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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