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Semaphore

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Old 06-14-2012
Semaphore
If I create a semaphore and then I fork a number of child processes then all the child process use that same semaphore.
Since the process address spaces are different rfom each other then how all the child process are able to access the same semaphore?
I understand that semaphore/mutex is at os level out of process context.
Am I correct?
Where in which part of type/memory is this semaphore located?
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LEARN ABOUT CENTOS

sem_init

SEM_INIT(3)						     Linux Programmer's Manual						       SEM_INIT(3)

NAME

       sem_init - initialize an unnamed semaphore

SYNOPSIS

       #include <semaphore.h>

       int sem_init(sem_t *sem, int pshared, unsigned int value);

       Link with -pthread.

DESCRIPTION

       sem_init() initializes the unnamed semaphore at the address pointed to by sem.  The value argument specifies the initial value for the sem-
       aphore.

       The pshared argument indicates whether this semaphore is to be shared between the threads of a process, or between processes.

       If pshared has the value 0, then the semaphore is shared between the threads of a process, and should be located at some  address  that	is
       visible to all threads (e.g., a global variable, or a variable allocated dynamically on the heap).

       If pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory (see shm_open(3),
       mmap(2), and shmget(2)).  (Since a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore.)   Any
       process that can access the shared memory region can operate on the semaphore using sem_post(3), sem_wait(3), etc.

       Initializing a semaphore that has already been initialized results in undefined behavior.

RETURN VALUE

       sem_init() returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.

ERRORS

       EINVAL value exceeds SEM_VALUE_MAX.

       ENOSYS pshared is nonzero, but the system does not support process-shared semaphores (see sem_overview(7)).

CONFORMING TO

       POSIX.1-2001.

NOTES

       Bizarrely,  POSIX.1-2001  does  not  specify  the value that should be returned by a successful call to sem_init().  POSIX.1-2008 rectifies
       this, specifying the zero return on success.

SEE ALSO

       sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)

COLOPHON

       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2012-05-13							       SEM_INIT(3)