__builtin_return_address returns null for index >0?

Question

I want to get the return address of the caller function. I\'m using __builtin_return_address() funtion, but if I give index value greater than 0 it is returning

Answer 1

In addition to compiler optimisation reasons (which IMO is the most likely reason for the issue you're facing), the GCC documentation states quite plainly:

Calling this function with a nonzero argument can have unpredictable effects, including crashing the calling program. As a result, calls that are considered unsafe are diagnosed when the -Wframe-address option is in effect. Such calls should only be made in debugging situations.

As Basile said, since it's a compiler builtin (read: very processor specific and a bad idea to use) the behaviour is exceptionally loosely defined (as it is not required by any standards and does not have to make any guarantees).

Just use backtrace(3), it's POSIX-compliant and doesn't rely on compiler builtins.

Answer 2

See this answer to a related question.

__builtin_return_address is GCC and processor specific (also available in some versions of Clang on some processors with some -lack of- optimizations), and documented as

On some machines it may be impossible to determine the return address of any function other than the current one

The compiler might optimize a function (e.g. when it is compiled with -fomit-frame-pointer, or for tail-calls, or by function inlining) without the relevant information.

So probably you are getting NULL because the information is not available!