K&R Exercise 5-5 strcpy Segmentation fault

November 27, 2021

im working my way through the K&R C programing language book and trying to implement my own version of the function strncpy according to the description in exercise 5 at chapter 5:

strncpy(s,t,n) copies at most n characters of t to s

I have tried to write the following:

#include <stdio.h>

void *strn_cpy(char *dest, const char *src, int n){
    while ((*dest++ = *src++) && n--);
}

int main(){
    char * s1 = "hello";
    char * s2 = "abc";
    strn_cpy(s1, s2, 2);
    printf("%s", t1);
}

The code above returns a Segmentation fault error and I cant seem to figure out why,
the way I understand the function is that with each iteration of the loop, the current value of *src is copied onto *dest, afterwards both pointers locations are incremented by 1. Then, if the value of *src is equal to ‘\0’ or if n equals to 0 then the loop breaks.
Thanks.

>Solution :

char *s = "some string" creates a pointer to a string literal that you cannot change. You’re getting a seg fault because you’re trying to write to a read only string. Change it to char s[number]

#include <stdio.h>

void *strn_cpy(char *dest, const char *src, int n) {
    while (n-- && (*dest++ = *src++));
}

int main() {
    char s1[6] = "hello";
    char *s2 = "abc";
    strn_cpy(s1, s2, 2);
    printf("%s", s1);
}