C++ - What is dangling pointer

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What is dangling pointer

Total Answers and Comments: 2

Last Update: October 30, 2009 Asked by: vijaymca

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Submitted by: sjappasab

pointers that do not point to a valid object of the appropriate type. Dangling pointers arise when an object is deleted or deallocated, without modifying the value of the pointer, so that the pointer still points to the memory location of the deallocated memory. As the system may reallocate the previously freed memory to another process, if the original program then dereferences the (now) dangling pointer, unpredictable behavior may result, as the memory may now contain completely different data. This is especially the case if the program writes data to memory pointed by a dangling pointer, as silent corruption of unrelated data may result, leading to subtle bugs that can be extremely difficult to find, or cause segmentation faults (*NIX) or general protection faults (Windows).

Above answer was rated as good by the following members:
mohinuddinkhan

June 25, 2008 06:36:13

sjappasab

Member Since: May 2008 Contribution: 1

RE: What is dangling pointer

pointers that do not point to a valid object of the appropriate type. Dangling pointers arise when an object is deleted or deallocated without modifying the value of the pointer so that the pointer still points to the memory location of the deallocated memory. As the system may reallocate the previously freed memory to another process if the original program then dereferences the (now) dangling pointer unpredictable behavior may result as the memory may now contain completely different data. This is especially the case if the program writes data to memory pointed by a dangling pointer as silent corruption of unrelated data may result leading to subtle bugs that can be extremely difficult to find or cause segmentation faults (*NIX) or general protection faults (Windows).

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Overall Rating: +1

October 30, 2009 00:57:30

mohinuddinkhan

Member Since: October 2009 Contribution: 2

RE: What is dangling pointer

Let us understand through the following code snippet
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define BLOCKSIZE 20;
int main()
{
char *ptr;
ptr (char *)malloc(BLOCKSIZE);
if(ptr! NULL)
strcpy(ptr "mohin khan");
printf("content s" ptr);
printf("naddress u" ptr);
free(ptr);
printf("nAfter free");
printf("ncontent s" ptr);
printf("naddress u" ptr);
}
output of the above code
content mohin khan
address 134520840 //(just a example)
After free
content
address 134520840 //same as the previous address

so we can conclude from the above code that
after free() also i keeps the address but content is deallocated.so now it is not pointing to any valid memory location.Hence ptr is now dangling pointer which probably having a address but not pointing to any valid memory location.
so it is good practice to assign NULL after freeing the allocated memory as below

ptr NULL;

in this way dangling problem will be solved.

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