What is Dangling Pointer with Cause and How to avoid it?

Dangling pointers in computer programming are pointers that pointing to a memory location that has been deleted (or freed).

Cause of dangling pointers

1. Return Local Variable in Function Call

#include<stdio.h>
#include<string.h>

char *getHello()
{
    char str[10];
    strcpy(str,"Hello!");
    return(str);
}

int main()
{
    //str falls out of scope
    //function call char *getHello() is now a dangling pointer 
    printf("%s", getHello());
}

2. Variable goes Out of Scope

#include<stdio.h>

int main()
{
    char **strPtr;
    {
        char *str = "Hello!";
        strPtr = &str;
    }
    // str falls out of scope 
    // strPtr is now a dangling pointer 
    printf("%s", *strPtr);
}

3. De-allocating or free variable memory

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

int main()
{
    char **strPtr;
    char *str = "Hello!";
	
    strPtr = &str;
    free(str);
    //strPtr now becomes a dangling pointer
	
    printf("%s", *strPtr);
}

Avoiding dangling pointer errors

We can avoid the dangling pointer errors by initialize pointer to NULL, after de-allocating memory, so that pointer will be no longer dangling. Assigning NULL value means pointer is not pointing to any memory location.

char **strPtr;
char *str = "Hello!";
	
strPtr = &str;

free (str);  /* strPtr now becomes a dangling pointer */
ptr = NULL;   /* strPtr is no more dangling pointer */