Converting char* to unsigned char*

Question

How do I copy a char* to a unsigned char* correctly in C. Following is my code

int main(int argc, char **argv)
{
    unsigned char *digest;

    digest = malloc(20 * sizeof(unsigned char));
    strncpy(digest, argv[2], 20);
    return 0;
}

I would like to correctly copy char* array to unsigned char* array. I get the following warning using the above code

warning: pointer targets in passing argument 1 of âstrncpyâ differ in signedness 

EDIT: Adding more information, My requirement is that the caller provide a SHA digest to the main function as a string on command line and the main function internally save it in the digest. SHA digest can be best represented using a unsigned char.

Now the catch is that I can't change the signature of the main function (** char) because the main function parses other arguments which it requires as char* and not unsigned char*.

Answer 1

To avoid the compiler warning, you simply need:

strncpy((char *)digest, argv[2], 20);

But avoiding the compiler warning is often not a good idea; it's telling you that there is a fundamental incompatibility. In this case, the incompatibility is that char has a range of -128 to +127 (typically), whereas unsigned char is 0 to +255.

Answer 2

You can't correctly copy it since there is difference in types, compiler warns you just about that.

If you need to copy raw bits of argv[2] array you should use memcpy function.

Answer 3

Cast the signedness away in the strncpy() call

strncpy((char*)digest, argv[2], 20);

or introduce another variable

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

int main(int argc, char **argv)
{
    unsigned char *digest;
    void *tmp;                   /* (void*) is compatible with both (char*) and (unsigned char*) */

    digest = malloc(20 * sizeof *digest);
    if (digest) {
        tmp = digest;
        if (argc > 2) strncpy(tmp, argv[2], 20);
        free(digest);
    } else {
        fprintf(stderr, "No memory.\n");
    }
    return 0;
}

---

Also note that malloc(20 * sizeof(unsigned char*)) is probably not what you want. I think you want malloc(20 * sizeof(unsigned char)), or, as by definition sizeof (unsigned char) is 1, malloc(20). If you really want to use the size of each element in the call, use the object itself, like in my code above.