Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!kepler1!jwu
From: jwu@kepler.com (Jasper Wu)
Newsgroups: comp.unix.programmer
Subject: problems with shmdt/shmat on SuSunOs 4.1.1
Message-ID: <593@kepler1.kepler.com>
Date: 7 Jun 91 17:43:47 GMT
Reply-To: jwu@kepler.com (Jasper Wu)
Organization: Kepler Financial Management, Ltd., Setauket, NY.
Lines: 47

I have a problem in using shmat/shmdt on Sun IPC runing SunOS 4.1.1.  When i 
tried to access a shared memory region created by another process, i got 
different behaviors on shmdt, depending on whether i set SHM_RDONLY flag or 
not when i did shmat.

Consider the following simlified codes: 

(error checking omitted for simplicity.)
(shmem created by another process with shmget(key,shmsize,IPC_CREATE|0666).)

	id = shmget(key, shmsize, perm);

(perm could be 0777, 0666, 0444, 0, 0644,.. no differences on the results.)

	addr = shmat(id, 0, 0);
	shmdt(addr);

"shmdt" succeeded as expected.  But if changed "shmat" line to:

	addr = shmat(id, 0, SHM_RDONLY);

then "shmdt" failed with EINVAL (Invalid argument).

Both shmat returned the same addr.(0xf7700000 when i tested it)


Manpage said:
     shmdt() will fail and not unmap the  shared  memory  segment
     if:

     EINVAL         shmaddr is not the data segment start address
                    of a shared memory segment.

Doesn't sound like the reason to me since i used addr returned by shmat and 
both shmat returned the same addr.


So, what's special about the SHM_RDONLY flag?  Is this the way it's supposed 
to work? Or perhaps it's a "feature" on SunOS 4.1.1  :-).

Any ideas would be appreciated.


-jasper

===========================
jasper wu   jwu@kepler.com