466 lines
9.2 KiB
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466 lines
9.2 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SEMGET</TITLE>
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</HEAD><BODY>
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<H1>SEMGET</H1>
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Section: Linux Programmer's Manual (2)<BR>Updated: 2019-08-02<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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semget - get a System V semaphore set identifier
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<PRE>
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<B>#include <<A HREF="file:///usr/include/sys/types.h">sys/types.h</A>></B>
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<B>#include <<A HREF="file:///usr/include/sys/ipc.h">sys/ipc.h</A>></B>
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<B>#include <<A HREF="file:///usr/include/sys/sem.h">sys/sem.h</A>></B>
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</PRE>
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<P>
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<B>int semget(key_t </B><I>key</I><B>,</B>
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<B>int </B><I>nsems</I><B>,</B>
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<B>int </B><I>semflg</I><B>);</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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The
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<B>semget</B>()
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system call returns the System V semaphore set identifier
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associated with the argument
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<I>key</I>.
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It may be used either to obtain the identifier of a previously created
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semaphore set (when
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<I>semflg</I>
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is zero and
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<I>key</I>
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does not have the value
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<B>IPC_PRIVATE</B>),
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or to create a new set.
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<P>
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A new set of
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<I>nsems</I>
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semaphores is created if
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<I>key</I>
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has the value
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<B>IPC_PRIVATE</B>
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or if no existing semaphore set is associated with
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<I>key</I>
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and
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<B>IPC_CREAT</B>
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is specified in
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<I>semflg</I>.
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<P>
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If
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<I>semflg</I>
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specifies both
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<B>IPC_CREAT</B>
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and
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<B>IPC_EXCL</B>
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and a semaphore set already exists for
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<I>key</I>,
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then
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<B>semget</B>()
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fails with
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<I>errno</I>
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set to
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<B>EEXIST</B>.
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(This is analogous to the effect of the combination
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<B>O_CREAT | O_EXCL</B>
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for
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2).)
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<P>
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Upon creation, the least significant 9 bits of the argument
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<I>semflg</I>
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define the permissions (for owner, group and others)
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for the semaphore set.
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These bits have the same format, and the same
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meaning, as the
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<I>mode</I>
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argument of
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<B><A HREF="/cgi-bin/man/man2html?2+open">open</A></B>(2)
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(though the execute permissions are
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not meaningful for semaphores, and write permissions mean permission
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to alter semaphore values).
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<P>
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When creating a new semaphore set,
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<B>semget</B>()
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initializes the set's associated data structure,
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<I>semid_ds</I>
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(see
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<B><A HREF="/cgi-bin/man/man2html?2+semctl">semctl</A></B>(2)),
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as follows:
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<DL COMPACT>
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<DT id="1"><DD>
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<I>sem_perm.cuid</I>
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and
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<I>sem_perm.uid</I>
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are set to the effective user ID of the calling process.
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<DT id="2"><DD>
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<I>sem_perm.cgid</I>
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and
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<I>sem_perm.gid</I>
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are set to the effective group ID of the calling process.
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<DT id="3"><DD>
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The least significant 9 bits of
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<I>sem_perm.mode</I>
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are set to the least significant 9 bits of
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<I>semflg</I>.
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<DT id="4"><DD>
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<I>sem_nsems</I>
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is set to the value of
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<I>nsems</I>.
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<DT id="5"><DD>
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<I>sem_otime</I>
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is set to 0.
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<DT id="6"><DD>
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<I>sem_ctime</I>
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is set to the current time.
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</DL>
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<P>
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The argument
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<I>nsems</I>
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can be 0
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(a don't care)
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when a semaphore set is not being created.
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Otherwise,
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<I>nsems</I>
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must be greater than 0
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and less than or equal to the maximum number of semaphores per semaphore set
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(<B>SEMMSL</B>).
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<P>
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If the semaphore set already exists, the permissions are
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verified.
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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If successful, the return value will be the semaphore set identifier
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(a nonnegative integer), otherwise, -1
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is returned, with
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<I>errno</I>
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indicating the error.
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<A NAME="lbAF"> </A>
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<H2>ERRORS</H2>
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On failure,
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<I>errno</I>
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will be set to one of the following:
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<DL COMPACT>
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<DT id="7"><B>EACCES</B>
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<DD>
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A semaphore set exists for
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<I>key</I>,
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but the calling process does not have permission to access the set,
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and does not have the
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<B>CAP_IPC_OWNER</B>
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capability in the user namespace that governs its IPC namespace.
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<DT id="8"><B>EEXIST</B>
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<DD>
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<B>IPC_CREAT</B>
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and
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<B>IPC_EXCL</B>
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were specified in
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<I>semflg</I>,
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but a semaphore set already exists for
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<I>key</I>.
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<DT id="9"><B>EINVAL</B>
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<DD>
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<I>nsems</I>
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is less than 0 or greater than the limit on the number
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of semaphores per semaphore set
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(<B>SEMMSL</B>).
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<DT id="10"><B>EINVAL</B>
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<DD>
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A semaphore set corresponding to
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<I>key</I>
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already exists, but
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<I>nsems</I>
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is larger than the number of semaphores in that set.
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<DT id="11"><B>ENOENT</B>
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<DD>
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No semaphore set exists for
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<I>key</I>
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and
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<I>semflg</I>
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did not specify
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<B>IPC_CREAT</B>.
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<DT id="12"><B>ENOMEM</B>
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<DD>
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A semaphore set has to be created but the system does not have
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enough memory for the new data structure.
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<DT id="13"><B>ENOSPC</B>
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<DD>
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A semaphore set has to be created but the system limit for the maximum
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number of semaphore sets
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(<B>SEMMNI</B>),
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or the system wide maximum number of semaphores
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(<B>SEMMNS</B>),
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would be exceeded.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>CONFORMING TO</H2>
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SVr4, POSIX.1-2001.
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<A NAME="lbAH"> </A>
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<H2>NOTES</H2>
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The inclusion of
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<I><<A HREF="file:///usr/include/sys/types.h">sys/types.h</A>></I>
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and
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<I><<A HREF="file:///usr/include/sys/ipc.h">sys/ipc.h</A>></I>
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isn't required on Linux or by any version of POSIX.
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However,
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some old implementations required the inclusion of these header files,
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and the SVID also documented their inclusion.
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Applications intended to be portable to such old systems may need
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to include these header files.
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<P>
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<B>IPC_PRIVATE</B>
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isn't a flag field but a
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<I>key_t</I>
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type.
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If this special value is used for
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<I>key</I>,
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the system call ignores all but the least significant 9 bits of
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<I>semflg</I>
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and creates a new semaphore set (on success).
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<A NAME="lbAI"> </A>
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<H3>Semaphore initialization</H3>
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The values of the semaphores in a newly created set are indeterminate.
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(POSIX.1-2001 and POSIX.1-2008 are explicit on this point,
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although POSIX.1-2008 notes that a future version of the standard
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may require an implementation to initialize the semaphores to 0.)
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Although Linux, like many other implementations,
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initializes the semaphore values to 0,
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a portable application cannot rely on this:
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it should explicitly initialize the semaphores to the desired values.
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<P>
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Initialization can be done using
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<B><A HREF="/cgi-bin/man/man2html?2+semctl">semctl</A></B>(2)
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<B>SETVAL</B>
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or
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<B>SETALL</B>
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operation.
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Where multiple peers do not know who will be the first to
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initialize the set, checking for a nonzero
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<I>sem_otime</I>
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in the associated data structure retrieved by a
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<B><A HREF="/cgi-bin/man/man2html?2+semctl">semctl</A></B>(2)
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<B>IPC_STAT</B>
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operation can be used to avoid races.
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<A NAME="lbAJ"> </A>
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<H3>Semaphore limits</H3>
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The following limits on semaphore set resources affect the
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<B>semget</B>()
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call:
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<DL COMPACT>
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<DT id="14"><B>SEMMNI</B>
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<DD>
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System-wide limit on the number of semaphore sets.
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On Linux systems before version 3.19,
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the default value for this limit was 128.
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Since Linux 3.19,
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the default value is 32,000.
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On Linux, this limit can be read and modified via the fourth field of
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<I>/proc/sys/kernel/sem</I>.
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<DT id="15"><B>SEMMSL</B>
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<DD>
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Maximum number of semaphores per semaphore ID.
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On Linux systems before version 3.19,
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the default value for this limit was 250.
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Since Linux 3.19,
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the default value is 32,000.
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On Linux, this limit can be read and modified via the first field of
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<I>/proc/sys/kernel/sem</I>.
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<DT id="16"><B>SEMMNS</B>
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<DD>
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System-wide limit on the number of semaphores: policy dependent
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(on Linux, this limit can be read and modified via the second field of
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<I>/proc/sys/kernel/sem</I>).
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Note that the number of semaphores system-wide
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is also limited by the product of
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<B>SEMMSL</B>
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and
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<B>SEMMNI</B>.
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</DL>
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<A NAME="lbAK"> </A>
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<H2>BUGS</H2>
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The name choice
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<B>IPC_PRIVATE</B>
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was perhaps unfortunate,
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<B>IPC_NEW</B>
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would more clearly show its function.
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<A NAME="lbAL"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?2+semctl">semctl</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?2+semop">semop</A></B>(2),
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<B><A HREF="/cgi-bin/man/man2html?3+ftok">ftok</A></B>(3),
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<B><A HREF="/cgi-bin/man/man2html?7+capabilities">capabilities</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+sem_overview">sem_overview</A></B>(7),
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<B><A HREF="/cgi-bin/man/man2html?7+sysvipc">sysvipc</A></B>(7)
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<A NAME="lbAM"> </A>
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<H2>COLOPHON</H2>
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This page is part of release 5.05 of the Linux
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<I>man-pages</I>
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="17"><A HREF="#lbAB">NAME</A><DD>
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<DT id="18"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="19"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="20"><A HREF="#lbAE">RETURN VALUE</A><DD>
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<DT id="21"><A HREF="#lbAF">ERRORS</A><DD>
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<DT id="22"><A HREF="#lbAG">CONFORMING TO</A><DD>
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<DT id="23"><A HREF="#lbAH">NOTES</A><DD>
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<DL>
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<DT id="24"><A HREF="#lbAI">Semaphore initialization</A><DD>
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<DT id="25"><A HREF="#lbAJ">Semaphore limits</A><DD>
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</DL>
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<DT id="26"><A HREF="#lbAK">BUGS</A><DD>
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<DT id="27"><A HREF="#lbAL">SEE ALSO</A><DD>
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<DT id="28"><A HREF="#lbAM">COLOPHON</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:34 GMT, March 31, 2021
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