Make a copy of a char*

Question

I have a function that accepts a char* as one of its parameters. I need to manipulate it, but leave the original char* intact. Essentially, I want to create a working copy of this char*. It seems like this should be easy, but I am really struggling.

My first (naive) attempt was to create another char* and set it equal to the original:

char* linkCopy = link;

This doesn't work, of course, because all I did was cause them to point to the same place.

Should I use strncpy to accomplish this?

I have tried the following, but it causes a crash:

char linkCopy[sizeof(link)] = strncpy(linkCopy, link, sizeof(link));

Am I missing something obvious...?

EDIT: My apologies, I was trying to simplify the examples, but I left some of the longer variable names in the second example. Fixed.

Answer 1

The sizeof will give you the size of the pointer. Which is often 4 or 8 depending on your processor/compiler, but not the size of the string pointed to. You can use strlen and strcpy:

// +1 because of '\0' at the end
char * copy = malloc(strlen(original) + 1); 
strcpy(copy, original);
...
free(copy); // at the end, free it again.

I've seen some answers propose use of strdup, but that's a posix function, and not part of C.

Answer 2

You might want to take a look at the strdup (man strdup) function:

char *linkCopy = strdup(link);

/* Do some work here */

free(linkCopy);

Edit: And since you need it to be standard C, do as others have pointed out:

char *linkCopy = malloc(strlen(link) + 1);
/* Note that strncpy is unnecessary here since you know both the size
 * of the source and destination buffers
 */
strcpy(linkCopy, link);

/* Do some work */

free(linkCopy);

---

Since strdup() is not in ANSI/ISO standard C, if it's not available in your compiler's runtime, go ahead and use this:

/*
**  Portable, public domain strdup() originally by Bob Stout
*/

#include <stdlib.h>
#include <string.h>

char* strdup(const char* str)
{
      char* newstr = (char*) malloc( strlen( str) + 1);

      if (newstr) {
          strcpy( newstr, str);
      }

      return newstr;
}