TIL: A Reason To Use bool in C

2025-03-16

While making notes for my students I came upon something that changed my view of bool in C. Feature introduced in C99 via stdbool.h header that hide ugly _Bool name behind the bool macro, only to be replaced in C23 by proper keyword (thanks N2934).

bool vs int

The standard value for truth in most of the C code I seen is int. However we are 25 years after C99 and I think that every compiler had time to catch up and there is no need for pure C89 code bases (I would love to be proven wrong). I think that this what it: just a more appropiate usage of types. However I was proven wrong when I discovered that type conversion differs for bool and int:

$ cat test.c
#include <stdio.h>
#include <stdbool.h>

int main()
{
  printf("as int:  %d\n", (int)0.5);
  printf("as bool: %d\n", (bool)0.5);
  return 0;
}
$ cc test.c -o test -std=c99 && ./test
as int:  0
as bool: 1
$ cc test.c -o test -std=c23 && ./test
as int:  0
as bool: 1
$ cp test.c test.cc && c++ test.cc -o test && ./test
as int:  0
as bool: 1

The difference is a suprise, however a welcome one. It allows to match the behaviour of logical tests:

$ cat test.c
#include <stdio.h>

int main()
{
  printf("as int:  %d\n", (int)0.5);
  printf("as bool: %d\n", (0.5 ? 1 : 0));
  return 0;
}
$ cc test.c -o test -std=c89 && ./test
as int:  0
as bool: 1
$ cc test.c -o test -std=c23 && ./test
as int:  0
as bool: 1

I recommend teaching to everyone. After 10 years of coding I discovered something new in the 40 years old „simple” language.