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 <a name="Stack-Checking"></a>
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 <p>
 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> &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="Specifying-How-Stack-Checking-is-Done"></a>
 <h4 class="subsection">17.10.3 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
 &ldquo;probe&rdquo; 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 &ldquo;guard pages&rdquo; 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 75 words
 with the <code>setjmp</code>/<code>longjmp</code>-based exception handling mechanism and
 8192 bytes with other exception handling mechanisms should be adequate for
 most machines.
 </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>

 <hr>
 <div class="header">
 <p>
 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> &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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	<a name="Stack-Checking"></a>
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	<p>
	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>
	</div>
	<hr>
	<a name="Specifying-How-Stack-Checking-is-Done"></a>
	<h4 class="subsection">17.10.3 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 75 words
	with the <code>setjmp</code>/<code>longjmp</code>-based exception handling mechanism and
	8192 bytes with other exception handling mechanisms should be adequate for
	most machines.
	</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>

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	<p>
	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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