Converting hex char array to ascii char array

Question

Is there any function that can convert hex char array to ascii char array, something like this:

'''

      char array_input[] = "48656C6C6F3";//Input hex sequence

      char array_output[] = "Hello"; // Output ASCII

'''

and that can accept input sequences that contain more than 300 letters.

something like:

'''

      char array_input[] = "08010000013feb55f000000000000f74000f0ea850209a690000940000120000001e09010002000300040016014703f0001504c8000c0900730a00460b00501300464306d7440000b5000bb60007422e9f180000cd0386ce000107c700000000f10000601a46000001344800000bb84900000bb84a00000bb84c00000000024e0000000000000000cf000001";

'''

Answer 1

Parse 2 hex digits at a time

Walk the input string looking for 2 hex digits. Form the ASCII value and then append to the output array. Append a final null character .

void foo(char *array_output, const char *array_input) {
  unsigned value;
  while (sscanf(array_input, "%2x", &value) == 1) {
    *array_output++ = value;
    array_input += 2;
  } 
  *array_output = '\0';
}

Pathological strings with white-space or signs like " A12-3" need additional code to handle.

---

Alternative: use is...() to test for hex digits. Then form a compound literal s to pass to strtol() .

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>

void foo2(char *array_output, const char *array_input) {
  const unsigned char *h = (const unsigned char *) array_input;
  while (isxdigit(h[0]) && isxdigit(h[1])) {
    char s[3] = { h[0], h[1], '\0' };
    *array_output++ = (char) strtol(s, 0, 16);
    h += 2;
  }
  *array_output = '\0';
}

Answer 2

Here is an efficient alternative solution without any library calls:

void hextostr(char *dest, const char *src) {
    static unsigned char const val[(unsigned char)-1] = {
        ['0'] =  0+1, ['1'] =  1+1, ['2'] =  2+1, ['3'] =  3+1, ['4'] =  4+1,
        ['5'] =  5+1, ['6'] =  6+1, ['7'] =  7+1, ['8'] =  8+1, ['9'] =  9+1,
        ['a'] = 10+1, ['b'] = 11+1, ['c'] = 12+1, ['d'] = 13+1, ['e'] = 14+1, ['f'] = 15+1,
        ['A'] = 10+1, ['B'] = 11+1, ['C'] = 12+1, ['D'] = 13+1, ['E'] = 14+1, ['F'] = 15+1,
    };
    const unsigned char *p = (const unsigned char *)src;
    while (val[p[0]] && val[p[1]]) {
        *dest++ = (char)(val[p[0]] * 16 + val[p[1]] - 17);
        p += 2;
    }
    *dest = '\0';
}