Use After Free

memory

heap use after free

Description

The program uses memory after it has been freed, which can lead to unpredictable behavior.

Common Causes

• Accessing memory through a pointer after calling free on it
• Storing pointers to freed memory and using them later
• Complex data structures with dangling pointers

Example of Error

Error Code

int main() {
    int *ptr = malloc(sizeof(int));
    *ptr = 10;
    free(ptr);
    printf("%d\n", *ptr);  // Use after free
    return 0;
}

Solution

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

Corrected Code

int main() {
    int *ptr = malloc(sizeof(int));
    if (ptr != NULL) {
        *ptr = 10;
        free(ptr);
        ptr = NULL;  // Set to NULL after freeing
    }

    // Check before using
    if (ptr != NULL) {
        printf("%d\n", *ptr);
    } else {
        printf("Pointer is NULL\n");
    }

    return 0;
}

Additional Tips

• Always set pointers to NULL after freeing them
• Check if a pointer is NULL before dereferencing it
• Be careful with complex data structures that might contain freed pointers
• Use tools like AddressSanitizer or Valgrind to detect use-after-free errors