Dangling pointer example confusion

Question

Why isn't the following example correct? Why doesn't it demonstrate a dangling pointer? My teacher said it doesn't show the dangling pointer. Thanks in advance!

int X = 32;
int *p = &X;
free(p);
*p = 32; //<------Shouldn't this line cause dangling pointer ???

Same thing here. Why doesn't the following example demonstrate a memory leak?

void function(int x){
   int *p = &x;
   *p = 32;
   //shouln't this code show a warning as p was not freed?
}

you are trying to free memory that is on the stack. memory leaks happen from the free store (heap) — ForeverStudent, Nov 23 '15 at 16:08
Please read the [man page](http://linux.die.net/man/3/free) of `free()` first. — Sourav Ghosh, Nov 23 '15 at 16:15
in the first example, this line: `free(p);` is an error because 'p' was not set via a call to `malloc()` or `calloc()` or `realloc()` — user3629249, Nov 25 '15 at 19:18
failing to free some allocated memory will not cause a compiler to raise a warning/error. However, some tool, for instance `valgrind` will raise an error. However, getting the address of something is not allocating memory. Suggest speaking with your instructor to get a clarification on allocating/freeing memory and on values on the stack. — user3629249, Nov 25 '15 at 19:21

Giorgi Moniava · Accepted Answer · 2015-11-23T17:03:31.427

To cite Wikipedia:

Dangling pointer and wild pointers in computer programming are pointers that do not point to a valid object of the appropriate type.

Also you should only free memory which was allocated by malloc or similar allocation functions -it seems that is your confusion in both cases. Basically none of your examples need free.

An example of dangling pointer would be:

{
   char *ptr = NULL;

   {
       char c;
       ptr = &c;
   } 
     // c falls out of scope 
     // ptr is now a dangling pointer 
}

Also if you had example like:

int *p = malloc(sizeof(int));
*p = 9;
free(p); // now p is dangling

score 2 · Answer 2 · answered Nov 23 '15 at 16:03

2

Because X is not allocated on heap you cannot free p. To free you must use malloc, calloc or realloc.

Similarly, in second part again variable is on stack which will be automatically cleaned.

answered Nov 23 '15 at 16:03

Shiv

1,912
1
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score 2 · Answer 3 · answered Nov 23 '15 at 16:04

First code

This is undefined behavior.

N1256 7.20.3.2 The free function

If ptr is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by the calloc, malloc, or realloc function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.

Second code

This code won't cause memory leak by itself because it doesn't throw away any allocated buffer.

Dangling pointer example confusion

3 Answers3

First code

Second code

Linked