Using free() corrupts char array data in C

Question

The problem:

I need to malloc a struct to populate a char *[64] array. This array gets corrupted when I free the struct. Specifically the first index. How should I deal with this?

int main(void) {
char *names[64];
uint32_t aCount = 0;
uint32_t count = 0;
vkEnumerateInstanceExtensionProperties(NULL,&count, NULL);
VkExtensionProperties *extension_names = malloc(sizeof(VkExtensionProperties) * count);
vkEnumerateInstanceExtensionProperties(NULL,&count,extension_names);

for(uint32_t i = 0; i < count; i++) {
    names[aCount++] = extension_names[i].extensionName;
}

printf("First extension available: %s\n",names[0]);

free(extension_names);

printf("First extension available: %s\n",names[0]);
return 0;}

Here is the result:

Before free()

First extension available: VK_KHR_device_group_creation

After free()

First extension available: ���yUU

Answer 1

You can use strdup to make copies of the strings and solve the "use after free" problem you are having:

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

#define UNUSED(x) \
    ((void)(x))

#define VK_MAX_EXTENSION_NAME_SIZE 256

typedef struct VkExtensionProperties {
    char        extensionName[VK_MAX_EXTENSION_NAME_SIZE];
    uint32_t    specVersion;
} VkExtensionProperties;

void vkEnumerateInstanceExtensionProperties(void *unused, 
        uint32_t *count, VkExtensionProperties *result) {
    UNUSED(unused);
    *count = 64;
    if (result) {
        for (int index = 0; index < *count; index++) {
            snprintf(result[index].extensionName, 
                    sizeof(result->extensionName), 
                    "extension%03d", 
                    index);
        }
    }
}

int main()
{
    char *names[64];
    uint32_t aCount = 0;
    uint32_t count = 0;
    vkEnumerateInstanceExtensionProperties(NULL, &count, NULL);
    VkExtensionProperties *extension_names = malloc(sizeof(VkExtensionProperties) * count);
    vkEnumerateInstanceExtensionProperties(NULL, &count, extension_names);

    for (uint32_t i = 0; i < count; i++) {
        names[aCount++] = strdup(extension_names[i].extensionName);
    }
    printf("First extension available: %s\n", names[0]);

    free(extension_names);

    printf("First extension available: %s\n", names[0]);

    return 0;
}

Output

First extension available: extension000
First extension available: extension000

I don't have Vulcan installed, so I simulated the behavior of the function you called.

Helpful GCC Flags

While I have your attention, don't forget to compile your code with -Wall -Werror to help you fix problems at compile time:

$ gcc -Wall -Werror -o program program.c