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#include  <stdio.h>
 
#ifdef DEBUG
#define CHECK(expr) foo(expr)
#else
#define CHECK(expr)
#endif /* DEBUG */
 
int bar (int x, int y, int z)
{
    //do nothing
    return 0;
}
 
int main(void)
{
    int a= 0, b = 0, c = 0;
    int x = bar(a, b, c); /* bar has to be called for both DEBUG begin defined and undefined */ 
    CHECK(x);
    printf("Hello, world!\n");
    return 0;
}
 

prog.c: In function 'main':
prog.c:18:9: error: unused variable 'x' [-Werror=unused-variable]
     int x = bar(a, b, c); /* bar has to be called for both DEBUG begin defined and undefined */ 
         ^
cc1: all warnings being treated as errors

Standard output is empty