.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
-.\" and Copyright 2004, 2005 Michael Kerrisk <mtk-manpages@gmx.net>
+.\" 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.
.\" 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.\" manual page may be incorrect or out-of-date.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
.\" 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.
-.\"
+.\"
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+.\" %%%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@gmx.net>
+.\" Modified 20 Dec 2001, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified 21 Dec 2001, aeb
-.\" Modified 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
+.\" 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@gmx.net>
+.\" 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@gmx.net>
+.\" 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.
+.\" 2018-03-20, dbueso: Added SEM_STAT_ANY description.
.\"
-.TH SEMCTL 2 2004-11-10 "Linux 2.6.9" "Linux Programmer's Manual"
+.TH SEMCTL 2 2019-08-02 "Linux" "Linux Programmer's Manual"
.SH NAME
-semctl \- semaphore control operations
+semctl \- System V semaphore control operations
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/ipc.h>
.B #include <sys/sem.h>
-.sp
+.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 semaphore set identified by
+on the System\ V semaphore set identified by
.IR semid ,
or on the
.IR semnum -th
When there are four, the fourth has the type
.IR "union semun" .
The \fIcalling program\fP must define this union as follows:
-
-.nf
+.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) */
+ struct seminfo *__buf; /* Buffer for IPC_INFO
+ (Linux-specific) */
};
-.in -4n
-.fi
+.EE
+.in
.PP
The
.I semid_ds
-data structure is defined in <sys/sem.h> as follows:
-.nf
+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
+ struct ipc_perm sem_perm; /* Ownership and permissions */
time_t sem_otime; /* Last semop time */
time_t sem_ctime; /* Last change time */
- unsigned short sem_nsems; /* No. of semaphores in set */
+ unsigned long sem_nsems; /* No. of semaphores in set */
};
-.in -4n
-.fi
+.EE
+.in
.PP
The
.I ipc_perm
-structure is defined in <sys/ipc.h> as follows
+structure is defined as follows
(the highlighted fields are settable using
.BR IPC_SET ):
.PP
-.nf
.in +4n
+.EX
struct ipc_perm {
- key_t key; /* Key supplied to semget() */
- 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 */
+ 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 */
+ unsigned short __seq; /* Sequence number */
};
-.in -4n
-.fi
+.EE
+.in
.PP
Valid values for
.I cmd
are:
-.TP 12
+.TP 10
.B IPC_STAT
Copy information from the kernel data structure associated with
.I semid
updating also its
.I sem_ctime
member.
-The following members of the structure are updated:
+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
+or creator
.RI ( sem_perm.cuid )
of the semaphore set, or the caller must be privileged.
The argument
.B IPC_RMID
Immediately remove the semaphore set,
awakening all processes blocked in
-.BR semop ()
+.BR semop (2)
calls on the set (with an error return and
.I errno
set to
.I semnum
is ignored.
.TP
-.BR IPC_INFO " (Linux specific)"
-Returns information about system-wide semaphore limits and
+.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 _GNU_SOURCE feature test macro is defined:
-.nf
-.in +2n
-
+if the
+.B _GNU_SOURCE
+feature test macro is defined:
+.IP
+.in +4n
+.EX
struct seminfo {
- int semmap; /* # of entries in semaphore map;
- unused */
- int semmni; /* Max. # of semaphore sets */
- int semmns; /* Max. # of semaphores in all
+ 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 max. # of undo
- structures; unused */
- int semmsl; /* Max. # of semaphores in a set */
- int semopm; /* Max. # of operations for semop() */
- int semume; /* Max. # of undo entries per
- process; unused */
- int semusz; /* size of struct sem_undo */
+ 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
+ int semaem; /* Max. value that can be recorded for
semaphore adjustment (SEM_UNDO) */
};
-
-.in -2n
-.fi
-The
+.EE
+.in
+.IP
+The
.IR semmsl ,
.IR semmns ,
.IR semopm ,
settings can be changed via
.IR /proc/sys/kernel/sem ;
see
-.BR proc (5)
+.BR proc (5)
for details.
.TP
-.BR SEM_INFO " (Linux specific)"
-Returns a
+.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
+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
+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
+field returns the total number of semaphores in all semaphore sets
on the system.
.TP
-.BR SEM_STAT " (Linux specific)"
-Returns a
-.I semid_ds
+.BR SEM_STAT " (Linux-specific)"
+Return a
+.I semid_ds
structure as for
.BR IPC_STAT .
However, the
the kernel's internal array that maintains information about
all semaphore sets on the system.
.TP
+.BR SEM_STAT_ANY " (Linux-specific, since Linux 4.17)"
+Return a
+.I seminfo
+structure containing the same information as for
+.BR SEM_STAT .
+However,
+.I sem_perm.mode
+is not checked for read access for
+.IR semid
+meaning that any user can employ this operation (just as any user may read
+.IR /proc/sysvipc/sem
+to obtain the same information).
+.TP
.B GETALL
Return
.B semval
(i.e., the current value)
for all semaphores of the set into
-.IB arg.array .
+.IR arg.array .
The argument
.I semnum
is ignored.
The calling process must have read permission on the semaphore set.
.TP
.B GETNCNT
-The system call returns the value of
+Return the value of
.B semncnt
-(i.e., the number of processes waiting for the value of
-this semaphore to increase) for the
+for the
.IR semnum \-th
semaphore of the set
-(i.e. the number of processes waiting for an increase of
+(i.e., the number of processes waiting for an increase of
.B semval
for the
.IR semnum \-th
The calling process must have read permission on the semaphore set.
.TP
.B GETPID
-The system call returns the value of
+Return 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
-.BR semop ()
-call for the
-.IR semnum \-th
-semaphore of the set).
+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
-The system call returns the value of
+Return the value of
.B semval
for the
.IR semnum \-th
The calling process must have read permission on the semaphore set.
.TP
.B GETZCNT
-The system call returns the value of
+Return the value of
.B semzcnt
-(i.e., the number of processes waiting for the value of
-this semaphore to become zero) for the
+for the
.IR semnum \-th
semaphore of the set
-(i.e. the number of processes waiting for
+(i.e., the number of processes waiting for
.B semval
of the
.IR semnum \-th
Set
.B semval
for all semaphores of the set using
-.IB arg.array ,
+.IR arg.array ,
updating also the
.I sem_ctime
member of the
.BR semop (2))
are cleared for altered semaphores in all processes.
If the changes to semaphore values would permit blocked
-.BR semop ()
+.BR semop (2)
calls in other processes to proceed, then those processes are woken up.
The argument
.I semnum
Set the value of
.B semval
to
-.IB arg.val
+.I arg.val
for the
.IR semnum \-th
semaphore of the set, updating also the
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 ()
+.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
+.SH RETURN VALUE
+On failure,
.BR semctl ()
returns \-1
with
.I errno
indicating the error.
-
-Otherwise the system call returns a nonnegative value depending on
+.PP
+Otherwise, the system call returns a nonnegative value depending on
.I cmd
as follows:
-.TP 11
+.TP 10
.B GETNCNT
the value of
.BR semncnt .
kernel's internal array recording information about all
semaphore sets.
(This information can be used with repeated
-.B SEM_STAT
+.B SEM_STAT
+or
+.B SEM_STAT_ANY
operations to obtain information about all semaphore sets on the system.)
.TP
.B SEM_INFO
-As for
+as for
.BR IPC_INFO .
.TP
.B SEM_STAT
the identifier of the semaphore set whose index was given in
.IR semid .
-.LP
+.TP
+.B SEM_STAT_ANY
+as for
+.BR SEM_STAT .
+.PP
All other
.I cmd
values return 0 on success.
On failure,
.I errno
will be set to one of the following:
-.TP 11
+.TP
.B EACCES
The argument
.I cmd
.BR GETZCNT ,
.BR IPC_STAT ,
.BR SEM_STAT ,
+.BR SEM_STAT_ANY ,
.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.
+capability in the user namespace that governs its IPC namespace.
.TP
.B EFAULT
The address pointed to by
-.IB arg.buf
+.I arg.buf
or
-.IB arg.array
+.I arg.array
isn't accessible.
.TP
.B EIDRM
.IR semid .
Or: for a
.B SEM_STAT
-operation, the index value specified in
+operation, the index value specified in
.I semid
referred to an array slot that is currently unused.
.TP
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 ,
-.BR SEM_STAT
+.B SEM_STAT
and
.B SEM_INFO
operations are used by the
-.BR ipcs (8)
+.BR ipcs (1)
program to provide information on allocated resources.
-In the future these may modified 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,
+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 IPC_64 flag in
+(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 <sys/sem.h>, but POSIX.1-2001 requires
+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 <sys/sem.h>.
+is defined in \fI<sys/sem.h>\fP.
.PP
The following system limit on semaphore sets affects a
.BR semctl ()
call:
-.TP 11
+.TP
.B SEMVMX
Maximum value for
.BR semval :
implementation dependent (32767).
-.LP
-For greater portability it is best to always call
+.PP
+For greater portability, it is best to always call
.BR semctl ()
with four arguments.
-.LP
-Under Linux,
-.BR 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"
+.\"
+.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 ipc (7),
-.BR sem_overview (7)
+.BR sem_overview (7),
+.BR sysvipc (7)
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