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From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2007-08-07 03:41:37

K. Noel Belcourt wrote:
> I should have mentioned that I ran ipcs, here's the output.
> [~]$ ipcs -a
> ------ Shared Memory Segments --------
> key shmid owner perms bytes nattch status
> 0x00000000 0 root 777 94208 0
> 0x00000000 65537 root 777 94208 1
> ------ Semaphore Arrays --------
> key semid owner perms nsems
> ------ Message Queues --------
> key msqid owner perms used-bytes messages
> What kind of IPC object does this test create? Why is there no IPC
> object in the kernel data structures? I don't believe there's
> anything running on my system that would call ipcrm.

This depends on the system, but for the moment it creates a shared
memory with an anonymous mutex. Interprocess uses POSIX primitives (see
/dev/shm or /dev/sem for POSIX resources) instead of system V ones and
emulates them with memory mapped files in the temp directory if POSIX
shared memory is not provided (unistd.h does not define
POSIX_SHARED_MEMORY_OBJECTS > 0). Your stack points that the thread is
blocked on:

while(value == InitializingSegment || value == UninitializedSegment){
    value = detail::atomic_read32(patomic_word);

which means that the thread has found the shared memory created but the
flag to synchronize the thread opening the segment until the segment is
initialized by the thread that created the segment is not updated. This
could mean that the thread that created the segment has been killed
before initializing properly the segment.

> There's two
> copies of this test currently running on my system (gcc-3.4.3 and
> gcc-4.0.1), would that matter?
> 19303 kbelco 25 0 14944 1380 1188 R 98.9 0.0 40:00.53
> named_recursive
> 19328 kbelco 25 0 14988 1360 1176 R 98.9 0.0 34:25.73
> named_recursive

This is really nasty. You should only have one instance, since the test
is just single-threaded. This can be causing several problems. Can you
investigate a bit why is this happening?

I'm thinking about the use of shared memory + anonymous synchronization
objects to emulate named synchronization objects and I think I should
use native named semaphores when available in systems that provide them.
I think it will use less resources. Added to my to-do list.



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