I'm trying to create a function that converts a hex string into an array of hex bytes. Example: str = "1c01" -> hex_bytes = { 0x1c, 0x01 }.

When I try to print the hex values all I get are 0s. I'm thinking it's something to do with my pointers but I am not sure. Any help would be greatly appreciated.

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

const char *input_1 = "1c0111001f010100061a024b53535009181c";

unsigned int *str_to_hexbytes(const char *hex_str) {
    size_t len = strlen(hex_str);
    unsigned int *hex = malloc(sizeof(unsigned int)* len / 2);
    for(int i, j = 0; i < len; i  = 2, j  ) {
        char tmp[2];
        strncpy(tmp, hex_str   i, 2);
        hex[j] = strtol(tmp, NULL, 16);
    }
    return hex;
}

int main(void) {
    size_t len = strlen(input_1) / 2;
    unsigned int *hex = str_to_hexbytes(input_1);
    for (int i = 0; i < len; i  ) {
        printf("%x ", hex[i]);
    }
    return 0;
}

CodePudding user response：

tmp only has enough space to store the two characters you copy in. It does not have space for a null byte to terminate the string, and in fact strncpy won't write that null byte since it didn't find one in the two characters it read.

As a result, the strtol function reads past the end of the array, triggering undefined behavior.

Make tmp 3 characters long and add the null byte manually.

Also, you're only initializing j, not i, so make sure you do that as well.

for(int i = 0, j = 0; i < len; i  = 2, j  ) { 
    char tmp[3];
    strncpy(tmp, hex_str i, 2);
    tmp[2]=0;
    hex[j] = strtol(tmp, NULL, 16);
}

CodePudding user response：

You are not initializing i with 0. That might be your problem. int i, j = 0; only changes j's value to zero, i remains garbage since it is allocated from stack.

Also a few suggestions:

• Since you are using string's length in main too, you can only calculate it in main and send it to the function.
• You used a 'malloc' which requires you to call 'free' also. After you are done using the pointer call free(hex)
• Iterate until len - 1 since you are using one memory block ahead in your for loop's body.