man-pages/man2/semctl.2

.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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.\" 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>
.\" Modified Sun Feb 18 01:59:29 2001 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 20 Dec 2001, Michael Kerrisk <mtk-manpages@gmx.net>
.\" Modified 21 Dec 2001, aeb
.\" Modified 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added notes on CAP_IPC_OWNER requirement
.\" Modified 17 Jun 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
.\"
.TH SEMCTL 2 2004-06-17 "Linux 2.6.7" "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>
.sp
.BI "int semctl(int " semid ", int " semnum ", int " cmd ", ...);"
.fi
.SH DESCRIPTION
The function
.B semctl
performs the control operation specified by
.I cmd
on the semaphore set identified by
.IR semid ,
or on the
.IR semnum -th
semaphore of that set.
(Semaphores are numbered starting at 0.)
.PP
This function has three or four arguments. When there
are four, the call is
.BI semctl( semid , semnum , cmd , arg );
where the fourth argument
.I arg
has a type
.B union semun
defined as follows:

.nf
#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 *array;    /* array for GETALL, SETALL */
                                /* Linux specific part: */
      struct seminfo *__buf;    /* buffer for IPC_INFO */
};
#endif
.fi
.PP
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 effective user\-ID of the calling process must be that of the
super\-user, or match the creator or owner of the semaphore set.
The argument
.I semnum
is ignored.
.TP
.B IPC_RMID
Immediately remove the semaphore set and its data structures
awakening all waiting processes (with an error return and
.B errno
set to
.BR EIDRM ).
The effective user\-ID of the calling process must be that of the
super\-user, or match the 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 entries are cleared for altered semaphores 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 failure
.B semctl
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 .
.LP
All other
.I cmd
values return 0 on success.
.SH ERRORS
On failure,
.B errno
will be set to one of the following:
.TP 11
.B EACCES
The argument
.I cmd
has one of the values
.BR GETALL ,
.BR GETPID ,
.BR GETVAL ,
.BR GETNCNT ,
.BR GETZCNT ,
.BR IPC_STAT ,
.BR SEM_STAT ,
.BR SETALL ,
or
.B SETVAL
and the calling process does not have the required
permissions on the semaphore set and does not have the
.B CAP_IPC_OWNER
capability.
.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 but the effective user ID of the calling process is not the creator
(as found in
.IR sem_perm.cuid )
or the owner
(as found in
.IR sem_perm.uid )
of the semaphore set,
and the process does not have the
.B CAP_SYS_ADMIN
capability.
.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.
.LP
Various fields in a \fIstruct semid_ds\fP were shorts under Linux 2.2
and have become longs 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 a IPC_64 flag in
.IR cmd .)
.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).
.LP
For greater portability it is best to always call
.B semctl
with four arguments.
.LP
Under Linux, the function
.B semctl
is not a system call, but is implemented via the system call
.BR ipc (2).
.SH "CONFORMING TO"
SVr4, SVID.  SVr4 documents more error conditions EINVAL and EOVERFLOW.
.SH "SEE ALSO"
.BR ipc (2),
.BR semget (2),
.BR semop (2),
.BR ipc (5),
.BR capabilities (7)