.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
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.\" Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1 Jan 2002, Martin Schulze <joey@infodrom.org>
.\" Modified 4 Jan 2002, Michael Kerrisk <mtk16@ext.canterbury.ac.nz>
.\" Modified, 27 May 2004, Michael Kerrisk <mtk16@ext.canterbury.ac.nz>
.\"     Added notes on capability requirements
.\"
.TH SEMGET 2 2004-05-27 "Linux 2.6.6" "Linux Programmer's Manual"
.SH NAME
semget \- get a semaphore set identifier
.SH SYNOPSIS
.nf
.B
#include <sys/types.h>
.B
#include <sys/ipc.h>
.B
#include <sys/sem.h>
.fi
.sp
.BI "int semget(key_t " key ,
.BI "int " nsems ,
.BI "int " semflg );
.SH DESCRIPTION
This function returns the semaphore set identifier
associated with the argument
.IR key .
A new set of
.I nsems
semaphores is created if
.I key
has the value
.B IPC_PRIVATE
or if no existing semaphore set is associated to
.I key
and
.B IPC_CREAT
is asserted in
.I semflg
(i.e.
.IR semflg " &"
.B IPC_CREAT
isn't zero).
.PP
The presence in
.I semflg
of the fields
.B IPC_CREAT
and
.B IPC_EXCL
plays the same role, with respect to the existence
of the semaphore set, as the presence
of
.B O_CREAT
and
.B O_EXCL
in the mode argument of the
.BR open (2)
system call: i.e. the
.B semget
function fails if
.I semflg
asserts both
.B IPC_CREAT
and
.B IPC_EXCL
and a semaphore set already exists for
.IR key .
.PP
Upon creation, the low-order 9 bits of the argument
.I semflg
define the access permissions (for owner, group and others)
for the semaphore set.
These bits have the same format, and the same
meaning, as the mode argument in the
.BR open (2)
or
.BR creat (2)
system calls (though the execute permissions are
not meaningful for semaphores, and write permissions mean permission
to alter semaphore values).
.PP
When creating a new semaphore set,
.B semget
initializes the semaphore set's associated data structure
.B semid_ds
as follows:
.IP
.B sem_perm.cuid
and
.B sem_perm.uid
are set to the effective user\-ID of the calling process.
.IP
.B sem_perm.cgid
and
.B sem_perm.gid
are set to the effective group\-ID of the calling process.
.IP
The low-order 9 bits of
.B sem_perm.mode
are set to the low-order 9 bits of
.IR semflg .
.IP
.B sem_nsems
is set to the value of
.IR nsems .
.IP
.B sem_otime
is set to 0.
.IP
.B sem_ctime
is set to the current time.
.PP
The argument
.I nsems
can be
.B 0
(a don't care)
when a semaphore set is not being created.
Otherwise
.I nsems
must be greater than
.B 0
and less than or equal to the maximum number of semaphores per semaphore set
.RB ( SEMMSL ).
.PP
If the semaphore set already exists, the access permissions are
verified.
.\" and a check is made to see if it is marked for destruction.
.SH "RETURN VALUE"
If successful, the return value will be the semaphore set identifier
(a nonnegative integer), otherwise
.B \-1
is returned, with
.I errno
indicating the error.
.SH ERRORS
On failure
.I errno
will be set to one of the following:
.TP 11
.B EACCES
A semaphore set exists for
.IR key ,
but the calling process does not have permission to access the set,
and does not have the
.BR CAP_IPC_OWNER
capability.
.TP
.B EEXIST
A semaphore set exists for
.B key
and
.I semflg
was asserting both
.B IPC_CREAT
and
.BR IPC_EXCL .
.\" .TP
.\" .B EIDRM
.\" The semaphore set is marked to be deleted.
.TP
.B EINVAL
.IR nsems
is less than 0 or greater than the limit on the number
of semaphores per semaphore set
.RB ( SEMMSL ),
or a semaphore set corresponding to
.I key
already exists, and
.I nsems
is larger than the number of semaphores in that set.
.TP
.B ENOENT
No semaphore set exists for
.I key
and
.I semflg
wasn't asserting
.BR IPC_CREAT .
.TP
.B ENOMEM
A semaphore set has to be created but the system has not enough memory for
the new data structure.
.TP
.B ENOSPC
A semaphore set has to be created but the system limit for the maximum
number of semaphore sets
.RB ( SEMMNI ),
or the system wide maximum number of semaphores
.RB ( SEMMNS ),
would be exceeded.
.SH NOTES
.B IPC_PRIVATE
isn't a flag field but a
.B key_t
type.
If this special value is used for
.IR key ,
the system call ignores everything but the low-order 9 bits of
.I semflg
and creates a new semaphore set (on success).
.PP
The followings are limits on semaphore set resources affecting a
.B semget
call:
.TP 11
.B SEMMNI
System wide maximum number of semaphore sets: policy dependent.
.TP
.B SEMMSL
Maximum number of semaphores per semid: implementation dependent
(500 currently).
.TP
.B SEMMNS
System wide maximum number of semaphores: policy dependent.
Values greater than
.B SEMMSL * SEMMNI
makes it irrelevant.
.SH BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
would more clearly show its function.
.LP
The data structure associated with each semaphore in the set
isn't initialized by the system call.
In order to initialize those data structures, one has to execute a
subsequent call to
.BR semctl (2)
to perform a
.B SETVAL
or a
.B SETALL
command on the semaphore set.
.SH "CONFORMING TO"
SVr4, SVID.
SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN,
ERANGE, EFAULT.
.SH "SEE ALSO"
.BR semctl (2),
.BR semop (2),
.BR ftok (3),
.BR ipc (5),
.BR capabilities (7)