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 semaphore.

       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 vari‐
       able, 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 sema‐
       phore using sem_post(3), sem_wait(3), and so on.

       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)).

ATTRIBUTES
   Multithreading (see pthreads(7))
       The sem_init() function is thread-safe.

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 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.



Linux                                       2014-03-10                                SEM_INIT(3)