### 5.42 Getting the Return or Frame Address of a Function

These functions may be used to get information about the callers of a function.

— Built-in Function: void * __builtin_return_address (unsigned int 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. When inlining the expected behavior is that the function will return the address of the function that will be returned to. To work around this behavior use the `noinline` function attribute.

> 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` or a random value. In addition, `__builtin_frame_address` may be used to determine if the top of the stack has been reached.

> This function should only be used with a nonzero argument for debugging purposes.

— Built-in Function: void * __builtin_frame_address (unsigned int 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.

> On some machines it may be impossible to determine the frame 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` if the first frame pointer is properly initialized by the startup code.

> This function should only be used with a nonzero argument for debugging purposes.