840 lines
19 KiB
HTML
840 lines
19 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of SEMCTL</TITLE>
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</HEAD><BODY>
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<H1>SEMCTL</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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semctl - System V semaphore control operations
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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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<B>int semctl(int </B><I>semid</I><B>, int </B><I>semnum</I><B>, int </B><I>cmd</I><B>, ...);</B>
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</PRE>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>semctl</B>()
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performs the control operation specified by
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<I>cmd</I>
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on the System V semaphore set identified by
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<I>semid</I>,
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or on the
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<I>semnum</I>-th
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semaphore of that set.
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(The semaphores in a set are numbered starting at 0.)
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<P>
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This function has three or four arguments, depending on
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<I>cmd</I>.
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When there are four, the fourth has the type
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<I>union semun</I>.
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The <I>calling program</I> must define this union as follows:
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<P>
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union semun {
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<BR> int val; /* Value for SETVAL */
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<BR> struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */
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<BR> unsigned short *array; /* Array for GETALL, SETALL */
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<BR> struct seminfo *__buf; /* Buffer for IPC_INFO
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<BR> (Linux-specific) */
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};
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<P>
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The
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<I>semid_ds</I>
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data structure is defined in <I><<A HREF="file:///usr/include/sys/sem.h">sys/sem.h</A>></I> as follows:
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<P>
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struct semid_ds {
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<BR> struct ipc_perm sem_perm; /* Ownership and permissions */
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<BR> time_t sem_otime; /* Last semop time */
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<BR> time_t sem_ctime; /* Last change time */
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<BR> unsigned long sem_nsems; /* No. of semaphores in set */
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};
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<P>
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The
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<I>ipc_perm</I>
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structure is defined as follows
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(the highlighted fields are settable using
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<B>IPC_SET</B>):
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<P>
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struct ipc_perm {
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<BR> key_t __key; /* Key supplied to <A HREF="/cgi-bin/man/man2html?2+semget">semget</A>(2) */
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<BR> uid_t <B>uid</B>; /* Effective UID of owner */
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<BR> gid_t <B>gid</B>; /* Effective GID of owner */
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<BR> uid_t cuid; /* Effective UID of creator */
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<BR> gid_t cgid; /* Effective GID of creator */
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<BR> unsigned short <B>mode</B>; /* Permissions */
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<BR> unsigned short __seq; /* Sequence number */
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};
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<P>
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Valid values for
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<I>cmd</I>
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are:
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<DL COMPACT>
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<DT id="1"><B>IPC_STAT</B>
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<DD>
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Copy information from the kernel data structure associated with
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<I>semid</I>
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into the
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<I>semid_ds</I>
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structure pointed to by
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<I>arg.buf</I>.
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The argument
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<I>semnum</I>
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is ignored.
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The calling process must have read permission on the semaphore set.
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<DT id="2"><B>IPC_SET</B>
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<DD>
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Write the values of some members of the
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<I>semid_ds</I>
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structure pointed to by
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<I>arg.buf</I>
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to the kernel data structure associated with this semaphore set,
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updating also its
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<I>sem_ctime</I>
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member.
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The following members of the structure are updated:
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<I>sem_perm.uid</I>,
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<I>sem_perm.gid</I>,
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and (the least significant 9 bits of)
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<I>sem_perm.mode</I>.
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The effective UID of the calling process must match the owner
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(<I>sem_perm.uid</I>)
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or creator
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(<I>sem_perm.cuid</I>)
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of the semaphore set, or the caller must be privileged.
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The argument
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<I>semnum</I>
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is ignored.
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<DT id="3"><B>IPC_RMID</B>
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<DD>
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Immediately remove the semaphore set,
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awakening all processes blocked in
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<B><A HREF="/cgi-bin/man/man2html?2+semop">semop</A></B>(2)
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calls on the set (with an error return and
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<I>errno</I>
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set to
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<B>EIDRM</B>).
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The effective user ID of the calling process must
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match the creator or owner of the semaphore set,
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or the caller must be privileged.
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The argument
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<I>semnum</I>
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is ignored.
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<DT id="4"><B>IPC_INFO</B> (Linux-specific)
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<DD>
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Return information about system-wide semaphore limits and
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parameters in the structure pointed to by
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<I>arg.__buf</I>.
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This structure is of type
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<I>seminfo</I>,
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defined in
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<I><<A HREF="file:///usr/include/sys/sem.h">sys/sem.h</A>></I>
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if the
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<B>_GNU_SOURCE</B>
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feature test macro is defined:
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<DT id="5"><DD>
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struct seminfo {
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<BR> int semmap; /* Number of entries in semaphore
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<BR> map; unused within kernel */
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<BR> int semmni; /* Maximum number of semaphore sets */
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<BR> int semmns; /* Maximum number of semaphores in all
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<BR> semaphore sets */
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<BR> int semmnu; /* System-wide maximum number of undo
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<BR> structures; unused within kernel */
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<BR> int semmsl; /* Maximum number of semaphores in a
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<BR> set */
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<BR> int semopm; /* Maximum number of operations for
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<BR> <A HREF="/cgi-bin/man/man2html?2+semop">semop</A>(2) */
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<BR> int semume; /* Maximum number of undo entries per
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<BR> process; unused within kernel */
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<BR> int semusz; /* Size of struct sem_undo */
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<BR> int semvmx; /* Maximum semaphore value */
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<BR> int semaem; /* Max. value that can be recorded for
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<BR> semaphore adjustment (SEM_UNDO) */
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};
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<DT id="6"><DD>
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The
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<I>semmsl</I>,
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<I>semmns</I>,
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<I>semopm</I>,
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and
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<I>semmni</I>
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settings can be changed via
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<I>/proc/sys/kernel/sem</I>;
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see
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<B><A HREF="/cgi-bin/man/man2html?5+proc">proc</A></B>(5)
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for details.
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<DT id="7"><B>SEM_INFO</B> (Linux-specific)
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<DD>
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Return a
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<I>seminfo</I>
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structure containing the same information as for
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<B>IPC_INFO</B>,
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except that the following fields are returned with information
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about system resources consumed by semaphores: the
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<I>semusz</I>
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field returns the number of semaphore sets that currently exist
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on the system; and the
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<I>semaem</I>
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field returns the total number of semaphores in all semaphore sets
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on the system.
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<DT id="8"><B>SEM_STAT</B> (Linux-specific)
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<DD>
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Return a
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<I>semid_ds</I>
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structure as for
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<B>IPC_STAT</B>.
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However, the
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<I>semid</I>
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argument is not a semaphore identifier, but instead an index into
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the kernel's internal array that maintains information about
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all semaphore sets on the system.
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<DT id="9"><B>SEM_STAT_ANY</B> (Linux-specific, since Linux 4.17)
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<DD>
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Return a
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<I>seminfo</I>
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structure containing the same information as for
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<B>SEM_STAT</B>.
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However,
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<I>sem_perm.mode</I>
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is not checked for read access for
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<I>semid</I>
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meaning that any user can employ this operation (just as any user may read
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<I>/proc/sysvipc/sem</I>
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to obtain the same information).
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<DT id="10"><B>GETALL</B>
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<DD>
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Return
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<B>semval</B>
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(i.e., the current value)
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for all semaphores of the set into
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<I>arg.array</I>.
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The argument
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<I>semnum</I>
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is ignored.
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The calling process must have read permission on the semaphore set.
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<DT id="11"><B>GETNCNT</B>
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<DD>
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Return the value of
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<B>semncnt</B>
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for the
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<I>semnum</I>-th
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semaphore of the set
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(i.e., the number of processes waiting for an increase of
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<B>semval</B>
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for the
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<I>semnum</I>-th
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semaphore of the set).
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The calling process must have read permission on the semaphore set.
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<DT id="12"><B>GETPID</B>
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<DD>
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Return the value of
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<B>sempid</B>
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for the
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<I>semnum</I>-th
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semaphore of the set.
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This is the PID of the process that last performed an operation on
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that semaphore (but see NOTES).
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The calling process must have read permission on the semaphore set.
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<DT id="13"><B>GETVAL</B>
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<DD>
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Return the value of
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<B>semval</B>
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for the
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<I>semnum</I>-th
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semaphore of the set.
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The calling process must have read permission on the semaphore set.
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<DT id="14"><B>GETZCNT</B>
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<DD>
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Return the value of
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<B>semzcnt</B>
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for the
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<I>semnum</I>-th
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semaphore of the set
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(i.e., the number of processes waiting for
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<B>semval</B>
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of the
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<I>semnum</I>-th
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semaphore of the set to become 0).
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The calling process must have read permission on the semaphore set.
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<DT id="15"><B>SETALL</B>
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<DD>
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Set
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<B>semval</B>
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for all semaphores of the set using
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<I>arg.array</I>,
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updating also the
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<I>sem_ctime</I>
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member of the
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<I>semid_ds</I>
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structure associated with the set.
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Undo entries (see
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<B><A HREF="/cgi-bin/man/man2html?2+semop">semop</A></B>(2))
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are cleared for altered semaphores in all processes.
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If the changes to semaphore values would permit blocked
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<B><A HREF="/cgi-bin/man/man2html?2+semop">semop</A></B>(2)
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calls in other processes to proceed, then those processes are woken up.
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The argument
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<I>semnum</I>
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is ignored.
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The calling process must have alter (write) permission on
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the semaphore set.
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<DT id="16"><B>SETVAL</B>
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<DD>
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Set the value of
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<B>semval</B>
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to
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<I>arg.val</I>
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for the
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<I>semnum</I>-th
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semaphore of the set, updating also the
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<I>sem_ctime</I>
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member of the
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<I>semid_ds</I>
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structure associated with the set.
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Undo entries are cleared for altered semaphores in all processes.
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If the changes to semaphore values would permit blocked
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<B><A HREF="/cgi-bin/man/man2html?2+semop">semop</A></B>(2)
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calls in other processes to proceed, then those processes are woken up.
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The calling process must have alter permission on the semaphore set.
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</DL>
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<A NAME="lbAE"> </A>
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<H2>RETURN VALUE</H2>
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On failure,
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<B>semctl</B>()
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returns -1
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with
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<I>errno</I>
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indicating the error.
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<P>
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Otherwise, the system call returns a nonnegative value depending on
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<I>cmd</I>
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as follows:
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<DL COMPACT>
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<DT id="17"><B>GETNCNT</B>
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<DD>
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the value of
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<B>semncnt</B>.
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<DT id="18"><B>GETPID</B>
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<DD>
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the value of
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<B>sempid</B>.
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<DT id="19"><B>GETVAL</B>
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<DD>
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the value of
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<B>semval</B>.
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<DT id="20"><B>GETZCNT</B>
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<DD>
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the value of
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<B>semzcnt</B>.
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<DT id="21"><B>IPC_INFO</B>
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<DD>
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the index of the highest used entry in the
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kernel's internal array recording information about all
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semaphore sets.
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(This information can be used with repeated
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<B>SEM_STAT</B>
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or
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<B>SEM_STAT_ANY</B>
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operations to obtain information about all semaphore sets on the system.)
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<DT id="22"><B>SEM_INFO</B>
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<DD>
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as for
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<B>IPC_INFO</B>.
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<DT id="23"><B>SEM_STAT</B>
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<DD>
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the identifier of the semaphore set whose index was given in
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<I>semid</I>.
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<DT id="24"><B>SEM_STAT_ANY</B>
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<DD>
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as for
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<B>SEM_STAT</B>.
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</DL>
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<P>
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All other
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<I>cmd</I>
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values return 0 on success.
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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="25"><B>EACCES</B>
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<DD>
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The argument
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<I>cmd</I>
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has one of the values
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<B>GETALL</B>,
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<B>GETPID</B>,
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<B>GETVAL</B>,
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<B>GETNCNT</B>,
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<B>GETZCNT</B>,
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<B>IPC_STAT</B>,
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<B>SEM_STAT</B>,
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<B>SEM_STAT_ANY</B>,
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<B>SETALL</B>,
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or
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<B>SETVAL</B>
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and the calling process does not have the required
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permissions on the semaphore set 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="26"><B>EFAULT</B>
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<DD>
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The address pointed to by
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<I>arg.buf</I>
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or
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<I>arg.array</I>
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isn't accessible.
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<DT id="27"><B>EIDRM</B>
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<DD>
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The semaphore set was removed.
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<DT id="28"><B>EINVAL</B>
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<DD>
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Invalid value for
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<I>cmd</I>
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or
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<I>semid</I>.
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Or: for a
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<B>SEM_STAT</B>
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operation, the index value specified in
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<I>semid</I>
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referred to an array slot that is currently unused.
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<DT id="29"><B>EPERM</B>
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<DD>
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The argument
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<I>cmd</I>
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has the value
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<B>IPC_SET</B>
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or
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<B>IPC_RMID</B>
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but the effective user ID of the calling process is not the creator
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(as found in
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<I>sem_perm.cuid</I>)
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or the owner
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(as found in
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|
<I>sem_perm.uid</I>)
|
|
|
|
of the semaphore set,
|
|
and the process does not have the
|
|
<B>CAP_SYS_ADMIN</B>
|
|
|
|
capability.
|
|
<DT id="30"><B>ERANGE</B>
|
|
|
|
<DD>
|
|
The argument
|
|
<I>cmd</I>
|
|
|
|
has the value
|
|
<B>SETALL</B>
|
|
|
|
or
|
|
<B>SETVAL</B>
|
|
|
|
and the value to which
|
|
<B>semval</B>
|
|
|
|
is to be set (for some semaphore of the set) is less than 0
|
|
or greater than the implementation limit
|
|
<B>SEMVMX</B>.
|
|
|
|
</DL>
|
|
<A NAME="lbAG"> </A>
|
|
<H2>CONFORMING TO</H2>
|
|
|
|
POSIX.1-2001, POSIX.1-2008, SVr4.
|
|
|
|
<P>
|
|
|
|
POSIX.1 specifies the
|
|
|
|
<I>sem_nsems</I>
|
|
|
|
field of the
|
|
<I>semid_ds</I>
|
|
|
|
structure as having the type
|
|
<I>unsigned short</I>,
|
|
|
|
and the field is so defined on most other systems.
|
|
It was also so defined on Linux 2.2 and earlier,
|
|
but, since Linux 2.4, the field has the type
|
|
<I>unsigned long</I>.
|
|
|
|
<A NAME="lbAH"> </A>
|
|
<H2>NOTES</H2>
|
|
|
|
The inclusion of
|
|
<I><<A HREF="file:///usr/include/sys/types.h">sys/types.h</A>></I>
|
|
|
|
and
|
|
<I><<A HREF="file:///usr/include/sys/ipc.h">sys/ipc.h</A>></I>
|
|
|
|
isn't required on Linux or by any version of POSIX.
|
|
However,
|
|
some old implementations required the inclusion of these header files,
|
|
and the SVID also documented their inclusion.
|
|
Applications intended to be portable to such old systems may need
|
|
to include these header files.
|
|
|
|
|
|
<P>
|
|
|
|
The
|
|
<B>IPC_INFO</B>,
|
|
|
|
<B>SEM_STAT</B>
|
|
|
|
and
|
|
<B>SEM_INFO</B>
|
|
|
|
operations are used by the
|
|
<B><A HREF="/cgi-bin/man/man2html?1+ipcs">ipcs</A></B>(1)
|
|
|
|
program to provide information on allocated resources.
|
|
In the future these may modified or moved to a
|
|
<I>/proc</I>
|
|
|
|
filesystem interface.
|
|
<P>
|
|
|
|
Various fields in a <I>struct semid_ds</I> were typed as
|
|
<I>short</I>
|
|
|
|
under Linux 2.2
|
|
and have become
|
|
<I>long</I>
|
|
|
|
under Linux 2.4.
|
|
To take advantage of this,
|
|
a recompilation under glibc-2.1.91 or later should suffice.
|
|
(The kernel distinguishes old and new calls by an
|
|
<B>IPC_64</B>
|
|
|
|
flag in
|
|
<I>cmd</I>.)
|
|
|
|
<P>
|
|
|
|
In some earlier versions of glibc, the
|
|
<I>semun</I>
|
|
|
|
union was defined in <I><<A HREF="file:///usr/include/sys/sem.h">sys/sem.h</A>></I>, but POSIX.1 requires
|
|
|
|
that the caller define this union.
|
|
On versions of glibc where this union is <I>not</I> defined,
|
|
the macro
|
|
<B>_SEM_SEMUN_UNDEFINED</B>
|
|
|
|
is defined in <I><<A HREF="file:///usr/include/sys/sem.h">sys/sem.h</A>></I>.
|
|
<P>
|
|
|
|
The following system limit on semaphore sets affects a
|
|
<B>semctl</B>()
|
|
|
|
call:
|
|
<DL COMPACT>
|
|
<DT id="31"><B>SEMVMX</B>
|
|
|
|
<DD>
|
|
Maximum value for
|
|
<B>semval</B>:
|
|
|
|
implementation dependent (32767).
|
|
</DL>
|
|
<P>
|
|
|
|
For greater portability, it is best to always call
|
|
<B>semctl</B>()
|
|
|
|
with four arguments.
|
|
|
|
<A NAME="lbAI"> </A>
|
|
<H3>The sempid value</H3>
|
|
|
|
POSIX.1 defines
|
|
<I>sempid</I>
|
|
|
|
as the "process ID of [the] last operation" on a semaphore,
|
|
and explicitly notes that this value is set by a successful
|
|
<B><A HREF="/cgi-bin/man/man2html?2+semop">semop</A></B>(2)
|
|
|
|
call, with the implication that no other interface affects the
|
|
<I>sempid</I>
|
|
|
|
value.
|
|
<P>
|
|
|
|
While some implementations conform to the behavior specified in POSIX.1,
|
|
others do not.
|
|
(The fault here probably lies with POSIX.1 inasmuch as it likely failed
|
|
to capture the full range of existing implementation behaviors.)
|
|
Various other implementations
|
|
|
|
also update
|
|
<I>sempid</I>
|
|
|
|
for the other operations that update the value of a semaphore: the
|
|
<B>SETVAL</B>
|
|
|
|
and
|
|
<B>SETALL</B>
|
|
|
|
operations, as well as the semaphore adjustments performed
|
|
on process termination as a consequence of the use of the
|
|
<B>SEM_UNDO</B>
|
|
|
|
flag (see
|
|
<B><A HREF="/cgi-bin/man/man2html?2+semop">semop</A></B>(2)).
|
|
|
|
<P>
|
|
|
|
Linux also updates
|
|
<I>sempid</I>
|
|
|
|
for
|
|
<B>SETVAL</B>
|
|
|
|
operations and semaphore adjustments.
|
|
However, somewhat inconsistently, up to and including 4.5,
|
|
Linux did not update
|
|
<I>sempid</I>
|
|
|
|
for
|
|
<B>SETALL</B>
|
|
|
|
operations.
|
|
This was rectified
|
|
|
|
in Linux 4.6.
|
|
<A NAME="lbAJ"> </A>
|
|
<H2>SEE ALSO</H2>
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+ipc">ipc</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+semget">semget</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?2+semop">semop</A></B>(2),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?7+capabilities">capabilities</A></B>(7),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?7+sem_overview">sem_overview</A></B>(7),
|
|
|
|
<B><A HREF="/cgi-bin/man/man2html?7+sysvipc">sysvipc</A></B>(7)
|
|
|
|
<A NAME="lbAK"> </A>
|
|
<H2>COLOPHON</H2>
|
|
|
|
This page is part of release 5.05 of the Linux
|
|
<I>man-pages</I>
|
|
|
|
project.
|
|
A description of the project,
|
|
information about reporting bugs,
|
|
and the latest version of this page,
|
|
can be found at
|
|
<A HREF="https://www.kernel.org/doc/man-pages/.">https://www.kernel.org/doc/man-pages/.</A>
|
|
<P>
|
|
|
|
<HR>
|
|
<A NAME="index"> </A><H2>Index</H2>
|
|
<DL>
|
|
<DT id="32"><A HREF="#lbAB">NAME</A><DD>
|
|
<DT id="33"><A HREF="#lbAC">SYNOPSIS</A><DD>
|
|
<DT id="34"><A HREF="#lbAD">DESCRIPTION</A><DD>
|
|
<DT id="35"><A HREF="#lbAE">RETURN VALUE</A><DD>
|
|
<DT id="36"><A HREF="#lbAF">ERRORS</A><DD>
|
|
<DT id="37"><A HREF="#lbAG">CONFORMING TO</A><DD>
|
|
<DT id="38"><A HREF="#lbAH">NOTES</A><DD>
|
|
<DL>
|
|
<DT id="39"><A HREF="#lbAI">The sempid value</A><DD>
|
|
</DL>
|
|
<DT id="40"><A HREF="#lbAJ">SEE ALSO</A><DD>
|
|
<DT id="41"><A HREF="#lbAK">COLOPHON</A><DD>
|
|
</DL>
|
|
<HR>
|
|
This document was created by
|
|
<A HREF="/cgi-bin/man/man2html">man2html</A>,
|
|
using the manual pages.<BR>
|
|
Time: 00:05:34 GMT, March 31, 2021
|
|
</BODY>
|
|
</HTML>
|