Comparison of string lengths in C using strlen

Question

I am quite confused with the behavior of strlen,the for loop below never ends (without adding the break) when try it, while the i < -2 should return False in the first step.

Is it related to my compiler? What did I misunderstand?

#include <stdio.h>
#include <string.h>

int main()
{
    char a[] = "longstring";
    char b[] = "shortstr";

    printf("%d\n", strlen(b) - strlen(a)); // print -2

    for (int i = 0; i < strlen(b) - strlen(a); i++)
    {
        printf("why print this, %d  %d\n", i, strlen(b) - strlen(a));
        break;
    }
    return 0;
}

Output

-2
why print this, 0 -2

Answer 1

The conversion specifier in this call of printf

printf("%d\n", strlen(b) - strlen(a)); // print -2

is incorrect. The function strlen returns a value of the unsigned integer type size_t. So this expression strlen(b) - strlen(a) also has the type size_t. So you need to write either

printf("%d\n", ( int ) strlen(b) - ( int )strlen(a) ); // print -2

or

printf("%zu\n", strlen(b) - strlen(a)); // print a big positive value.

In the condition of the for loop

for (int i = 0; i < strlen(b) - strlen(a); i++)

the expression strlen(b) - strken(a) as it has been mentioned above has the unsigned integer type size_t. So its value can not be a negative and represents a big positive value.

Again instead you need to write

for (int i = 0; i < ( int )strlen(b) - ( int )strlen(a); i++)

Or you could write

for ( size_t i = 0; i + strlen( a ) < strlen(b); i++)