Dangling pointers

Learn the dangling pointer concept and safety.

What is a dangling pointer?

A dangling pointer still points to memory that has been freed. After free(p) the value of p is unchanged; it still holds the old address.

Example: int *p = malloc(sizeof(int)); *p = 42; free(p); *p = 99;. Here the assignment after free is undefined behavior. The memory no longer belongs to you; it may be reused for another allocation. Such bugs often don't crash immediately; that makes them sneaky. valgrind reports them as Invalid write.

Set to NULL after free

A simple rule to reduce dangling pointer risk: free(p); p = NULL;. After setting the address to NULL, accidental *p use crashes the program; that's better than silent corruption.

Example: free(arr); arr = NULL;. Now you can use if (arr != NULL) to check whether the memory is valid. While accessing a NULL pointer is undefined behavior, on modern systems it crashes immediately (segfault); that helps you catch the bug early. This pattern is standard for long-lived and shared pointers.

Exercise Safe free pattern

Allocate a single int with int *p = malloc(sizeof(int));. Assign *p = 42; and print it with printf("%d\n", *p). Free it with free(p);, then set p = NULL;. Inside if (p != NULL) put printf("still valid\n"); this must not print "still valid".

Show example solution

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

int main(void) {
    int *p = malloc(sizeof(int));
    *p = 42;
    printf("%d\n", *p);
    free(p);
    p = NULL;
    if (p != NULL) printf("still valid\n");
    return 0;
}

Test yourself

What is p's value after free(p)?
- Still holds the old address
- Is deleted
- Becomes 0
- Becomes NULL
Show answer

Still holds the old address — p becomes a dangling pointer.
Match dangling and safe patterns

free(p); *p = 5;

Dangling
free(p); p = NULL;

Safe
if (p) *p = 5;

Guarded
free(NULL)

Valid (no-op)

Set to NULL after free

int *p = malloc(sizeof(int));
*p = 42;
free(p);
p = ____;

Show answer

int *p = malloc(sizeof(int));
*p = 42;
free(p);
p = NULL;

Next lesson: Allocation patterns

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