What happens if malloc((size_t)NULL) in C

Question

What happen I write malloc((size_t)NULL) for dynamic memory allocation in C?

int main()
{
    char *ptr = malloc((size_t)NULL);
}

Is it allocate memory in heap section? or Is it undefined behavior?

Once you deal with the compiler warnings, you won't be running code like that. — Weather Vane, Apr 14 '17 at 06:58
This won't compile since malloc arguement needs to be numerical value. NULL is not a numerical value. char *ptr = malloc(sizeof(NULL)); allocates 8 bytes of memory. — Nguai al, Apr 14 '17 at 07:02
@M.M I get "warning C4047: 'function': 'size_t' differs in levels of indirection from 'void *'" and also "warning C4024: 'malloc': different types for formal and actual parameter 1" after adding `#include `. — Weather Vane, Apr 14 '17 at 07:04
This is undefined behaviour due to calling function returning `void*` without a prototype in scope — M.M, Apr 14 '17 at 07:05

Sourav Ghosh · Answer 1 · 2017-04-14T07:10:48.663

What happen I write malloc(NULL)..

Wait, stop, you do not write malloc(NULL), why would you?

The argument to malloc() is the "size" of the memory expected, that is not supposed to be a null pointer constant.

Quoting C11, chapter §7.22.3.4, (emphasis mine)

void *malloc(size_t size);
The malloc function allocates space for an object whose size is specified by size and whose value is indeterminate.

That said, for most cases, NULL is represented by integer constant value 0, so malloc(NULL) is equivalent to malloc(0), which again is implementation defined behavior.

Quoting chapter §7.22.3./P1

[...] If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

@M.M nopes, its implementation defined, kindly see the edit above. :) — Sourav Ghosh, Apr 14 '17 at 07:12

score 0 · Answer 2 · answered Apr 14 '17 at 06:59

0

This allocates the amount of memory defined by the constant NULL. If NULL == 0, then behavior is implementation defined.

answered Apr 14 '17 at 06:59

Erki Aring

2,032
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`NULL == 0` is always true – M.M Apr 14 '17 at 07:02
There is no indication in the posted code about the value of NULL. – Erki Aring Apr 14 '17 at 07:03

What happens if malloc((size_t)NULL) in C

2 Answers2