man2/semget.2

.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\" and Copyright (C) 2020 Michael Kerrisk <mtk.manpages@gmail.com>
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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 <mtk.manpages@gmail.com>
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Language and formatting clean-ups
.\"	Added notes on /proc files
.\"	Rewrote BUGS note about semget()'s failure to initialize
.\"		semaphore values
.\"
.TH SEMGET 2 2020-04-11 "Linux" "Linux Programmer's Manual"
.SH NAME
semget \- get a System V semaphore set identifier
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/ipc.h>
.B #include <sys/sem.h>
.fi
.PP
.BI "int semget(key_t " key ,
.BI "int " nsems ,
.BI "int " semflg );
.SH DESCRIPTION
The
.BR semget ()
system call returns the System\ V semaphore set identifier
associated with the argument
.IR key .
It may be used either to obtain the identifier of a previously created
semaphore set (when
.I semflg
is zero and
.I key
does not have the value
.BR IPC_PRIVATE ),
or to create a new set.
.PP
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 with
.I key
and
.B IPC_CREAT
is specified in
.IR semflg .
.PP
If
.I semflg
specifies both
.B IPC_CREAT
and
.B IPC_EXCL
and a semaphore set already exists for
.IR key ,
then
.BR semget ()
fails with
.I errno
set to
.BR EEXIST .
(This is analogous to the effect of the combination
.B O_CREAT | O_EXCL
for
.BR open (2).)
.PP
Upon creation, the least significant 9 bits of the argument
.I semflg
define the permissions (for owner, group and others)
for the semaphore set.
These bits have the same format, and the same
meaning, as the
.I mode
argument of
.BR open (2)
(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,
.BR semget ()
initializes the set's associated data structure,
.I semid_ds
(see
.BR semctl (2)),
as follows:
.IP \(bu 2
.I sem_perm.cuid
and
.I sem_perm.uid
are set to the effective user ID of the calling process.
.IP \(bu
.I sem_perm.cgid
and
.I sem_perm.gid
are set to the effective group ID of the calling process.
.IP \(bu
The least significant 9 bits of
.I sem_perm.mode
are set to the least significant 9 bits of
.IR semflg .
.IP \(bu
.I sem_nsems
is set to the value of
.IR nsems .
.IP \(bu
.I sem_otime
is set to 0.
.IP \(bu
.I sem_ctime
is set to the current time.
.PP
The argument
.I nsems
can be 0
(a don't care)
when a semaphore set is not being created.
Otherwise,
.I nsems
must be greater than 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 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, \-1
is returned, with
.I errno
indicating the error.
.SH ERRORS
On failure,
.I errno
will be set to one of the following:
.TP
.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
.B CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
.TP
.B EEXIST
.B IPC_CREAT
and
.BR IPC_EXCL
were specified in
.IR semflg ,
but a semaphore set already exists for
.IR key .
.\" .TP
.\" .B EIDRM
.\" The semaphore set is marked to be deleted.
.TP
.B EINVAL
.I nsems
is less than 0 or greater than the limit on the number
of semaphores per semaphore set
.RB ( SEMMSL ).
.TP
.B EINVAL
A semaphore set corresponding to
.I key
already exists, but
.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
did not specify
.BR IPC_CREAT .
.TP
.B ENOMEM
A semaphore set has to be created but the system does not have
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 CONFORMING TO
SVr4, POSIX.1-2001.
.\" SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN,
.\" ERANGE, EFAULT.
.SH NOTES
The inclusion of
.I <sys/types.h>
and
.I <sys/ipc.h>
isn't required on Linux or by any version of POSIX.
However,
some old implementations required the inclusion of these header files,
and the SVID also documented their inclusion.
Applications intended to be portable to such old systems may need
to include these header files.
.\" Like Linux, the FreeBSD man pages still document
.\" the inclusion of these header files.
.PP
.B IPC_PRIVATE
isn't a flag field but a
.I key_t
type.
If this special value is used for
.IR key ,
the system call ignores all but the least significant 9 bits of
.I semflg
and creates a new semaphore set (on success).
.\"
.SS Semaphore initialization
The values of the semaphores in a newly created set are indeterminate.
(POSIX.1-2001 and POSIX.1-2008 are explicit on this point,
although POSIX.1-2008 notes that a future version of the standard
may require an implementation to initialize the semaphores to 0.)
Although Linux, like many other implementations,
initializes the semaphore values to 0,
a portable application cannot rely on this:
it should explicitly initialize the semaphores to the desired values.
.\" In truth, every one of the many implementations that I've tested sets
.\" the values to zero, but I suppose there is/was some obscure
.\" implementation out there that does not.
.PP
Initialization can be done using
.BR semctl (2)
.B SETVAL
or
.B SETALL
operation.
Where multiple peers do not know who will be the first to
initialize the set, checking for a nonzero
.I sem_otime
in the associated data structure retrieved by a
.BR semctl (2)
.B IPC_STAT
operation can be used to avoid races.
.\"
.SS Semaphore limits
The following limits on semaphore set resources affect the
.BR semget ()
call:
.TP
.B SEMMNI
System-wide limit on the number of semaphore sets.
On Linux systems before version 3.19,
the default value for this limit was 128.
Since Linux 3.19,
.\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4
the default value is 32,000.
On Linux, this limit can be read and modified via the fourth field of
.IR /proc/sys/kernel/sem .
.\" This /proc file is not available in Linux 2.2 and earlier -- MTK
.TP
.B SEMMSL
Maximum number of semaphores per semaphore ID.
On Linux systems before version 3.19,
the default value for this limit was 250.
Since Linux 3.19,
.\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4
the default value is 32,000.
On Linux, this limit can be read and modified via the first field of
.IR /proc/sys/kernel/sem .
.TP
.B SEMMNS
System-wide limit on the number of semaphores: policy dependent
(on Linux, this limit can be read and modified via the second field of
.IR /proc/sys/kernel/sem ).
Note that the number of semaphores system-wide
is also limited by the product of
.B SEMMSL
and
.BR SEMMNI .
.SH BUGS
The name choice
.B IPC_PRIVATE
was perhaps unfortunate,
.B IPC_NEW
would more clearly show its function.
.SH EXAMPLE
The program shown below uses
.BR semget ()
to create a new semaphore set or retrieve the ID of an existing set.
It generates the
.I key
for
.BR semget ()
using
.BR ftok (3).
The first two command-line arguments are used as the
.I pathname
and
.I proj_id
arguments for
.BR ftok (3).
The third command-line argument is an integer that specifies the
.I nsems
argument for
.BR semget ().
Command-line options can be used to specify the
.BR IPC_CREAT
.RI ( \-c )
and
.BR IPC_EXCL
.RI ( \-x )
flags for the call to
.BR semget ().
The usage of this program is demonstrated below.
.PP
We first create two files that will be used to generate keys using
.BR ftok (3),
create two semaphore sets using those files, and then list the sets using
.BR ipcs (1):
.PP
.in +4n
.EX
$ \fBtouch mykey mykey2\fP
$ \fB./t_semget \-c mykey p 1\fP
ID = 9
$ \fB./t_semget \-c mykey2 p 2\fP
ID = 10
$ \fBipcs \-s\fP

\-\-\-\-\-\- Semaphore Arrays \-\-\-\-\-\-\-\-
key        semid      owner      perms      nsems
0x7004136d 9          mtk        600        1
0x70041368 10         mtk        600        2
.EE
.in
.PP
Next, we demonstrate that when
.BR semctl ()
is given the same
.I key
(as generated by the same arguments to
.BR ftok (3)),
it returns the ID of the already existing semaphore set:
.PP
.in +4n
.EX
$ \fB./t_semget \-c mykey p 1\fP
ID = 9
.EE
.in
.PP
Finally, we demonstrate the kind of collision that can occur when
.BR ftok (3)
is given different
.I pathname
arguments that have the same inode number:
.PP
.in +4n
.EX
$ \fBln mykey link\fP
$ \fBls \-i1 link mykey\fP
2233197 link
2233197 mykey
$ \fB./t_semget link p 1\fP       # Generates same key as \(aqmykey\(aq
ID = 9
.EE
.in
.PP
.SS Program source
\&
.nf
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

static void
usage(const char *pname)
{
    fprintf(stderr, "Usage: %s [\-cx] pathname proj\-id num\-sems\en",
            pname);
    fprintf(stderr, "    \-c           Use IPC_CREAT flag\en");
    fprintf(stderr, "    \-x           Use IPC_EXCL flag\en");
    exit(EXIT_FAILURE);
}

int
main(int argc, char *argv[])
{
    int semid, nsems, flags, opt;
    key_t key;

    flags = 0;
    while ((opt = getopt(argc, argv, "cx")) != \-1) {
        switch (opt) {
        case \(aqc\(aq: flags |= IPC_CREAT;   break;
        case \(aqx\(aq: flags |= IPC_EXCL;    break;
        default:  usage(argv[0]);
        }
    }

    if (argc != optind + 3)
        usage(argv[0]);

    key = ftok(argv[optind], argv[optind + 1][0]);
    if (key == \-1) {
        perror("ftok");
        exit(EXIT_FAILURE);
    }

    nsems = atoi(argv[optind + 2]);

    semid = semget(key, nsems, flags | 0600);
    if (semid == \-1) {
        perror("semget");
        exit(EXIT_FAILURE);
    }

    printf("ID = %d\en", semid);

    exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.BR semctl (2),
.BR semop (2),
.BR ftok (3),
.BR capabilities (7),
.BR sem_overview (7),
.BR sysvipc (7)