Implementing strcat using pointers - c

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?