checking that `malloc` succeeded in C

Question

I want to allocate memory using malloc and check that it succeeded. something like:

if (!(new_list=(vlist)malloc(sizeof (var_list))))
  return -1;

how do I check success?

Your code already does check success. On failure it returns -1, on success it carries on to the next line. I hope `vlist` is a typedef for `var_list*`. — Steve Jessop, Apr 09 '11 at 20:10

Etienne de Martel · Accepted Answer · 2011-04-10T19:00:21.890

31

malloc returns a null pointer on failure. So, if what you received isn't null, then it points to a valid block of memory.

Since NULL evaluates to false in an if statement, you can check it in a very straightforward manner:

value = malloc(...);
if(value)
{
    // value isn't null
}
else
{
    // value is null
}

edited Apr 10 '11 at 19:00

answered Apr 09 '11 at 19:51

Etienne de Martel

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Zero can be used wherever NULL is appropriate, but NULL is not zero. It's NULL. It does always evaluate to false, though. – Philip Apr 09 '11 at 23:38
@Phillip My bad, I wrongly assumed that C was like C++ in this regard. – Etienne de Martel Apr 10 '11 at 19:00
2

in C (unlike C++) `(void*)0` is a null pointer constant. That's because a `void*` pointer in C (unlike C++) can be implicitly converted to any other pointer type, so it's possible to allow NULL to have `void*` type. So in both languages NULL is an implementation-defined null pointer constant, but C implementations have more freedom. – Steve Jessop Apr 12 '11 at 10:39
On some platforms NULL might not be 0 but some addr like 0xFFEE0000 so to be on a safe side and portable I would go with an explicit if (value != NULL) – Alex D Jan 26 '17 at 00:24
1

@Alex `NULL` is a _null pointer constant_, and that is guaranteed to evaluate to zero. If your particular implementation doesn't comply with the standard, I feel sorry for you, but on sane platforms you don't need that kind of check. – Etienne de Martel Jan 26 '17 at 05:13
@EtiennedeMartel Yep, you are right. The null pointer value sometimes might be other then 0, but it's guaranteed to evaluate to zero. – Alex D Jan 26 '17 at 05:26
@EtiennedeMartel Actually I gave it a thought and now I think I know why I doesn't like it. Readability issue. value is not a boolean expression and I consider it's a bad habit (encapsulated for over 30 years now) write code like if(!value), that's why C++ has an explicit keyword nullptr exactly for that reason; and in other languages like java and c# you cannot write something like if(!value) for a pointer. Even in C, especially in embedded mixing up 0 (zero) and null pointer may lead to some unpleasant side effects. – Alex D Jan 26 '17 at 17:38

score 8 · Answer 2 · edited Oct 20 '16 at 11:49

8

Man page :

If successful, calloc(), malloc(), realloc(), reallocf(), and valloc() functions return a pointer to allocated memory. If there is an error, they return a NULL pointer and set errno to ENOMEM.

edited Oct 20 '16 at 11:49

NathanOliver

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answered Apr 09 '11 at 19:52

Spyros

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score 8 · Answer 3 · answered Apr 09 '11 at 19:53

8

new_list=(vlist)malloc(sizeof (var_list)
if (new_list != NULL) {
  /* succeeded */
} else {
  /* failed */
}

answered Apr 09 '11 at 19:53

jdehaan

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score 1 · Answer 4 · answered Oct 20 '16 at 12:03

The code you have already tests for error, although I normally write the assignment and check as two separate lines:

new_list = malloc(sizeof *new_list);
if (!new_list)
    /* error handling here */;

(Note two small changes - you shouldn't cast the return value, and we take the size from the variable rather than its type to reduce the chance of a mismatch).

If malloc() fails, it returns a null pointer, which is the only pointer value that is false.

The error handling you have is simply return -1; - how you handle that in the calling function is up to you, really.

checking that `malloc` succeeded in C

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