Why pointer is not null after free() (no copy pointer)

Question

I use only main() function. Not copying any pointer. Why free() function not get NULL value? Whay I can check my wariable is full or empty?

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

int main()
{
int *e = NULL;

 printf("before e=%p\n",(void *)e);
 e = malloc(sizeof(int));
 *e = 7;
 printf("after,  e=%p\n",(void *)e);
 if(e == NULL) { printf("Problemy z pamięcia\n"); return 0; }
 printf(" value = %d\n",*e);

 free(e);
 printf("after free,  e=%p\n",(void *)e);
 if(e == NULL) printf("is NULL\n");

return 1;
}

result

before e=0
after,  e=464027a0
 value = 7
after free,  e=0x7f82464027a0

why if(e==NULL) is not true? How do it?

because i need check it is 'e' release free() or not an my program, after free(). Sometimes i use free or not. Information about 'e' is random, empty is important for me after next lines — Alojzy Drabina, May 01 '18 at 05:33
`free` just frees the memory associated with the pointer. It does not set the variables value to NULL. You need to do that manually. — CannedMoose, May 01 '18 at 05:34
@AlojzyDrabina - Why do you think you need to check? Do you check that the compiler does arithmetic correctly? Just don't leak memory, and trust free. — StoryTeller - Unslander Monica, May 01 '18 at 05:35
@StoryTeller no my algorithm is more complicatet than this example — Alojzy Drabina, May 01 '18 at 05:36
All the more reason to be diligent and not leak memory. You are looking for a language/library solution to what is essentially a design/programming issue. — StoryTeller - Unslander Monica, May 01 '18 at 05:37

Basile Starynkevitch · Answer 1 · 2018-05-01T06:27:03.667

Because C has a call-by-value policy and free is a standard function. Every function call is not modifying its arguments, because formal parameters hold a copy of the argument passed at the call.

In practice, you could use the comma operator and systematically do

  free(e), e = NULL;

However, using a pointer (value) after it has been free-d is undefined behavior. Be scared of UB, so learn more about it.

You should compile with all warnings and debug info (so gcc -Wall -Wextra -g if using GCC) and learn to use the gdb debugger. You could use tools like valgrind or address sanitizers, since they are very helpful to hunt bugs such as memory leaks or buffer overflows.

BTW, your code is wrong. You don't test against failure of malloc (your if(e == NULL) is too late, and the preceding *e = 7; is likely to get a segmentation fault when malloc has failed) , and you really should code at least something like:

 e = malloc(sizeof(int));
 if (!e) { perror("malloc e"); exit(EXIT_FAILURE); };

The !e above could be replaced by e==NULL for readability.

`free(e), e = NULL`. Looks like a nice way of doing it. – J...S May 01 '18 at 05:41 — J...S, May 01 '18 at 05:41

Why pointer is not null after free() (no copy pointer)

1 Answers1