Void Pointer Error

Question

#include <iostream>

func(void *ptr)
{
 *ptr = NULL;          
}

int main()
{
 void *ptr = (int*)malloc(sizeof(int));   
 func(ptr);
 return 0;   
}

Can Some one Help me in Resolving this. I want to assign NULL to this Pointer. Do Not make any changes in Main().

Why do you need this? To change pointer in function call, you need to pass pointer to pointer. But I afraid you don't even understand usual pointers. — JIghtuse, Nov 21 '13 at 10:38
In C you should not cast the return of `malloc` (or any function returning a void pointer). You only need that in C++. — Some programmer dude, Nov 21 '13 at 10:38
void func(void *ptr) { ptr = NULL; } int main() { void *ptr = malloc(sizeof(int)); func(ptr); return 0; } — Rakesh_Kumar, Nov 21 '13 at 10:40
By the way, unless you `free` the allocated memory before calling `func` (after it's fixed) you will have a memory leak. — Some programmer dude, Nov 21 '13 at 10:40
to make changes in ptr declared in main...do like this void func(void **ptr) { *ptr = NULL; } int main() { void *ptr = malloc(sizeof(int)); func(&ptr); return 0; } — Rakesh_Kumar, Nov 21 '13 at 10:47
*(&ptr) = NULL; this is wht i was expecting.. some other way !! Hope fullyyy — Prag Rao, Nov 21 '13 at 10:54

Tony The Lion · Answer 1 · 2013-11-21T11:06:19.767

3

Pointers are passed by value. So change func(void *ptr) to func(void **ptr) and pass in func(&ptr)

edited Nov 21 '13 at 11:06

answered Nov 21 '13 at 10:38

Tony The Lion

1

Actually, once the signature is fixed, it should be `*ptr = NULL;` – StoryTeller - Unslander Monica Nov 21 '13 at 10:39
Well, if the OP passes the pointer by reference, then it indeed should be `*ptr = NULL`. :) – Some programmer dude Nov 21 '13 at 10:39
@StoryTeller: agree with you. – Deidrei Nov 21 '13 at 10:55
*(&ptr) = NULL; ............ – Prag Rao Nov 21 '13 at 10:56
Pointers are passed by value : NOT NECESSARY – Prag Rao Nov 21 '13 at 10:57

score 0 · Answer 2 · answered Nov 21 '13 at 10:38

0

use ptr = NULL; and don't cast the return of malloc.

answered Nov 21 '13 at 10:38

score 0 · Answer 3 · answered Nov 21 '13 at 11:08

0

Just Dont cast because C malloc returns void pointer by default. Then the program should be good to run

answered Nov 21 '13 at 11:08

Krishna Oza

Noo.. Still the same thing !!! I am not able to understand.. cant i do the *ptr = NULL. no one has given exxact reason.. instead of ignoring – Prag Rao Nov 21 '13 at 11:10

3 Answers3