strcpy using pointers

Question

I'm trying to write strcpy on my own using pointers and I get an error during runtime.

void str_cpy(char **destination, const char *source) {
//    char *s1 = *destination;

   while (*source != '\0') {
      **destination++ = *source++; //Get an error here
   }
   **destination = '\0';
}

I call the function as follows:

char *str = NULL;
str_cpy(&str, "String");

Is it not OK?

Thanks!

Answer 1

No, it's not okay. Why? Because str is a NULL pointer. It's pointing to nothing. When you try to write values into it, where will they go? It's not pointing to any allocated memory!

You first have to allocate memory for str. You can do:

char *str = malloc(strlen("String") + 1); // + 1 for the '\0' character at the end of C-style strings

Or you can do:

char str[256]; // make str large enough to hold 256 chars. Note that this is not as safe as the above version!

Also, destination should be a single pointer, not a double pointer. Well, it's not technically wrong to use a double pointer, it's just unnecessary.

Optionally, you can allocate the memory in the str_cpy function, like so:

void str_cpy(char **destination, const char *source) {
    *destination = malloc(strlen(source) + 1);
    // ... continue as normal

Answer 2

For simplicity's sake, this can be done in one line in a function.

void mystrcpy(char *dest, const char *src) {
  while (*dest++ = *src++);
}

This being said, you do need to allocate memory for dest beforehand using malloc or just simply by having a character array like char dest[256].