mirror of https://github.com/mkerrisk/man-pages
107 lines
3.1 KiB
Groff
107 lines
3.1 KiB
Groff
'\" t
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.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.TH SEM_INIT 3 2006-03-25 "Linux" "Linux Programmer's Manual"
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.SH NAME
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sem_init \- initialize an unnamed semaphore
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.SH SYNOPSIS
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.nf
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.B #include <semaphore.h>
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.sp
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.BI "int sem_init(sem_t *" sem ", int " pshared ", unsigned int " value );
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.fi
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.sp
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Link with \fI\-lrt\fP or \fI\-pthread\fP.
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.SH DESCRIPTION
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.BR sem_init ()
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initializes the unnamed semaphore at the address pointed to by
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.IR sem .
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The
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.I value
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argument specifies the initial value for the semaphore.
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The
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.I pshared
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argument indicates whether this semaphore is to be shared
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between the threads of a process, or between processes.
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If
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.I pshared
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has the value 0,
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then the semaphore is shared between the threads of a process,
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and should be located at some address that is visible to all threads
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(e.g., a global variable, or a variable allocated dynamically on
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the heap).
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If
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.I pshared
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is non-zero, then the semaphore is shared between processes,
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and should be located in a region of shared memory (see
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.BR shm_open (3),
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.BR mmap (2),
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and
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.BR shmget (2)).
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(Since a child created by
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.BR fork (2)
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inherits its parent's memory mappings, it can also access the semaphore.)
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Any process that can access the shared memory region
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can operate on the semaphore using
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.BR sem_post (3),
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.BR sem_wait (3),
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etc.
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Initializing a semaphore that has already been initialized
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results in undefined behavior.
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.SH RETURN VALUE
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.BR sem_init ()
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returns 0 on success;
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on error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EINVAL
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.I value
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exceeds
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.BR SEM_VALUE_MAX .
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.TP
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.B ENOSYS
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.I pshared
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is non-zero,
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but the system does not support process-shared semaphores (see
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.BR sem_overview (7)).
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.SH CONFORMING TO
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POSIX.1-2001.
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.SH NOTES
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Bizarrely, POSIX.1-2001 does not specify the value that should
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be returned by a successful call to
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.BR sem_init ().
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However, virtually every implementation returns zero in this case.
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.SH "SEE ALSO"
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.BR sem_destroy (3),
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.BR sem_post (3),
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.BR sem_wait (3),
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.BR sem_overview (7)
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