Re: Question on the current behaviour of malloc () on Linux

[linux-os (Dick Johnson)]

On Wed, 21 Dec 2005, Jie Zhang wrote:

> Hi,
>
> I first asked this question on uClinux mailing list. My first question
> is <http://mailman.uclinux.org/pipermail/uclinux-dev/2005-December/036042.html>.
> Although I found this issue on uClinux, it's also can be demostrated
> on Linux. This is a small program:
>

Another FAQ....

> $> cat test2.c
> #include <stdio.h>
> #include <stdlib.h>
>
> int
> main ()
> {
>  char *p;
>  int i, j;
>  for (i = 0;; i++)
>    {
>      p = (char *) malloc (8 * 1024 * 1024);
>      if (p)
>        for (j = 0; j < 8 * 1024 * 1024; j++)
>          p[j] = 0x55;
>      else
>        {
>          printf ("%d fail\n", i);
>          break;
>        }
>    }
>  return 0;
> }
>
> When I run it on my Linux notebook, it will be killed. I expect to see
> it prints out   "fail".

Your expectations are not based upon any logic, only wishes.

>
> Thanks,
> Jie
>


To make your wishes come true execute:
     echo "1" >/proc/sys/vm/overcommit_memory
... as a super-user.

That will make malloc() fail when there isn't any more virtual
memory.

An even shorter program...

main ()
{
     for(;;)
         *(long *)malloc(0x0010000) = 1;
}

This will seg-fault because malloc will eventually return NULL and,
yawn, nothing except your program gets killed.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.55 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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