Chaining 2 Strings in C - c

Question

I want to chain 2 strings in C. The function I use called concat()
First I defined something like this and it worked
char* concat(const char *s1, const char *s2)
{
char* result = malloc (15);
int lengh1 = simple_strlen (s1);
int lengh2 = simple_strlen (s2);
int i=0,j;
for ( i = 0 ;i < lengh1;i++){
if (i!=lengh1-1)
result[i]=s1[i];
else{
result[i]=s1[i];
for ( j=i+1 ; j< lengh1+lengh2;j++){
result[j] = s2[j-i-1];
}
}
}
return result;
}
But then I was asked to do it without malloc() so I defined something like this:
char* concat( char *result, const char *s2)
{
int lengh1 = simple_strlen (result);
int lengh2 = simple_strlen (s2);
int i=0;
for ( i = 0 ;i < lengh2;i++){
result[i+lengh1]=s2[i];
}
return result;
}
but it has segmentation fault
example:
int main(int argc , char* argv[], char* envp[])
{
printf(concat( "hello", "world"));/*output expected "helloworld"*/
return 0;
}

Answer 1

There are multiple problems in your code:
in the malloc version, the space allocated for the destination string is hard coded at 15 instead of computing it as lengh1 + lengh2 + 1, enough space for both strings and the trailing null byte.
you do not set the null terminator at the end of the destination string in both versions.
in your version without malloc, you must provide a large enough array as the destination for concat(). A string constant cannot be modified. A simple solution is to pass the destination buffer and the source strings as separate arguments.
Here are modified versions:
char *concat(const char *s1, const char *s2) {
int length1 = simple_strlen(s1);
int length2 = simple_strlen(s2);
char *result = malloc(length1 + length2 + 1);
int i, j;
for (i = 0; i < length1; i++) {
result[i] = s1[i];
}
for (i = 0; i < length2; i++) {
result[length1 + i] = s2[1];
}
result[length1 + length2] = '\0';
return result;
}
Without malloc():
#include <string.h>
char *concat(char *dest, const char *s1, const char *s2) {
char *p = dest;
while (*s1)
*p++ = *s1++;
while (*s2)
*p++ = *s2++;
*p = '\0';
return dest;
}
int main() {
char buf[100];
/* output expected "helloworld" */
printf("%s\n", concat(buf, "hello", "world"));
return 0;
}

Answer 2

As mentioned by the prior commenters, you need to allocate memory somewhere to store the concatenated string. If you are not allowed to allocate on the heap via malloc, then you can do something like this:
#include "stdafx.h"
# include <string.h>
char* concat( char * result , const char *s1, const char *s2)
{
int lengh1 = strlen(s1);
int lengh2 = strlen(s2);
int i = 0;
for (i = 0; i < lengh1; i++) {
result[i] = s1[i];
}
for (i = 0; i < lengh2; i++) {
result[i+ lengh1] = s2[i];
}
return result;
}
int main()
{
char mybuffer[100];
memset(mybuffer, 0, sizeof(mybuffer));
printf(concat( mybuffer,"hello", "world"));/*output expected "helloworld"*/
return 0;
}