string - my own strcat function using C errors

I'm basically coding my very own string functions in C.

I've been trying to do the strcat function using pointers and cannot seem to understand whether I should be allocating memory using malloc or leaving it up to the heap.

char *my_strcat(const char *s1, const char *s2)
{


    const unsigned char *p1 = (const unsigned char *)s1;
    const unsigned char *p2 = (const unsigned char *)s2;


    unsigned char *string;
        //string = malloc(strlen(s1) + strlen(s2) + 1);
    while (*p1 != '\0')
{
        *string = *p1;
        string++;
        p1++;

        if(*p1 == '\0')
        {

            while(*p2 != '\0')
            {
                *string = *p2;
                string++;
                p2++;
            }
        }
    }
    return (char *)string;  
}

Any tips on more efficiently performing this task or things I'm doing wrong would be great!

Cheers

EDIT

OK so I got a working solution but just wondering after I use malloc where should I free() it?

char *my_strcat(const char *s1, const char *s2)
{


    const unsigned char *p1 = (const unsigned char *)s1;
    const unsigned char *p2 = (const unsigned char *)s2;

    char *string = malloc(sizeof(char *));
    char *res = string;


    while (*p1 != '\0')
{
        *string = *p1;
        string++;
        p1++;
    }
    while (*p2 != '\0')
    {
        *string = *p2;
        string++;

        p2++;
}
    *string = '\0'; 

    return (char *)res; 
}

Answer

First, I assume that the allocation is commented out by mistake.

• You need to save the pointer that you allocate, and return it. Otherwise, you're returning a pointer string, which points at the end of the concatenation result


• You are not terminating the resultant string; you need to add *string = '\0'


• You should move the second loop to the outside of the first loop, and drop the if condition around it: if the first loop has terminated, you know that *p1 points to \0

---

char *string = malloc(strlen(s1) + strlen(s2) + 1);
char *res = string;

for (; *p1 ; *string++ = *p1++);
for (; *p2 ; *string++ = *p2++);
*string = '\0';
return res;