jwu@kepler.com (Jasper Wu) (06/08/91)
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
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jasper wu jwu@kepler.com