.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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.\" Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond <esr@thyrsus.com>
.TH SEMGET 2 "November 1, 1993" "Linux 0.99.13" "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
The function returns the semaphore set identifier
associated to the value of the argument
.IR key .
A new set of
.I nsems
semaphores is created if
.I key
has value
.B IPC_PRIVATE
or
.I key
isn't
.BR IPC_PRIVATE ,
no existing semaphore set is associated to
.IR key ,
and
.B IPC_CREAT
is asserted in
.I semflg
(i.e.
.IR semflg " &"
.B IPC_CREAT
isn't zero).
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 lower 9 bits of the argument
.I semflg
define the access permissions (for owner, group and others)
to the semaphore set in the same format, and with the same
meaning, as for the access permissions parameter in the
.BR open (2)
or
.BR creat (2)
system calls (though the execute permissions are not used by the system,
and write permissions, for a semaphore set, effectively means alter
permissions).
.PP
Furthermore, while creating,
the system call initializes the system semaphore set 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 lowest order 9 bits of
.B sem_perm.mode
are set to the lowest order 9 bit 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 the system call isn't a create one.
Otherwise
.I nsems
must be greater than
.B 0
and less or equal to the maximum number of semaphores per semid,
.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 positive integer), otherwise
.B \-1
with
.B errno
indicating the error.
.SH ERRORS
For a failing return,
.B errno
will be set to one among the following values:
.TP 11
.B EACCES
A semaphore set exists for
.IR key ,
but the calling process has no access permissions to the set.
.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 as to be deleted.
.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 lowest 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
Use of
.B IPC_PRIVATE
doesn't inhibit to other processes the access to the allocated
semaphore set.
.PP
As for the files, there is currently no intrinsic way for a process to ensure
exclusive access to a semaphore set.
Asserting both
.B IPC_CREAT
and
.B IPC_EXCL
in
.I semflg
only ensures (on success) that a new semaphore set will be created,
it doesn't imply exclusive access to the semaphore set.
.PP
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 EINVAL, EFBIG,
E2BIG, EAGAIN, ERANGE, EFAULT.
.SH "SEE ALSO"
.BR ftok (3),
.BR ipc (5),
.BR semctl (2),
.BR semop (2)