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From: chris@mimsy.UUCP
Newsgroups: comp.lang.c,comp.unix.questions
Subject: Re: Question on large arrays in C
Message-ID: <5389@mimsy.UUCP>
Date: Wed, 11-Feb-87 23:49:50 EST
Article-I.D.: mimsy.5389
Posted: Wed Feb 11 23:49:50 1987
Date-Received: Thu, 12-Feb-87 23:30:26 EST
References: <1051@uwmacc.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 38
Keywords: C unix
Xref: utgpu comp.lang.c:987 comp.unix.questions:991
Summary: resource limit: stacksize 512K

In article <1051@uwmacc.UUCP> jwp@uwmacc.UUCP (Jeffrey W Percival) writes:
>I am running 4.3BSD on a MicroVax II. ...

with array declarations that require 20480*5*8 = 819200 bytes.

>When I run it, I get "Segmentation violation".

>If I move the 5 array declarations up above main(),
>under the define statement, the program works OK.
>What is wrong with the program as listed above?

Nothing.  This is a `feature' of a large virtual address space with
demand paging.  The system cannot tell whether many stack pointer
alterations are proper, so it uses a heuristic.  If the stack
pointer has been moved less than 512 kilobytes, the stack allocation
is a controlled one and is allowed.  If it has moved by more than
512K, it is assumed to be accidental, and the program is sent a
SIGSEGV.

This heuristic has an obvious flaw.  To fix it, Berkeley made the
limit not really 512K, but rather the `stacksize' resource limit
for the process.  This is set to 512K by init, and inherited by
all children.  It can be changed with the setrlimit() system call,
or with the C shell's built-in `limit' command:

	limit stacksize 1m

will raise it to one megabyte;

	unlimit stacksize

will raise it to the kernel's configured maximum, probably 16M.

Incidentally, on all machines with which I have worked, large arrays
are more efficiently accessed when static than when on the stack.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris@mimsy.umd.edu