Dangling Pointers in C Language

Categories: C language

The most common bugs related to pointers and memory management is dangling/wild pointers. Sometimes the programmer fails to initialize the pointer with a valid address, then this type of initialized pointer is known as a dangling pointer in C.

Dangling pointer occurs at the time of the object destruction when the object is deleted or de-allocated from memory without modifying the value of the pointer. In this case, the pointer is pointing to the memory, which is de-allocated. The dangling pointer can point to the memory, which contains either the program code or the code of the operating system. If we assign the value to this pointer, then it overwrites the value of the program code or operating system instructions; in such cases, the program will show the undesirable result or may even crash. If the memory is re-allocated to some other process, then we dereference the dangling pointer will cause the segmentation faults.


In the above figure, we can observe that the Pointer 3 is a dangling pointer. Pointer 1 and Pointer 2 are the pointers that point to the allocated objects, i.e., Object 1 and Object 2, respectively. Pointer 3 is a dangling pointer as it points to the de-allocated object.

Let's understand the dangling pointer through some C programs.

Using free() function to de-allocate the memory.

#include <stdio.h>  

int main()  

{  

   int *ptr=(int *)malloc(sizeof(int));  

   int a=560;  

   ptr=&a;  

   free(ptr);  

   return 0;  

}  

In the above code, we have created two variables, i.e., *ptr and a where 'ptr' is a pointer and 'a' is a integer variable. The *ptr is a pointer variable which is created with the help of malloc() function. As we know that malloc() function returns void, so we use int * to convert void pointer into int pointer.


In the above code, we did the following steps:

1. First, we declare the pointer variable named 'str'.

2. In the inner scope, we declare a character variable. The str pointer contains the address of the variable 'a'.

3. When the control comes out of the inner scope, 'a' variable will no longer be available, so str points to the de-allocated memory. It means that the str pointer becomes the dangling pointer.

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