.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)

.\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>

.\"

.\" %%%LICENSE_START(VERBATIM)

.\" Permission is granted to make and distribute verbatim copies of this

.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

.\"

.\" Permission is granted to copy and distribute modified versions of this

.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

.\" permission notice identical to this one.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date.  The author(s) assume no

.\" responsibility for errors or omissions, or for damages resulting from

.\" the use of the information contained herein.  The author(s) may not

.\" have taken the same level of care in the production of this manual,

.\" which is licensed free of charge, as they might when working

.\" professionally.

.\"

.\" Formatted or processed versions of this manual, if unaccompanied by

.\" the source, must acknowledge the copyright and authors of this work.

.\" %%%LICENSE_END

.\"

.\" 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@gmail.com>

.\" Modified 21 Dec 2001, aeb

.\" Modified 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>

.\"     Added notes on CAP_IPC_OWNER requirement

.\" Modified 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>

.\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID

.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>

.\"	Language and formatting clean-ups

.\"	Rewrote semun text

.\"	Added semid_ds and ipc_perm structure definitions

.\" 2005-08-02, mtk: Added IPC_INFO, SEM_INFO, SEM_STAT descriptions.

.\"

.TH SEMCTL 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

semctl \- System V semaphore control operations

.SH SYNOPSIS

.nf

.B #include <sys/types.h>

.B #include <sys/ipc.h>

.B #include <sys/sem.h>

.PP

.BI "int semctl(int " semid ", int " semnum ", int " cmd ", ...);"

.fi

.SH DESCRIPTION

.BR semctl ()

performs the control operation specified by

.I cmd

on the System\ V semaphore set identified by

.IR semid ,

or on the

.IR semnum -th

semaphore of that set.

(The semaphores in a set are numbered starting at 0.)

.PP

This function has three or four arguments, depending on

.IR cmd .

When there are four, the fourth has the type

.IR "union semun" .

The \fIcalling program\fP must define this union as follows:

.PP

.in +4n

.EX

union semun {

    int              val;    /* Value for SETVAL */

    struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */

    unsigned short  *array;  /* Array for GETALL, SETALL */

    struct seminfo  *__buf;  /* Buffer for IPC_INFO

                                (Linux-specific) */

};

.EE

.in

.PP

The

.I semid_ds

data structure is defined in \fI<sys/sem.h>\fP as follows:

.PP

.in +4n

.EX

struct semid_ds {

    struct ipc_perm sem_perm;  /* Ownership and permissions */

    time_t          sem_otime; /* Last semop time */

    time_t          sem_ctime; /* Last change time */

    unsigned long   sem_nsems; /* No. of semaphores in set */

};

.EE

.in

.PP

The

.I ipc_perm

structure is defined as follows

(the highlighted fields are settable using

.BR IPC_SET ):

.PP

.in +4n

.EX

struct ipc_perm {

    key_t          __key; /* Key supplied to semget(2) */

    uid_t          \fBuid\fP;   /* Effective UID of owner */

    gid_t          \fBgid\fP;   /* Effective GID of owner */

    uid_t          cuid;  /* Effective UID of creator */

    gid_t          cgid;  /* Effective GID of creator */

    unsigned short \fBmode\fP;  /* Permissions */

    unsigned short __seq; /* Sequence number */

};

.EE

.in

.PP

Valid values for

.I cmd

are:

.TP 10

.B IPC_STAT

Copy information from the kernel data structure associated with

.I semid

into the

.I semid_ds

structure pointed to by

.IR arg.buf .

The argument

.I semnum

is ignored.

The calling process must have read permission on the semaphore set.

.TP

.B IPC_SET

Write the values of some members of the

.I semid_ds

structure pointed to by

.I arg.buf

to the kernel data structure associated with this semaphore set,

updating also its

.I sem_ctime

member.

The following members of the structure are updated:

.IR sem_perm.uid ,

.IR sem_perm.gid ,

and (the least significant 9 bits of)

.IR sem_perm.mode .

The effective UID of the calling process must match the owner

.RI ( sem_perm.uid )

or creator

.RI ( sem_perm.cuid )

of the semaphore set, or the caller must be privileged.

The argument

.I semnum

is ignored.

.TP

.B IPC_RMID

Immediately remove the semaphore set,

awakening all processes blocked in

.BR semop (2)

calls on the set (with an error return and

.I errno

set to

.BR EIDRM ).

The effective user ID of the calling process must

match the creator or owner of the semaphore set,

or the caller must be privileged.

The argument

.I semnum

is ignored.

.TP

.BR IPC_INFO " (Linux-specific)"

Return information about system-wide semaphore limits and

parameters in the structure pointed to by

.IR arg.__buf .

This structure is of type

.IR seminfo ,

defined in

.I <sys/sem.h>

if the

.B _GNU_SOURCE

feature test macro is defined:

.IP

.in +4n

.EX

struct  seminfo {

    int semmap;  /* Number of entries in semaphore

                    map; unused within kernel */

    int semmni;  /* Maximum number of semaphore sets */

    int semmns;  /* Maximum number of semaphores in all

                    semaphore sets */

    int semmnu;  /* System-wide maximum number of undo

                    structures; unused within kernel */

    int semmsl;  /* Maximum number of semaphores in a

                    set */

    int semopm;  /* Maximum number of operations for

                    semop(2) */

    int semume;  /* Maximum number of undo entries per

                    process; unused within kernel */

    int semusz;  /* Size of struct sem_undo */

    int semvmx;  /* Maximum semaphore value */

    int semaem;  /* Max. value that can be recorded for

                    semaphore adjustment (SEM_UNDO) */

};

.EE

.in

.IP

The

.IR semmsl ,

.IR semmns ,

.IR semopm ,

and

.I semmni

settings can be changed via

.IR /proc/sys/kernel/sem ;

see

.BR proc (5)

for details.

.TP

.BR SEM_INFO " (Linux-specific)"

Return a

.I seminfo

structure containing the same information as for

.BR IPC_INFO ,

except that the following fields are returned with information

about system resources consumed by semaphores: the

.I semusz

field returns the number of semaphore sets that currently exist

on the system; and the

.I semaem

field returns the total number of semaphores in all semaphore sets

on the system.

.TP

.BR SEM_STAT " (Linux-specific)"

Return a

.I semid_ds

structure as for

.BR IPC_STAT .

However, the

.I semid

argument is not a semaphore identifier, but instead an index into

the kernel's internal array that maintains information about

all semaphore sets on the system.

.TP

.B GETALL

Return

.B semval

(i.e., the current value)

for all semaphores of the set into

.IR arg.array .

The argument

.I semnum

is ignored.

The calling process must have read permission on the semaphore set.

.TP

.B GETNCNT

Return 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 permission on the semaphore set.

.TP

.B GETPID

Return the value of

.B sempid

for the

.IR semnum \-th

semaphore of the set.

This is the PID of the process that last performed an operation on

that semaphore (but see NOTES).

The calling process must have read permission on the semaphore set.

.TP

.B GETVAL

Return the value of

.B semval

for the

.IR semnum \-th

semaphore of the set.

The calling process must have read permission on the semaphore set.

.TP

.B GETZCNT

Return 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 permission on the semaphore set.

.TP

.B SETALL

Set

.B semval

for all semaphores of the set using

.IR arg.array ,

updating also the

.I sem_ctime

member of the

.I semid_ds

structure associated with the set.

Undo entries (see

.BR semop (2))

are cleared for altered semaphores in all processes.

If the changes to semaphore values would permit blocked

.BR semop (2)

calls in other processes to proceed, then those processes are woken up.

The argument

.I semnum

is ignored.

The calling process must have alter (write) permission on

the semaphore set.

.TP

.B SETVAL

Set the value of

.B semval

to

.I arg.val

for the

.IR semnum \-th

semaphore of the set, updating also the

.I sem_ctime

member of the

.I semid_ds

structure associated with the set.

Undo entries are cleared for altered semaphores in all processes.

If the changes to semaphore values would permit blocked

.BR semop (2)

calls in other processes to proceed, then those processes are woken up.

The calling process must have alter permission on the semaphore set.

.SH RETURN VALUE

On failure,

.BR semctl ()

returns \-1

with

.I errno

indicating the error.

.PP

Otherwise, the system call returns a nonnegative value depending on

.I cmd

as follows:

.TP 10

.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 .

.TP

.B IPC_INFO

the index of the highest used entry in the

kernel's internal array recording information about all

semaphore sets.

(This information can be used with repeated

.B SEM_STAT

operations to obtain information about all semaphore sets on the system.)

.TP

.B SEM_INFO

as for

.BR IPC_INFO .

.TP

.B SEM_STAT

the identifier of the semaphore set whose index was given in

.IR semid .

.PP

All other

.I cmd

values return 0 on success.

.SH ERRORS

On failure,

.I errno

will be set to one of the following:

.TP

.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 in the user namespace that governs its IPC namespace.

.TP

.B EFAULT

The address pointed to by

.I arg.buf

or

.I arg.array

isn't accessible.

.TP

.B EIDRM

The semaphore set was removed.

.TP

.B EINVAL

Invalid value for

.I cmd

or

.IR semid .

Or: for a

.B SEM_STAT

operation, the index value specified in

.I semid

referred to an array slot that is currently unused.

.TP

.B EPERM

The argument

.I cmd

has the value

.B IPC_SET

or

.B IPC_RMID

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 the value

.B SETALL

or

.B SETVAL

and the value to which

.B semval

is to be set (for some semaphore of the set) is less than 0

or greater than the implementation limit

.BR SEMVMX .

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4.

.\" SVr4 documents more error conditions EINVAL and EOVERFLOW.

.PP

POSIX.1 specifies the

.\" POSIX.1-2001, POSIX.1-2008

.I sem_nsems

field of the

.I semid_ds

structure as having the type

.IR "unsigned\ short" ,

and the field is so defined on most other systems.

It was also so defined on Linux 2.2 and earlier,

but, since Linux 2.4, the field has the type

.IR "unsigned\ long" .

.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

The

.BR IPC_INFO ,

.B SEM_STAT

and

.B SEM_INFO

operations are used by the

.BR ipcs (1)

program to provide information on allocated resources.

In the future these may modified or moved to a

.I /proc

filesystem interface.

.PP

Various fields in a \fIstruct semid_ds\fP were typed as

.I short

under Linux 2.2

and have become

.I long

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 an

.B IPC_64

flag in

.IR cmd .)

.PP

In some earlier versions of glibc, the

.I semun

union was defined in \fI<sys/sem.h>\fP, but POSIX.1 requires

.\" POSIX.1-2001, POSIX.1-2008

that the caller define this union.

On versions of glibc where this union is \fInot\fP defined,

the macro

.B _SEM_SEMUN_UNDEFINED

is defined in \fI<sys/sem.h>\fP.

.PP

The following system limit on semaphore sets affects a

.BR semctl ()

call:

.TP

.B SEMVMX

Maximum value for

.BR semval :

implementation dependent (32767).

.PP

For greater portability, it is best to always call

.BR semctl ()

with four arguments.

.\"

.SS The sempid value

POSIX.1 defines

.I sempid

as the "process ID of [the] last operation" on a semaphore,

and explicitly notes that this value is set by a successful

.BR semop (2)

call, with the implication that no other interface affects the

.I sempid

value.

.PP

While some implementations conform to the behavior specified in POSIX.1,

others do not.

(The fault here probably lies with POSIX.1 inasmuch as it likely failed

to capture the full range of existing implementation behaviors.)

Various other implementations

.\" At least OpenSolaris (and, one supposes, older Solaris) and Darwin

also update

.I sempid

for the other operations that update the value of a semaphore: the

.B SETVAL

and

.B SETALL

operations, as well as the semaphore adjustments performed

on process termination as a consequence of the use of the

.B SEM_UNDO

flag (see

.BR semop (2)).

.PP

Linux also updates

.I sempid

for

.BR SETVAL

operations and semaphore adjustments.

However, somewhat inconsistently, up to and including 4.5,

Linux did not update

.I sempid

for

.BR SETALL

operations.

This was rectified

.\" commit a5f4db877177d2a3d7ae62a7bac3a5a27e083d7f

in Linux 4.6.

.SH SEE ALSO

.BR ipc (2),

.BR semget (2),

.BR semop (2),

.BR capabilities (7),

.BR sem_overview (7),

.BR svipc (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.