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From: tim@ism780c.UUCP (Tim Smith)
Newsgroups: net.lang.c,net.unix
Subject: Re: Microport and SCO Xenix Memory Managment Problems
Message-ID: <4104@ism780c.UUCP>
Date: Fri, 24-Oct-86 23:41:05 EST
Article-I.D.: ism780c.4104
Posted: Fri Oct 24 23:41:05 1986
Date-Received: Sun, 26-Oct-86 03:37:42 EST
References: <646@chinet.UUCP>
Reply-To: tim@ism780c.UUCP (Tim Smith)
Organization: Interactive Systems Corp., Santa Monica, CA
Lines: 49
Keywords: sbrk, brk, Microport, SCO Xenix
Xref: watmath net.lang.c:10816 net.unix:9701

sbrk has to behave funny because of program that assume a linear
address space.  Many program that want N bytes do the following:

		char * ptr;
		ptr = sbrk(0);
		brk(ptr+N);

To make these work correctly, sbrk(0) has to return the start of
the next segment.

For example, here is how brk and sbrk work on the AT&T pc6300+ (
which is a good indication of how AT&T thinks things should work
on 80286 based machines )

	( all byte counts and addresses are rounded to a multiple
	  of 512 )

	In small model, everything looks like a normal unix system

	In large model,

		sbrk(0) returns start of next data segment

		sbrk(N) N > 0 allocates N bytes from next data segment

		sbrk(N) N < 0 does this:
			N = -N
			while ( N >= size of last segment )
				N = N - size of last segment
				remove last segment
			remove N bytes from last segment

		brk(addr) ( where addr == byte N of segment S )
			if addr in current last segment, then set
			segment size to N

			if addr in next segment, then same as sbrk(N)

			if addr in an allocated part of a previous segment
			then throw away everything above addr

			else  error
-- 
member, all HASA divisions              POELOD  ECBOMB
                                        --------------
                                               ^-- Secret Satanic Message

Tim Smith       USENET: sdcrdcf!ism780c!tim   Compuserve: 72257,3706
                Delphi or GEnie: mnementh