Double Free

memory

double free or corruption

Description

The program attempts to free memory that has already been freed.

Common Causes

• Calling free on the same pointer twice
• Freeing a pointer that was not allocated with malloc/calloc
• Freeing a pointer that has been modified since allocation

Example of Error

Error Code

int main() {
    int *ptr = malloc(sizeof(int));
    free(ptr);
    free(ptr);  // Double free
    return 0;
}

Solution

Set pointers to NULL after freeing them and check before freeing.

Corrected Code

int main() {
    int *ptr = malloc(sizeof(int));
    if (ptr != NULL) {
        free(ptr);
        ptr = NULL;  // Set to NULL after freeing
    }
    
    // If we try to free again, check first
    if (ptr != NULL) {
        free(ptr);
        ptr = NULL;
    }

    return 0;
}

Additional Tips

• Always set pointers to NULL after freeing them
• Check if a pointer is NULL before freeing it
• Use tools like Valgrind to detect memory errors
• Consider using a memory management library for complex applications