freeing malloc'd memory causes other malloc'd memory to garbage

Question

I'm trying to learn C, and one of the things I'm finding tricky is strings and manipulating them. I think I understand the basics of it, but I've taken for granted a lot of what might go into strings in JS or PHP (where I'm coming from).

I'm trying now to write a function that explodes a string into an array, based on a delimiter, using strtok. Similar to PHP's implementation of explode().

Here's the code:

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

char **explode(char *input, char delimiter) {
    char **output;
    char *token;
    char *string = malloc(sizeof(char) * strlen(input));
    char delimiter_str[2] = {delimiter, '\0'};
    int i;
    int delim_count = 0;

    for (i = 0; i < strlen(input); i++) {
        string[i] = input[i];
        if (input[i] == delimiter) {
            delim_count++;
        }
    }
    string[strlen(input)] = '\0';

    output = malloc(sizeof(char *) * (delim_count + 1));
    token = strtok(string, delimiter_str);

    i = 0;
    while (token != NULL) {
        output[i] = token;
        token = strtok(NULL, delimiter_str);
        i++;
    }

    // if i uncomment this line, output gets all messed up
    // free(string);

    return output;
}

int main() {
    char **row = explode("id,username,password", ',');
    int i;

    for (i = 0; i < 3; i++) {
        printf("%s\n", row[i]);
    }

    free(row);
    return 0;
}

The question I have is why if I try to free(string) in the function, the output gets messed up, and if I'm doing this incorrectly in the first place. I believe I'm just not mapping out the memory properly in my head and that's why I'm not understanding the issue.

Answer 1

you misunderstand what strtok does, It does not make new strings, it is simply returning a pointer to different parts of the original string. If you then free that string all the pointers you stored become invalid. I think you need

while (token != NULL) {
    output[i] = strdup(token);
    token = strtok(NULL, delimiter_str);
    i++;
}

strdup will allocated and copy a new string for you