sem_init


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  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), 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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