creating a wrapper for strncpy to insert terminating null

Question

Hello,

I have decided to make a wrapper for strncpy as my source code requires me to do a lot of string copies. And I want to ensure that the string is terminated if the source is equal or greater than the destination.

As this code will be used in production, so I just want to see if there are any potential dangers using this wrapper.

I have never done wrappers before so tyring to make itI am trying to make it perfect.

Many thanks for any advice,

/* Null terminate a string after coping */
char* strncpy_wrapper(char *dest, const char* source, 
          const size_t dest_size, const size_t source_size)
{
    strncpy(dest, source, dest_size);
    /* 
     * Compare the different length, if source is greater
     * or equal to the destination terminate with a null.
     */
    if(source_size >= dest_size)
    {
     dest[dest_size - 1] = '\0';
    }

    return dest;
}

==== Edited updated ====

/* Null terminate a string after coping */
char* strncpy_wrapper(char *dest, const char* source, 
          const size_t dest_size)
{
    strncpy(dest, source, dest_size);
    /* 
     * If the destination is greater than zero terminate with a null.
     */
    if(dest_size > 0)
    {
     dest[dest_size - 1] = '\0';
    }
    else
    {
     dest[0] = '\0'; /* Return empty string if the destination is zero length */
    }

    return dest;
}

Answer 1

You don't need to check that source is greater than destination just always make the last char in the dest a '\0'.

Answer 2

Check dest_size before accessing the array or you'll get into trouble:

if (dest_size > 0) {
    dest[dest_size - 1] = '\0';
}

---

Actually, now that I think of it, it's probably better to just die:

if (dest_size == 0) {
    fputs(stderr, "strncpy(x,y,0)");
    exit(1);
}

Otherwise, you'll have the same problem as with the original strncpy(), that dest may not be terminated if dest_size is 0.

Answer 3

If you don't have any constraints on the allocation of the buffer, strndup could be more appropriate.

It will allocate and copy at most len chars, and always terminate with NULL.

Answer 4

if dest_size is the number of characters to be copied and source_size is the number of characters in source string.
you should try this:

 size_t numchars = dest_size > source_size ? source_size : dest_size;
 strncpy(dest,source,numchars) ;
 dest[numchars] = 0 ;

Answer 5

You should investigate the semi-standard BSD functions strlcpy() and strlcat().

You should consider whether the ISO TR24731 functions such as strncpy_s() are appropriate and available where you need them.