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    Command:

    sem_init

    
    
    

    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  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

       Multithreading (see pthreads(7))
           The sem_init() function is thread-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.
    
    
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