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 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

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