shared memory under SysV

[Ed Schwalenberg]

    Date: Wed, 11 May 88 01:50 EST
    From: EE784 at sscvax.mcmaster.ca

    Under uPort sysV/AT (a system V, rev. 2 port to IBM AT and clones [swapping,
    not paging:-) ]) shared memory is supported for, among other things, using
    the memory-mapped video RAM.  The man pages for the various shm(2) calls
    give the following synopses:
    ...
    and say the following about the return value of shmat:

	    Upon successful completion, the return value is as follows:
		    Shmat returns the data segment start address of the
		    attached shared memory segment.
		    [stuff about detaching deleted]
	    Otherwise, a value of -1 is returned and errno is set to
	    indicate the error.

    Since shared memory is available only under the large model, pointers are
    32 bits.  There is no apparent reason that a pointer cannot assume the
    equivalent of -1 for valid addresses (although trying to use it will
    probably generate a memory exception, since I certainly don't have that
    much physical memory).  My initial temptation was to assume the
    documentation was wrong and that a NULL pointer should be returned if
    shmat fails; however, the doc'n went out of its way to indicate that it
    should be -1 (presumably -1L) on failure.

    So, the questions are:
	    1) what is the proper return value of shmat on failure;
At least in the AT&T/Intel/Interactive System V for the 386, you can only
attach shared memory segments on 4Meg boundaries.  NULL is on a 4Meg
boundary, but -1 is not.  Normally, of course, your text segment lives at
0, but perhaps you can make an executable with the text segment elsewhere,
and use segment 0 for shared memory.

	    2) if it is -1, is it int or long (should be long, I believe).
    Any other insights cheerfully accepted.

    --mike borza    <EE784.SSCvax.McMaster.CA>