sem_init(3)

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

6       sem_init - initialize 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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13       Link with -pthread.
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DESCRIPTION

16       sem_init()  initializes the unnamed semaphore at the address pointed to
17       by sem.  The value argument specifies the initial value for  the  sema‐
18       phore.
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20       The  pshared  argument indicates whether this semaphore is to be shared
21       between the threads of a process, or between processes.
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23       If pshared has the value 0, then the semaphore is  shared  between  the
24       threads  of  a  process,  and should be located at some address that is
25       visible to all threads (e.g., a global variable, or  a  variable  allo‐
26       cated dynamically on the heap).
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28       If  pshared is nonzero, then the semaphore is shared between processes,
29       and should be located in a region of shared  memory  (see  shm_open(3),
30       mmap(2),  and  shmget(2)).   (Since a child created by fork(2) inherits
31       its parent's memory mappings, it can also access the  semaphore.)   Any
32       process  that  can  access  the shared memory region can operate on the
33       semaphore using sem_post(3), sem_wait(3), and so on.
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35       Initializing a semaphore that has already been initialized  results  in
36       undefined behavior.
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RETURN VALUE

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

43       EINVAL value exceeds SEM_VALUE_MAX.
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45       ENOSYS pshared is nonzero, but the system  does  not  support  process-
46              shared semaphores (see sem_overview(7)).
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ATTRIBUTES

49       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
50       tributes(7).
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52       ┌───────────┬───────────────┬─────────┐
53       │Interface  │ Attribute     │ Value   │
54       ├───────────┼───────────────┼─────────┤
55       │sem_init() │ Thread safety │ MT-Safe │
56       └───────────┴───────────────┴─────────┘

CONFORMING TO

58       POSIX.1-2001.
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NOTES

61       Bizarrely, POSIX.1-2001 does not specify the value that should  be  re‐
62       turned  by  a  successful  call  to sem_init().  POSIX.1-2008 rectifies
63       this, specifying the zero return on success.
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EXAMPLES

66       See shm_open(3) and sem_wait(3).
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COLOPHON

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73       description  of  the project, information about reporting bugs, and the
74       latest    version    of    this    page,    can     be     found     at
75       https://www.kernel.org/doc/man-pages/.
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79Linux                             2020-08-13                       SEM_INIT(3)