How to turn a hex string into an unsigned char array?

Question

For example, I have a cstring \"E8 48 D8 FF FF 8B 0D\" (including spaces) which needs to be converted into the equivalent unsigned char array {0xE8,0x48,0

Answer 1

For a pure C implementation I think you can persuade sscanf(3) to do what you what. I believe this should be portable (including the slightly dodgy type coercion to appease the compiler) so long as your input string is only ever going to contain two-character hex values.

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


char hex[] = "E8 48 D8 FF FF 8B 0D";
char *p;
int cnt = (strlen(hex) + 1) / 3; // Whether or not there's a trailing space
unsigned char *result = (unsigned char *)malloc(cnt), *r;
unsigned char c;

for (p = hex, r = result; *p; p += 3) {
    if (sscanf(p, "%02X", (unsigned int *)&c) != 1) {
        break; // Didn't parse as expected
    }
    *r++ = c;
}