CopyRight GPL mmc Mike Chirico

System V Semaphores:

(This document is current in rought draft format. Specific programs
used to verify results can be found at the following location:

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

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:

       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
	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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Chirico img 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 All consulting work must include a donation to Logo Logo