C programming strcat using pointer

Question

I am a beginner in C. I wanted to make strcat function using pointers. I made it but don't know what is wrong with it. I used gcc compiler and it gave segmentation fault output.

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

char scat(char *,char *);

void main()
{
    char *s="james";
    char *t="bond";

    char *q=scat(s,t);
    while(*q!='\0') printf("the concatenated string is %c",*q);
}

char *scat(char *s,char *t)
{
    char *p=s; 
    while(*p!='\0'){
        p++;
    } 
    while(*t!='\0'){
        *p=*t;
        p++;
        t++;
    }
    return p-s-t;
}

Answer 1

This one works:

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

char *scat(char *,char *);                 /* 1: your prototype was wrong */

void main()
{
    char *s="james";
    char *t="bond";

    char *q=scat(s,t);   
    printf("cat: %s\n", q);               /* 2: you can use %s to print a string */
    free(q);
}

char *scat(char *s,char *t)
{
    char *p=malloc(strlen(s)+strlen(t)+1);    /* 3: you will have to reserve memory to hold the copy. */
    int ptr =0, temp = 0;                   /* 4 initialise some helpers */

    while(s[temp]!='\0'){                  /* 5. use the temp to "walk" over string 1 */
        p[ptr++] = s[temp++];
    }
    temp=0;
    while(t[temp]!='\0'){                   /* and string two */
        p[ptr++]=t[temp++];
    }
    return p;
}

Answer 2

You have to allocate new space to copy at the end of s. Otherwise, your while loo[ will go in memory you don't have access to.

You shoul learn about malloc() here.