.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SEMCTL" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" semctl 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
semctl \- XSI semaphore control operations
.SH SYNOPSIS
.LP
\fB#include\ <sys/sem.h>
.br
.sp
int semctl(int\fP \fIsemid\fP\fB, int\fP \fIsemnum\fP\fB, int\fP \fIcmd\fP\fB,
\&...); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIsemctl\fP() function operates on XSI semaphores (see the Base
Definitions volume of IEEE\ Std\ 1003.1-2001, Section 4.15, Semaphore).
It is unspecified whether this function interoperates
with the realtime interprocess communication facilities defined in
\fIRealtime\fP .
.LP
The \fIsemctl\fP() function provides a variety of semaphore control
operations as specified by \fIcmd\fP. The fourth argument
is optional and depends upon the operation requested. If required,
it is of type \fBunion semun\fP, which the application shall
explicitly declare:
.sp
.RS
.nf

\fBunion semun {
    int val;
    struct semid_ds *buf;
    unsigned short  *array;
} arg;
\fP
.fi
.RE
.LP
The following semaphore control operations as specified by \fIcmd\fP
are executed with respect to the semaphore specified by
\fIsemid\fP and \fIsemnum\fP. The level of permission required for
each operation is shown with each command; see \fIXSI Interprocess
Communication\fP . The symbolic names for the values of \fIcmd\fP
are
defined in the \fI<sys/sem.h>\fP header:
.TP 7
GETVAL
Return the value of \fIsemval\fP; see \fI<sys/sem.h>\fP. Requires
read
permission.
.TP 7
SETVAL
Set the value of \fIsemval\fP to \fIarg.val\fP, where \fIarg\fP is
the value of the fourth argument to \fIsemctl\fP(). When
this command is successfully executed, the \fIsemadj\fP value corresponding
to the specified semaphore in all processes is
cleared. Requires alter permission; see \fIXSI Interprocess Communication\fP
\&.
.TP 7
GETPID
Return the value of \fIsempid\fP. Requires read permission.
.TP 7
GETNCNT
Return the value of \fIsemncnt\fP. Requires read permission.
.TP 7
GETZCNT
Return the value of \fIsemzcnt\fP. Requires read permission.
.sp
.LP
The following values of \fIcmd\fP operate on each \fIsemval\fP in
the set of semaphores:
.TP 7
GETALL
Return the value of \fIsemval\fP for each semaphore in the semaphore
set and place into the array pointed to by
\fIarg.array\fP, where \fIarg\fP is the fourth argument to \fIsemctl\fP().
Requires read permission.
.TP 7
SETALL
Set the value of \fIsemval\fP for each semaphore in the semaphore
set according to the array pointed to by \fIarg.array\fP,
where \fIarg\fP is the fourth argument to \fIsemctl\fP(). When this
command is successfully executed, the \fIsemadj\fP values
corresponding to each specified semaphore in all processes are cleared.
Requires alter permission.
.sp
.LP
The following values of \fIcmd\fP are also available:
.TP 7
IPC_STAT
Place the current value of each member of the \fBsemid_ds\fP data
structure associated with \fIsemid\fP into the structure
pointed to by \fIarg.buf\fP, where \fIarg\fP is the fourth argument
to \fIsemctl\fP(). The contents of this structure are
defined in \fI<sys/sem.h>\fP. Requires read permission.
.TP 7
IPC_SET
Set the value of the following members of the \fBsemid_ds\fP data
structure associated with \fIsemid\fP to the corresponding
value found in the structure pointed to by \fIarg.buf\fP, where \fIarg\fP
is the fourth argument to \fIsemctl\fP(): 
.sp
.RS
.nf

\fBsem_perm.uid
sem_perm.gid
sem_perm.mode
\fP
.fi
.RE
.LP
The mode bits specified in \fIIPC General Description\fP are copied
into the
corresponding bits of the \fIsem_perm.mode\fP associated with \fIsemid\fP.
The stored values of any other bits are
unspecified.
.LP
This command can only be executed by a process that has an effective
user ID equal to either that of a process with appropriate
privileges or to the value of \fIsem_perm.cuid\fP or \fIsem_perm.uid\fP
in the \fBsemid_ds\fP data structure associated with
\fIsemid\fP.
.TP 7
IPC_RMID
Remove the semaphore identifier specified by \fIsemid\fP from the
system and destroy the set of semaphores and \fBsemid_ds\fP
data structure associated with it. This command can only be executed
by a process that has an effective user ID equal to either
that of a process with appropriate privileges or to the value of \fIsem_perm.cuid\fP
or \fIsem_perm.uid\fP in the \fBsemid_ds\fP
data structure associated with \fIsemid\fP.
.sp
.SH RETURN VALUE
.LP
If successful, the value returned by \fIsemctl\fP() depends on \fIcmd\fP
as follows:
.TP 7
GETVAL
The value of \fIsemval\fP.
.TP 7
GETPID
The value of \fIsempid\fP.
.TP 7
GETNCNT
The value of \fIsemncnt\fP.
.TP 7
GETZCNT
The value of \fIsemzcnt\fP.
.TP 7
All others
0.
.sp
.LP
Otherwise, \fIsemctl\fP() shall return -1 and set \fIerrno\fP to indicate
the error.
.SH ERRORS
.LP
The \fIsemctl\fP() function shall fail if:
.TP 7
.B EACCES
Operation permission is denied to the calling process; see \fIXSI
Interprocess
Communication\fP .
.TP 7
.B EINVAL
The value of \fIsemid\fP is not a valid semaphore identifier, or the
value of \fIsemnum\fP is less than 0 or greater than or
equal to \fIsem_nsems\fP, or the value of \fIcmd\fP is not a valid
command.
.TP 7
.B EPERM
The argument \fIcmd\fP is equal to IPC_RMID or IPC_SET and the effective
user ID of the calling process is not equal to that
of a process with appropriate privileges and it is not equal to the
value of \fIsem_perm.cuid\fP or \fIsem_perm.uid\fP in the
data structure associated with \fIsemid\fP.
.TP 7
.B ERANGE
The argument \fIcmd\fP is equal to SETVAL or SETALL and the value
to which \fIsemval\fP is to be set is greater than the
system-imposed maximum.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
The fourth parameter in the SYNOPSIS section is now specified as \fB"..."\fP
in order to avoid a clash with the ISO\ C
standard when referring to the union \fIsemun\fP (as defined in Issue
3) and for backwards-compatibility.
.LP
The POSIX Realtime Extension defines alternative interfaces for interprocess
communication. Application developers who need to
use IPC should design their applications so that modules using the
IPC routines described in \fIXSI Interprocess Communication\fP can
be easily modified to use the alternative
interfaces.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIXSI Interprocess Communication\fP, \fIRealtime\fP, \fIsemget\fP(),
\fIsemop\fP(), \fIsem_close\fP(), \fIsem_destroy\fP(), \fIsem_getvalue\fP(),
\fIsem_init\fP(), \fIsem_open\fP(), \fIsem_post\fP(), \fIsem_unlink\fP(),
\fIsem_wait\fP(), the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<sys/sem.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .