Search code examples
cbinaryuint32

Why is the value of this uint32_t -2147483648?

Using gcc compiler, why is this uint32_t -2147483648 when I set the MSB? It's unsigned - shouldn't it be positive?

#include <stdio.h>
#include <stdint.h>

#define BIT_SET(a,b) ((a) |= (1<<(b)))

int main()
{
    uint32_t var = 0;
    BIT_SET(var, 31);

    printf("%d\n", var); //prints -2147483648

    return 0;
}
Solution

  • Type of var is uint32_t. But you are printing it using %d which is undefined behaviour. Use PRIu32 from <inttypes.h> to print uint32_t:

    printf("%"PRIu32"\n",var);