Date: Wed, 28 Jan 2009 13:30:06 +0100 From: Christoph Mallon <christoph.mallon@gmx.de> To: Channa <channa.kad@gmail.com> Cc: freebsd-current@freebsd.org Subject: Re: Jemalloc SEGV for 1MB chunk Message-ID: <49804FCE.7090405@gmx.de> In-Reply-To: <515c64960901280425y642a190ka31409cfc2a2fd8f@mail.gmail.com> 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>
next in thread | previous in thread | raw e-mail | index | archive | help
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@.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?49804FCE.7090405>