Why is there a segmentation fault in C? - c

Question

I am trying to solve this:
https://www.reddit.com/r/dailyprogrammer/comments/ffxabb/20200309_challenge_383_easy_necklace_matching/
where you check if words can be rearranged by moving the last letter of one to the front(can be done multiple times) and I keep getting a segmentation fault on line 37. Does anyone know why?
#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
void shift(string, int);
bool same_necklace(string a, string b)
{
if(strlen(a) != strlen(b))
{
printf("false\n");
return false;
}
int n = strlen(a);
for(int i = 0; i < n; i++)
{
shift(a, n);
if(strcmp(a, b) == 0)
{
printf("true\n");
return true;
}
}
printf("false\n");
return false;
}
void shift(string a, int n)
{
char output[n];
for(int i = 0; i < n; i++)
{
output[i] = a[(i+1) % n];
}
for(int i = 0; i < n; i++)
{
a[i] = output[i];
}
}
int main(void)
{
same_necklace("nicole", "icolen");
same_necklace("nicole", "lenico");
same_necklace("nicole", "coneli");
same_necklace("aabaaaaabaab", "aabaabaabaaa");
same_necklace("abc", "cba");
same_necklace("xxyyy", "xxxyy");
same_necklace("xyxxz", "xxyxz");
same_necklace("x", "x");
same_necklace("x", "xx");
same_necklace("x", "");
same_necklace("", "");
}

Answer 1

You're trying to store values to a string literal, which is read-only memory on modern OSes.
To fix it make a copy of the string you're going to shift before-hand. Remember to make room for the null-terminator and free your memory after you're done:
#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
void shift(string, int);
bool same_necklace(string a, string b)
{
int n = strlen(a);
a = memcpy(malloc(n + 1), a, n + 1);
if(strlen(a) != strlen(b))
{
printf("false\n");
return false;
}
for(int i = 0; i < n; i++)
{
shift(a, n);
if(strcmp(a, b) == 0)
{
printf("true\n");
return true;
}
}
printf("false\n");
free(a);
return false;
}
void shift(string a, int n)
{
char output[n];
for(int i = 0; i < n; i++)
{
output[i] = a[(i+1) % n];
}
for(int i = 0; i < n; i++)
{
a[i] = output[i];
}
}
int main(void)
{
same_necklace("nicole", "icolen");
same_necklace("nicole", "lenico");
same_necklace("nicole", "coneli");
same_necklace("aabaaaaabaab", "aabaabaabaaa");
same_necklace("abc", "cba");
same_necklace("xxyyy", "xxxyy");
same_necklace("xyxxz", "xxyxz");
same_necklace("x", "x");
same_necklace("x", "xx");
same_necklace("x", "");
same_necklace("", "");
}