or separately specify <samp>-Wunused-parameter</samp>.
</p>
</dd>
<dt><code>-Wuninitialized</code></dt>
<dd><a name="index-Wuninitialized"></a>
<a name="index-Wno_002duninitialized"></a>
<p>Warn if an automatic variable is used without first being initialized
or if a variable may be clobbered by a <code>setjmp</code> call. In C++,
warn if a non-static reference or non-static <code>const</code> member
appears in a class without constructors.
</p>
<p>If you want to warn about code that uses the uninitialized value of the
variable in its own initializer, use the <samp>-Winit-self</samp> option.
</p>
<p>These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables that are uninitialized or clobbered as a whole. They do
not occur for variables or elements declared <code>volatile</code>. Because
these warnings depend on optimization, the exact variables or elements
for which there are warnings depends on the precise optimization
options and version of GCC used.
</p>
<p>Note that there may be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the warnings
are printed.
</p>
</dd>
<dt><code>-Winvalid-memory-model</code></dt>
<dd><a name="index-Winvalid_002dmemory_002dmodel"></a>
<a name="index-Wno_002dinvalid_002dmemory_002dmodel"></a>
<p>Warn for invocations of <a href="_005f_005fatomic-Builtins.html#g_t_005f_005fatomic-Builtins">__atomic Builtins</a>, <a href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins">__sync Builtins</a>,
and the C11 atomic generic functions with a memory consistency argument
that is either invalid for the operation or outside the range of values
of the <code>memory_order</code> enumeration. For example, since the
<code>__atomic_store</code> and <code>__atomic_store_n</code> built-ins are only
defined for the relaxed, release, and sequentially consistent memory
orders the following code is diagnosed:
</p>
<div class="smallexample">
<pre class="smallexample">void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
</pre></div>
<p><samp>-Winvalid-memory-model</samp> is enabled by default.
</p>
</dd>
<dt><code>-Wmaybe-uninitialized</code></dt>
<dd><a name="index-Wmaybe_002duninitialized"></a>
<a name="index-Wno_002dmaybe_002duninitialized"></a>
<p>For an automatic (i.e.&nbsp;local) variable, if there exists a path from the
function entry to a use of the variable that is initialized, but there exist
some other paths for which the variable is not initialized, the compiler
emits a warning if it cannot prove the uninitialized paths are not
executed at run time.
</p>
<p>These warnings are only possible in optimizing compilation, because otherwise
GCC does not keep track of the state of variables.
</p>
<p>These warnings are made optional because GCC may not be able to determine when
the code is correct in spite of appearing to have an error. Here is one
example of how this can happen:
</p>
<div class="smallexample">
<pre class="smallexample">{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
</pre></div>
<p>If the value of <code>y</code> is always 1, 2 or 3, then <code>x</code> is
always initialized, but GCC doesn&rsquo;t know this. To suppress the
warning, you need to provide a default case with assert(0) or
similar code.
</p>
<a name="index-longjmp-warnings"></a>
<p>This option also warns when a non-volatile automatic variable might be
changed by a call to <code>longjmp</code>.
The compiler sees only the calls to <code>setjmp</code>. It cannot know
where <code>longjmp</code> will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a warning
even when there is in fact no problem because <code>longjmp</code> cannot
in fact be called at the place that would cause a problem.
</p>
<p>Some spurious warnings can be avoided if you declare all the functions
you use that never return as <code>noreturn</code>. See <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>.
</p>
<p>This warning is enabled by <samp>-Wall</samp> or <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wunknown-pragmas</code></dt>
<dd><a name="index-Wunknown_002dpragmas"></a>
<a name="index-Wno_002dunknown_002dpragmas"></a>
<a name="index-warning-for-unknown-pragmas"></a>
<a name="index-unknown-pragmas_002c-warning"></a>
<a name="index-pragmas_002c-warning-of-unknown"></a>
<p>Warn when a <code>#pragma</code> directive is encountered that is not understood by
GCC. If this command-line option is used, warnings are even issued
for unknown pragmas in system header files. This is not the case if
the warnings are only enabled by the <samp>-Wall</samp> command-line option.
</p>
</dd>
<dt><code>-Wno-pragmas</code></dt>
<dd><a name="index-Wno_002dpragmas"></a>
<a name="index-Wpragmas"></a>
<p>Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
<samp>-Wunknown-pragmas</samp>.
</p>
</dd>
<dt><code>-Wstrict-aliasing</code></dt>
<dd><a name="index-Wstrict_002daliasing"></a>
<a name="index-Wno_002dstrict_002daliasing"></a>
<p>This option is only active when <samp>-fstrict-aliasing</samp> is active.
It warns about code that might break the strict aliasing rules that the
compiler is using for optimization. The warning does not catch all
cases, but does attempt to catch the more common pitfalls. It is
included in <samp>-Wall</samp>.
It is equivalent to <samp>-Wstrict-aliasing=3</samp>
</p>
</dd>
<dt><code>-Wstrict-aliasing=n</code></dt>
<dd><a name="index-Wstrict_002daliasing_003dn"></a>
<p>This option is only active when <samp>-fstrict-aliasing</samp> is active.
It warns about code that might break the strict aliasing rules that the
compiler is using for optimization.
Higher levels correspond to higher accuracy (fewer false positives).
Higher levels also correspond to more effort, similar to the way <samp>-O</samp>
works.
<samp>-Wstrict-aliasing</samp> is equivalent to <samp>-Wstrict-aliasing=3</samp>.
</p>
<p>Level 1: Most aggressive, quick, least accurate.
Possibly useful when higher levels
do not warn but <samp>-fstrict-aliasing</samp> still breaks the code, as it has very few
false negatives. However, it has many false positives.
Warns for all pointer conversions between possibly incompatible types,
even if never dereferenced. Runs in the front end only.
</p>
<p>Level 2: Aggressive, quick, not too precise.
May still have many false positives (not as many as level 1 though),
and few false negatives (but possibly more than level 1).
Unlike level 1, it only warns when an address is taken. Warns about
incomplete types. Runs in the front end only.
</p>
<p>Level 3 (default for <samp>-Wstrict-aliasing</samp>):
Should have very few false positives and few false
negatives. Slightly slower than levels 1 or 2 when optimization is enabled.
Takes care of the common pun+dereference pattern in the front end:
<code>*(int*)&amp;some_float</code>.
If optimization is enabled, it also runs in the back end, where it deals
with multiple statement cases using flow-sensitive points-to information.
Only warns when the converted pointer is dereferenced.
Does not warn about incomplete types.
</p>
</dd>
<dt><code>-Wstrict-overflow</code></dt>
<dt><code>-Wstrict-overflow=<var>n</var></code></dt>
<dd><a name="index-Wstrict_002doverflow"></a>
<a name="index-Wno_002dstrict_002doverflow"></a>
<p>This option is only active when signed overflow is undefined.
It warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does not
warn about all cases where the code might overflow: it only warns
about cases where the compiler implements some optimization. Thus
this warning depends on the optimization level.
</p>
<p>An optimization that assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such that
overflow never does, in fact, occur. Therefore this warning can
easily give a false positive: a warning about code that is not
actually a problem. To help focus on important issues, several
warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a loop
requires, in particular when determining whether a loop will be
executed at all.
</p>
<dl compact="compact">
<dt><code>-Wstrict-overflow=1</code></dt>
<dd><p>Warn about cases that are both questionable and easy to avoid. For
example the compiler simplifies
<code>x + 1 &gt; x</code> to <code>1</code>. This level of
<samp>-Wstrict-overflow</samp> is enabled by <samp>-Wall</samp>; higher levels
are not, and must be explicitly requested.
</p>
</dd>
<dt><code>-Wstrict-overflow=2</code></dt>
<dd><p>Also warn about other cases where a comparison is simplified to a
constant. For example: <code>abs (x) &gt;= 0</code>. This can only be
simplified when signed integer overflow is undefined, because
<code>abs (INT_MIN)</code> overflows to <code>INT_MIN</code>, which is less than
zero. <samp>-Wstrict-overflow</samp> (with no level) is the same as
<samp>-Wstrict-overflow=2</samp>.
</p>
</dd>
<dt><code>-Wstrict-overflow=3</code></dt>
<dd><p>Also warn about other cases where a comparison is simplified. For
example: <code>x + 1 &gt; 1</code> is simplified to <code>x &gt; 0</code>.
</p>
</dd>
<dt><code>-Wstrict-overflow=4</code></dt>
<dd><p>Also warn about other simplifications not covered by the above cases.
For example: <code>(x * 10) / 5</code> is simplified to <code>x * 2</code>.
</p>
</dd>
<dt><code>-Wstrict-overflow=5</code></dt>
<dd><p>Also warn about cases where the compiler reduces the magnitude of a
constant involved in a comparison. For example: <code>x + 2 &gt; y</code> is
simplified to <code>x + 1 &gt;= y</code>. This is reported only at the
highest warning level because this simplification applies to many
comparisons, so this warning level gives a very large number of
false positives.
</p></dd>
</dl>
</dd>
<dt><code>-Wstringop-overflow</code></dt>
<dt><code>-Wstringop-overflow=<var>type</var></code></dt>
<dd><a name="index-Wstringop_002doverflow"></a>
<a name="index-Wno_002dstringop_002doverflow"></a>
<p>Warn for calls to string manipulation functions such as <code>memcpy</code> and
<code>strcpy</code> that are determined to overflow the destination buffer. The
optional argument is one greater than the type of Object Size Checking to
perform to determine the size of the destination. See <a href="Object-Size-Checking.html#Object-Size-Checking">Object Size Checking</a>.
The argument is meaningful only for functions that operate on character arrays
but not for raw memory functions like <code>memcpy</code> which always make use
of Object Size type-0. The option also warns for calls that specify a size
in excess of the largest possible object or at most <code>SIZE_MAX / 2</code> bytes.
The option produces the best results with optimization enabled but can detect
a small subset of simple buffer overflows even without optimization in
calls to the GCC built-in functions like <code>__builtin_memcpy</code> that
correspond to the standard functions. In any case, the option warns about
just a subset of buffer overflows detected by the corresponding overflow
checking built-ins. For example, the option will issue a warning for
the <code>strcpy</code> call below because it copies at least 5 characters
(the string <code>&quot;blue&quot;</code> including the terminating NUL) into the buffer
of size 4.
</p>
<div class="smallexample">
<pre class="smallexample">enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = &quot;blue&quot;; break;
case purple: str = &quot;purple&quot;; break;
case yellow: str = &quot;yellow&quot;; break;
}
return strcpy (buf, str); // warning here
}
</pre></div>
<p>Option <samp>-Wstringop-overflow=2</samp> is enabled by default.
</p>
<dl compact="compact">
<dt><code>-Wstringop-overflow</code></dt>
<dt><code>-Wstringop-overflow=1</code></dt>
<dd><a name="index-Wstringop_002doverflow-1"></a>
<a name="index-Wno_002dstringop_002doverflow-1"></a>
<p>The <samp>-Wstringop-overflow=1</samp> option uses type-zero Object Size Checking
to determine the sizes of destination objects. This is the default setting
of the option. At this setting the option will not warn for writes past
the end of subobjects of larger objects accessed by pointers unless the
size of the largest surrounding object is known. When the destination may
be one of several objects it is assumed to be the largest one of them. On
Linux systems, when optimization is enabled at this setting the option warns
for the same code as when the <code>_FORTIFY_SOURCE</code> macro is defined to
a non-zero value.
</p>
</dd>
<dt><code>-Wstringop-overflow=2</code></dt>
<dd><p>The <samp>-Wstringop-overflow=2</samp> option uses type-one Object Size Checking
to determine the sizes of destination objects. At this setting the option
will warn about overflows when writing to members of the largest complete
objects whose exact size is known. It will, however, not warn for excessive
writes to the same members of unknown objects referenced by pointers since
they may point to arrays containing unknown numbers of elements.
</p>
</dd>
<dt><code>-Wstringop-overflow=3</code></dt>
<dd><p>The <samp>-Wstringop-overflow=3</samp> option uses type-two Object Size Checking
to determine the sizes of destination objects. At this setting the option
warns about overflowing the smallest object or data member. This is the
most restrictive setting of the option that may result in warnings for safe
code.
</p>
</dd>
<dt><code>-Wstringop-overflow=4</code></dt>
<dd><p>The <samp>-Wstringop-overflow=4</samp> option uses type-three Object Size Checking
to determine the sizes of destination objects. At this setting the option
will warn about overflowing any data members, and when the destination is
one of several objects it uses the size of the largest of them to decide
whether to issue a warning. Similarly to <samp>-Wstringop-overflow=3</samp> this
setting of the option may result in warnings for benign code.
</p></dd>
</dl>
</dd>
<dt><code>-Wstringop-truncation</code></dt>
<dd><a name="index-Wstringop_002dtruncation"></a>
<a name="index-Wno_002dstringop_002dtruncation"></a>
<p>Warn for calls to bounded string manipulation functions such as <code>strncat</code>,
<code>strncpy</code>, and <code>stpncpy</code> that may either truncate the copied string
or leave the destination unchanged.
</p>
<p>In the following example, the call to <code>strncat</code> specifies a bound that
is less than the length of the source string. As a result, the copy of
the source will be truncated and so the call is diagnosed. To avoid the
warning use <code>bufsize - strlen (buf) - 1)</code> as the bound.
</p>
<div class="smallexample">
<pre class="smallexample">void append (char *buf, size_t bufsize)
{
strncat (buf, &quot;.txt&quot;, 3);
}
</pre></div>
<p>As another example, the following call to <code>strncpy</code> results in copying
to <code>d</code> just the characters preceding the terminating NUL, without
appending the NUL to the end. Assuming the result of <code>strncpy</code> is
necessarily a NUL-terminated string is a common mistake, and so the call
is diagnosed. To avoid the warning when the result is not expected to be
NUL-terminated, call <code>memcpy</code> instead.
</p>
<div class="smallexample">
<pre class="smallexample">void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}
</pre></div>
<p>In the following example, the call to <code>strncpy</code> specifies the size
of the destination buffer as the bound. If the length of the source
string is equal to or greater than this size the result of the copy will
not be NUL-terminated. Therefore, the call is also diagnosed. To avoid
the warning, specify <code>sizeof buf - 1</code> as the bound and set the last
element of the buffer to <code>NUL</code>.
</p>
<div class="smallexample">
<pre class="smallexample">void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
&hellip;
}
</pre></div>
<p>In situations where a character array is intended to store a sequence
of bytes with no terminating <code>NUL</code> such an array may be annotated
with attribute <code>nonstring</code> to avoid this warning. Such arrays,
however, are not suitable arguments to functions that expect
<code>NUL</code>-terminated strings. To help detect accidental misuses of
such arrays GCC issues warnings unless it can prove that the use is
safe. See <a href="Common-Variable-Attributes.html#Common-Variable-Attributes">Common Variable Attributes</a>.
</p>
<p>Option <samp>-Wstringop-truncation</samp> is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wsuggest-attribute=<span class="roman">[</span>pure<span class="roman">|</span>const<span class="roman">|</span>noreturn<span class="roman">|</span>format<span class="roman">|</span>cold<span class="roman">|</span>malloc<span class="roman">]</span></code></dt>
<dd><a name="index-Wsuggest_002dattribute_003d"></a>
<a name="index-Wno_002dsuggest_002dattribute_003d"></a>
<p>Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.
</p>
<dl compact="compact">
<dt><code>-Wsuggest-attribute=pure</code></dt>
<dt><code>-Wsuggest-attribute=const</code></dt>
<dt><code>-Wsuggest-attribute=noreturn</code></dt>
<dt><code>-Wsuggest-attribute=malloc</code></dt>
<dd><a name="index-Wsuggest_002dattribute_003dpure"></a>
<a name="index-Wno_002dsuggest_002dattribute_003dpure"></a>
<a name="index-Wsuggest_002dattribute_003dconst"></a>
<a name="index-Wno_002dsuggest_002dattribute_003dconst"></a>
<a name="index-Wsuggest_002dattribute_003dnoreturn"></a>
<a name="index-Wno_002dsuggest_002dattribute_003dnoreturn"></a>
<a name="index-Wsuggest_002dattribute_003dmalloc"></a>
<a name="index-Wno_002dsuggest_002dattribute_003dmalloc"></a>
<p>Warn about functions that might be candidates for attributes
<code>pure</code>, <code>const</code> or <code>noreturn</code> or <code>malloc</code>. The compiler
only warns for functions visible in other compilation units or (in the case of
<code>pure</code> and <code>const</code>) if it cannot prove that the function returns
normally. A function returns normally if it doesn&rsquo;t contain an infinite loop or
return abnormally by throwing, calling <code>abort</code> or trapping. This analysis
requires option <samp>-fipa-pure-const</samp>, which is enabled by default at
<samp>-O</samp> and higher. Higher optimization levels improve the accuracy
of the analysis.
</p>
</dd>
<dt><code>-Wsuggest-attribute=format</code></dt>
<dt><code>-Wmissing-format-attribute</code></dt>
<dd><a name="index-Wsuggest_002dattribute_003dformat"></a>
<a name="index-Wmissing_002dformat_002dattribute"></a>
<a name="index-Wno_002dsuggest_002dattribute_003dformat"></a>
<a name="index-Wno_002dmissing_002dformat_002dattribute"></a>
<a name="index-Wformat-2"></a>
<a name="index-Wno_002dformat-1"></a>
<p>Warn about function pointers that might be candidates for <code>format</code>
attributes. Note these are only possible candidates, not absolute ones.
GCC guesses that function pointers with <code>format</code> attributes that
are used in assignment, initialization, parameter passing or return
statements should have a corresponding <code>format</code> attribute in the
resulting type. I.e. the left-hand side of the assignment or
initialization, the type of the parameter variable, or the return type
of the containing function respectively should also have a <code>format</code>
attribute to avoid the warning.
</p>
<p>GCC also warns about function definitions that might be
candidates for <code>format</code> attributes. Again, these are only
possible candidates. GCC guesses that <code>format</code> attributes
might be appropriate for any function that calls a function like
<code>vprintf</code> or <code>vscanf</code>, but this might not always be the
case, and some functions for which <code>format</code> attributes are
appropriate may not be detected.
</p>
</dd>
<dt><code>-Wsuggest-attribute=cold</code></dt>
<dd><a name="index-Wsuggest_002dattribute_003dcold"></a>
<a name="index-Wno_002dsuggest_002dattribute_003dcold"></a>
<p>Warn about functions that might be candidates for <code>cold</code> attribute. This
is based on static detection and generally will only warn about functions which
always leads to a call to another <code>cold</code> function such as wrappers of
C++ <code>throw</code> or fatal error reporting functions leading to <code>abort</code>.
</p></dd>
</dl>
</dd>
<dt><code>-Wsuggest-final-types</code></dt>
<dd><a name="index-Wno_002dsuggest_002dfinal_002dtypes"></a>
<a name="index-Wsuggest_002dfinal_002dtypes"></a>
<p>Warn about types with virtual methods where code quality would be improved
if the type were declared with the C++11 <code>final</code> specifier,
or, if possible,
declared in an anonymous namespace. This allows GCC to more aggressively
devirtualize the polymorphic calls. This warning is more effective with link
time optimization, where the information about the class hierarchy graph is
more complete.
</p>
</dd>
<dt><code>-Wsuggest-final-methods</code></dt>
<dd><a name="index-Wno_002dsuggest_002dfinal_002dmethods"></a>
<a name="index-Wsuggest_002dfinal_002dmethods"></a>
<p>Warn about virtual methods where code quality would be improved if the method
were declared with the C++11 <code>final</code> specifier,
or, if possible, its type were
declared in an anonymous namespace or with the <code>final</code> specifier.
This warning is
more effective with link-time optimization, where the information about the
class hierarchy graph is more complete. It is recommended to first consider
suggestions of <samp>-Wsuggest-final-types</samp> and then rebuild with new
annotations.
</p>
</dd>
<dt><code>-Wsuggest-override</code></dt>
<dd><p>Warn about overriding virtual functions that are not marked with the override
keyword.
</p>
</dd>
<dt><code>-Walloc-zero</code></dt>
<dd><a name="index-Wno_002dalloc_002dzero"></a>
<a name="index-Walloc_002dzero"></a>
<p>Warn about calls to allocation functions decorated with attribute
<code>alloc_size</code> that specify zero bytes, including those to the built-in
forms of the functions <code>aligned_alloc</code>, <code>alloca</code>, <code>calloc</code>,
<code>malloc</code>, and <code>realloc</code>. Because the behavior of these functions
when called with a zero size differs among implementations (and in the case
of <code>realloc</code> has been deprecated) relying on it may result in subtle
portability bugs and should be avoided.
</p>
</dd>
<dt><code>-Walloc-size-larger-than=<var>n</var></code></dt>
<dd><p>Warn about calls to functions decorated with attribute <code>alloc_size</code>
that attempt to allocate objects larger than the specified number of bytes,
or where the result of the size computation in an integer type with infinite
precision would exceed <code>SIZE_MAX / 2</code>. The option argument <var>n</var>
may end in one of the standard suffixes designating a multiple of bytes
such as <code>kB</code> and <code>KiB</code> for kilobyte and kibibyte, respectively,
<code>MB</code> and <code>MiB</code> for megabyte and mebibyte, and so on.
<samp>-Walloc-size-larger-than=</samp><var>PTRDIFF_MAX</var> is enabled by default.
Warnings controlled by the option can be disabled by specifying <var>n</var>
of <var>SIZE_MAX</var> or more.
See <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>.
</p>
</dd>
<dt><code>-Walloca</code></dt>
<dd><a name="index-Wno_002dalloca"></a>
<a name="index-Walloca"></a>
<p>This option warns on all uses of <code>alloca</code> in the source.
</p>
</dd>
<dt><code>-Walloca-larger-than=<var>n</var></code></dt>
<dd><p>This option warns on calls to <code>alloca</code> that are not bounded by a
controlling predicate limiting its argument of integer type to at most
<var>n</var> bytes, or calls to <code>alloca</code> where the bound is unknown.
Arguments of non-integer types are considered unbounded even if they
appear to be constrained to the expected range.
</p>
<p>For example, a bounded case of <code>alloca</code> could be:
</p>
<div class="smallexample">
<pre class="smallexample">void func (size_t n)
{
void *p;
if (n &lt;= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}
</pre></div>
<p>In the above example, passing <code>-Walloca-larger-than=1000</code> would not
issue a warning because the call to <code>alloca</code> is known to be at most
1000 bytes. However, if <code>-Walloca-larger-than=500</code> were passed,
the compiler would emit a warning.
</p>
<p>Unbounded uses, on the other hand, are uses of <code>alloca</code> with no
controlling predicate constraining its integer argument. For example:
</p>
<div class="smallexample">
<pre class="smallexample">void func ()
{
void *p = alloca (n);
f (p);
}
</pre></div>
<p>If <code>-Walloca-larger-than=500</code> were passed, the above would trigger
a warning, but this time because of the lack of bounds checking.
</p>
<p>Note, that even seemingly correct code involving signed integers could
cause a warning:
</p>
<div class="smallexample">
<pre class="smallexample">void func (signed int n)
{
if (n &lt; 500)
{
p = alloca (n);
f (p);
}
}
</pre></div>
<p>In the above example, <var>n</var> could be negative, causing a larger than
expected argument to be implicitly cast into the <code>alloca</code> call.
</p>
<p>This option also warns when <code>alloca</code> is used in a loop.
</p>
<p>This warning is not enabled by <samp>-Wall</samp>, and is only active when
<samp>-ftree-vrp</samp> is active (default for <samp>-O2</samp> and above).
</p>
<p>See also <samp>-Wvla-larger-than=<var>n</var></samp>.
</p>
</dd>
<dt><code>-Warray-bounds</code></dt>
<dt><code>-Warray-bounds=<var>n</var></code></dt>
<dd><a name="index-Wno_002darray_002dbounds"></a>
<a name="index-Warray_002dbounds"></a>
<p>This option is only active when <samp>-ftree-vrp</samp> is active
(default for <samp>-O2</samp> and above). It warns about subscripts to arrays
that are always out of bounds. This warning is enabled by <samp>-Wall</samp>.
</p>
<dl compact="compact">
<dt><code>-Warray-bounds=1</code></dt>
<dd><p>This is the warning level of <samp>-Warray-bounds</samp> and is enabled
by <samp>-Wall</samp>; higher levels are not, and must be explicitly requested.
</p>
</dd>
<dt><code>-Warray-bounds=2</code></dt>
<dd><p>This warning level also warns about out of bounds access for
arrays at the end of a struct and for arrays accessed through
pointers. This warning level may give a larger number of
false positives and is deactivated by default.
</p></dd>
</dl>
</dd>
<dt><code>-Wattribute-alias</code></dt>
<dd><p>Warn about declarations using the <code>alias</code> and similar attributes whose
target is incompatible with the type of the alias. See <a href="Function-Attributes.html#Function-Attributes">Declaring Attributes of Functions</a>.
</p>
</dd>
<dt><code>-Wbool-compare</code></dt>
<dd><a name="index-Wno_002dbool_002dcompare"></a>
<a name="index-Wbool_002dcompare"></a>
<p>Warn about boolean expression compared with an integer value different from
<code>true</code>/<code>false</code>. For instance, the following comparison is
always false:
</p><div class="smallexample">
<pre class="smallexample">int n = 5;
&hellip;
if ((n &gt; 1) == 2) { &hellip; }
</pre></div>
<p>This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wbool-operation</code></dt>
<dd><a name="index-Wno_002dbool_002doperation"></a>
<a name="index-Wbool_002doperation"></a>
<p>Warn about suspicious operations on expressions of a boolean type. For
instance, bitwise negation of a boolean is very likely a bug in the program.
For C, this warning also warns about incrementing or decrementing a boolean,
which rarely makes sense. (In C++, decrementing a boolean is always invalid.
Incrementing a boolean is invalid in C++17, and deprecated otherwise.)
</p>
<p>This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wduplicated-branches</code></dt>
<dd><a name="index-Wno_002dduplicated_002dbranches"></a>
<a name="index-Wduplicated_002dbranches"></a>
<p>Warn when an if-else has identical branches. This warning detects cases like
</p><div class="smallexample">
<pre class="smallexample">if (p != NULL)
return 0;
else
return 0;
</pre></div>
<p>It doesn&rsquo;t warn when both branches contain just a null statement. This warning
also warn for conditional operators:
</p><div class="smallexample">
<pre class="smallexample"> int i = x ? *p : *p;
</pre></div>
</dd>
<dt><code>-Wduplicated-cond</code></dt>
<dd><a name="index-Wno_002dduplicated_002dcond"></a>
<a name="index-Wduplicated_002dcond"></a>
<p>Warn about duplicated conditions in an if-else-if chain. For instance,
warn for the following code:
</p><div class="smallexample">
<pre class="smallexample">if (p-&gt;q != NULL) { &hellip; }
else if (p-&gt;q != NULL) { &hellip; }
</pre></div>
</dd>
<dt><code>-Wframe-address</code></dt>
<dd><a name="index-Wno_002dframe_002daddress"></a>
<a name="index-Wframe_002daddress"></a>
<p>Warn when the &lsquo;<samp>__builtin_frame_address</samp>&rsquo; or &lsquo;<samp>__builtin_return_address</samp>&rsquo;
is called with an argument greater than 0. Such calls may return indeterminate
values or crash the program. The warning is included in <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wno-discarded-qualifiers <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wno_002ddiscarded_002dqualifiers"></a>
<a name="index-Wdiscarded_002dqualifiers"></a>
<p>Do not warn if type qualifiers on pointers are being discarded.
Typically, the compiler warns if a <code>const char *</code> variable is
passed to a function that takes a <code>char *</code> parameter. This option
can be used to suppress such a warning.
</p>
</dd>
<dt><code>-Wno-discarded-array-qualifiers <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wno_002ddiscarded_002darray_002dqualifiers"></a>
<a name="index-Wdiscarded_002darray_002dqualifiers"></a>
<p>Do not warn if type qualifiers on arrays which are pointer targets
are being discarded. Typically, the compiler warns if a
<code>const int (*)[]</code> variable is passed to a function that
takes a <code>int (*)[]</code> parameter. This option can be used to
suppress such a warning.
</p>
</dd>
<dt><code>-Wno-incompatible-pointer-types <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wno_002dincompatible_002dpointer_002dtypes"></a>
<a name="index-Wincompatible_002dpointer_002dtypes"></a>
<p>Do not warn when there is a conversion between pointers that have incompatible
types. This warning is for cases not covered by <samp>-Wno-pointer-sign</samp>,
which warns for pointer argument passing or assignment with different
signedness.
</p>
</dd>
<dt><code>-Wno-int-conversion <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wno_002dint_002dconversion"></a>
<a name="index-Wint_002dconversion"></a>
<p>Do not warn about incompatible integer to pointer and pointer to integer
conversions. This warning is about implicit conversions; for explicit
conversions the warnings <samp>-Wno-int-to-pointer-cast</samp> and
<samp>-Wno-pointer-to-int-cast</samp> may be used.
</p>
</dd>
<dt><code>-Wno-div-by-zero</code></dt>
<dd><a name="index-Wno_002ddiv_002dby_002dzero"></a>
<a name="index-Wdiv_002dby_002dzero"></a>
<p>Do not warn about compile-time integer division by zero. Floating-point
division by zero is not warned about, as it can be a legitimate way of
obtaining infinities and NaNs.
</p>
</dd>
<dt><code>-Wsystem-headers</code></dt>
<dd><a name="index-Wsystem_002dheaders"></a>
<a name="index-Wno_002dsystem_002dheaders"></a>
<a name="index-warnings-from-system-headers"></a>
<a name="index-system-headers_002c-warnings-from"></a>
<p>Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the assumption
that they usually do not indicate real problems and would only make the
compiler output harder to read. Using this command-line option tells
GCC to emit warnings from system headers as if they occurred in user
code. However, note that using <samp>-Wall</samp> in conjunction with this
option does <em>not</em> warn about unknown pragmas in system
headers&mdash;for that, <samp>-Wunknown-pragmas</samp> must also be used.
</p>
</dd>
<dt><code>-Wtautological-compare</code></dt>
<dd><a name="index-Wtautological_002dcompare"></a>
<a name="index-Wno_002dtautological_002dcompare"></a>
<p>Warn if a self-comparison always evaluates to true or false. This
warning detects various mistakes such as:
</p><div class="smallexample">
<pre class="smallexample">int i = 1;
&hellip;
if (i &gt; i) { &hellip; }
</pre></div>
<p>This warning also warns about bitwise comparisons that always evaluate
to true or false, for instance:
</p><div class="smallexample">
<pre class="smallexample">if ((a &amp; 16) == 10) { &hellip; }
</pre></div>
<p>will always be false.
</p>
<p>This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wtrampolines</code></dt>
<dd><a name="index-Wtrampolines"></a>
<a name="index-Wno_002dtrampolines"></a>
<p>Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and is
used to call the nested function indirectly. For some targets, it is
made up of data only and thus requires no special treatment. But, for
most targets, it is made up of code and thus requires the stack to be
made executable in order for the program to work properly.
</p>
</dd>
<dt><code>-Wfloat-equal</code></dt>
<dd><a name="index-Wfloat_002dequal"></a>
<a name="index-Wno_002dfloat_002dequal"></a>
<p>Warn if floating-point values are used in equality comparisons.
</p>
<p>The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you need
to compute (by analyzing the code, or in some other way) the maximum or
likely maximum error that the computation introduces, and allow for it
when performing comparisons (and when producing output, but that&rsquo;s a
different problem). In particular, instead of testing for equality, you
should check to see whether the two values have ranges that overlap; and
this is done with the relational operators, so equality comparisons are
probably mistaken.
</p>
</dd>
<dt><code>-Wtraditional <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wtraditional"></a>
<a name="index-Wno_002dtraditional"></a>
<p>Warn about certain constructs that behave differently in traditional and
ISO C. Also warn about ISO C constructs that have no traditional C
equivalent, and/or problematic constructs that should be avoided.
</p>
<ul>
<li> Macro parameters that appear within string literals in the macro body.
In traditional C macro replacement takes place within string literals,
but in ISO C it does not.
</li><li> In traditional C, some preprocessor directives did not exist.
Traditional preprocessors only considered a line to be a directive
if the &lsquo;<samp>#</samp>&rsquo; appeared in column 1 on the line. Therefore
<samp>-Wtraditional</samp> warns about directives that traditional C
understands but ignores because the &lsquo;<samp>#</samp>&rsquo; does not appear as the
first character on the line. It also suggests you hide directives like
<code>#pragma</code> not understood by traditional C by indenting them. Some
traditional implementations do not recognize <code>#elif</code>, so this option
suggests avoiding it altogether.
</li><li> A function-like macro that appears without arguments.
</li><li> The unary plus operator.
</li><li> The &lsquo;<samp>U</samp>&rsquo; integer constant suffix, or the &lsquo;<samp>F</samp>&rsquo; or &lsquo;<samp>L</samp>&rsquo; floating-point
constant suffixes. (Traditional C does support the &lsquo;<samp>L</samp>&rsquo; suffix on integer
constants.) Note, these suffixes appear in macros defined in the system
headers of most modern systems, e.g. the &lsquo;<samp>_MIN</samp>&rsquo;/&lsquo;<samp>_MAX</samp>&rsquo; macros in <code>&lt;limits.h&gt;</code>.
Use of these macros in user code might normally lead to spurious
warnings, however GCC&rsquo;s integrated preprocessor has enough context to
avoid warning in these cases.
</li><li> A function declared external in one block and then used after the end of
the block.
</li><li> A <code>switch</code> statement has an operand of type <code>long</code>.
</li><li> A non-<code>static</code> function declaration follows a <code>static</code> one.
This construct is not accepted by some traditional C compilers.
</li><li> The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only issued if
the base of the constant is ten. I.e. hexadecimal or octal values, which
typically represent bit patterns, are not warned about.
</li><li> Usage of ISO string concatenation is detected.
</li><li> Initialization of automatic aggregates.
</li><li> Identifier conflicts with labels. Traditional C lacks a separate
namespace for labels.
</li><li> Initialization of unions. If the initializer is zero, the warning is
omitted. This is done under the assumption that the zero initializer in
user code appears conditioned on e.g. <code>__STDC__</code> to avoid missing
initializer warnings and relies on default initialization to zero in the
traditional C case.
</li><li> Conversions by prototypes between fixed/floating-point values and vice
versa. The absence of these prototypes when compiling with traditional
C causes serious problems. This is a subset of the possible
conversion warnings; for the full set use <samp>-Wtraditional-conversion</samp>.
</li><li> Use of ISO C style function definitions. This warning intentionally is
<em>not</em> issued for prototype declarations or variadic functions
because these ISO C features appear in your code when using
libiberty&rsquo;s traditional C compatibility macros, <code>PARAMS</code> and
<code>VPARAMS</code>. This warning is also bypassed for nested functions
because that feature is already a GCC extension and thus not relevant to
traditional C compatibility.
</li></ul>
</dd>
<dt><code>-Wtraditional-conversion <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wtraditional_002dconversion"></a>
<a name="index-Wno_002dtraditional_002dconversion"></a>
<p>Warn if a prototype causes a type conversion that is different from what
would happen to the same argument in the absence of a prototype. This
includes conversions of fixed point to floating and vice versa, and
conversions changing the width or signedness of a fixed-point argument
except when the same as the default promotion.
</p>
</dd>
<dt><code>-Wdeclaration-after-statement <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wdeclaration_002dafter_002dstatement"></a>
<a name="index-Wno_002ddeclaration_002dafter_002dstatement"></a>
<p>Warn when a declaration is found after a statement in a block. This
construct, known from C++, was introduced with ISO C99 and is by default
allowed in GCC. It is not supported by ISO C90. See <a href="Mixed-Declarations.html#Mixed-Declarations">Mixed Declarations</a>.
</p>
</dd>
<dt><code>-Wshadow</code></dt>
<dd><a name="index-Wshadow"></a>
<a name="index-Wno_002dshadow"></a>
<p>Warn whenever a local variable or type declaration shadows another
variable, parameter, type, class member (in C++), or instance variable
(in Objective-C) or whenever a built-in function is shadowed. Note
that in C++, the compiler warns if a local variable shadows an
explicit typedef, but not if it shadows a struct/class/enum.
Same as <samp>-Wshadow=global</samp>.
</p>
</dd>
<dt><code>-Wno-shadow-ivar <span class="roman">(Objective-C only)</span></code></dt>
<dd><a name="index-Wno_002dshadow_002divar"></a>
<a name="index-Wshadow_002divar"></a>
<p>Do not warn whenever a local variable shadows an instance variable in an
Objective-C method.
</p>
</dd>
<dt><code>-Wshadow=global</code></dt>
<dd><a name="index-Wshadow_003dlocal"></a>
<p>The default for <samp>-Wshadow</samp>. Warns for any (global) shadowing.
</p>
</dd>
<dt><code>-Wshadow=local</code></dt>
<dd><a name="index-Wshadow_003dlocal-1"></a>
<p>Warn when a local variable shadows another local variable or parameter.
This warning is enabled by <samp>-Wshadow=global</samp>.
</p>
</dd>
<dt><code>-Wshadow=compatible-local</code></dt>
<dd><a name="index-Wshadow_003dcompatible_002dlocal"></a>
<p>Warn when a local variable shadows another local variable or parameter
whose type is compatible with that of the shadowing variable. In C++,
type compatibility here means the type of the shadowing variable can be
converted to that of the shadowed variable. The creation of this flag
(in addition to <samp>-Wshadow=local</samp>) is based on the idea that when
a local variable shadows another one of incompatible type, it is most
likely intentional, not a bug or typo, as shown in the following example:
</p>
<div class="smallexample">
<pre class="smallexample">for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i &lt; N; ++i)
{
...
}
...
}
</pre></div>
<p>Since the two variable <code>i</code> in the example above have incompatible types,
enabling only <samp>-Wshadow=compatible-local</samp> will not emit a warning.
Because their types are incompatible, if a programmer accidentally uses one
in place of the other, type checking will catch that and emit an error or
warning. So not warning (about shadowing) in this case will not lead to
undetected bugs. Use of this flag instead of <samp>-Wshadow=local</samp> can
possibly reduce the number of warnings triggered by intentional shadowing.
</p>
<p>This warning is enabled by <samp>-Wshadow=local</samp>.
</p>
</dd>
<dt><code>-Wlarger-than=<var>len</var></code></dt>
<dd><a name="index-Wlarger_002dthan_003dlen"></a>
<a name="index-Wlarger_002dthan_002dlen"></a>
<p>Warn whenever an object of larger than <var>len</var> bytes is defined.
</p>
</dd>
<dt><code>-Wframe-larger-than=<var>len</var></code></dt>
<dd><a name="index-Wframe_002dlarger_002dthan"></a>
<p>Warn if the size of a function frame is larger than <var>len</var> bytes.
The computation done to determine the stack frame size is approximate
and not conservative.
The actual requirements may be somewhat greater than <var>len</var>
even if you do not get a warning. In addition, any space allocated
via <code>alloca</code>, variable-length arrays, or related constructs
is not included by the compiler when determining
whether or not to issue a warning.
</p>
</dd>
<dt><code>-Wno-free-nonheap-object</code></dt>
<dd><a name="index-Wno_002dfree_002dnonheap_002dobject"></a>
<a name="index-Wfree_002dnonheap_002dobject"></a>
<p>Do not warn when attempting to free an object that was not allocated
on the heap.
</p>
</dd>
<dt><code>-Wstack-usage=<var>len</var></code></dt>
<dd><a name="index-Wstack_002dusage"></a>
<p>Warn if the stack usage of a function might be larger than <var>len</var> bytes.
The computation done to determine the stack usage is conservative.
Any space allocated via <code>alloca</code>, variable-length arrays, or related
constructs is included by the compiler when determining whether or not to
issue a warning.
</p>
<p>The message is in keeping with the output of <samp>-fstack-usage</samp>.
</p>
<ul>
<li> If the stack usage is fully static but exceeds the specified amount, it&rsquo;s:
<div class="smallexample">
<pre class="smallexample"> warning: stack usage is 1120 bytes
</pre></div>
</li><li> If the stack usage is (partly) dynamic but bounded, it&rsquo;s:
<div class="smallexample">
<pre class="smallexample"> warning: stack usage might be 1648 bytes
</pre></div>
</li><li> If the stack usage is (partly) dynamic and not bounded, it&rsquo;s:
<div class="smallexample">
<pre class="smallexample"> warning: stack usage might be unbounded
</pre></div>
</li></ul>
</dd>
<dt><code>-Wno-pedantic-ms-format <span class="roman">(MinGW targets only)</span></code></dt>
<dd><a name="index-Wno_002dpedantic_002dms_002dformat"></a>
<a name="index-Wpedantic_002dms_002dformat"></a>
<p>When used in combination with <samp>-Wformat</samp>
and <samp>-pedantic</samp> without GNU extensions, this option
disables the warnings about non-ISO <code>printf</code> / <code>scanf</code> format
width specifiers <code>I32</code>, <code>I64</code>, and <code>I</code> used on Windows targets,
which depend on the MS runtime.
</p>
</dd>
<dt><code>-Waligned-new</code></dt>
<dd><a name="index-Waligned_002dnew"></a>
<a name="index-Wno_002daligned_002dnew"></a>
<p>Warn about a new-expression of a type that requires greater alignment
than the <code>alignof(std::max_align_t)</code> but uses an allocation
function without an explicit alignment parameter. This option is
enabled by <samp>-Wall</samp>.
</p>
<p>Normally this only warns about global allocation functions, but
<samp>-Waligned-new=all</samp> also warns about class member allocation
functions.
</p>
</dd>
<dt><code>-Wplacement-new</code></dt>
<dt><code>-Wplacement-new=<var>n</var></code></dt>
<dd><a name="index-Wplacement_002dnew"></a>
<a name="index-Wno_002dplacement_002dnew"></a>
<p>Warn about placement new expressions with undefined behavior, such as
constructing an object in a buffer that is smaller than the type of
the object. For example, the placement new expression below is diagnosed
because it attempts to construct an array of 64 integers in a buffer only
64 bytes large.
</p><div class="smallexample">
<pre class="smallexample">char buf [64];
new (buf) int[64];
</pre></div>
<p>This warning is enabled by default.
</p>
<dl compact="compact">
<dt><code>-Wplacement-new=1</code></dt>
<dd><p>This is the default warning level of <samp>-Wplacement-new</samp>. At this
level the warning is not issued for some strictly undefined constructs that
GCC allows as extensions for compatibility with legacy code. For example,
the following <code>new</code> expression is not diagnosed at this level even
though it has undefined behavior according to the C++ standard because
it writes past the end of the one-element array.
</p><div class="smallexample">
<pre class="smallexample">struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s-&gt;a[0]);
new (s-&gt;a)int [32]();
</pre></div>
</dd>
<dt><code>-Wplacement-new=2</code></dt>
<dd><p>At this level, in addition to diagnosing all the same constructs as at level
1, a diagnostic is also issued for placement new expressions that construct
an object in the last member of structure whose type is an array of a single
element and whose size is less than the size of the object being constructed.
While the previous example would be diagnosed, the following construct makes
use of the flexible member array extension to avoid the warning at level 2.
</p><div class="smallexample">
<pre class="smallexample">struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s-&gt;a[0]);
new (s-&gt;a)int [32]();
</pre></div>
</dd>
</dl>
</dd>
<dt><code>-Wpointer-arith</code></dt>
<dd><a name="index-Wpointer_002darith"></a>
<a name="index-Wno_002dpointer_002darith"></a>
<p>Warn about anything that depends on the &ldquo;size of&rdquo; a function type or
of <code>void</code>. GNU C assigns these types a size of 1, for
convenience in calculations with <code>void *</code> pointers and pointers
to functions. In C++, warn also when an arithmetic operation involves
<code>NULL</code>. This warning is also enabled by <samp>-Wpedantic</samp>.
</p>
</dd>
<dt><code>-Wpointer-compare</code></dt>
<dd><a name="index-Wpointer_002dcompare"></a>
<a name="index-Wno_002dpointer_002dcompare"></a>
<p>Warn if a pointer is compared with a zero character constant. This usually
means that the pointer was meant to be dereferenced. For example:
</p>
<div class="smallexample">
<pre class="smallexample">const char *p = foo ();
if (p == '\0')
return 42;
</pre></div>
<p>Note that the code above is invalid in C++11.
</p>
<p>This warning is enabled by default.
</p>
</dd>
<dt><code>-Wtype-limits</code></dt>
<dd><a name="index-Wtype_002dlimits"></a>
<a name="index-Wno_002dtype_002dlimits"></a>
<p>Warn if a comparison is always true or always false due to the limited
range of the data type, but do not warn for constant expressions. For
example, warn if an unsigned variable is compared against zero with
<code>&lt;</code> or <code>&gt;=</code>. This warning is also enabled by
<samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wcomment</code></dt>
<dt><code>-Wcomments</code></dt>
<dd><a name="index-Wcomment"></a>
<a name="index-Wcomments"></a>
<p>Warn whenever a comment-start sequence &lsquo;<samp>/*</samp>&rsquo; appears in a &lsquo;<samp>/*</samp>&rsquo;
comment, or whenever a backslash-newline appears in a &lsquo;<samp>//</samp>&rsquo; comment.
This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wtrigraphs</code></dt>
<dd><a name="index-Wtrigraphs"></a>
<a name="Wtrigraphs"></a><p>Warn if any trigraphs are encountered that might change the meaning of
the program. Trigraphs within comments are not warned about,
except those that would form escaped newlines.
</p>
<p>This option is implied by <samp>-Wall</samp>. If <samp>-Wall</samp> is not
given, this option is still enabled unless trigraphs are enabled. To
get trigraph conversion without warnings, but get the other
<samp>-Wall</samp> warnings, use &lsquo;<samp>-trigraphs -Wall -Wno-trigraphs</samp>&rsquo;.
</p>
</dd>
<dt><code>-Wundef</code></dt>
<dd><a name="index-Wundef"></a>
<a name="index-Wno_002dundef"></a>
<p>Warn if an undefined identifier is evaluated in an <code>#if</code> directive.
Such identifiers are replaced with zero.
</p>
</dd>
<dt><code>-Wexpansion-to-defined</code></dt>
<dd><a name="index-Wexpansion_002dto_002ddefined"></a>
<p>Warn whenever &lsquo;<samp>defined</samp>&rsquo; is encountered in the expansion of a macro
(including the case where the macro is expanded by an &lsquo;<samp>#if</samp>&rsquo; directive).
Such usage is not portable.
This warning is also enabled by <samp>-Wpedantic</samp> and <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wunused-macros</code></dt>
<dd><a name="index-Wunused_002dmacros"></a>
<p>Warn about macros defined in the main file that are unused. A macro
is <em>used</em> if it is expanded or tested for existence at least once.
The preprocessor also warns if the macro has not been used at the
time it is redefined or undefined.
</p>
<p>Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.
</p>
<p><em>Note:</em> If a macro is actually used, but only used in skipped
conditional blocks, then the preprocessor reports it as unused. To avoid the
warning in such a case, you might improve the scope of the macro&rsquo;s
definition by, for example, moving it into the first skipped block.
Alternatively, you could provide a dummy use with something like:
</p>
<div class="smallexample">
<pre class="smallexample">#if defined the_macro_causing_the_warning
#endif
</pre></div>
</dd>
<dt><code>-Wno-endif-labels</code></dt>
<dd><a name="index-Wno_002dendif_002dlabels"></a>
<a name="index-Wendif_002dlabels"></a>
<p>Do not warn whenever an <code>#else</code> or an <code>#endif</code> are followed by text.
This sometimes happens in older programs with code of the form
</p>
<div class="smallexample">
<pre class="smallexample">#if FOO
&hellip;
#else FOO
&hellip;
#endif FOO
</pre></div>
<p>The second and third <code>FOO</code> should be in comments.
This warning is on by default.
</p>
</dd>
<dt><code>-Wbad-function-cast <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wbad_002dfunction_002dcast"></a>
<a name="index-Wno_002dbad_002dfunction_002dcast"></a>
<p>Warn when a function call is cast to a non-matching type.
For example, warn if a call to a function returning an integer type
is cast to a pointer type.
</p>
</dd>
<dt><code>-Wc90-c99-compat <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wc90_002dc99_002dcompat"></a>
<a name="index-Wno_002dc90_002dc99_002dcompat"></a>
<p>Warn about features not present in ISO C90, but present in ISO C99.
For instance, warn about use of variable length arrays, <code>long long</code>
type, <code>bool</code> type, compound literals, designated initializers, and so
on. This option is independent of the standards mode. Warnings are disabled
in the expression that follows <code>__extension__</code>.
</p>
</dd>
<dt><code>-Wc99-c11-compat <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wc99_002dc11_002dcompat"></a>
<a name="index-Wno_002dc99_002dc11_002dcompat"></a>
<p>Warn about features not present in ISO C99, but present in ISO C11.
For instance, warn about use of anonymous structures and unions,
<code>_Atomic</code> type qualifier, <code>_Thread_local</code> storage-class specifier,
<code>_Alignas</code> specifier, <code>Alignof</code> operator, <code>_Generic</code> keyword,
and so on. This option is independent of the standards mode. Warnings are
disabled in the expression that follows <code>__extension__</code>.
</p>
</dd>
<dt><code>-Wc++-compat <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wc_002b_002b_002dcompat"></a>
<p>Warn about ISO C constructs that are outside of the common subset of
ISO C and ISO C++, e.g. request for implicit conversion from
<code>void *</code> to a pointer to non-<code>void</code> type.
</p>
</dd>
<dt><code>-Wc++11-compat <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wc_002b_002b11_002dcompat"></a>
<p>Warn about C++ constructs whose meaning differs between ISO C++ 1998
and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords
in ISO C++ 2011. This warning turns on <samp>-Wnarrowing</samp> and is
enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wc++14-compat <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wc_002b_002b14_002dcompat"></a>
<p>Warn about C++ constructs whose meaning differs between ISO C++ 2011
and ISO C++ 2014. This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wc++17-compat <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wc_002b_002b17_002dcompat"></a>
<p>Warn about C++ constructs whose meaning differs between ISO C++ 2014
and ISO C++ 2017. This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wcast-qual</code></dt>
<dd><a name="index-Wcast_002dqual"></a>
<a name="index-Wno_002dcast_002dqual"></a>
<p>Warn whenever a pointer is cast so as to remove a type qualifier from
the target type. For example, warn if a <code>const char *</code> is cast
to an ordinary <code>char *</code>.
</p>
<p>Also warn when making a cast that introduces a type qualifier in an
unsafe way. For example, casting <code>char **</code> to <code>const char **</code>
is unsafe, as in this example:
</p>
<div class="smallexample">
<pre class="smallexample"> /* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = &quot;string&quot;;
/* Now char** pointer points to read-only memory. */
**p = 'b';
</pre></div>
</dd>
<dt><code>-Wcast-align</code></dt>
<dd><a name="index-Wcast_002dalign"></a>
<a name="index-Wno_002dcast_002dalign"></a>
<p>Warn whenever a pointer is cast such that the required alignment of the
target is increased. For example, warn if a <code>char *</code> is cast to
an <code>int *</code> on machines where integers can only be accessed at
two- or four-byte boundaries.
</p>
</dd>
<dt><code>-Wcast-align=strict</code></dt>
<dd><a name="index-Wcast_002dalign_003dstrict"></a>
<p>Warn whenever a pointer is cast such that the required alignment of the
target is increased. For example, warn if a <code>char *</code> is cast to
an <code>int *</code> regardless of the target machine.
</p>
</dd>
<dt><code>-Wcast-function-type</code></dt>
<dd><a name="index-Wcast_002dfunction_002dtype"></a>
<a name="index-Wno_002dcast_002dfunction_002dtype"></a>
<p>Warn when a function pointer is cast to an incompatible function pointer.
In a cast involving function types with a variable argument list only
the types of initial arguments that are provided are considered.
Any parameter of pointer-type matches any other pointer-type. Any benign
differences in integral types are ignored, like <code>int</code> vs. <code>long</code>
on ILP32 targets. Likewise type qualifiers are ignored. The function
type <code>void (*) (void)</code> is special and matches everything, which can
be used to suppress this warning.
In a cast involving pointer to member types this warning warns whenever
the type cast is changing the pointer to member type.
This warning is enabled by <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wwrite-strings</code></dt>
<dd><a name="index-Wwrite_002dstrings"></a>
<a name="index-Wno_002dwrite_002dstrings"></a>
<p>When compiling C, give string constants the type <code>const
char[<var>length</var>]</code> so that copying the address of one into a
non-<code>const</code> <code>char *</code> pointer produces a warning. These
warnings help you find at compile time code that can try to write
into a string constant, but only if you have been very careful about
using <code>const</code> in declarations and prototypes. Otherwise, it is
just a nuisance. This is why we did not make <samp>-Wall</samp> request
these warnings.
</p>
<p>When compiling C++, warn about the deprecated conversion from string
literals to <code>char *</code>. This warning is enabled by default for C++
programs.
</p>
</dd>
<dt><code>-Wcatch-value</code></dt>
<dt><code>-Wcatch-value=<var>n</var> <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wcatch_002dvalue"></a>
<a name="index-Wno_002dcatch_002dvalue"></a>
<p>Warn about catch handlers that do not catch via reference.
With <samp>-Wcatch-value=1</samp> (or <samp>-Wcatch-value</samp> for short)
warn about polymorphic class types that are caught by value.
With <samp>-Wcatch-value=2</samp> warn about all class types that are caught
by value. With <samp>-Wcatch-value=3</samp> warn about all types that are
not caught by reference. <samp>-Wcatch-value</samp> is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wclobbered</code></dt>
<dd><a name="index-Wclobbered"></a>
<a name="index-Wno_002dclobbered"></a>
<p>Warn for variables that might be changed by <code>longjmp</code> or
<code>vfork</code>. This warning is also enabled by <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wconditionally-supported <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wconditionally_002dsupported"></a>
<a name="index-Wno_002dconditionally_002dsupported"></a>
<p>Warn for conditionally-supported (C++11 [intro.defs]) constructs.
</p>
</dd>
<dt><code>-Wconversion</code></dt>
<dd><a name="index-Wconversion"></a>
<a name="index-Wno_002dconversion"></a>
<p>Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like <code>abs (x)</code> when
<code>x</code> is <code>double</code>; conversions between signed and unsigned,
like <code>unsigned ui = -1</code>; and conversions to smaller types, like
<code>sqrtf (M_PI)</code>. Do not warn for explicit casts like <code>abs
((int) x)</code> and <code>ui = (unsigned) -1</code>, or if the value is not
changed by the conversion like in <code>abs (2.0)</code>. Warnings about
conversions between signed and unsigned integers can be disabled by
using <samp>-Wno-sign-conversion</samp>.
</p>
<p>For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that never use a type conversion
operator: conversions to <code>void</code>, the same type, a base class or a
reference to them. Warnings about conversions between signed and
unsigned integers are disabled by default in C++ unless
<samp>-Wsign-conversion</samp> is explicitly enabled.
</p>
</dd>
<dt><code>-Wno-conversion-null <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wconversion_002dnull"></a>
<a name="index-Wno_002dconversion_002dnull"></a>
<p>Do not warn for conversions between <code>NULL</code> and non-pointer
types. <samp>-Wconversion-null</samp> is enabled by default.
</p>
</dd>
<dt><code>-Wzero-as-null-pointer-constant <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wzero_002das_002dnull_002dpointer_002dconstant"></a>
<a name="index-Wno_002dzero_002das_002dnull_002dpointer_002dconstant"></a>
<p>Warn when a literal &lsquo;<samp>0</samp>&rsquo; is used as null pointer constant. This can
be useful to facilitate the conversion to <code>nullptr</code> in C++11.
</p>
</dd>
<dt><code>-Wsubobject-linkage <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wsubobject_002dlinkage"></a>
<a name="index-Wno_002dsubobject_002dlinkage"></a>
<p>Warn if a class type has a base or a field whose type uses the anonymous
namespace or depends on a type with no linkage. If a type A depends on
a type B with no or internal linkage, defining it in multiple
translation units would be an ODR violation because the meaning of B
is different in each translation unit. If A only appears in a single
translation unit, the best way to silence the warning is to give it
internal linkage by putting it in an anonymous namespace as well. The
compiler doesn&rsquo;t give this warning for types defined in the main .C
file, as those are unlikely to have multiple definitions.
<samp>-Wsubobject-linkage</samp> is enabled by default.
</p>
</dd>
<dt><code>-Wdangling-else</code></dt>
<dd><a name="index-Wdangling_002delse"></a>
<a name="index-Wno_002ddangling_002delse"></a>
<p>Warn about constructions where there may be confusion to which
<code>if</code> statement an <code>else</code> branch belongs. Here is an example of
such a case:
</p>
<div class="smallexample">
<pre class="smallexample">{
if (a)
if (b)
foo ();
else
bar ();
}
</pre></div>
<p>In C/C++, every <code>else</code> branch belongs to the innermost possible
<code>if</code> statement, which in this example is <code>if (b)</code>. This is
often not what the programmer expected, as illustrated in the above
example by indentation the programmer chose. When there is the
potential for this confusion, GCC issues a warning when this flag
is specified. To eliminate the warning, add explicit braces around
the innermost <code>if</code> statement so there is no way the <code>else</code>
can belong to the enclosing <code>if</code>. The resulting code
looks like this:
</p>
<div class="smallexample">
<pre class="smallexample">{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
</pre></div>
<p>This warning is enabled by <samp>-Wparentheses</samp>.
</p>
</dd>
<dt><code>-Wdate-time</code></dt>
<dd><a name="index-Wdate_002dtime"></a>
<a name="index-Wno_002ddate_002dtime"></a>
<p>Warn when macros <code>__TIME__</code>, <code>__DATE__</code> or <code>__TIMESTAMP__</code>
are encountered as they might prevent bit-wise-identical reproducible
compilations.
</p>
</dd>
<dt><code>-Wdelete-incomplete <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wdelete_002dincomplete"></a>
<a name="index-Wno_002ddelete_002dincomplete"></a>
<p>Warn when deleting a pointer to incomplete type, which may cause
undefined behavior at runtime. This warning is enabled by default.
</p>
</dd>
<dt><code>-Wuseless-cast <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wuseless_002dcast"></a>
<a name="index-Wno_002duseless_002dcast"></a>
<p>Warn when an expression is casted to its own type.
</p>
</dd>
<dt><code>-Wempty-body</code></dt>
<dd><a name="index-Wempty_002dbody"></a>
<a name="index-Wno_002dempty_002dbody"></a>
<p>Warn if an empty body occurs in an <code>if</code>, <code>else</code> or <code>do
while</code> statement. This warning is also enabled by <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wenum-compare</code></dt>
<dd><a name="index-Wenum_002dcompare"></a>
<a name="index-Wno_002denum_002dcompare"></a>
<p>Warn about a comparison between values of different enumerated types.
In C++ enumerated type mismatches in conditional expressions are also
diagnosed and the warning is enabled by default. In C this warning is
enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wextra-semi <span class="roman">(C++, Objective-C++ only)</span></code></dt>
<dd><a name="index-Wextra_002dsemi"></a>
<a name="index-Wno_002dextra_002dsemi"></a>
<p>Warn about redundant semicolon after in-class function definition.
</p>
</dd>
<dt><code>-Wjump-misses-init <span class="roman">(C, Objective-C only)</span></code></dt>
<dd><a name="index-Wjump_002dmisses_002dinit"></a>
<a name="index-Wno_002djump_002dmisses_002dinit"></a>
<p>Warn if a <code>goto</code> statement or a <code>switch</code> statement jumps
forward across the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only warns about
variables that are initialized when they are declared. This warning is
only supported for C and Objective-C; in C++ this sort of branch is an
error in any case.
</p>
<p><samp>-Wjump-misses-init</samp> is included in <samp>-Wc++-compat</samp>. It
can be disabled with the <samp>-Wno-jump-misses-init</samp> option.
</p>
</dd>
<dt><code>-Wsign-compare</code></dt>
<dd><a name="index-Wsign_002dcompare"></a>
<a name="index-Wno_002dsign_002dcompare"></a>
<a name="index-warning-for-comparison-of-signed-and-unsigned-values"></a>
<a name="index-comparison-of-signed-and-unsigned-values_002c-warning"></a>
<a name="index-signed-and-unsigned-values_002c-comparison-warning"></a>
<p>Warn when a comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to unsigned.
In C++, this warning is also enabled by <samp>-Wall</samp>. In C, it is
also enabled by <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wsign-conversion</code></dt>
<dd><a name="index-Wsign_002dconversion"></a>
<a name="index-Wno_002dsign_002dconversion"></a>
<p>Warn for implicit conversions that may change the sign of an integer
value, like assigning a signed integer expression to an unsigned
integer variable. An explicit cast silences the warning. In C, this
option is enabled also by <samp>-Wconversion</samp>.
</p>
</dd>
<dt><code>-Wfloat-conversion</code></dt>
<dd><a name="index-Wfloat_002dconversion"></a>
<a name="index-Wno_002dfloat_002dconversion"></a>
<p>Warn for implicit conversions that reduce the precision of a real value.
This includes conversions from real to integer, and from higher precision
real to lower precision real values. This option is also enabled by
<samp>-Wconversion</samp>.
</p>
</dd>
<dt><code>-Wno-scalar-storage-order</code></dt>
<dd><a name="index-_002dWno_002dscalar_002dstorage_002dorder"></a>
<a name="index-_002dWscalar_002dstorage_002dorder"></a>
<p>Do not warn on suspicious constructs involving reverse scalar storage order.
</p>
</dd>
<dt><code>-Wsized-deallocation <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wsized_002ddeallocation"></a>
<a name="index-Wno_002dsized_002ddeallocation"></a>
<p>Warn about a definition of an unsized deallocation function
</p><div class="smallexample">
<pre class="smallexample">void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
</pre></div>
<p>without a definition of the corresponding sized deallocation function
</p><div class="smallexample">
<pre class="smallexample">void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
</pre></div>
<p>or vice versa. Enabled by <samp>-Wextra</samp> along with
<samp>-fsized-deallocation</samp>.
</p>
</dd>
<dt><code>-Wsizeof-pointer-div</code></dt>
<dd><a name="index-Wsizeof_002dpointer_002ddiv"></a>
<a name="index-Wno_002dsizeof_002dpointer_002ddiv"></a>
<p>Warn for suspicious divisions of two sizeof expressions that divide
the pointer size by the element size, which is the usual way to compute
the array size but won&rsquo;t work out correctly with pointers. This warning
warns e.g. about <code>sizeof (ptr) / sizeof (ptr[0])</code> if <code>ptr</code> is
not an array, but a pointer. This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wsizeof-pointer-memaccess</code></dt>
<dd><a name="index-Wsizeof_002dpointer_002dmemaccess"></a>
<a name="index-Wno_002dsizeof_002dpointer_002dmemaccess"></a>
<p>Warn for suspicious length parameters to certain string and memory built-in
functions if the argument uses <code>sizeof</code>. This warning triggers for
example for <code>memset (ptr, 0, sizeof (ptr));</code> if <code>ptr</code> is not
an array, but a pointer, and suggests a possible fix, or about
<code>memcpy (&amp;foo, ptr, sizeof (&amp;foo));</code>. <samp>-Wsizeof-pointer-memaccess</samp>
also warns about calls to bounded string copy functions like <code>strncat</code>
or <code>strncpy</code> that specify as the bound a <code>sizeof</code> expression of
the source array. For example, in the following function the call to
<code>strncat</code> specifies the size of the source string as the bound. That
is almost certainly a mistake and so the call is diagnosed.
</p><div class="smallexample">
<pre class="smallexample">void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, &quot;.text&quot;, sizeof &quot;.text&quot;);
&hellip;
}
</pre></div>
<p>The <samp>-Wsizeof-pointer-memaccess</samp> option is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wsizeof-array-argument</code></dt>
<dd><a name="index-Wsizeof_002darray_002dargument"></a>
<a name="index-Wno_002dsizeof_002darray_002dargument"></a>
<p>Warn when the <code>sizeof</code> operator is applied to a parameter that is
declared as an array in a function definition. This warning is enabled by
default for C and C++ programs.
</p>
</dd>
<dt><code>-Wmemset-elt-size</code></dt>
<dd><a name="index-Wmemset_002delt_002dsize"></a>
<a name="index-Wno_002dmemset_002delt_002dsize"></a>
<p>Warn for suspicious calls to the <code>memset</code> built-in function, if the
first argument references an array, and the third argument is a number
equal to the number of elements, but not equal to the size of the array
in memory. This indicates that the user has omitted a multiplication by
the element size. This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wmemset-transposed-args</code></dt>
<dd><a name="index-Wmemset_002dtransposed_002dargs"></a>
<a name="index-Wno_002dmemset_002dtransposed_002dargs"></a>
<p>Warn for suspicious calls to the <code>memset</code> built-in function, if the
second argument is not zero and the third argument is zero. This warns e.g.&nbsp;about <code>memset (buf, sizeof buf, 0)</code> where most probably
<code>memset (buf, 0, sizeof buf)</code> was meant instead. The diagnostics
is only emitted if the third argument is literal zero. If it is some
expression that is folded to zero, a cast of zero to some type, etc.,
it is far less likely that the user has mistakenly exchanged the arguments
and no warning is emitted. This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Waddress</code></dt>
<dd><a name="index-Waddress"></a>
<a name="index-Wno_002daddress"></a>
<p>Warn about suspicious uses of memory addresses. These include using
the address of a function in a conditional expression, such as
<code>void func(void); if (func)</code>, and comparisons against the memory
address of a string literal, such as <code>if (x == &quot;abc&quot;)</code>. Such
uses typically indicate a programmer error: the address of a function
always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function
call; and comparisons against string literals result in unspecified
behavior and are not portable in C, so they usually indicate that the
programmer intended to use <code>strcmp</code>. This warning is enabled by
<samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wlogical-op</code></dt>
<dd><a name="index-Wlogical_002dop"></a>
<a name="index-Wno_002dlogical_002dop"></a>
<p>Warn about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a
bit-wise operator is likely to be expected. Also warns when
the operands of a logical operator are the same:
</p><div class="smallexample">
<pre class="smallexample">extern int a;
if (a &lt; 0 &amp;&amp; a &lt; 0) { &hellip; }
</pre></div>
</dd>
<dt><code>-Wlogical-not-parentheses</code></dt>
<dd><a name="index-Wlogical_002dnot_002dparentheses"></a>
<a name="index-Wno_002dlogical_002dnot_002dparentheses"></a>
<p>Warn about logical not used on the left hand side operand of a comparison.
This option does not warn if the right operand is considered to be a boolean
expression. Its purpose is to detect suspicious code like the following:
</p><div class="smallexample">
<pre class="smallexample">int a;
&hellip;
if (!a &gt; 1) { &hellip; }
</pre></div>
<p>It is possible to suppress the warning by wrapping the LHS into
parentheses:
</p><div class="smallexample">
<pre class="smallexample">if ((!a) &gt; 1) { &hellip; }
</pre></div>
<p>This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Waggregate-return</code></dt>
<dd><a name="index-Waggregate_002dreturn"></a>
<a name="index-Wno_002daggregate_002dreturn"></a>
<p>Warn if any functions that return structures or unions are defined or
called. (In languages where you can return an array, this also elicits
a warning.)
</p>
</dd>
<dt><code>-Wno-aggressive-loop-optimizations</code></dt>
<dd><a name="index-Wno_002daggressive_002dloop_002doptimizations"></a>
<a name="index-Waggressive_002dloop_002doptimizations"></a>
<p>Warn if in a loop with constant number of iterations the compiler detects
undefined behavior in some statement during one or more of the iterations.
</p>
</dd>
<dt><code>-Wno-attributes</code></dt>
<dd><a name="index-Wno_002dattributes"></a>
<a name="index-Wattributes"></a>
<p>Do not warn if an unexpected <code>__attribute__</code> is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This does not stop errors for incorrect use of supported
attributes.
</p>
</dd>
<dt><code>-Wno-builtin-declaration-mismatch</code></dt>
<dd><a name="index-Wno_002dbuiltin_002ddeclaration_002dmismatch"></a>
<a name="index-Wbuiltin_002ddeclaration_002dmismatch"></a>
<p>Warn if a built-in function is declared with the wrong signature or
as non-function.
This warning is enabled by default.
</p>
</dd>
<dt><code>-Wno-builtin-macro-redefined</code></dt>
<dd><a name="index-Wno_002dbuiltin_002dmacro_002dredefined"></a>
<a name="index-Wbuiltin_002dmacro_002dredefined"></a>
<p>Do not warn if certain built-in macros are redefined. This suppresses
warnings for redefinition of <code>__TIMESTAMP__</code>, <code>__TIME__</code>,
<code>__DATE__</code>, <code>__FILE__</code>, and <code>__BASE_FILE__</code>.
</p>
</dd>
<dt><code>-Wstrict-prototypes <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wstrict_002dprototypes"></a>
<a name="index-Wno_002dstrict_002dprototypes"></a>
<p>Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted without
a warning if preceded by a declaration that specifies the argument
types.)
</p>
</dd>
<dt><code>-Wold-style-declaration <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wold_002dstyle_002ddeclaration"></a>
<a name="index-Wno_002dold_002dstyle_002ddeclaration"></a>
<p>Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
<code>static</code> are not the first things in a declaration. This warning
is also enabled by <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wold-style-definition <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wold_002dstyle_002ddefinition"></a>
<a name="index-Wno_002dold_002dstyle_002ddefinition"></a>
<p>Warn if an old-style function definition is used. A warning is given
even if there is a previous prototype.
</p>
</dd>
<dt><code>-Wmissing-parameter-type <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wmissing_002dparameter_002dtype"></a>
<a name="index-Wno_002dmissing_002dparameter_002dtype"></a>
<p>A function parameter is declared without a type specifier in K&amp;R-style
functions:
</p>
<div class="smallexample">
<pre class="smallexample">void foo(bar) { }
</pre></div>
<p>This warning is also enabled by <samp>-Wextra</samp>.
</p>
</dd>
<dt><code>-Wmissing-prototypes <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wmissing_002dprototypes"></a>
<a name="index-Wno_002dmissing_002dprototypes"></a>
<p>Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. Use this option to detect global functions
that do not have a matching prototype declaration in a header file.
This option is not valid for C++ because all function declarations
provide prototypes and a non-matching declaration declares an
overload rather than conflict with an earlier declaration.
Use <samp>-Wmissing-declarations</samp> to detect missing declarations in C++.
</p>
</dd>
<dt><code>-Wmissing-declarations</code></dt>
<dd><a name="index-Wmissing_002ddeclarations"></a>
<a name="index-Wno_002dmissing_002ddeclarations"></a>
<p>Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype.
Use this option to detect global functions that are not declared in
header files. In C, no warnings are issued for functions with previous
non-prototype declarations; use <samp>-Wmissing-prototypes</samp> to detect
missing prototypes. In C++, no warnings are issued for function templates,
or for inline functions, or for functions in anonymous namespaces.
</p>
</dd>
<dt><code>-Wmissing-field-initializers</code></dt>
<dd><a name="index-Wmissing_002dfield_002dinitializers"></a>
<a name="index-Wno_002dmissing_002dfield_002dinitializers"></a>
<a name="index-W-1"></a>
<a name="index-Wextra-1"></a>
<a name="index-Wno_002dextra-1"></a>
<p>Warn if a structure&rsquo;s initializer has some fields missing. For
example, the following code causes such a warning, because
<code>x.h</code> is implicitly zero:
</p>
<div class="smallexample">
<pre class="smallexample">struct s { int f, g, h; };
struct s x = { 3, 4 };
</pre></div>
<p>This option does not warn about designated initializers, so the following
modification does not trigger a warning:
</p>
<div class="smallexample">
<pre class="smallexample">struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
</pre></div>
<p>In C this option does not warn about the universal zero initializer
&lsquo;<samp>{ 0 }</samp>&rsquo;:
</p>
<div class="smallexample">
<pre class="smallexample">struct s { int f, g, h; };
struct s x = { 0 };
</pre></div>
<p>Likewise, in C++ this option does not warn about the empty { }
initializer, for example:
</p>
<div class="smallexample">
<pre class="smallexample">struct s { int f, g, h; };
s x = { };
</pre></div>
<p>This warning is included in <samp>-Wextra</samp>. To get other <samp>-Wextra</samp>
warnings without this one, use <samp>-Wextra -Wno-missing-field-initializers</samp>.
</p>
</dd>
<dt><code>-Wno-multichar</code></dt>
<dd><a name="index-Wno_002dmultichar"></a>
<a name="index-Wmultichar"></a>
<p>Do not warn if a multicharacter constant (&lsquo;<samp>'FOOF'</samp>&rsquo;) is used.
Usually they indicate a typo in the user&rsquo;s code, as they have
implementation-defined values, and should not be used in portable code.
</p>
</dd>
<dt><code>-Wnormalized=<span class="roman">[</span>none<span class="roman">|</span>id<span class="roman">|</span>nfc<span class="roman">|</span>nfkc<span class="roman">]</span></code></dt>
<dd><a name="index-Wnormalized_003d"></a>
<a name="index-Wnormalized"></a>
<a name="index-Wno_002dnormalized"></a>
<a name="index-NFC"></a>
<a name="index-NFKC"></a>
<a name="index-character-set_002c-input-normalization"></a>
<p>In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when characters
outside the basic ASCII character set are used, you can have two
different character sequences that look the same. To avoid confusion,
the ISO 10646 standard sets out some <em>normalization rules</em> which
when applied ensure that two sequences that look the same are turned into
the same sequence. GCC can warn you if you are using identifiers that
have not been normalized; this option controls that warning.
</p>
<p>There are four levels of warning supported by GCC. The default is
<samp>-Wnormalized=nfc</samp>, which warns about any identifier that is
not in the ISO 10646 &ldquo;C&rdquo; normalized form, <em>NFC</em>. NFC is the
recommended form for most uses. It is equivalent to
<samp>-Wnormalized</samp>.
</p>
<p>Unfortunately, there are some characters allowed in identifiers by
ISO C and ISO C++ that, when turned into NFC, are not allowed in
identifiers. That is, there&rsquo;s no way to use these symbols in portable
ISO C or C++ and have all your identifiers in NFC.
<samp>-Wnormalized=id</samp> suppresses the warning for these characters.
It is hoped that future versions of the standards involved will correct
this, which is why this option is not the default.
</p>
<p>You can switch the warning off for all characters by writing
<samp>-Wnormalized=none</samp> or <samp>-Wno-normalized</samp>. You should
only do this if you are using some other normalization scheme (like
&ldquo;D&rdquo;), because otherwise you can easily create bugs that are
literally impossible to see.
</p>
<p>Some characters in ISO 10646 have distinct meanings but look identical
in some fonts or display methodologies, especially once formatting has
been applied. For instance <code>\u207F</code>, &ldquo;SUPERSCRIPT LATIN SMALL
LETTER N&rdquo;, displays just like a regular <code>n</code> that has been
placed in a superscript. ISO 10646 defines the <em>NFKC</em>
normalization scheme to convert all these into a standard form as
well, and GCC warns if your code is not in NFKC if you use
<samp>-Wnormalized=nfkc</samp>. This warning is comparable to warning
about every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming environment
cannot be fixed to display these characters distinctly.
</p>
</dd>
<dt><code>-Wno-deprecated</code></dt>
<dd><a name="index-Wno_002ddeprecated"></a>
<a name="index-Wdeprecated"></a>
<p>Do not warn about usage of deprecated features. See <a href="Deprecated-Features.html#Deprecated-Features">Deprecated Features</a>.
</p>
</dd>
<dt><code>-Wno-deprecated-declarations</code></dt>
<dd><a name="index-Wno_002ddeprecated_002ddeclarations"></a>
<a name="index-Wdeprecated_002ddeclarations"></a>
<p>Do not warn about uses of functions (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>),
variables (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>), and types (see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>) marked as deprecated by using the <code>deprecated</code>
attribute.
</p>
</dd>
<dt><code>-Wno-overflow</code></dt>
<dd><a name="index-Wno_002doverflow"></a>
<a name="index-Woverflow"></a>
<p>Do not warn about compile-time overflow in constant expressions.
</p>
</dd>
<dt><code>-Wno-odr</code></dt>
<dd><a name="index-Wno_002dodr"></a>
<a name="index-Wodr"></a>
<p>Warn about One Definition Rule violations during link-time optimization.
Requires <samp>-flto-odr-type-merging</samp> to be enabled. Enabled by default.
</p>
</dd>
<dt><code>-Wopenmp-simd</code></dt>
<dd><a name="index-Wopenm_002dsimd"></a>
<p>Warn if the vectorizer cost model overrides the OpenMP
simd directive set by user. The <samp>-fsimd-cost-model=unlimited</samp>
option can be used to relax the cost model.
</p>
</dd>
<dt><code>-Woverride-init <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Woverride_002dinit"></a>
<a name="index-Wno_002doverride_002dinit"></a>
<a name="index-W-2"></a>
<a name="index-Wextra-2"></a>
<a name="index-Wno_002dextra-2"></a>
<p>Warn if an initialized field without side effects is overridden when
using designated initializers (see <a href="Designated-Inits.html#Designated-Inits">Designated
Initializers</a>).
</p>
<p>This warning is included in <samp>-Wextra</samp>. To get other
<samp>-Wextra</samp> warnings without this one, use <samp>-Wextra
-Wno-override-init</samp>.
</p>
</dd>
<dt><code>-Woverride-init-side-effects <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Woverride_002dinit_002dside_002deffects"></a>
<a name="index-Wno_002doverride_002dinit_002dside_002deffects"></a>
<p>Warn if an initialized field with side effects is overridden when
using designated initializers (see <a href="Designated-Inits.html#Designated-Inits">Designated
Initializers</a>). This warning is enabled by default.
</p>
</dd>
<dt><code>-Wpacked</code></dt>
<dd><a name="index-Wpacked"></a>
<a name="index-Wno_002dpacked"></a>
<p>Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable <code>f.x</code> in <code>struct bar</code>
is misaligned even though <code>struct bar</code> does not itself
have the packed attribute:
</p>
<div class="smallexample">
<pre class="smallexample">struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
</pre></div>
</dd>
<dt><code>-Wpacked-bitfield-compat</code></dt>
<dd><a name="index-Wpacked_002dbitfield_002dcompat"></a>
<a name="index-Wno_002dpacked_002dbitfield_002dcompat"></a>
<p>The 4.1, 4.2 and 4.3 series of GCC ignore the <code>packed</code> attribute
on bit-fields of type <code>char</code>. This has been fixed in GCC 4.4 but
the change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4.
For example there is no longer a 4-bit padding between field <code>a</code>
and <code>b</code> in this structure:
</p>
<div class="smallexample">
<pre class="smallexample">struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
</pre></div>
<p>This warning is enabled by default. Use
<samp>-Wno-packed-bitfield-compat</samp> to disable this warning.
</p>
</dd>
<dt><code>-Wpacked-not-aligned <span class="roman">(C, C++, Objective-C and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wpacked_002dnot_002daligned"></a>
<a name="index-Wno_002dpacked_002dnot_002daligned"></a>
<p>Warn if a structure field with explicitly specified alignment in a
packed struct or union is misaligned. For example, a warning will
be issued on <code>struct S</code>, like, <code>warning: alignment 1 of
'struct S' is less than 8</code>, in this code:
</p>
<div class="smallexample">
<pre class="smallexample">struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};
</pre></div>
<p>This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wpadded</code></dt>
<dd><a name="index-Wpadded"></a>
<a name="index-Wno_002dpadded"></a>
<p>Warn if padding is included in a structure, either to align an element
of the structure or to align the whole structure. Sometimes when this
happens it is possible to rearrange the fields of the structure to
reduce the padding and so make the structure smaller.
</p>
</dd>
<dt><code>-Wredundant-decls</code></dt>
<dd><a name="index-Wredundant_002ddecls"></a>
<a name="index-Wno_002dredundant_002ddecls"></a>
<p>Warn if anything is declared more than once in the same scope, even in
cases where multiple declaration is valid and changes nothing.
</p>
</dd>
<dt><code>-Wno-restrict</code></dt>
<dd><a name="index-Wrestrict"></a>
<a name="index-Wno_002drestrict"></a>
<p>Warn when an object referenced by a <code>restrict</code>-qualified parameter
(or, in C++, a <code>__restrict</code>-qualified parameter) is aliased by another
argument, or when copies between such objects overlap. For example,
the call to the <code>strcpy</code> function below attempts to truncate the string
by replacing its initial characters with the last four. However, because
the call writes the terminating NUL into <code>a[4]</code>, the copies overlap and
the call is diagnosed.
</p>
<div class="smallexample">
<pre class="smallexample">void foo (void)
{
char a[] = &quot;abcd1234&quot;;
strcpy (a, a + 4);
&hellip;
}
</pre></div>
<p>The <samp>-Wrestrict</samp> option detects some instances of simple overlap
even without optimization but works best at <samp>-O2</samp> and above. It
is included in <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wnested-externs <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wnested_002dexterns"></a>
<a name="index-Wno_002dnested_002dexterns"></a>
<p>Warn if an <code>extern</code> declaration is encountered within a function.
</p>
</dd>
<dt><code>-Wno-inherited-variadic-ctor</code></dt>
<dd><a name="index-Winherited_002dvariadic_002dctor"></a>
<a name="index-Wno_002dinherited_002dvariadic_002dctor"></a>
<p>Suppress warnings about use of C++11 inheriting constructors when the
base class inherited from has a C variadic constructor; the warning is
on by default because the ellipsis is not inherited.
</p>
</dd>
<dt><code>-Winline</code></dt>
<dd><a name="index-Winline"></a>
<a name="index-Wno_002dinline"></a>
<p>Warn if a function that is declared as inline cannot be inlined.
Even with this option, the compiler does not warn about failures to
inline functions declared in system headers.
</p>
<p>The compiler uses a variety of heuristics to determine whether or not
to inline a function. For example, the compiler takes into account
the size of the function being inlined and the amount of inlining
that has already been done in the current function. Therefore,
seemingly insignificant changes in the source program can cause the
warnings produced by <samp>-Winline</samp> to appear or disappear.
</p>
</dd>
<dt><code>-Wno-invalid-offsetof <span class="roman">(C++ and Objective-C++ only)</span></code></dt>
<dd><a name="index-Wno_002dinvalid_002doffsetof"></a>
<a name="index-Winvalid_002doffsetof"></a>
<p>Suppress warnings from applying the <code>offsetof</code> macro to a non-POD
type. According to the 2014 ISO C++ standard, applying <code>offsetof</code>
to a non-standard-layout type is undefined. In existing C++ implementations,
however, <code>offsetof</code> typically gives meaningful results.
This flag is for users who are aware that they are
writing nonportable code and who have deliberately chosen to ignore the
warning about it.
</p>
<p>The restrictions on <code>offsetof</code> may be relaxed in a future version
of the C++ standard.
</p>
</dd>
<dt><code>-Wint-in-bool-context</code></dt>
<dd><a name="index-Wint_002din_002dbool_002dcontext"></a>
<a name="index-Wno_002dint_002din_002dbool_002dcontext"></a>
<p>Warn for suspicious use of integer values where boolean values are expected,
such as conditional expressions (?:) using non-boolean integer constants in
boolean context, like <code>if (a &lt;= b ? 2 : 3)</code>. Or left shifting of signed
integers in boolean context, like <code>for (a = 0; 1 &lt;&lt; a; a++);</code>. Likewise
for all kinds of multiplications regardless of the data type.
This warning is enabled by <samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wno-int-to-pointer-cast</code></dt>
<dd><a name="index-Wno_002dint_002dto_002dpointer_002dcast"></a>
<a name="index-Wint_002dto_002dpointer_002dcast"></a>
<p>Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size is
an error. <samp>Wint-to-pointer-cast</samp> is enabled by default.
</p>
</dd>
<dt><code>-Wno-pointer-to-int-cast <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wno_002dpointer_002dto_002dint_002dcast"></a>
<a name="index-Wpointer_002dto_002dint_002dcast"></a>
<p>Suppress warnings from casts from a pointer to an integer type of a
different size.
</p>
</dd>
<dt><code>-Winvalid-pch</code></dt>
<dd><a name="index-Winvalid_002dpch"></a>
<a name="index-Wno_002dinvalid_002dpch"></a>
<p>Warn if a precompiled header (see <a href="Precompiled-Headers.html#Precompiled-Headers">Precompiled Headers</a>) is found in
the search path but cannot be used.
</p>
</dd>
<dt><code>-Wlong-long</code></dt>
<dd><a name="index-Wlong_002dlong"></a>
<a name="index-Wno_002dlong_002dlong"></a>
<p>Warn if <code>long long</code> type is used. This is enabled by either
<samp>-Wpedantic</samp> or <samp>-Wtraditional</samp> in ISO C90 and C++98
modes. To inhibit the warning messages, use <samp>-Wno-long-long</samp>.
</p>
</dd>
<dt><code>-Wvariadic-macros</code></dt>
<dd><a name="index-Wvariadic_002dmacros"></a>
<a name="index-Wno_002dvariadic_002dmacros"></a>
<p>Warn if variadic macros are used in ISO C90 mode, or if the GNU
alternate syntax is used in ISO C99 mode. This is enabled by either
<samp>-Wpedantic</samp> or <samp>-Wtraditional</samp>. To inhibit the warning
messages, use <samp>-Wno-variadic-macros</samp>.
</p>
</dd>
<dt><code>-Wvarargs</code></dt>
<dd><a name="index-Wvarargs"></a>
<a name="index-Wno_002dvarargs"></a>
<p>Warn upon questionable usage of the macros used to handle variable
arguments like <code>va_start</code>. This is default. To inhibit the
warning messages, use <samp>-Wno-varargs</samp>.
</p>
</dd>
<dt><code>-Wvector-operation-performance</code></dt>
<dd><a name="index-Wvector_002doperation_002dperformance"></a>
<a name="index-Wno_002dvector_002doperation_002dperformance"></a>
<p>Warn if vector operation is not implemented via SIMD capabilities of the
architecture. Mainly useful for the performance tuning.
Vector operation can be implemented <code>piecewise</code>, which means that the
scalar operation is performed on every vector element;
<code>in parallel</code>, which means that the vector operation is implemented
using scalars of wider type, which normally is more performance efficient;
and <code>as a single scalar</code>, which means that vector fits into a
scalar type.
</p>
</dd>
<dt><code>-Wno-virtual-move-assign</code></dt>
<dd><a name="index-Wvirtual_002dmove_002dassign"></a>
<a name="index-Wno_002dvirtual_002dmove_002dassign"></a>
<p>Suppress warnings about inheriting from a virtual base with a
non-trivial C++11 move assignment operator. This is dangerous because
if the virtual base is reachable along more than one path, it is
moved multiple times, which can mean both objects end up in the
moved-from state. If the move assignment operator is written to avoid
moving from a moved-from object, this warning can be disabled.
</p>
</dd>
<dt><code>-Wvla</code></dt>
<dd><a name="index-Wvla"></a>
<a name="index-Wno_002dvla"></a>
<p>Warn if a variable-length array is used in the code.
<samp>-Wno-vla</samp> prevents the <samp>-Wpedantic</samp> warning of
the variable-length array.
</p>
</dd>
<dt><code>-Wvla-larger-than=<var>n</var></code></dt>
<dd><p>If this option is used, the compiler will warn on uses of
variable-length arrays where the size is either unbounded, or bounded
by an argument that can be larger than <var>n</var> bytes. This is similar
to how <samp>-Walloca-larger-than=<var>n</var></samp> works, but with
variable-length arrays.
</p>
<p>Note that GCC may optimize small variable-length arrays of a known
value into plain arrays, so this warning may not get triggered for
such arrays.
</p>
<p>This warning is not enabled by <samp>-Wall</samp>, and is only active when
<samp>-ftree-vrp</samp> is active (default for <samp>-O2</samp> and above).
</p>
<p>See also <samp>-Walloca-larger-than=<var>n</var></samp>.
</p>
</dd>
<dt><code>-Wvolatile-register-var</code></dt>
<dd><a name="index-Wvolatile_002dregister_002dvar"></a>
<a name="index-Wno_002dvolatile_002dregister_002dvar"></a>
<p>Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate reads
and/or writes to register variables. This warning is enabled by
<samp>-Wall</samp>.
</p>
</dd>
<dt><code>-Wdisabled-optimization</code></dt>
<dd><a name="index-Wdisabled_002doptimization"></a>
<a name="index-Wno_002ddisabled_002doptimization"></a>
<p>Warn if a requested optimization pass is disabled. This warning does
not generally indicate that there is anything wrong with your code; it
merely indicates that GCC&rsquo;s optimizers are unable to handle the code
effectively. Often, the problem is that your code is too big or too
complex; GCC refuses to optimize programs when the optimization
itself is likely to take inordinate amounts of time.
</p>
</dd>
<dt><code>-Wpointer-sign <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wpointer_002dsign"></a>
<a name="index-Wno_002dpointer_002dsign"></a>
<p>Warn for pointer argument passing or assignment with different signedness.
This option is only supported for C and Objective-C. It is implied by
<samp>-Wall</samp> and by <samp>-Wpedantic</samp>, which can be disabled with
<samp>-Wno-pointer-sign</samp>.
</p>
</dd>
<dt><code>-Wstack-protector</code></dt>
<dd><a name="index-Wstack_002dprotector"></a>
<a name="index-Wno_002dstack_002dprotector"></a>
<p>This option is only active when <samp>-fstack-protector</samp> is active. It
warns about functions that are not protected against stack smashing.
</p>
</dd>
<dt><code>-Woverlength-strings</code></dt>
<dd><a name="index-Woverlength_002dstrings"></a>
<a name="index-Wno_002doverlength_002dstrings"></a>
<p>Warn about string constants that are longer than the &ldquo;minimum
maximum&rdquo; length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the
standard&rsquo;s minimum limit, but very portable programs should avoid
using longer strings.
</p>
<p>The limit applies <em>after</em> string constant concatenation, and does
not count the trailing NUL. In C90, the limit was 509 characters; in
C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.
</p>
<p>This option is implied by <samp>-Wpedantic</samp>, and can be disabled with
<samp>-Wno-overlength-strings</samp>.
</p>
</dd>
<dt><code>-Wunsuffixed-float-constants <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><a name="index-Wunsuffixed_002dfloat_002dconstants"></a>
<p>Issue a warning for any floating constant that does not have
a suffix. When used together with <samp>-Wsystem-headers</samp> it
warns about such constants in system header files. This can be useful
when preparing code to use with the <code>FLOAT_CONST_DECIMAL64</code> pragma
from the decimal floating-point extension to C99.
</p>
</dd>
<dt><code>-Wno-designated-init <span class="roman">(C and Objective-C only)</span></code></dt>
<dd><p>Suppress warnings when a positional initializer is used to initialize
a structure that has been marked with the <code>designated_init</code>
attribute.
</p>
</dd>
<dt><code>-Whsa</code></dt>
<dd><p>Issue a warning when HSAIL cannot be emitted for the compiled function or
OpenMP construct.
</p>
</dd>
</dl>
<hr>
<div class="header">
<p>
Next: <a href="Debugging-Options.html#Debugging-Options" accesskey="n" rel="next">Debugging Options</a>, Previous: <a href="Diagnostic-Message-Formatting-Options.html#Diagnostic-Message-Formatting-Options" accesskey="p" rel="prev">Diagnostic Message Formatting Options</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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