Alternate implementation of stpncpy

stpncpy.c

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

// Copy at most len bytes, including NUL terminating byte
// if src is too long don't terminate
// if src is shorter than len, fill remainder with zeroes
// return pointer to terminator in dst
// if no terminator, return dst + len (address after last byte written)
//
// This is not the fastest way to implement stpncpy, and it is only
// used where it is missing (mingw/windows)
char *__stpncpy(char *dst, const char *src, size_t len)
{
    for (int i = 0; i < len; ++i)
    {
        const char copy_byte = src[i];
        dst[i] = copy_byte;
        if (copy_byte == '\0')
        {
            // Zero fill and return:
            for (int j = i+1; j < len; ++j)
            {
                dst[j] = '\0';
            }
            return dst + i;
        }
    }
    return dst + len;
}

bool string_equal(const char *const a, const char *const b)
{
    return (strcmp(a, b) == 0);
}

bool memory_equal(const void *a, const void *b, size_t n)
{
    return (memcmp(a, b, n) == 0);
}

#define test_str(str, num)                            \
    {                                                 \
        printf("stpncpy(\"%s\", %d):    ", str, num); \
        char buf_a[1000];                             \
        memset(buf_a, 0, 1000);                       \
        buf_a[500] = buf_a[0] = 'X';                  \
        char buf_b[1000];                             \
        memset(buf_b, 0, 1000);                       \
        buf_b[500] = buf_b[0] = 'X';                  \
        char *r_a = stpncpy(buf_a, str, num);         \
        char *r_b = stpncpy_windows(buf_b, str, num); \
        if (!string_equal(buf_a, buf_b))              \
        {                                             \
            printf("'%s' != '%s'\n", buf_a, buf_b);   \
        }                                             \
        else                                          \
        {                                             \
            printf("'%s' == '%s'\n", buf_a, buf_b);   \
        }                                             \
        assert(string_equal(buf_a, buf_b));           \
        assert(memory_equal(buf_a, buf_b, 1000));     \
        r_b = stpncpy_windows(buf_a, str, num);       \
        assert(r_a == r_b);                           \
    }

#define test_loop(str, num)           \
    {                                 \
        for (int i = 0; i < num; ++i) \
        {                             \
            test_str(str, i);         \
        }                             \
    }

int main()
{
    test_loop("", 16);
    test_loop(" ", 16);
    test_loop("     ", 16);
    test_loop("\t\t ", 16);
    test_loop("Hello, world!", 16);
}