Creating C substrings: looping with assignment operator VS strncopy, which is better?

Question

This might be somewhat pointless, but I'm curious what you guys think about it. I'm iterating over a string with pointers and want to pull a short substring out of it (placing the substring into a pre-allocated temporary array). Are there any reasons to use assignment over strncopy, or vice-versa? I.e.

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

int main()
{   char orig[]  = "Hello. I am looking for Molly.";

    /* Strings to store the copies
     * Pretend that strings had some prior value, ensure null-termination */
    char cpy1[4] = "huh\0";
    char cpy2[4] = "huh\0";

    /* Pointer to simulate iteration over a string */
    char *startptr = orig + 2;
    int length = 3;
    int i;

    /* Using strncopy */
    strncpy(cpy1, startptr, length);

    /* Using assignment operator */
    for (i = 0; i < length; i++)
    {   cpy2[i] = *(startptr + i); 
    }   

    /* Display Results */
    printf("strncpy result:\n");
    printf("%s\n\n", cpy1);
    printf("loop result:\n");
    printf("%s\n", cpy2);
}   

It seems to me that strncopy is both less typing and more easily readable, but I've seen people advocate looping instead. Is there a difference? Does it even matter? Assume that this is for small values of i (0 < i < 5), and null-termination is assured.

Refs: Strings in c, how to get subString, How to get substring in C, Difference between strncpy and memcpy?

Answer 1

While this may seem counter-intuitive, there are more optimized ways to copy a string than by using the assignment operator in a loop. For instance, IA-32 provides the REP prefix for MOVS, STOS, CMPS etc for string handling, and these can be much faster than a loop that copies one char at a time. The implementation of strncpy or strcpy may choose to use such hardware-optimized code to achieve better performance.