How can I convert hex encoded string to string in C efficiently

I need to convert hex encoded string like this:

char hstr[9] = "61626364"; // characters abcd\0

Into

"abcd" // characters as hex: 0x61 0x62 0x63 0x64
       // hex "digits" a-f are always lowercase

At this moment I wrote this function:

#include <stdlib.h>

void htostr(char* hexstr, char* str) {
    int len = strlen(hexstr);

    for (int i = 0; i < len/2; i++) // edit: fixed bounds
    {
        char input[3] = { hexstr[2 * i], hexstr[2 * i + 1], 0 };
        *(str + i) = (char)strtol(input, NULL, 16);
    }
}

I'm using strtol function to do the job.

I feel I'm wasting 3 bytes of memory for input array and some processor time for copying two bytes and terminating with 0, because strtol function has no parameter like "length".

The code is supposed to run on a pretty busy microcontroller, the strings are quite long (it would be a good idea to free up the memory used by hexstr as soon as possible).

The question is: is there more efficient way to do this without writing my own converter from scratch?

By "from scratch" I mean low level conversion without using functions standard library.

Solution

Instead of copying two characters and using strtol you could create a function that converts the characters 0 .. 9 and A .. F to an int (0x0 to 0xF).

#include <ctype.h>

int toval(char ch) {
    if (isdigit((unsigned char)ch)) return ch - '0';
    return toupper((unsigned char)ch) - 'A' + 0x10;
}

Then looping over the string and adding up the result will be pretty straight forward:

void htostr(char *wr, const char *rd) {
    for (; rd[0] != '\0' && rd[1] != '\0'; rd += 2, ++wr) {
        // multiply the first with 0x10 and add the value of the second
        *wr = toval(rd[0]) * 0x10 + toval(rd[1]);
    }
    *wr = '\0'; // null terminate
}

Example usage:

#include <stdio.h>

int main() {
    char hstr[] = "61626364";
    char res[1 + sizeof hstr / 2];

    htostr(res, hstr);

    printf(">%s<\n", res);
}