Question

#include <stdio.h>
int main(void)
{
    printf("%d", sizeof(signed int) > -1);
    return 0;
}

the result is 0 (FALSE).
how can it be?
Im using 64bit ubuntu linux so the result should be (4 > -1) => 1 => True.

Answer 1

sizeof(signed int) has type size_t, which is an unsigned type. When you make a comparison between a signed and an unsigned value [and the unsigned value’s type is at least as large as the signed value’s type], the signed value is converted to unsigned before the comparison. This conversion causes -1 to become the largest possible value of the unsigned type. In other words, it is as if you wrote

#include <limits.h>
/* ... */
printf("%d", sizeof(signed int) > SIZE_MAX);

You can make gcc warn when you make this mistake, but it’s not on by default or even in -Wall: you need -Wextra or more specifically -Wsign-compare. (This warning can produce a great many false positives, but I think it’s useful to turn on for new code.)