Implementing strcat using pointers

Question

While doing some programs on strings, I have come across this little problem. The question which was asked to me was this - Write a pointer version of the function strcat(s,t) which copies the string t to the end of s. I wrote the program as this -

#include<stdio.h>
void strcat(char *s, char *t);
int main()
{
    char *s1, *s2;
    printf("enter the first string\n");
    scanf("%s",s1);
    printf("Enter the second string\n");
    scanf("%s",s2);
    strcat(s1,s2);
    printf("Strings concatenated\n");
    printf("%s",s1);
    return 0;
}
void strcat(char *s, char *t)
{   
    while(*s++)
       ;
    while(*s++ = *t++)
               ;
}

I know i have done something(or many things) terribly wrong. Because whenever i try to run this code- it gives me segmentation fault. Like this-

Enter the first string

Hello

Enter the second string

Segmentation fault (core dumped)

It would be really helpful if someone points me out the flaw/flaws of my implementation. Thanks in advance.

Thank you very much guys, for such quick responses. But seems that wasn't the only problem. After writing the program like this-

#include<stdio.h>
void strcat(char *s, char *t);
int main()
{
    char s1[20], s2[20];
    printf("enter the first string\n");
    scanf("%s",s1);
    printf("Enter the second string\n");
    scanf("%s",s2);
    strcat(s1,s2);
    printf("Strings concatenated\n");
    printf("%s",s1);
    return 0;
}
void strcat(char *s, char *t)
{   
    while(*s++)
        ;
    while(*s++ = *t++)
        ;
}

It runs like this.

Enter the first string

Hello

Enter the second string

There

Hello

It only prints the first string i have entered. Now i think i have made some mistake on that strcat function too.

Answer 1

1) In the main() , you have to allocate memory for both s1 and s2 pointers

char *s1=malloc(100*sizeof(char)), *s2=malloc(100*sizeof(char));
scanf("%99s",s1); //the "%99s" allow to avoid buffer overflow

And if you use gcc and your gcc>2.7 then you can use "%ms" in the scanf() in this way:

scanf("%ms",&s1);

with the "%ms" , the scanf() will allocate memory for s1 pointer

2) and you have to add s-- in

while(*s++)
    ;
s--; // add s-- here
while(*s++ = *t++)
    ;

because the s pointer is pointing in the next element of '\\0' element. the s pointer should be pointed in the '\\0' element before starting copy the second string

Answer 2

You don't allocate memory for s1 , s1 (or initialized with array), Value of both s1 , s1 are garbage .

char *s1, *s2;
printf("enter the first string\n");
scanf("%s",s1);

This causes an undefined behavior.

Suggestion use either:

#define SIZE  1024
char s1[SIZE], s2[SIZE];

or dynamically allocate memory using calloc()/maloc() functions:

char *s1, *s2;
s1 = malloc(SIZE);
s2 = malloc(SIZE);

and lately free() memory explicitly when work done with s1 , s2 .

Additionally instead of unsafe scanf() use fgets() function to avoid buffer overflow error. read Reading a line using scanf() not good?