Is there any difference between this two macros?

Question

I find no difference between these two macros except the parentheses surrounding the macro in the first one.

Is it a matter of readability or is it a way to deal with the priority of operators?

#define TAM_ARRAY(a) (sizeof(a)/sizeof(*a))
#define TAM_ARRAY2(a) sizeof(a)/sizeof(*a)

Answer 1

Yes, there is a difference. The parentheses are needed so that the result of the macro is evaluated as a single expression, rather than being mixed with the surrounding context where the macro is used.

The output of this program:

#include <stdio.h>

#define TAM_ARRAY(a) (sizeof(a)/sizeof(*a))
#define TAM_ARRAY2(a) sizeof(a)/sizeof(*a)

int main(void)
{
    int x[4];
    printf("%zu\n", 13 % TAM_ARRAY (x));
    printf("%zu\n", 13 % TAM_ARRAY2(x));
}

is, in a C implementation where int is four bytes:

1
3

because:

• 13 % TAM_ARRAY (x) is replaced by 13 % (sizeof(x)/sizeof(*x)), which groups to 13 % (sizeof(x) / sizeof(*x)), which evaluates to 13 % (16 / 4), then 13 % 4, then 1.
• 13 % TAM_ARRAY2(x) is replaced by 13 % sizeof(x)/sizeof(*x), which groups to (13 % sizeof(x)) / sizeof(*x), which evaluates to (13 % 16) / 4, then 13 / 4, then 3.

Along these lines, TAM_ARRAY would be better to include parentheses around the second instance of a as well:

#define TAM_ARRAY(a) (sizeof(a)/sizeof(*(a)))