Parse hex chars string to bytes

Very simple task as for interview: to parse hex chars into bytes. Example: "123Ad" -> "\x12\x3A\x0D". This procedure can be helpfull if you want to transmit some encoding string (in UTF8, UTF16, mbcs, etc) between different processes/OS and it's probably to get broken bytes, so best solution will be to use such hex chars (it's ASCII [0-9a-fA-F] only).

Implementation in C may use sscanf() which will be slow or does not. Here is an example without sscanf:

#include <stdio.h>
#include "algutils.h"

int parse_hex_str(const char *s, int slen, char *out) {
#define __ord(c) \
    ((c) >= '0' && (c) <= '9'? (c) - '0' : \
     (c) >= 'a' && (c) <= 'f'? (c) - 'a' + 10 : \
     (c) >= 'A' && (c) <= 'F'? (c) - 'A' + 10 : -1)

    register int b, t, i;
    for (i=0; i<slen; i++) {
        t = __ord(s[i]);
        if (-1 == t) return (1);
        if (i%2) out[i/2] = (b << 4) | t;
        else     b = t;
    }
    if (i%2) out[i/2] = b;
    return (0);

#undef __ord
}

/*************************************************************/
int main() {
    unsigned char ins[] = "1230aAF0D";
    unsigned char out[20] = {0};
    int res = parse_hex_str(ins, sizeof(ins) - 1, out);
    printf("input = %s\n", ins);
    printf("res=%d\n", res);
    PRINTARRX(out, 10, "%02X");
    return (0);
}

Compilation:

gcc -ggdb -std=c99 -o hex.exe hex.c

Output:

input = 1230aAF0D
res=0
{ 12 30 AA F0 0D 00 00 00 00 00 }