Python's binascii.unhexlify function in C

Question

I'm building a program that takes input as if it is a bare MAC address and turn it into a binary string. I'm doing this on a embedded system so there is no STD. I have been trying something similar to this question but after 2 days I haven't achieved anything, I'm really bad with these kind of things.

What I wanted is output to be equal to goal, take this into consideration:

#include <stdio.h>

int main() {
    const char* goal = "\xaa\xbb\xcc\xdd\xee\xff";
    printf("Goal: %s\n", goal);

    char* input = "aabbccddeeff";
    printf("Input: %s\n", input);

    char* output = NULL;
    // Magic code here

    if (output == goal) {
        printf("Did work! Yay!");
    } else {
        printf("Did not work, keep trying");
    }
}

Thanks, this is for a personal project and I really want to finish it

You need to allocate space to store your output; char *output = NULL: doesn't do that. You need to use strcmp() to compare strings; using == doesn't work (it compares two pointer values; unless you do output = goal; (or goal = output;), the comparison will fail). You've not even shown a basic attempt to convert aa into \xaa. — Jonathan Leffler
– Jonathan Leffler, Commented Mar 6, 2018 at 15:36
Possible duplicate of How to convert a char array to a byte array? — gre_gor
– gre_gor, Commented Mar 6, 2018 at 15:49

Jean-François Fabre · Accepted Answer · 2018-03-06 15:29:32Z

2

First, your comparison should use strcmp else it'll be always wrong.

Then, I would read the string 2-char by 2-char and convert each "digit" to its value (0-15), then compose the result with shifting

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

// helper function to convert a char 0-9 or a-f to its decimal value (0-16)
// if something else is passed returns 0...
int a2v(char c)
{
    if ((c>='0')&&(c<='9'))
    {
        return c-'0';
    }
    if ((c>='a')&&(c<='f'))
    {
        return c-'a'+10;
    }
    else return 0;
}

int main() {
    const char* goal = "\xaa\xbb\xcc\xdd\xee\xff";
    printf("Goal: %s\n", goal);

    const char* input = "aabbccddeeff";
    int i;

    char output[strlen(input)/2 + 1];
    char *ptr = output;

    for (i=0;i<strlen(input);i+=2)
    {

       *ptr++ = (a2v(input[i])<<4) + a2v(input[i]);
    }
    *ptr = '\0';
    printf("Goal: %s\n", output);

    if (strcmp(output,goal)==0) {
        printf("Did work! Yay!");
    } else {
        printf("Did not work, keep trying");
    }
}

answered Mar 6, 2018 at 15:29

Jean-François Fabre♦

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1 Comment

jamezrin Over a year ago

Thank you so much, I tested it and it works flawlessly, finally!

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