Date: Wed, 28 Jan 2009 18:07:18 +0530 From: Channa <channa.kad@gmail.com> To: Christoph Mallon <christoph.mallon@gmx.de> Cc: freebsd-current@freebsd.org Subject: Re: Jemalloc SEGV for 1MB chunk Message-ID: <515c64960901280437hd4f7b2dx36a1774afe2f5646@mail.gmail.com> In-Reply-To: <49804FCE.7090405@gmx.de> References: <515c64960901280339m17fa9309v2e1bc3f55454ab@mail.gmail.com> <49804597.6040303@gmx.de> <515c64960901280401w1e1d08bfx29adc124bc749c4a@mail.gmail.com> <E1LS9Pa-000NBW-Ct@kabab.cs.huji.ac.il> <515c64960901280425y642a190ka31409cfc2a2fd8f@mail.gmail.com> <49804FCE.7090405@gmx.de>
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Hi,
Thanks for the reply.
Does the same behaviour seen on FreeBSD environment?
I am using FreeBSD libraries to test the test code.
So according to you its better to NULL terminate in the test code to avoid
this undefined behaviour?
Thanks ,
Channa
On 28/01/2009, Christoph Mallon <christoph.mallon@gmx.de> wrote:
> Channa schrieb:
>
> > Hi,
> > Thanks for the reply.
> >
> > I understand , after terminating the string with NULL character no SEGV is
> seen.
> >
> > But if i change the request size to a value less than 1MB for eg: 4096
> Bytes,
> >
> > As in the below test code:
> >
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <string.h>
> >
> > int main()
> > {
> > int i;
> > char *buf;
> > size_t size = 4096 ;
> >
> > buf = malloc(size);
> > for (i = 0; i < size; i++)
> > buf[i] = 'a';
> > printf("The length of buff is : %d\n",strlen(buf));
> > free(buf);
> > return 0;
> > }
> >
> > I dont see any issues, without terminating the string with NULL
> > character the test code works fine. The issue is seen only for size
> > 1MB exactly.
> >
> > Can anyone explain this behaviour?
> >
>
> Undefined behaviour and (bad) luck. You are reading random garbage from
> memory. For a large allocation like 1MB you get page aligned memory and the
> page after the allocation is very likely not mapped, so you get a segfault
> when you try to access it.
> This really does not belong on freebsd-current@.
>
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