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Old 01-28-2006
semaphores
Hi there,

Could someone please confirm which POSIX semaphore routines should be used for a multiprocess (and not multithreaded) environment?

sys/sem.h definitely works. but the routines, semget, semctl, semop are pretty unwieldy. So, I am looking for an easier way out.

From the man pages it also appears that routines in semaphore.h should work for multiprocesses.

#include <semaphore.h>
int sem_init(sem_t *sem, int pshared, unsigned int value);

The man pages say:
"If the pshared argument has a non-zero value, then the semaphore is shared between processes; in this case, any process that can access the semaphore sem can use sem for performing sem_wait(), sem_trywait(), sem_post(), and sem_destroy() operations."

However, the I am not able to achieve mutual exclusion with this. There are no compilation or runtime errors. The danm thing just doesn't work. Could somebody take a guess what I might be doing wrong?

regards,
Quantum Teleporter
 
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LEARN ABOUT NETBSD

semget

SEMGET(2)						      BSD System Calls Manual							 SEMGET(2)

NAME

     semget -- get set of semaphores

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/sem.h>

     int
     semget(key_t key, int nsems, int semflg);

DESCRIPTION

     The semget() system call returns the semaphore identifier associated with key.

     A new set containing nsems semaphores is created if either key is equal to IPC_PRIVATE, or key does not have a semaphore set associated with
     it and the IPC_CREAT bit is set in semflg.  If both the IPC_CREAT bit and the IPC_EXCL bit are set in semflg, and key has a semaphore set
     associated with it already, the operation will fail.

     If a new set of semaphores is created, the data structure associated with it (the semid_ds structure, see semctl(2)) is initialized as fol-
     lows:

     o	 sem_perm.cuid and sem_perm.uid are set to the effective uid of the calling process.

     o	 sem_perm.gid and sem_perm.cgid are set to the effective gid of the calling process.

     o	 sem_perm.mode is set to the lower 9 bits of semflg.

     o	 sem_nsems is set to the value of nsems.

     o	 sem_ctime is set to the current time.

     o	 sem_otime is set to 0.

RETURN VALUES

     semget() returns a non-negative semaphore identifier if successful.  Otherwise, -1 is returned and errno is set to reflect the error.

ERRORS

     [EACCES]		The caller has no permission to access a semaphore set already associated with key.

     [EEXIST]		Both IPC_CREAT and IPC_EXCL are set in semflg, and a semaphore set is already associated with key.

     [EINVAL]		nsems is less than 0 or greater than the system limit for the number in a semaphore set.

			A semaphore set associated with key exists, but has fewer semaphores than the number specified in nsems.

     [ENOSPC]		A new set of semaphores could not be created because the system limit for the number of semaphores or the number of sema-
			phore sets has been reached.

     [ENOENT]		IPC_CREAT is not set in semflg and no semaphore set associated with key was found.

SEE ALSO

     ipcs(1), semctl(2), semop(2), ftok(3)

STANDARDS

     The semget system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5'').

HISTORY

     Semaphores appeared in the first release of AT&T System V UNIX.

BSD
								   May 13, 2004 							       BSD