C: How to free memory after realloc for string arrays - c

Question

I'm learning C and want to create a simple function which keeps track of a list of paths.
I start with a default path ["/bin/"] and have a function "change_paths" which updates this list based on some input, so for example ["/bin/"] -> ["/temp/", "/auxillary/"].
However, I encounter problems when I try to free my memory at the end. Specifically, fsanitize tells me that the 6 bytes from my default PATH paths[0] = malloc(6 * sizeof(char)); are not freed properly.
What should I do to correctly free this memory if I use change_paths arbitrarily many times with arbitrarily many inputs (including possibly 0 new PATHs)? Is this structure appropriate for storing data with this mechanism?
Note: I've edited this code for brevity, so even though the line "char** new_paths = {"/temp/", "/auxillary/"};" is not technically correct, please think of it as working. I just want to give an example and I'm not sure how to define a char** manually.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void change_paths(int *num_paths, char*** paths, int num_new_paths, char** new_paths){
*paths = realloc(*paths, num_new_paths * sizeof(char*));
for (int i = 0; i < num_new_paths; i++){
*paths[i] = malloc((strlen(new_paths[i]) + 1) * sizeof(char));
strcpy(*paths[i], new_paths[i]);
}
}
int main(int argc, char *argv[]){
// A Default PATH
char** paths = malloc(1 * sizeof(char*));
paths[0] = malloc(6 * sizeof(char)); // <- HOW TO FREE THIS AFTER REALLOC OF paths?
strcpy(paths[0], "/bin/");
int num_paths = 1;
// A new set of PATHs
int num_new_paths = 2;
char** new_paths = {"/temp/", "/auxillary/"};
change_paths(&num_paths, &paths, num_new_paths, new_paths);
// Free memory
for(int i = 0; i < num_new_paths; i++){
free(paths[i]);
}
free(paths);
return 0;
}

Answer 1

You need to keep track of the number of old path entries, and free each one before you overwrite the reallocated space with new path entries.
void change_paths(int *num_paths, char ***paths, int num_new_paths, char **new_paths)
{
for (int i = 0; i < *num_paths; i++)
free((*paths)[i]);
*paths = realloc(*paths, num_new_paths * sizeof((*paths)[0]));
for (int i = 0; i < num_new_paths; i++)
{
(*paths)[i] = malloc((strlen(new_paths[i]) + 1));
strcpy((*paths)[i], new_paths[i]);
}
*num_paths = num_new_paths;
}
Consider using strdup() in the second loop:
void change_paths(int *num_paths, char ***paths, int num_new_paths, char **new_paths)
{
for (int i = 0; i < *num_paths; i++)
free((*paths)[i]);
*paths = realloc(*paths, num_new_paths * sizeof((*paths)[0]));
for (int i = 0; i < num_new_paths; i++)
(*paths)[i] = strdup(new_paths[i]));
*num_paths = num_new_paths;
}
Also, don't forget to error check memory allocations and handle them appropriately. Note that the idiom:
old_space = realloc(old_space, new_size);
leaks memory if the reallocation fails. It is important to use:
void *new_space = realloc(old_space, new_size);
if (new_space == NULL)
…deal with error…
old_space = new_space;
That leads to this code:
void change_paths(int *num_paths, char ***paths, int num_new_paths, char **new_paths)
{
for (int i = 0; i < *num_paths; i++)
free((*paths)[i]);
*num_paths = 0; // In case realloc fails
void *new_space = realloc(*paths, num_new_paths * sizeof((*paths)[0]));
if (new_space == NULL)
return;
*paths = new_space;
for (int i = 0; i < *num_paths; i++)
free((*paths)[i]);
for (int i = 0; i < num_new_paths; i++)
(*paths)[i] = strdup(new_paths[i]));
*num_paths = num_new_paths;
}
Note that if strdup() fails, some of the entries in the array may be null pointers. You could use other error handling for failures in strdup().
Warning: no compiler was consulted about the validity of any of this code.