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cc-preprocessor

Why does an empty preprocessor command still evaluate to something?

Please take a look at this small code snippet:

#include <stdio.h>

#define BAR

int main(int argc, char *argv[])
{
    int foo = 1;
    
    foo = BAR(1, 2, 3);
    
    printf("%d\n", foo);
    
    return 0;
}

Since BAR is defined as an empty macro, I'd estimate the line foo = BAR(1, 2, 3); to be evaluated as

foo = ;

and cause a syntax error. However, that doesn't happen. In fact, the code above compiles just fine and for some reason foo will be set to 3, i.e. the last parameter to the BAR macro. I can't make any sense of this. Can somebody shed some light onto why it is behaving like this? Is this defined behaviour in the C language?

Solution

  • You've defined BAR as a "plain" macro, not a function-like macro, so it doesn't match BAR(1, 2, 3), only BAR, so it expands to foo = (1, 2, 3).

    To get what you expected, change the definition to a function-like macro:

    #define BAR(...)