malloc with size 0

malloc with size 0

[Suma C]

Hi all

When  a 0 is passed to malloc, I get a valid pointer instead of a NULL
.What does this mean?

thanks  in advance

regards
suma

PS:
code:
#include <stdio.h>
#include <stdlib.h>

int main()
{
        char  *ptr = NULL;
        ptr = malloc(0);
        printf("ptr :%u \n",ptr);
        strcpy(ptr,"abcdefghijkl");
        printf("%s\n",ptr);  //no seg fault here????
        return 0;
}

Re: malloc with size 0

[Hareesh Nagarajan]

On 12/22/05, Suma C <gsumac@gmail.com> wrote:

> When  a 0 is passed to malloc, I get a valid pointer instead of a NULL
> .What does this mean?

A search on Google would have given you this thread:
http://www.theone.ru/lists/comp.lang.c/msg00504.html

hth,

./hareesh

Re: malloc with size 0

[Suma C]

http://www.ussg.iu.edu/hypermail/linux/kernel/0011.0/1078.html   got
this one...the thread discusses the same situation

I guess it talks about malloc allocating  a minimum memory (the header
space !!??)...
and so the next question will be: wt about the unique pointer which is
given in the man page!!!


regards
suma

PS
snip from the link above...
All true, but the reason it "works" is that malloc WILL allocate some memory,
even if it's only a few bytes of header.:

       | | (other memory block controled by malloc/free...)
       |-------|
       | header|
       | | - address returned to program
       | next |
       | header| (next memory block...)
Now the strcpy may have copied the string "fffff" over the next header.
The copy worked, the printf worked (its buffers were already allocated...)
BUT... If you allocate more memory via malloc, you will get an error
(eventually). I believe malloc(0) allocates 4 bytes as a minimum, though
this particular call IS undefined. You also did not check to see if
malloc did return something (It did, or you would have gotten a segmentation
fault from writing to location 0 with strcpy).


On 12/22/05, Hareesh Nagarajan <hareesh.nagarajan@gmail.com> wrote:
> On 12/22/05, Suma C <gsumac@gmail.com> wrote:
>
> > When  a 0 is passed to malloc, I get a valid pointer instead of a NULL
> > .What does this mean?
>
> A search on Google would have given you this thread:
> http://www.theone.ru/lists/comp.lang.c/msg00504.html
>
> hth,
>
> ./hareesh
>

Re: malloc with size 0

[Neil Horman]

On Thu, Dec 22, 2005 at 10:13:57AM +0530, Suma C wrote:
> Hi all
> 
> When  a 0 is passed to malloc, I get a valid pointer instead of a NULL
> .What does this mean?
> 
> thanks  in advance
> 
> regards
> suma
> 
> PS:
> code:
> #include <stdio.h>
> #include <stdlib.h>
> 
> int main()
> {
>         char  *ptr = NULL;
>         ptr = malloc(0);
>         printf("ptr :%u \n",ptr);
>         strcpy(ptr,"abcdefghijkl");
>         printf("%s\n",ptr);  //no seg fault here????
>         return 0;
> }
> -
> To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

http://www.opengroup.org/onlinepubs/000095399/toc.htm
If you pass 0 into malloc, you will get back NULL or a unique pointer suitable
for passing to free.  There is no guarantee about the amount of memory you can
write to beyond the pointer, and it is in fact not allocated to you (allocation
of 0).  As such, _dont_write_to_it_, as a subseuqent allocation may be
destroyed.  All the pointer is good for (if you get one back) is being passed to
free.

Regards
Neil

-- 
/***************************************************
 *Neil Horman
 *Software Engineer
 *gpg keyid: 1024D / 0x92A74FA1 - http://pgp.mit.edu
 ***************************************************/

RE: malloc with size 0

[mail-lists]

Hi,

This was  a strange problem I came across about 2 years ago.
Some systems return NULL some return a valid result.
I would treat it as a case where you would not try to alloc a size of 0 its
kind of unpredictable depending on lib version and which system your on.

	James

> -----Original Message-----
> From: linux-c-programming-owner@vger.kernel.org [mailto:linux-c-
> programming-owner@vger.kernel.org] On Behalf Of Suma C
> Sent: 22 December 2005 04:44
> To: C programming list
> Subject: malloc with size 0
> 
> Hi all
> 
> When  a 0 is passed to malloc, I get a valid pointer instead of a NULL
> .What does this mean?
> 
> thanks  in advance
> 
> regards
> suma
> 
> PS:
> code:
> #include <stdio.h>
> #include <stdlib.h>
> 
> int main()
> {
>         char  *ptr = NULL;
>         ptr = malloc(0);
>         printf("ptr :%u \n",ptr);
>         strcpy(ptr,"abcdefghijkl");
>         printf("%s\n",ptr);  //no seg fault here????
>         return 0;
> }
> -
> To unsubscribe from this list: send the line "unsubscribe linux-c-
> programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html