change FREE preprocessor define to also put null into the pointer

Question

I have a preprocessor define:

#include "stdio.h"
#define FREE(ptr) free(ptr)

And I want to know if it is safe to change it to:

#include "stdio.h"
#define FREE(ptr) free(ptr); \
        (ptr) = null;

for code safe.

I think that because free is returning void so it can't be chained as shown at that question so it will be safe, but I'm not sure.

I'm also not sure what this has to do with the linked question, your macro doesn't `return` anything (and it also doesn't chain function calls) — UnholySheep, Sep 24 '20 at 09:56

Antti Haapala -- Слава Україні · Answer 1 · 2020-09-24T10:03:49.333

2

Your original is missing a semicolon, and this usage would not work after a control statement (if, else, while, for...) if not inside a block statement { ... }.

Since free does not return a value, for maximal compatibility (excepting the function pointer use, or use within comma operator. freeing the return value of a function, incrementing in call to free and other dubious tricks...) you could use the very common trick of wrapping into do { } while (0). The do { } while (0) will eat the following semicolon, and work alike even in nested if...else.

Therefore:

#define FREE(ptr) do { free(ptr); ptr = 0; } while (0)

edited Sep 24 '20 at 10:03

answered Sep 24 '20 at 09:58

Antti Haapala -- Слава Україні

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good answer, but still I will have the double evaluation problem that @chux - Reinstate Monica mentioned. – arye Sep 24 '20 at 10:57

score 1 · Answer 2 · answered Sep 24 '20 at 09:58

1

Fails in at least 3 ways:

#define FREE(ptr) free(ptr) \
    (ptr) = null;

null not define.

Not tied together. Consider

if (test) FREE(ptr);

is like

if (test) free(ptr);
ptr = null;

Double evaluation

FREE(foo());

is like

free(foo());
foo() = null;

answered Sep 24 '20 at 09:58

chux - Reinstate Monica

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for double evaluation perhaps should show the increment operator... – Antti Haapala -- Слава Україні Sep 24 '20 at 10:04
It would be good to show a solution for the problems. – Bodo Sep 24 '20 at 10:06
@AnttiHaapala Edit as you wish, IAC. your answer is a good one, setting aside double evaluation. – chux - Reinstate Monica Sep 24 '20 at 10:06
@Bodo `FREE(ptr)` is hiding an assignment. IMO, there is no _good_ solution. – chux - Reinstate Monica Sep 24 '20 at 10:08
I think there won't be double evaluation, but compilation error, no? it is bit better because there won't be problems we don't know about. – arye Sep 24 '20 at 11:00
@arye With the macro using `ptr` twice in `free(ptr)` and `(ptr) = ...`, `ptr` is evaluated twice. If a compilation error or not depends on `ptr`. – chux - Reinstate Monica Sep 24 '20 at 14:04
is there is an example where the double evaluation is bad (in current scenario) and ptr is not rvalue? I can think just on some weired scenarios that should not be at ggod code – arye Sep 26 '20 at 18:42
@arye The current scenario does not have any code using `FREE(), so I am unable to find any poor examples. – chux - Reinstate Monica Sep 26 '20 at 20:52
by current scenario I mean with the macro I suggested in the question, or better with the macro @AnttiHaapala suggested. this macro should be OK in any system and scenario that will use it, assuming proper use of the stdlib free function – arye Sep 29 '20 at 06:21

Jabberwocky · Answer 3 · 2020-09-24T11:18:40.800

You probably want this:

#define FREE(ptr) do {free(ptr); \
        (ptr) = NULL;} while (0)

instead of this:

#define FREE(ptr) free(ptr) \
        (ptr) = null;

...
int* a = malloc(10);
printf(" a = %p\n", (void*)a);
FREE(a);
printf(" a = %p\n", (void*)a);
...

This code snippet should print something similar to this:

a = 01bb95d9
a = 00000000

Read this for more information about the do... while(0) stuff.

A cleaner solution would be to put this into a function like this:

void FREE(void **ptr)
{
  free(*ptr);
  *ptr = NULL;
}

and call it like this:

int *a = malloc(10);
FREE(&a);
// a is NULL here

change FREE preprocessor define to also put null into the pointer

3 Answers3