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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Variables (Debugging with GDB)</title>
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<meta name="description" content="Variables (Debugging with GDB)">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Data.html#Data" rel="up" title="Data">
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<link href="Arrays.html#Arrays" rel="next" title="Arrays">
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<link href="Ambiguous-Expressions.html#Ambiguous-Expressions" rel="prev" title="Ambiguous Expressions">
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</head>
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<body lang="en">
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<a name="Variables"></a>
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<div class="header">
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<p>
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Next: <a href="Arrays.html#Arrays" accesskey="n" rel="next">Arrays</a>, Previous: <a href="Ambiguous-Expressions.html#Ambiguous-Expressions" accesskey="p" rel="prev">Ambiguous Expressions</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Program-Variables"></a>
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<h3 class="section">10.3 Program Variables</h3>
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<p>The most common kind of expression to use is the name of a variable
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in your program.
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</p>
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<p>Variables in expressions are understood in the selected stack frame
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(see <a href="Selection.html#Selection">Selecting a Frame</a>); they must be either:
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</p>
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<ul>
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<li> global (or file-static)
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</li></ul>
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<p>or
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</p>
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<ul>
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<li> visible according to the scope rules of the
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programming language from the point of execution in that frame
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</li></ul>
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<p>This means that in the function
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</p>
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<div class="smallexample">
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<pre class="smallexample">foo (a)
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int a;
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{
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bar (a);
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{
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int b = test ();
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bar (b);
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}
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}
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</pre></div>
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<p>you can examine and use the variable <code>a</code> whenever your program is
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executing within the function <code>foo</code>, but you can only use or
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examine the variable <code>b</code> while your program is executing inside
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the block where <code>b</code> is declared.
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</p>
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<a name="index-variable-name-conflict"></a>
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<p>There is an exception: you can refer to a variable or function whose
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scope is a single source file even if the current execution point is not
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in this file. But it is possible to have more than one such variable or
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function with the same name (in different source files). If that
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happens, referring to that name has unpredictable effects. If you wish,
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you can specify a static variable in a particular function or file by
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using the colon-colon (<code>::</code>) notation:
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</p>
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<a name="index-colon_002dcolon_002c-context-for-variables_002ffunctions"></a>
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<a name="index-_003a_003a_002c-context-for-variables_002ffunctions"></a>
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<div class="smallexample">
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<pre class="smallexample"><var>file</var>::<var>variable</var>
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<var>function</var>::<var>variable</var>
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</pre></div>
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<p>Here <var>file</var> or <var>function</var> is the name of the context for the
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static <var>variable</var>. In the case of file names, you can use quotes to
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make sure <small>GDB</small> parses the file name as a single word—for example,
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to print a global value of <code>x</code> defined in <samp>f2.c</samp>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) p 'f2.c'::x
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</pre></div>
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<p>The <code>::</code> notation is normally used for referring to
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static variables, since you typically disambiguate uses of local variables
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in functions by selecting the appropriate frame and using the
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simple name of the variable. However, you may also use this notation
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to refer to local variables in frames enclosing the selected frame:
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</p>
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<div class="smallexample">
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<pre class="smallexample">void
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foo (int a)
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{
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if (a < 10)
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bar (a);
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else
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process (a); /* Stop here */
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}
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int
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bar (int a)
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{
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foo (a + 5);
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}
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</pre></div>
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<p>For example, if there is a breakpoint at the commented line,
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here is what you might see
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when the program stops after executing the call <code>bar(0)</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) p a
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$1 = 10
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(gdb) p bar::a
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$2 = 5
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(gdb) up 2
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#2 0x080483d0 in foo (a=5) at foobar.c:12
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(gdb) p a
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$3 = 5
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(gdb) p bar::a
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$4 = 0
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</pre></div>
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<a name="index-C_002b_002b-scope-resolution"></a>
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<p>These uses of ‘<samp>::</samp>’ are very rarely in conflict with the very
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similar use of the same notation in C<tt>++</tt>. When they are in
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conflict, the C<tt>++</tt> meaning takes precedence; however, this can be
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overridden by quoting the file or function name with single quotes.
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</p>
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<p>For example, suppose the program is stopped in a method of a class
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that has a field named <code>includefile</code>, and there is also an
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include file named <samp>includefile</samp> that defines a variable,
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<code>some_global</code>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) p includefile
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$1 = 23
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(gdb) p includefile::some_global
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A syntax error in expression, near `'.
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(gdb) p 'includefile'::some_global
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$2 = 27
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</pre></div>
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<a name="index-wrong-values"></a>
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<a name="index-variable-values_002c-wrong"></a>
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<a name="index-function-entry_002fexit_002c-wrong-values-of-variables"></a>
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<a name="index-optimized-code_002c-wrong-values-of-variables"></a>
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<blockquote>
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<p><em>Warning:</em> Occasionally, a local variable may appear to have the
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wrong value at certain points in a function—just after entry to a new
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scope, and just before exit.
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</p></blockquote>
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<p>You may see this problem when you are stepping by machine instructions.
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This is because, on most machines, it takes more than one instruction to
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set up a stack frame (including local variable definitions); if you are
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stepping by machine instructions, variables may appear to have the wrong
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values until the stack frame is completely built. On exit, it usually
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also takes more than one machine instruction to destroy a stack frame;
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after you begin stepping through that group of instructions, local
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variable definitions may be gone.
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</p>
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<p>This may also happen when the compiler does significant optimizations.
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To be sure of always seeing accurate values, turn off all optimization
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when compiling.
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</p>
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<a name="index-_0060_0060No-symbol-_0022foo_0022-in-current-context_0027_0027"></a>
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<p>Another possible effect of compiler optimizations is to optimize
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unused variables out of existence, or assign variables to registers (as
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opposed to memory addresses). Depending on the support for such cases
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offered by the debug info format used by the compiler, <small>GDB</small>
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might not be able to display values for such local variables. If that
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happens, <small>GDB</small> will print a message like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">No symbol "foo" in current context.
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</pre></div>
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<p>To solve such problems, either recompile without optimizations, or use a
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different debug info format, if the compiler supports several such
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formats. See <a href="Compilation.html#Compilation">Compilation</a>, for more information on choosing compiler
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options. See <a href="C.html#C">C and C<tt>++</tt></a>, for more information about debug
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info formats that are best suited to C<tt>++</tt> programs.
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</p>
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<p>If you ask to print an object whose contents are unknown to
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<small>GDB</small>, e.g., because its data type is not completely specified
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by the debug information, <small>GDB</small> will say ‘<samp><incomplete
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type></samp>’. See <a href="Symbols.html#Symbols">incomplete type</a>, for more about this.
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</p>
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<a name="index-no-debug-info-variables"></a>
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<p>If you try to examine or use the value of a (global) variable for
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which <small>GDB</small> has no type information, e.g., because the program
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includes no debug information, <small>GDB</small> displays an error message.
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See <a href="Symbols.html#Symbols">unknown type</a>, for more about unknown types. If you
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cast the variable to its declared type, <small>GDB</small> gets the
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variable’s value using the cast-to type as the variable’s type. For
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example, in a C program:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> (gdb) p var
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'var' has unknown type; cast it to its declared type
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(gdb) p (float) var
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$1 = 3.14
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</pre></div>
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<p>If you append <kbd>@entry</kbd> string to a function parameter name you get its
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value at the time the function got called. If the value is not available an
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error message is printed. Entry values are available only with some compilers.
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Entry values are normally also printed at the function parameter list according
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to <a href="Print-Settings.html#set-print-entry_002dvalues">set print entry-values</a>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">Breakpoint 1, d (i=30) at gdb.base/entry-value.c:29
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29 i++;
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(gdb) next
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30 e (i);
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(gdb) print i
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$1 = 31
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(gdb) print i@entry
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$2 = 30
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</pre></div>
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<p>Strings are identified as arrays of <code>char</code> values without specified
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signedness. Arrays of either <code>signed char</code> or <code>unsigned char</code> get
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printed as arrays of 1 byte sized integers. <code>-fsigned-char</code> or
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<code>-funsigned-char</code> <small>GCC</small> options have no effect as <small>GDB</small>
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defines literal string type <code>"char"</code> as <code>char</code> without a sign.
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For program code
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</p>
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<div class="smallexample">
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<pre class="smallexample">char var0[] = "A";
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signed char var1[] = "A";
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</pre></div>
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<p>You get during debugging
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</p><div class="smallexample">
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<pre class="smallexample">(gdb) print var0
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$1 = "A"
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(gdb) print var1
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$2 = {65 'A', 0 '\0'}
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Arrays.html#Arrays" accesskey="n" rel="next">Arrays</a>, Previous: <a href="Ambiguous-Expressions.html#Ambiguous-Expressions" accesskey="p" rel="prev">Ambiguous Expressions</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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