process shared semaphores?
deischen at freebsd.org
Thu Dec 3 01:03:46 UTC 2009
On Wed, 2 Dec 2009, Andrew Gallatin wrote:
> Daniel Eischen [deischen at freebsd.org] wrote:
>> On Wed, 2 Dec 2009, Andrew Gallatin wrote:
>>> The man page for sem_init(3) says:
>>> A non-zero value for pshared specifies a
>>> shared semaphore that can be used by multiple processes, which this
>>> implementation is not capable of.
>>> Is this still correct? I'm asking, both because it seems strange to
>>> not return an error if the implementation does not support pshared
>>> semaphores, and because the threads library seems to expect
>>> it to work. Eg:
>>> _sem_init(sem_t *sem, int pshared, unsigned int value)
>>> semid_t semid;
>>> semid = (semid_t)SEM_USER;
>>> if ((pshared != 0) && (ksem_init(&semid, value) != 0))
>>> return (-1);
>>> So is the man page out of date, or is the userspace code future-proof
>>> for when the kernel catches up?
>> The code should probably return -1 and ENOTSUP.
>> Why don't you use named semaphores if you want
>> process shared (sem_open)? Shouldn't those work?
> To be honest, I didn't know they even existed. I'm
> mostly a driver guy, and know little about user-space.
> I'm trying to keep up FreeBSD support on a project that
> is being developed mainly on Linux. I've suggested them
> to our main developer.
> In the meantime, I'd like to understand what's going on under the
> hood, and why what we're doing now on Linux (semaphore resides in
> shared memory allocated with shm_open) wouldn't work. It looks like
> it should work, since with pshared semaphores, it just passes
> everything through to ksem*. Is problem that the kernel doesn't
> really know about different processes using it? Eg, it has only seen a
> ksem_init() from the server, which did the sem_init(), and it needs
> the ksem_open() to know about other processes using it?
We had this same discussion last time. You have a
short memory, don't you? :-) :-)
The sem_t in FreeBSD is a pointer to a malloc'd struct
(see sem_alloc() in libc/gen/sem.c). A pointer to malloc'd
memory cannot be shared across processes (unless they are
all children I suppose).
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