Side effects in generic expressions

Question

I'm doing some experiments with the new _Generic keyword and stumbled upon a special case regarding multiple evaluations. See the following:

#include <stdio.h>  #define write_char(c) _Generic(c, char: putchar, const char: putchar)(c)  int main(void) {     const char *s = "foo";      write_char(*s++);     write_char(*s++);     write_char(*s++);      putchar('\n'); } 

This compiles fine and produces the expected result with GCC:

$ gcc -std=c11 -Wall plusplus.c -o plusplus $ ./plusplus  foo 

On the other hand, Clang outputs a big honking warning:

$ clang -std=c11 plusplus.c -o plusplus plusplus.c:9:18: warning: multiple unsequenced modifications to 's'       [-Wunsequenced]     write_char(*s++);                  ^~ plusplus.c:3:32: note: expanded from macro 'write_char' #define write_char(c) _Generic(c, char: putchar, const char: putchar)(c)  ... 

Yet the result is as expected:

$ ./plusplus foo 

I checked the draft of the standard, which says (at p. 97 of the PDF):

The controlling expression of a generic selection is not evaluated.

This seems to precisely address the problem of side-effects in macros (e.g., MIN and MAX).

Now, can I safely ignore Clang's warning, or am I wrong?

Answer 1

As I mentioned in comments, you posted the question about two weeks after the bug was fixed in Clangs trunk. See revision rL223266 (December 3 2014). The fix is included in Clang 3.6.

Now, can I safely ignore Clang's warning, or am I wrong?

We already know that you're right, so here is a way to ignore pragmas in Clang for the future:

#pragma clang diagnostic push #pragma clang diagnostic ignored "-Wunsequenced"  write_char(*s++);  #pragma clang diagnostic pop 

To do not repeat it on every use of the macro, you could put _Pragma in its body:

#define write_char(c) \     _Pragma("clang diagnostic push") \     _Pragma("clang diagnostic ignored \"-Wunsequenced\"") \     _Generic(c, char: putchar, const char: putchar)(c) \     _Pragma("clang diagnostic pop")