asBytes.c

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

typedef unsigned char uint8_t;

void printBits(uint8_t num){
    int size = sizeof(uint8_t);
    unsigned int maxPow = 1<<(size*8-1);
    int i=0,j;
    for(;i<size;++i){
        for(;i<size*8;++i){
            // print last bit and shift left.
            printf("%u",num&maxPow ? 1 : 0);
            num = num<<1;
        }
    }
}

uint8_t * asBytes(char *  s) {
    int nelements=strlen(s)/2;
    uint8_t * result = (uint8_t *) calloc(nelements,sizeof(uint8_t));    
    for(int i=0;i<nelements;i++){
        result[i]=(uint8_t)strtol((char[]){s[i*2],s[i*2+1]}, NULL, 16);
    }
    return result;
}

 
main()
{
    int i;
    char * test = "0123456789abcdef";
    uint8_t * result = asBytes(test);
    for(i=0;i<strlen(test)/2;i++)
    {    
        printBits(result[i]);
        printf(" ");
    }
    printf("\n");

    return 0;
}