when use macro in c compound statement

Question

I write an example to show my problem, here it is to use nested macros to make calls to check the number is positive and odd. I know this is redundant to use this call, but as I said previously, it just show that if using macro in nested way, there is some problem:

#include <stdio.h>
#include <stdlib.h>

#define num_is_positive_odd(num)                           \
({                                                         \
    int __rc = 0;                                          \
    int __num = (num);                                     \
                                                           \
    printf("%s:%d: check linenum\n", __func__, __LINE__);  \
    if (num_is_positive(__num) && num_is_odd(__num))       \
        __rc = 1;                                          \
    __rc;                                                  \
 })

#define num_is_positive(num)                  \
({                                            \
    int __rc = 0;                             \
    int __num = (num);                        \
                                              \
    if (__num > 0) {                          \
        printf("%s: number %d is positive\n", \
               __func__, __num);              \
        __rc = 1;                             \
    }                                         \
    __rc;                                     \
 })

#define num_is_odd(num)                       \
({                                            \
    int __rc = 0;                             \
    int __num = (num);                        \
                                              \
    if (__num / 2) {                          \
        printf("%s: number %d is odd\n",      \
               __func__, __num);              \
        __rc = 1;                             \
    }                                         \
    __rc;                                     \
 })



int main()
{
    int num = 4;

    if (num_is_positive_odd(num++))
        printf("%s: number %d is positive odd\n", __func__, num);

    exit(0);
}

When compile it using command: gcc -Wunused-variable chk_comps.c

it shows error:

chk_comps.c: In function ‘main’:
chk_comps.c:7:9: warning: unused variable ‘__num’ [-Wunused-variable]
     int __num = (num);                                     \
         ^
chk_comps.c:47:9: note: in expansion of macro ‘num_is_positive_odd’
     if (num_is_positive_odd(num++))
         ^

`

Could somebody help explain why and how to fix it? thanks.

Answer 1

This is using a GCC extension called statement expressions — so the rules are specific to GCC (and probably Clang emulating GCC).

If you run gcc -E, you can see that the raw output for main() (with the void added by me) is:

# 41 "gccm43.c"
int main(void)
{
    int num = 4;

    if (({ int __rc = 0; int __num = (num++); printf("%s:%d: check linenum\n", __func__, 45); if (({ int __rc = 0; int __num = (__num); if (__num > 0) { printf("%s: number %d is positive\n", __func__, __num); __rc = 1; } __rc; }) && ({ int __rc = 0; int __num = (__num); if (__num / 2) { printf("%s: number %d is odd\n", __func__, __num); __rc = 1; } __rc; })) __rc = 1; __rc; }))
        printf("%s: number %d is positive odd\n", __func__, num);
    return 0;
}

which, when manually formatted (a variation on the theme of 'purgatory'), might look like:

# 41 "gccm43.c"
int main(void)
{
    int num = 4;

    if (({ int __rc = 0;
           int __num = (num++);
           printf("%s:%d: check linenum\n", __func__, 45);
           if (({ int __rc = 0;
                  int __num = (__num);
                  if (__num > 0)
                  {
                      printf("%s: number %d is positive\n", __func__, __num);
                      __rc = 1;
                  }
                  __rc;
                }) &&
                ({ int __rc = 0; 
                   int __num = (__num);
                   if (__num / 2)
                   {
                       printf("%s: number %d is odd\n", __func__, __num);
                       __rc = 1;
                   }
                   __rc;
                 }
               ))
           __rc = 1;
           __rc;
         }
       ))
        printf("%s: number %d is positive odd\n", __func__, num);
    return 0;
}

The lines int __num = (__num); are problematic; you are initializing the variable with itself, which doesn't really do a good job (the value is indeterminate before and after the initialization). You also used (__num / 2) to determine whether __num is odd, which is an odd way to detect oddness; you should use (__num % 2).

It is also now evident why the compiler warns that (one of the) __num (variables) is not used. The outer declaration assigns num++ to __num, but the initialized variable is never used because the inner occurrences of int __num = (__num); refer to themselves and not to the outer __num, so it isn't used.

You'd do better with static inline functions — something like this:

#include <stdio.h>
#include <stdlib.h>

static inline int num_is_positive(int num)
{
    int rc = 0;

    if (num > 0)
    {
        printf("%s: number %d is positive\n", __func__, num);
        rc = 1;
    }
    return rc;
}

static inline int num_is_odd(int num)
{
    int rc = 0;

    if (num % 2)        // BUG fixed
    {
        printf("%s: number %d is odd\n", __func__, num);
        rc = 1;
    }
    return rc;
}

static inline int num_is_positive_odd(int num)
{
    int rc = 0;

    printf("%s:%d: check linenum\n", __func__, __LINE__);
    if (num_is_positive(num) && num_is_odd(num))
        rc = 1;
    return rc;
}

int main(void)
{
    int num = 4;

    if (num_is_positive_odd(num++))
        printf("%s: number %d is positive odd\n", __func__, num);

    return 0;
}