what can be faulty in below code, gives segmentation fault in c

Question

My code gives me segmentation fault. I feels that problem in function encrypt_playfair() inside encryption part. actually it is code of Playfair Cipher. My full code is as following. Please help me to understand problem like this.

#include <stdio.h>
#include <stdlib.h>
#define SIZE 5
#define VVV 97
#define _MAX 26

void print(char *mat){
    int k,l;
    for(k=0;k<SIZE;k++)
    {
        for(l=0;l<SIZE;l++)
        {
            printf("%c",*(mat+k*SIZE+l));
        }
        printf("\n");
    }

}

void find_cord(int *x, int *y, char *mat, int value){
    int k,l;
    if(value==106){
        value=105;
    }
    for(k=0;k<SIZE;k++)
    {
        for(l=0;l<SIZE;l++)
        {
            if(*(mat+k*SIZE+l)==value){
                *x=k;
                *y=l;
                return;
            }
        }
    }
}

void encrypt_playfair(char *pt, char *key){
    int i=0;
    char *mat;
    mat=(char *)malloc(sizeof(char)*SIZE*SIZE);
    int k,l;
    int alph[_MAX]={0};

    while(*(key+i)!='\0'){
        *(mat+i)=(*(key+i));
        alph[*(key+i)-VVV]=1;
        i++;
    }
    for(i=VVV;i<_MAX+VVV;i++)
    {
        if(alph[i-VVV]==1){
            continue;
        }
        for(k=0;k<SIZE;k++)
        {
            for(l=0;l<SIZE;l++)
            {

                if(*(mat+k*SIZE+l)!='\0'){
                    continue;
                }
                *(mat+k*SIZE+l)=i;
                if(i==105){
                    i++;    // i(105) and j(106) with each other
                }
                alph[i-VVV]=1;
                k=6;
                break;
            }
        }
    }

    print(mat);

    // encryption
    i=0;
    int *x1,*x2,*y1,*y2;
    x1=(int *)malloc(sizeof(int));
    y1=(int *)malloc(sizeof(int));
    x2=(int *)malloc(sizeof(int));
    y2=(int *)malloc(sizeof(int));
    while(*(pt+i)!='\0'){
        find_cord(x1, y1, mat, *(pt+i));
        find_cord(x2, y2, mat, *(pt+i+1));
        printf("cordinates: %d %d %d %d\n",*x1,*y1,*x2,*y2);
        if((*x1)==(*x2)){
            *(pt+i)=*(mat+(*x1)*SIZE+((*y1+1)%SIZE));
            *(pt+i+1)=*(mat+(*x2)*SIZE+((*y2+1)%SIZE));
        }
        else if((*x1)==(*x2)){
            *(pt+i)=*(mat+((*x1+1)%SIZE)*SIZE+(*y1));
            *(pt+i+1)=*(mat+((*x2+1)%SIZE)*SIZE+(*y2));
        }
        else{
            *(pt+i)=*(mat+(*x1)*SIZE+(*y2));
            *(pt+i+1)=*(mat+(*x2)*SIZE+(*y1));
        }
        i+=2;
    }
}

void main(){
    char *msg="helxlo";
    char *key="monarchy";
    printf("%c\n",105);
    encrypt_playfair(msg,key);
    printf("%s\n",msg);
}

Answer 1

The segmentation fault results from an attempt to modify a constant string. To avoid it, change:

  char *msg="helxlo";
  char *key="monarchy";

to

  char msg[]="helxlo";
  char key[]="monarchy";