Using and returning output in C macro

GCC has a feature called statement expressions

So if define macro like

#define FOO(A) ({int retval; retval = do_something(A); retval;})

then you will be able to use it like

foo = FOO(bar);

This is relatively complicated code, there is not much reason to have it in a macro. Make it inline (C99) or static (C89) or both if you really want to place it in a header file. With any reasonable compiler this then should result in the same efficiency as a macro.