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<a name="Stack-Checking"></a>
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<a name="Specifying-How-Stack-Checking-is-Done"></a>
<h4 class="subsection">18.9.3 Specifying How Stack Checking is Done</h4>
<p>GCC will check that stack references are within the boundaries of the
stack, if the option <samp>-fstack-check</samp> is specified, in one of
three ways:
</p>
<ol>
<li> If the value of the <code>STACK_CHECK_BUILTIN</code> macro is nonzero, GCC
will assume that you have arranged for full stack checking to be done
at appropriate places in the configuration files. GCC will not do
other special processing.
</li><li> If <code>STACK_CHECK_BUILTIN</code> is zero and the value of the
<code>STACK_CHECK_STATIC_BUILTIN</code> macro is nonzero, GCC will assume
that you have arranged for static stack checking (checking of the
static stack frame of functions) to be done at appropriate places
in the configuration files. GCC will only emit code to do dynamic
stack checking (checking on dynamic stack allocations) using the third
approach below.
</li><li> If neither of the above are true, GCC will generate code to periodically
“probe” the stack pointer using the values of the macros defined below.
</li></ol>
<p>If neither STACK_CHECK_BUILTIN nor STACK_CHECK_STATIC_BUILTIN is defined,
GCC will change its allocation strategy for large objects if the option
<samp>-fstack-check</samp> is specified: they will always be allocated
dynamically if their size exceeds <code>STACK_CHECK_MAX_VAR_SIZE</code> bytes.
</p>
<dl>
<dt><a name="index-STACK_005fCHECK_005fBUILTIN"></a>Macro: <strong>STACK_CHECK_BUILTIN</strong></dt>
<dd><p>A nonzero value if stack checking is done by the configuration files in a
machine-dependent manner. You should define this macro if stack checking
is required by the ABI of your machine or if you would like to do stack
checking in some more efficient way than the generic approach. The default
value of this macro is zero.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fCHECK_005fSTATIC_005fBUILTIN"></a>Macro: <strong>STACK_CHECK_STATIC_BUILTIN</strong></dt>
<dd><p>A nonzero value if static stack checking is done by the configuration files
in a machine-dependent manner. You should define this macro if you would
like to do static stack checking in some more efficient way than the generic
approach. The default value of this macro is zero.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fCHECK_005fPROBE_005fINTERVAL_005fEXP"></a>Macro: <strong>STACK_CHECK_PROBE_INTERVAL_EXP</strong></dt>
<dd><p>An integer specifying the interval at which GCC must generate stack probe
instructions, defined as 2 raised to this integer. You will normally
define this macro so that the interval be no larger than the size of
the “guard pages” at the end of a stack area. The default value
of 12 (4096-byte interval) is suitable for most systems.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fCHECK_005fMOVING_005fSP"></a>Macro: <strong>STACK_CHECK_MOVING_SP</strong></dt>
<dd><p>An integer which is nonzero if GCC should move the stack pointer page by page
when doing probes. This can be necessary on systems where the stack pointer
contains the bottom address of the memory area accessible to the executing
thread at any point in time. In this situation an alternate signal stack
is required in order to be able to recover from a stack overflow. The
default value of this macro is zero.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fCHECK_005fPROTECT"></a>Macro: <strong>STACK_CHECK_PROTECT</strong></dt>
<dd><p>The number of bytes of stack needed to recover from a stack overflow, for
languages where such a recovery is supported. The default value of 4KB/8KB
with the <code>setjmp</code>/<code>longjmp</code>-based exception handling mechanism and
8KB/12KB with other exception handling mechanisms should be adequate for most
architectures and operating systems.
</p></dd></dl>
<p>The following macros are relevant only if neither STACK_CHECK_BUILTIN
nor STACK_CHECK_STATIC_BUILTIN is defined; you can omit them altogether
in the opposite case.
</p>
<dl>
<dt><a name="index-STACK_005fCHECK_005fMAX_005fFRAME_005fSIZE"></a>Macro: <strong>STACK_CHECK_MAX_FRAME_SIZE</strong></dt>
<dd><p>The maximum size of a stack frame, in bytes. GCC will generate probe
instructions in non-leaf functions to ensure at least this many bytes of
stack are available. If a stack frame is larger than this size, stack
checking will not be reliable and GCC will issue a warning. The
default is chosen so that GCC only generates one instruction on most
systems. You should normally not change the default value of this macro.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fCHECK_005fFIXED_005fFRAME_005fSIZE"></a>Macro: <strong>STACK_CHECK_FIXED_FRAME_SIZE</strong></dt>
<dd><p>GCC uses this value to generate the above warning message. It
represents the amount of fixed frame used by a function, not including
space for any callee-saved registers, temporaries and user variables.
You need only specify an upper bound for this amount and will normally
use the default of four words.
</p></dd></dl>
<dl>
<dt><a name="index-STACK_005fCHECK_005fMAX_005fVAR_005fSIZE"></a>Macro: <strong>STACK_CHECK_MAX_VAR_SIZE</strong></dt>
<dd><p>The maximum size, in bytes, of an object that GCC will place in the
fixed area of the stack frame when the user specifies
<samp>-fstack-check</samp>.
GCC computed the default from the values of the above macros and you will
normally not need to override that default.
</p></dd></dl>
<dl>
<dt><a name="index-TARGET_005fSTACK_005fCLASH_005fPROTECTION_005fFINAL_005fDYNAMIC_005fPROBE"></a>Target Hook: <em>bool</em> <strong>TARGET_STACK_CLASH_PROTECTION_FINAL_DYNAMIC_PROBE</strong> <em>(rtx <var>residual</var>)</em></dt>
<dd><p>Some targets make optimistic assumptions about the state of stack probing when they emit their prologues. On such targets a probe into the end of any dynamically allocated space is likely required for safety against stack clash style attacks. Define this variable to return nonzero if such a probe is required or zero otherwise. You need not define this macro if it would always have the value zero.
</p></dd></dl>
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Next: <a href="Frame-Registers.html#Frame-Registers" accesskey="n" rel="next">Frame Registers</a>, Previous: <a href="Exception-Handling.html#Exception-Handling" accesskey="p" rel="prev">Exception Handling</a>, Up: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="u" rel="up">Stack and Calling</a> [<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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