.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one
.\" 
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\" 
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" Modified Tue Oct 22 17:53:56 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Fri Jun 19 10:59:15 1998 by Andries Brouwer <aeb@cwi.nl>
.TH SEMCTL 2 "November 1, 1993" "Linux 0.99.13" "Linux Programmer's Manual" 
.SH NAME
semctl \- semaphore control operations
.SH SYNOPSIS
.nf
.B
#include <sys/types.h>
.B
#include <sys/ipc.h>
.B
#include <sys/sem.h>

#if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)
/* union semun is defined by including <sys/sem.h> */
#else
/* according to X/OPEN we have to define it ourselves */
union semun {
        int val;                    /* value for SETVAL */
        struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */
        unsigned short int *array;  /* array for GETALL, SETALL */
        struct seminfo *__buf;      /* buffer for IPC_INFO */
};
#endif
.fi
.sp
.BI "int semctl (int " semid ,
.BI "int " semnum ,
.BI "int " cmd ,
.BI "union semun " arg )
.SH DESCRIPTION
The function performs the control operation specified by
.I cmd
on the semaphore set (or on the
.IR semnum -th
semaphore of the set) identified by
.IR semid .
The first semaphore of the set is indicated by a value
.B 0
for
.IR semnum .
.PP
.sp
Legal values for
.I cmd
are
.TP 12
.B IPC_STAT
Copy info from the semaphore set data structure
into the structure pointed to by
.IB arg .buf \fR.
The argument
.I semnum
is ignored.
The calling process must have read access privileges on the semaphore set.
.TP
.B IPC_SET
Write the values of some members of the
.B semid_ds
structure pointed to by
.IB arg .buf
to the semaphore set data structure, updating also its
.B sem_ctime
member.
Considered members from the user supplied
.B "struct semid_ds"
pointed to by
.IB arg .buf
are
.nf
.sp
.ft B
	sem_perm.uid
	sem_perm.gid
	sem_perm.mode	\fR/* only lowest 9-bits */\fP
.fi
.ft R
.sp
The calling process effective user\-ID must be one among super\-user,
creator or owner of the semaphore set.
The argument
.I semnum
is ignored.
.TP
.B IPC_RMID
Remove immediately the semaphore set and its data structures
awakening all waiting processes (with an error return and
.B errno
set to
.BR EIDRM ).
The calling process effective user\-ID must be one among super\-user,
creator or owner of the semaphore set.
The argument
.I semnum
is ignored.
.TP
.B GETALL
Return
.B semval
for all semaphores of the set into
.IB arg .array \fR.
The argument
.I semnum
is ignored.
The calling process must have read access privileges on the semaphore set.
.TP
.B GETNCNT
The system call returns the value of
.B semncnt
for the
.IR semnum \-th
semaphore of the set
(i.e. the number of processes waiting for an increase of
.B semval
for the
.IR semnum \-th
semaphore of the set).
The calling process must have read access privileges on the semaphore set.
.TP
.B GETPID
The system call returns the value of
.B sempid
for the
.IR semnum \-th
semaphore of the set
(i.e. the pid of the process that executed the last
.B semop
call for the
.IR semnum \-th
semaphore of the set).
The calling process must have read access privileges on the semaphore set.
.TP
.B GETVAL
The system call returns the value of
.B semval
for the
.IR semnum \-th
semaphore of the set.
The calling process must have read access privileges on the semaphore set.
.TP
.B GETZCNT
The system call returns the value of
.B semzcnt
for the
.IR semnum \-th
semaphore of the set
(i.e. the number of processes waiting for
.B semval
of the
.IR semnum \-th
semaphore of the set to become 0).
The calling process must have read access privileges on the semaphore set.
.TP
.B SETALL
Set
.B semval
for all semaphores of the set using
.IB arg .array ,
updating also the
.B sem_ctime
member of the
.B semid_ds
structure associated to the set.
Undo entries are cleared for altered semaphores in all processes.
Processes sleeping on the wait queue are awakened if some
.B semval
becomes 0 or increases.
The argument
.I semnum
is ignored.
The calling process must have alter access privileges on the semaphore set.
.TP
.B SETVAL
Set the value of
.B semval
to
.IB arg .val
for the
.IR semnum \-th
semaphore of the set, updating also the
.B sem_ctime
member of the
.B semid_ds
structure associated to the set.
Undo entry is cleared for altered semaphore in all processes.
Processes sleeping on the wait queue are awakened if
.B semval
becomes 0 or increases.
The calling process must have alter access privileges on the semaphore set.
.SH "RETURN VALUE"
On fail the system call returns
.B \-1
with
.B errno
indicating the error.
Otherwise the system call returns a nonnegative value depending on
.I cmd
as follows:
.TP 11
.B GETNCNT
the value of
.BR semncnt .
.TP
.B GETPID
the value of
.BR sempid .
.TP
.B GETVAL
the value of
.BR semval .
.TP
.B GETZCNT
the value of
.BR semzcnt .
.SH ERRORS
For a failing return,
.B errno
will be set to one among the following values:
.TP 11
.B EACCES
The calling process has no access permissions needed to execute
.IR cmd .
.TP
.B EFAULT
The address pointed to by
.IB arg .buf
or
.IB arg .array
isn't accessible.
.TP
.B EIDRM
The semaphore set was removed.
.TP
.B EINVAL
Invalid value for
.I cmd
or
.IR semid .
.TP
.B EPERM
The argument
.I cmd
has value
.B IPC_SET
or
.B IPC_RMID
but the calling process effective user\-ID has insufficient
privileges to execute the command.
.TP
.B ERANGE
The argument
.I cmd
has value
.B SETALL
or
.B SETVAL
and the value to which
.B semval
has to be set (for some semaphore of the set) is less than 0
or greater than the implementation value
.BR SEMVMX .
.SH NOTES
The
.BR IPC_INFO ,
.BR SEM_STAT
and
.B SEM_INFO
control calls are used by the
.BR ipcs (8)
program to provide information on allocated resources.
In the future these can be modified as needed or moved to a proc file system
interface.
.PP
The following system limit on semaphore sets affects a
.B semctl
call:
.TP 11
.B SEMVMX
Maximum value for
.BR semval :
implementation dependent (32767).
.SH "CONFORMING TO"
SVr4, SVID.  SVr4 documents more error conditions EINVAL and EOVERFLOW.
.SH "SEE ALSO"
.BR ipc (5),
.BR shmget (2),
.BR shmat (2),
.BR shmdt (2)