semaphore concept Post: 302134201

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Top Forums UNIX for Advanced & Expert Users semaphore concept Post 302134201 by porter on Wednesday 29th of August 2007 05:31:59 PM

08-29-2007

Registered User

Quote:

Originally Posted by maverix

I have used lockf() and flock() system calls and it seems to work fine.

However, I want to use semaphores to get the same effect. FYI...I am doing these just to understand and learn the concepts. Its not a live/industry project.

One thing to be aware of is the type of thing you are guarding determines the type of semphore.

If it's a file, then file locking is preferred, this way even if the file is on a NFS volume, the locking and sharing will still work.

If the thing is in a single process and the sharing is between threads then using pthread_mutex/pthread_cond et al.

If the thing is shared memory or similar that is being shared by processes that are on the same machine then you can use the kernel provided semahores.

I suggest reading anything by W.Richard Stevens on the subject.

porter

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semaphore.h

semaphore.h(3HEAD)						      Headers							semaphore.h(3HEAD)

NAME

       semaphore.h, semaphore - semaphores

SYNOPSIS

       #include <semaphore.h>

DESCRIPTION

       The  <semaphore.h>  header  defines  the sem_t type, used in performing semaphore operations. The semaphore can be implemented using a file
       descriptor, in which case applications are able to open up at least a total of {OPEN_MAX} files and semaphores. The  symbol  SEM_FAILED	is
       defined (see sem_open(3C)).

       Inclusion  of  the <semaphore.h> header can make visible symbols defined in the headers <fcntl.h> and <sys/types.h>. See fcntl.h(3HEAD) and
       types.h(3HEAD).

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Committed 		   |
       +-----------------------------+-----------------------------+
       |Standard		     |See standards(5). 	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       fcntl.h(3HEAD),	types.h(3HEAD),  sem_destroy(3C),   sem_getvalue(3C),	sem_init(3C),	sem_open(3C),	sem_post(3C),	sem_timedwait(3C),
       sem_unlink(3C), sem_wait(3C), attributes(5), standards(5)

SunOS 5.11							    5 Feb 2008							semaphore.h(3HEAD)