Question

I have a variable length string where each character represents a hex digit. I could iterate through the characters and use a case statement to convert it to hex but I feel like there has to be a standard library function that will handle this. Is there any such thing?

Example of what I want to do. "17bf59c" -> int intarray[7] = { 1, 7, 0xb, 0xf, 5, 9, 0xc}

Answer 1

No, there’s no such function, probably because (and now I’m guessing, I’m not a C standard library architect by a long stretch) it’s something that’s quite easy to put together from existing functions. Here’s one way of doing it decently:

int * string_to_int_array(const char *string, size_t length)
{
  int *out = malloc(length * sizeof *out);
  if(out != NULL)
  {
    size_t i;
    for(i = 0; i < length; i++)
    {
      const char here = tolower(string[i]);
      out[i] = (here <= '9') ? (here - '\0') : (10 + (here - 'a'));
    }
  }
  return out;
}

Note: the above is untested.

Also note things that maybe aren’t obvious, but still subtly important (in my opinion):

1. Use const for pointer arguments that are treated as “read only” by the function.
2. Don’t repeat the type that out is pointing at, use sizeof *out.
3. Don’t cast the return value of malloc() in C.
4. Check that malloc() succeeded before using the memory.
5. Don’t hard-code ASCII values, use character constants.
6. The above still assumes an encoding where ‘a’..’f’ are contigous, and would likely break on e.g. EBCDIC. You get what you pay for, sometimes. 🙂