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(gcc-295.info)Return Address

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Getting the Return or Frame Address of a Function
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   These functions may be used to get information about the callers of a
function.

`__builtin_return_address (LEVEL)'
     This function returns the return address of the current function,
     or of one of its callers.  The LEVEL argument is number of frames
     to scan up the call stack.  A value of `0' yields the return
     address of the current function, a value of `1' yields the return
     address of the caller of the current function, and so forth.

     The LEVEL argument must be a constant integer.

     On some machines it may be impossible to determine the return
     address of any function other than the current one; in such cases,
     or when the top of the stack has been reached, this function will
     return `0'.

     This function should only be used with a non-zero argument for
     debugging purposes.

`__builtin_frame_address (LEVEL)'
     This function is similar to `__builtin_return_address', but it
     returns the address of the function frame rather than the return
     address of the function.  Calling `__builtin_frame_address' with a
     value of `0' yields the frame address of the current function, a
     value of `1' yields the frame address of the caller of the current
     function, and so forth.

     The frame is the area on the stack which holds local variables and
     saved registers.  The frame address is normally the address of the
     first word pushed on to the stack by the function.  However, the
     exact definition depends upon the processor and the calling
     convention.  If the processor has a dedicated frame pointer
     register, and the function has a frame, then
     `__builtin_frame_address' will return the value of the frame
     pointer register.

     The caveats that apply to `__builtin_return_address' apply to this
     function as well.