Preprocessing #if

[AnishaKaul]

#include <stdio.h>

struct g {} f;

int main ()
{
#if '\0'
  printf ("\nTomato\n");
#elif 10
  printf ("\nPotato %u\n", f);
#elif f
  printf ("\nLadyfinger\n");
#endif
}

The output of the above code is:

anisha@linux-uitj:~/junk/preprocessor> gcc hello.c
anisha@linux-uitj:~/junk/preprocessor> ./a.out

Potato 562924280

This proves that f has a non zero value, so I modified the above code as follows:

#include <stdio.h>

struct g {} f;
int main ()
{
#if 0
  printf ("\nTomato\n");
#elif 0
  printf ("\nPotato %u\n", f);
#elif f
 printf ("\nLadyfinger\n");
#endif
}

Now the first two if / else conditions fail since they are false but shouldn't the third get executed since f has the non-zero value ???

But the output is nil:

anisha@linux-uitj:~/junk/preprocessor> gcc hello.c
anisha@linux-uitj:~/junk/preprocessor> ./a.out
anisha@linux-uitj:~/junk/preprocessor>

Why it is so ?

[msh]

Because f is neither a macro, an integer constant or a character constant.

GCC #if and #elif documentation

[AnishaKaul]

Because f is neither a macro, an integer constant or a character constant.

GCC #if and #elif documentation

Thank you for the wonderful link. It has cleared my doubt