Function to convert hex string to ASCII values causes segfault

Question

I am trying to modify the code in this answer

https://stackoverflow.com/a/5403170/3657941

so that I have a function that converts a hex string to ASCII values which are stored in an output buffer.

I tried to use snprintf but I am getting a segmentation fault. In GDB I am getting

st=error reading variable: Cannot access memory at address 0x726f6445>
out=error reading variable: Cannot access memory at address 0x726f6441>

Here is my code:

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

int hex_to_int(char c)
{
    if (c >= 97)
        c = c - 32;
    int first = c / 16 - 3;
    int second = c % 16;
    int result = first * 10 + second;
    if (result > 9) result--;
    return result;
}

int hex_to_ascii(char c, char d){
        int high = hex_to_int(c) * 16;
        int low = hex_to_int(d);
        return high+low;
}

void hs(const char *st, char *out)
{
        //const char* st = "6665646F7261";
        int length = strlen(st);
        int i;
        char buf = 0;
        int offset = 0;

        for(i = 0; i < length; i++){
                if(i % 2 != 0){
                        //printf("%c", hex_to_ascii(buf, st[i]));

                        offset += snprintf(out + offset, 255 - offset, ":%c", hex_to_ascii(buf, st[i]));
                }else{
                        buf = st[i];
                }
        }

}

int main(){

        char str;
        hs("6665646F7261",&str);
        printf("%c\n", str);
}

I expect to send the function hs input "6665646F7261" in hex and get back the ASCII values in "fedora".

Answer 1

This converts the hex pairs into ASCII values and stores them in a buffer:

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

#define TARGET_SIZE(input) \
    ((strlen(input) / 2) + 1)

void convert(char *buffer, char *input) {
    // This version is a little better but we still rely on the caller
    // to give us a large enough buffer
    char storage[3];
    int offset = 0;
    assert(strlen(input) % 2 == 0);
    for (int index = 0; index < strlen(input); index+= 2) {
        storage[0] = input[index];
        storage[1] = input[index + 1];
        storage[2] = 0;
        // Now we handle upper and lower case hex values
        buffer[offset] = strtoul(storage, NULL, 16);
        offset++;
    }
    buffer[offset] = 0;
}

int main(void) {
    char fedora[] = "6665646F7261";
    char buffer[TARGET_SIZE(fedora)];
    convert(buffer, fedora);
    printf("%s\n", buffer);
    return 0;
}

Output

fedora