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 -lrt or -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  pro-
       cesses.

       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 3.25 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                                                      2008-07-27                                                SEM_INIT(3)