Dangling Pointers in C

The term "dangling pointers" in C is used to describe the behavior of a pointer when its target (the variable it is pointing to) has been deallocated or is no longer accessible. In other words, a dangling pointer in C is a pointer that doesn’t point to a valid variable of the appropriate type.

Why Do We Get Dangling Pointers in C?

Working with a dangling pointer can lead to unpredicted behavior in a C program and sometimes it may result in the program crashing. The situation of dangling pointers can occur due to the following reasons −

De-allocation of memory
Accessing an out-of-bounds memory location
When a variable goes out of scope

Let's analyze each of these three situations with the help of examples.

De-allocation of Memory

A pointer holds the address of a variable. If the target variable is deallocated or freed, then its pointer becomes a dangling pointer. Trying to access a pointer whose target variable has been deallocated results in garbage situations.

Let us use malloc() to create an integer variable and store its address in an integer pointer.

int *x = (int *) malloc(sizeof(int));
*x = 100;

Here, the pointer is referring to a valid location in the memory. Let us release the memory pointed by "x" using the free() function.

free(x);

Now, "x" stores an address that is no longer valid. Hence, if we try to dereference it, the compiler shows a certain garbage value.

Example

The following example shows how we end up getting dangling pointers in a C program −

#include <stdio.h>

int main(){

   int *x = (int *) malloc(sizeof(int));
   *x = 100;
   printf("x: %d\n", *x);

   free (x);
   printf("x: %d\n", *x);
}

Output

Run the code and check its output −

x: 100
x: 11665744

Accessing an Out-of-Bounds Memory Location

We know that a function can return a pointer. If it returns a pointer to any local variable inside the function, it results in a dangling pointer in the outer scope, as the location it points to is no longer valid.

Example

Take a look at the following code −

#include <stdio.h>

int * function();

int main(){

   int *x = function();
   printf("x: %d", *x);
   return 0;
}

int * function(){
   int a =100;
   return &a;
}

Output

When compiled, the following warning is displayed at the "return &a" statement in the function −

warning: function returns address of local variable [-Wreturn-local-addr]

If you run the program despite the warning, you get the following error −

Segmentation fault (core dumped)

When you get this error, it means that the program is trying to access a memory location that is out of bounds.

When a Variable Goes Out of Scope

The same reason applies when a variable declared in an inner block is accessed outside it. In the following example, we have a variable inside a block and its address is stored in a pointer variable.

However, outside the block, the pointer becomes a dangling pointer as its target is out of bounds.

Example

The following program shows how we get a dangling pointer when its base variable goes out of scope −

#include <stdio.h>

int main(){
   int *ptr;{
      int a = 10;
      ptr = &a;
   }

   // 'a' is now out of scope
   // ptr is a dangling pointer now
   printf("%d", ptr);

   return 0;
}

Output

It will display a garbage value −

How to Fix Dangling Pointers?

C doesn’t have the feature of automatic garbage collection, so we need to carefully manage the dynamically allocated memory.

To fix the issue of dangling pointers or to avoid them altogether, you need to apply proper memory management and try to avoid situations where you may end up getting dangling pointers.

Here are some general guidelines that you can follow to avoid dangling pointers −

Always ensure that pointers are set to NULL after the memory is deallocated. It will clearly signify that the pointer is no longer pointing to a valid memory location.
Avoid accessing a variable or a memory location that has gone out of scope.
Do not return pointers to local variables because such local variables will go out of scope when the function returns.

By following these guidelines, you can reduce the chances of getting dangling pointers in your code.