CopyRight GPL mmc Mike Chirico mchirico@users.sourceforge.net
System V Semaphores:
(This document is current in rought draft format. Specific programs
used to verify results can be found at the following location:
http://cvs.sourceforge.net/viewcvs.py/cpearls/cpearls/src/development/semaphore/test/
)
System V Semaphores are counting semaphores. Under Linux they should only be used
for processes, or threads in separate processes that need to communicate. On the
other hand Posix semaphores should only be used for multiple threads under a single
process. See, there's a need for both.
Process 0: <--- SystemV between --> Process 1:
thread 0 thread 0
thread 1 thread 1
thread 2 thread 2
(use Posix semaphores (Posix within ^
for these threads. However, |
when these threads want to Posix
communicate with Process 1's |
threads, use SystemV semaphores) v )
Posix semaphores block on 0 and can go negative or positive. Under Linux,
there is no way to write an Ftok value. There is no sem_open command.
Sem_open is only used for named semaphores. Linux (2.4) only supports
memory-based semaphores. Memory-based semaphores are initialize by
sem_init.
There is an array of values "semval".
B2 stands for block sem_op at time 2
U is unblock
semval: current value of the semaphore. Array size is 3 for these examples
semnct: number of threads waiting for the semval to be greater than its current value
semzcnt: number of threads waiting for for semval to be ZERO.
semadj: adjustment value that's updated when SEM_UNDO flag is set. When this flag is
set the following occurs:
semadj += abs(sem_op)
if ( prog_terminates)
semval = semadj;
This is NOT the same as reversing the operation of the semaphore as some textbooks
incorrect state. Note the Linux man page:
BUGS
When a process terminates, its set of associated semadj structures is
used to undo the effect of all of the semaphore operations it performed
with the SEM_UNDO flag. This raises a difficulty: if one (or more) of
these semaphore adjustments would result in an attempt to decrease a
semaphore's value below zero, what should an implementation do? One
possible approach would be to block until all the semaphore adjustments
could be performed. This is however undesirable since it could force
process termination to block for arbitrarily long periods. Another
possibility is that such semaphore adjustments could be ignored alto-
gether (somewhat analogously to failing when IPC_NOWAIT is specified
for a semaphore operation). Linux adopts a third approach: decreasing
the semaphore value as far as possible (i.e., to zero) and allowing
process termination to proceed immediately.
The following has been created using 3 semaphores, going through 2 states. 2x3=6. That's
why there are six zero's for state 2: 0 0 0 0 0 0. This translates to
sem_num 0 is 0
sem_num 1 is 0
sem_num 2 is 0
second state to jump next in the same
process
sem_num 0 is 0
sem_num 1 is 0
sem_num 2 is 0
Time semval sem_op
1 1 0 0
2 0 0 0 0 0 0
3 1 0 0 B2
4 -1 1 0 0 0 0
5 0 1 0 B4
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Mike Chirico, a father of triplets (all girls) lives outside of
Philadelphia, PA, USA. He has worked with Linux since 1996, has a Masters
in Computer Science and Mathematics from Villanova University, and has
worked in computer-related jobs from Wall Street to the University of
Pennsylvania. His hero is Paul Erdos, a brilliant number theorist who was
known for his open collaboration with others.
Mike's notes page is souptonuts. For
open source consulting needs, please send an email to
mchirico@gmail.com. All consulting work must include a donation to
SourceForge.net.
