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<a name="Return-Address"></a>
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<div class="header">
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<p>
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Next: <a href="Vector-Extensions.html#Vector-Extensions" accesskey="n" rel="next">Vector Extensions</a>, Previous: <a href="Function-Names.html#Function-Names" accesskey="p" rel="prev">Function Names</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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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<hr>
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<a name="Getting-the-Return-or-Frame-Address-of-a-Function"></a>
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<h3 class="section">6.48 Getting the Return or Frame Address of a Function</h3>
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<p>These functions may be used to get information about the callers of a
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function.
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</p>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005freturn_005faddress"></a>Built-in Function: <em>void *</em> <strong>__builtin_return_address</strong> <em>(unsigned int <var>level</var>)</em></dt>
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<dd><p>This function returns the return address of the current function, or of
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one of its callers. The <var>level</var> argument is number of frames to
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scan up the call stack. A value of <code>0</code> yields the return address
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of the current function, a value of <code>1</code> yields the return address
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of the caller of the current function, and so forth. When inlining
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the expected behavior is that the function returns the address of
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the function that is returned to. To work around this behavior use
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the <code>noinline</code> function attribute.
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</p>
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<p>The <var>level</var> argument must be a constant integer.
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</p>
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<p>On some machines it may be impossible to determine the return address of
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any function other than the current one; in such cases, or when the top
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of the stack has been reached, this function returns <code>0</code> or a
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random value. In addition, <code>__builtin_frame_address</code> may be used
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to determine if the top of the stack has been reached.
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</p>
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<p>Additional post-processing of the returned value may be needed, see
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<code>__builtin_extract_return_addr</code>.
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</p>
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<p>Calling this function with a nonzero argument can have unpredictable
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effects, including crashing the calling program. As a result, calls
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that are considered unsafe are diagnosed when the <samp>-Wframe-address</samp>
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option is in effect. Such calls should only be made in debugging
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situations.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fextract_005freturn_005faddr"></a>Built-in Function: <em>void *</em> <strong>__builtin_extract_return_addr</strong> <em>(void *<var>addr</var>)</em></dt>
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<dd><p>The address as returned by <code>__builtin_return_address</code> may have to be fed
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through this function to get the actual encoded address. For example, on the
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31-bit S/390 platform the highest bit has to be masked out, or on SPARC
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platforms an offset has to be added for the true next instruction to be
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executed.
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</p>
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<p>If no fixup is needed, this function simply passes through <var>addr</var>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005ffrob_005freturn_005faddress"></a>Built-in Function: <em>void *</em> <strong>__builtin_frob_return_address</strong> <em>(void *<var>addr</var>)</em></dt>
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<dd><p>This function does the reverse of <code>__builtin_extract_return_addr</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fframe_005faddress"></a>Built-in Function: <em>void *</em> <strong>__builtin_frame_address</strong> <em>(unsigned int <var>level</var>)</em></dt>
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<dd><p>This function is similar to <code>__builtin_return_address</code>, but it
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returns the address of the function frame rather than the return address
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of the function. Calling <code>__builtin_frame_address</code> with a value of
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<code>0</code> yields the frame address of the current function, a value of
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<code>1</code> yields the frame address of the caller of the current function,
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and so forth.
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</p>
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<p>The frame is the area on the stack that holds local variables and saved
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registers. The frame address is normally the address of the first word
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pushed on to the stack by the function. However, the exact definition
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depends upon the processor and the calling convention. If the processor
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has a dedicated frame pointer register, and the function has a frame,
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then <code>__builtin_frame_address</code> returns the value of the frame
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pointer register.
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</p>
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<p>On some machines it may be impossible to determine the frame address of
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any function other than the current one; in such cases, or when the top
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of the stack has been reached, this function returns <code>0</code> if
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the first frame pointer is properly initialized by the startup code.
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</p>
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<p>Calling this function with a nonzero argument can have unpredictable
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effects, including crashing the calling program. As a result, calls
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that are considered unsafe are diagnosed when the <samp>-Wframe-address</samp>
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option is in effect. Such calls should only be made in debugging
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situations.
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</p></dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Vector-Extensions.html#Vector-Extensions" accesskey="n" rel="next">Vector Extensions</a>, Previous: <a href="Function-Names.html#Function-Names" accesskey="p" rel="prev">Function Names</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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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