cause of Segmentation error not known

Question

I have seen related queries but that did not help https://gist.github.com/arshsherdill/174fe2945f3d43f3b33e1c307f7fde48

On running as ./vignere bacon. it shows segmentation fault i am unable to find what causes this error. Even debugging did not helped

#include<stdio.h>
#include<cs50.h>
#include<ctype.h>
#include<string.h>

int key_len;
//function to check that all characters in argv[1] are valid characters
int check(string c)
{
    key_len=strlen(c);
    for(int i=0;i<key_len;i++)
    {
        if(!isalpha(c[i]))
            return 1;
    }
    return 0;
}
//funvtion to change characters as done in caesar cipher
char caesar(char c,int k)
{
    if(isupper(c))
    return ((((c-'A')+k)%26)+'A');
    if(islower(c))
    return ((((c-'a')+k)%26)+'a');
    else 
    return 0;
}

int main(int argc, string argv[])
{
    string k="";
    if(argc==2)
    {
        if(check(argv[1])==1)
            return 1;

        for(int j=0;j<key_len;j++)
        {
            //I have stored values of key to be used later
            k[j] = tolower(argv[1][j])-'a';
        }
        //plaintext
        string p = GetString();
        int len_text = strlen(p);
        //ciphertext
        string c="";

        int l=0;//key index

        for(int r=0;r<len_text;r++)
        {
            if(isalpha(p[r])){
                //caesar shift function
                c[r]=caesar(p[r],k[l]);

            //key is used only when the p[r] is a character
                if(l<key_len-1){
                    l++;
                }
                else
                    l=0;

            }
            else
            {
                c[r]=p[r];

            }

        }
        printf("%s\n",c);
    }





    //if conditions are not met
    else
        return 1;
}

Answer 1

This string k=""; is the root of the seg fault. Declaring k a "string literal", in other words setting it in the declaration, makes it "read only", or constant. Therefore, it is immutable; it cannot be changed. It is technically this line k[j] = tolower(argv[1][j])-'a'; that gives the seg fault.