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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  sema-
     phore.

     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  allo-
     cated 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.