C Macro List size resolving but not entering for loop

May 29, 2023

I have the following code and when I try to use the sizeof list a as a condition in for loop it does not enter it, but if I hard code it, it enters the loop. Why is this behaviour happening?

#include <limits.h>
#include <malloc.h>
#include <stdio.h>

#define MY_VAL (sizeof(a) / sizeof(a[0]))
#define MY_NEW_VAL 5

int a[] = {1, 2, 3, 4, 5, 6, 7};

int main() {
  int i;
  for (i = -1; i <= (MY_VAL - 2); i++) {
    printf("In loop 1\n");
  }
  for (i = -1; i <= MY_NEW_VAL; i++) {
    printf("In loop 2\n");
  }
}

output –>

In loop 2
In loop 2
In loop 2
In loop 2
In loop 2
In loop 2
In loop 2

My compiler is clang version 15.0.7, running linux as my os. Just in case this is a compiler specific behaviour.

I am aware this is not something to be used in a real world use case, just found this behaviour to be odd. Please explain.

>Solution :

sizeof is of type size_t, which is unsigned, whereas your i is signed. C is converting i to a size_t, which makes the -1 a large positive number — greater than MY_VAL - 2. So the loop never runs. If you had had sufficient warnings enabled, the compiler would have told you about this possible problem.

MY_NEW_VAL on the other hand is an unspecified type, so C matches it to i. So that one works.

Since you want to start at -1, you need to make the limit signed: for (i = -1; i <= (int)MY_VAL - 2; i++). Then it works.