.\" 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.
.\"
.\" 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>
.\" 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 2009-09-27 "Linux" "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 */
struct seminfo *__buf; /* Buffer for IPC_INFO
(Linux-specific) */
};
+.EE
.in
-.fi
.PP
The
.I semid_ds
data structure is defined in \fI<sys/sem.h>\fP as follows:
-.nf
+.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 short sem_nsems; /* No. of semaphores in set */
+ unsigned long sem_nsems; /* No. of semaphores in set */
};
+.EE
.in
-.fi
.PP
The
.I ipc_perm
-structure is defined in \fI<sys/ipc.h>\fP 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(2) */
uid_t \fBuid\fP; /* Effective UID of owner */
unsigned short \fBmode\fP; /* Permissions */
unsigned short __seq; /* Sequence number */
};
+.EE
.in
-.fi
.PP
Valid values for
.I cmd
is ignored.
.TP
.BR IPC_INFO " (Linux-specific)"
-Returns information about system-wide semaphore limits and
+Return information about system-wide semaphore limits and
parameters in the structure pointed to by
.IR arg.__buf .
This structure is of type
if the
.B _GNU_SOURCE
feature test macro is defined:
-.nf
+.IP
.in +4n
-
+.EX
struct seminfo {
int semmap; /* Number of entries in semaphore
map; unused within kernel */
int semaem; /* Max. value that can be recorded for
semaphore adjustment (SEM_UNDO) */
};
-
+.EE
.in
-.fi
+.IP
The
.IR semmsl ,
.IR semmns ,
for details.
.TP
.BR SEM_INFO " (Linux-specific)"
-Returns a
+Return a
.I seminfo
structure containing the same information as for
.BR IPC_INFO ,
on the system.
.TP
.BR SEM_STAT " (Linux-specific)"
-Returns a
+Return a
.I semid_ds
structure as for
.BR IPC_STAT .
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
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
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 (2)
-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
for the
.IR semnum \-th
.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 non-negative value depending on
+.PP
+Otherwise, the system call returns a nonnegative value depending on
.I cmd
as follows:
-.TP 12
+.TP 10
.B GETNCNT
the value of
.BR semncnt .
semaphore sets.
(This information can be used with repeated
.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.
.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
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"
-SVr4, POSIX.1-2001.
+.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 (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
+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
.PP
In some earlier versions of glibc, the
.I semun
-union was defined in \fI<sys/sem.h>\fP, 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
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.
-.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 sem_overview (7),
-.BR svipc (7)
+.BR sysvipc (7)
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