C: macro produced warning "statement with no effect"

I have following code in C:

int do_something(void);

#ifdef SOMETHING
#define DO_SOMETHING() do_something()
#else
#define DO_SOMETHING() 0
#endif

This code produced warning "statement with no effect" when compiled without SOMETHING defined. I am trying to fix it, but there is one problem - code which uses this macro sometimes checks that "return value" and sometimes ignores it. Because of this I cannot use the easiest solution - casting to void in macro itself.

Is it possible to write macro which allows to compare "returned value" and does not produce this warning when it is ignored?

I use gcc to compile my code.

---

Three possible solutions:

Define a do_nothing function, which will get optimized out by gcc:

int do_something(void);
int do_nothing(void) { return 0; } 

#ifdef SOMETHING
#define DO_SOMETHING() do_something()
#else
#define DO_SOMETHING() do_nothing()
#endif

Or, modify the do_something implementation to move the #ifdef check there

int do_something(void)
{
  #ifndef SOMETHING
  return 0;
  #endif

// Your implementation here
}

You can also ignore the warning using #pragma directives.

By the way, which version of gcc and with which flags are you compiling? gcc -Wall -pedantic with GCC 4.9 doesn't produce the warning.

---

#include <stdio.h>

int do_something(void);

#ifdef SOMETHING
#define DO_SOMETHING() do_something()
#define DO_SOMETHING_ASSIGN() do_something()
#else
#define DO_SOMETHING() 
#define DO_SOMETHING_ASSIGN() 0
#endif

int do_something(void)
{
    static int cnt=0;
    /* code for do something here */
    ++cnt;
    printf("do_something has been called %d %s!n",cnt,
        (cnt==1)?"time":"times");   
    return cnt;
}

void main(void)
{
    int x;

    DO_SOMETHING();

    x=DO_SOMETHING_ASSIGN();    

    printf("%d %dn",x,DO_SOMETHING_ASSIGN());

    puts("end!!!");
}

I hope this is usefull to you!

If you save this code as main.c, you may compile it with:

gcc main.c -o main

when you run main, you obtain the following output:

0 0
end!!!

If you compile it with:

gcc main.c -DSOMETHING -o main

When you run main, you obtain the following output:

do_something has been called 1 time!
do_something has been called 2 times!
do_something has been called 3 times!
2 3
end!!!

链接地址: http://www.djcxy.com/p/73620.html

上一篇: 什么是JavaBean？

下一篇: C：宏产生警告“声明无效”