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), 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

       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                      │ Attribute     │ Value   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │sem_init()                                                     │ Thread safety │ MT-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.

EXAMPLES

       See shm_open(3) and sem_wait(3).

SEE ALSO

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

COLOPHON

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