sem_init(3)

1SEM_INIT(3)                Linux Programmer's Manual               SEM_INIT(3)
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NAME

6       sem_init - initialise an unnamed semaphore
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SYNOPSIS

9       #include <semaphore.h>
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11       int sem_init(sem_t *sem, int pshared, unsigned int value);
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DESCRIPTION

14       sem_init()  initialises the unnamed semaphore at the address pointed to
15       by sem.  The value argument specifies the initial value for  the  sema‐
16       phore.
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18       The  pshared  argument indicates whether this semaphore is to be shared
19       between the threads of a process, or between processes.
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21       If pshared has the value 0, then the semaphore is  shared  between  the
22       threads  of  a  process,  and should be located at some address that is
23       visible to all threads (e.g., a global variable, or  a  variable  allo‐
24       cated dynamically on the heap).
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26       If pshared is non-zero, then the semaphore is shared between processes,
27       and should be located in a region of shared  memory  (see  shm_open(3),
28       mmap(2),  and  shmget(2)).   (Since a child created by fork(2) inherits
29       its parent's memory mappings, it can also access the  semaphore.)   Any
30       process  that  can  access  the shared memory region can operate on the
31       semaphore using sem_post(3), sem_wait(3), etc.
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33       Initialising a semaphore that has already been initialised  results  in
34       undefined behaviour.
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RETURN VALUE

37       sem_init() returns 0 on success; on error, -1 is returned, and errno is
38       set to indicate the error.
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ERRORS

41       EINVAL value exceeds SEM_VALUE_MAX.
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43       ENOSYS pshared is non-zero, but the system does  not  support  process-
44              shared semaphores (see sem_overview(7)).
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CONFORMING TO

47       POSIX.1-2001.
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NOTES

50       Bizarrely,  POSIX.1-2001  does  not  specify  the  value that should be
51       returned by a successful call to sem_init().  However, virtually  every
52       implementation returns zero in this case.
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