malloc (0)

[Bhaskar]

Hey can any one tell me what malloc 0 does, it assigns space in memory
i know that but is it usable and how many bytes does it allocate ,
allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

[Kiru Sengal]

Bhaskar wrote:

Hey can any one tell me what malloc 0 does, it assigns space in memory
i know that but is it usable and how many bytes does it allocate ,
allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

I believe it returns NULL.
You're asking for 0 (no) memory to be allocated by doing malloc(0).

[Lew Pitcher]

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Hash: SHA1
Bhaskar wrote:

Hey can any one tell me what malloc 0 does,

ITYM
malloc(0);

And, the answer is yes and no.

it assigns space in memory i know that

Well, you know wrong, then

but is it usable and how many bytes does it allocate ,

FWIW, you ask for 0 bytes, you might just get zero bytes

allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

The C standards (actually, the draft) says

7.20.3 Memory management functions
[snip]
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.

The key phrase is "implementa tion defined". That means that we can't
answer your questions (or even grant that your assertions are correct
or not) without knowing the details of the C implementation you are
using. As I read it, so long as your C compiler/environment's
implementation documents it, malloc(0) could legitimately launch
Russian ICBM missles at your house or cause daemons to fly out of your
nose (the dreaded nasal daemons). Or, it could manipulate its
underlying storage management components so as to remove zero
user-accessable bytes of storage from the appropriate pool of free
storage and place it in a pool of used storage (such an operation would
likely involve an internal allocation of 0+n bytes, where the n
represents the data management overhead).

HTH

- --
Lew Pitcher

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[Antoine Junod]

"Kiru Sengal" <ki*********@gm ail.comwrites:

Bhaskar wrote:

Hey can any one tell me what malloc 0 does, it assigns space in memory
i know that but is it usable and how many bytes does it allocate ,
allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

I believe it returns NULL.

I don't think so:

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

int main(int argc, char **argv){
int *i = (int *)malloc(0);

printf("%p\n", (void *)i);
printf("%p\n", NULL);
return 0;
}

output me:
toto@epia ~ $ ./a.out
0x804a050
(nil)

[Harald van Dijk]

Lew Pitcher wrote:

Bhaskar wrote:

Hey can any one tell me what malloc 0 does,

ITYM
malloc(0);

And, the answer is yes and no.

it assigns space in memory i know that

Well, you know wrong, then

but is it usable and how many bytes does it allocate ,

FWIW, you ask for 0 bytes, you might just get zero bytes

allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

The C standards (actually, the draft) says

7.20.3 Memory management functions
[snip]
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.

The key phrase is "implementa tion defined". That means that we can't
answer your questions (or even grant that your assertions are correct
or not) without knowing the details of the C implementation you are
using. As I read it, so long as your C compiler/environment's
implementation documents it, malloc(0) could legitimately launch
Russian ICBM missles at your house or cause daemons to fly out of your
nose (the dreaded nasal daemons).

"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."

Unless either returning a null pointer by itself in a user function, or
calling malloc with a nonzero size, is allowed to launch missiles, no,
malloc(0) isn't allowed to launch missiles either. Either it just
returns a null pointer, or it behaves (almost) as, for example,
malloc(1).

[Malcolm]

"Bhaskar" <bh***********@ gmail.comwrote in message
news:11******** **************@ 75g2000cwc.goog legroups.com...

Hey can any one tell me what malloc 0 does, it assigns space in memory
i know that but is it usable and how many bytes does it allocate ,
allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

It can either return a pointer to a block of no length, or it can return a
null pointer.

A block of no length might seem a slightly Hindu concept, but it is not that
difficult for anyone to understand, even if they don't apprecuiate all the
subtleties of zero..
Imagine a control block of 8 bytes which gieves information about the memory
allocated, so that free() can return it properly to the pool. malloc returns
a pointer to the meory immediately after this block.

Now image we allocate block A, of 4 bytes, and block B od zero bytes, the
block C of 4 bytes.

AAAAAAAAaaaaBBB BBBBBCCCCCCCCcc cc
^ ^ ^

The Chinese hats represent the pointers returned. When we call free(),
internally 8 bytes are subtracted and this tells the system where the
control information is.


--
Buy my book 12 Common Atheist Arguments (refuted)
$1.25 download or $7.20 paper, available www.lulu.com/bgy1mm

[Keith Thompson]

"Lew Pitcher" <lp******@sympa tico.cawrites:
[...]

The key phrase is "implementa tion defined". That means that we can't
answer your questions (or even grant that your assertions are correct
or not) without knowing the details of the C implementation you are
using.

Correct.

As I read it, so long as your C compiler/environment's
implementation documents it, malloc(0) could legitimately launch
Russian ICBM missles at your house or cause daemons to fly out of your
nose (the dreaded nasal daemons).

[snip]

No. It's implementation-defined behavior, not undefined behavior.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Nils O. Selåsdal]

Keith Thompson wrote:

"Lew Pitcher" <lp******@sympa tico.cawrites:
[...]

>The key phrase is "implementa tion defined". That means that we can't
answer your questions (or even grant that your assertions are correct
or not) without knowing the details of the C implementation you are
using.

Correct.

> As I read it, so long as your C compiler/environment's
implementati on documents it, malloc(0) could legitimately launch
Russian ICBM missles at your house or cause daemons to fly out of your
nose (the dreaded nasal daemons).

[snip]

No. It's implementation-defined behavior, not undefined behavior.

And in either case, the return value of malloc(0) is safe to pass
to free.

[Clever Monkey]

Nils O. Selåsdal wrote:

Keith Thompson wrote:

>"Lew Pitcher" <lp******@sympa tico.cawrites:
[...]

>>The key phrase is "implementa tion defined". That means that we can't
answer your questions (or even grant that your assertions are correct
or not) without knowing the details of the C implementation you are
using.

Correct.

>> As I read it, so long as your C compiler/environment's
implementatio n documents it, malloc(0) could legitimately launch
Russian ICBM missles at your house or cause daemons to fly out of your
nose (the dreaded nasal daemons).

[snip]

No. It's implementation-defined behavior, not undefined behavior.

And in either case, the return value of malloc(0) is safe to pass
to free.

True, but this is really about the standard behaviour for free(), rather
than anything specific to malloc() or its return. That is, the standard
says that when passing a null pointer to free() it does nothing.

[Joe Wright]

Bhaskar wrote:

Hey can any one tell me what malloc 0 does, it assigns space in memory
i know that but is it usable and how many bytes does it allocate ,
allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

Try this.

#include <stdio.h>
#include <stdlib.h>
int main(void) {
void *vp = malloc(0);
printf("malloc( 0) returned %p\n", vp);
return 0;
}

Here at home,

malloc(0) returned 902a0

vp is non-null and so I assume that malloc did what I asked and
allocated 0 bytes successfully. Do you think I should free(vp) to avoid
a leak?

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---