control may reach end of non-void function

The last iteration of a non-void function must be a return. (your function return an int)

In some cases your code will not use a return. You need one so try to find where ;).

strncmp(s1, s2, n) compares up to n characters from the strings pointed to by s1 and s2. It stops if it finds a difference and returns a negative value if the character from s1, converted to an unsigned char is less than the corresponding character from s2 (also converted to an unsigned char) and a positive value it is is greater. Subtracting the values of the differing characters is a handy way to compute a return value with the correct sign.

If no difference was found before the end of both strings or n characters have been processed, whichever comes first, it returns 0.

Your code has some small issues:

• the pointer arguments should be declared with type const char * and n with type size_t.
• similarly, i should be defined as a size_t, defining it as an int forces you to use a cast in the comparison and will cause undefined behavior for strings with an identical prefix longer than INT_MAX and a large enough n argument.
• you should not dereference s1[i] nor s2[i] if n has reached 0, so the test on n should be performed first.
• the difference should be computed as diff = (unsigned char)s1[i] - (unsigned char)s2[i];
• note also that on some very exotic platforms, where sizeof(char) == sizeof(int), the subtraction cannot be used as it may wrap around and produce incorrect results.
• it is preferable to not use names reserved for standard functions for your own versions and it may cause clashes with the compiler's intrinsics.

Here is a modified version:

#include <stdio.h>

int my_strncmp(const char *s1, const char *s2, size_t n) {
    for (size_t i = 0; i < n; i++) {
        int diff = (unsigned char)s1[i] - (unsigned char)s2[i];
        if (diff != 0 || s1[i] == '\0')
            return diff;
    }
    return 0;
}

int main() {
    char s11[] = "abcs";
    char s12[] = "zzfs";
    size_t n = 3;
    printf("   strncmp() -> %d\n", strncmp(s11, s12, n));
    printf("my_strncmp() -> %d\n", my_strncmp(s11, s12, n));
    return 0;
}

The output values may differ but should have the same sign or should both be null.

The characters in the first non-matching positions are cast to unsigned char and then compared numerically - if that character in s1 is less than the corresponding character in s2, then a negative number is returned; if it's greater, a positive number is returned.