man2/semctl.2

.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" 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.
.\" 2018-03-20, dbueso: Added SEM_STAT_ANY description.
.\"
.TH SEMCTL 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
semctl \- System V semaphore control operations
.SH SYNOPSIS
.nf
.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; /* Creation time/time of last
                                  modification via semctl() */
    unsigned long   sem_nsems; /* No. of semaphores in set */
};
.EE
.in
.PP
The fields of the
.I semid_ds
structure are as follows:
.TP 11
.I sem_perm
This is an
.I ipc_perm
structure (see below) that specifies the access permissions on the semaphore
set.
.TP
.I sem_otime
Time of last
.BR semop (2)
system call.
.TP
.I sem_ctime
Time of creation of semaphore set or time of last
.BR semctl ()
.BR IPCSET ,
.BR SETVAL ,
or
.BR SETALL
operation.
.TP
.I sem_nsems
Number of semaphores in the set.
Each semaphore of the set is referenced by a nonnegative integer
ranging from
.B 0
to
.IR sem_nsems\-1 .
.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
The least significant 9 bits of the
.I mode
field of the
.I ipc_perm
structure define the access permissions for the shared memory segment.
The permission bits are as follows:
.TS
l l.
0400	Read by user
0200	Write by user
0040	Read by group
0020	Write by group
0004	Read by others
0002	Write by others
.TE
.PP
In effect, "write" means "alter" for a semaphore set.
Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
.PP
Valid values for
.I cmd
are:
.TP
.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.
.IP
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 .
.IP
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
.BR SEM_STAT_ANY " (Linux-specific, since Linux 4.17)"
Return a
.I semid_ds
structure 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
.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
.B semncnt
value for the
.IR semnum \-th
semaphore of the set
(i.e., the number of processes waiting for the semaphore's value to increase).
The calling process must have read permission on the semaphore set.
.TP
.B GETPID
Return the
.B sempid
value 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
.B semval
(i.e., the semaphore value) 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
.B semzcnt
value for the
.IR semnum \-th
semaphore of the set
(i.e., the number of processes waiting for the semaphore value to become 0).
The calling process must have read permission on the semaphore set.
.TP
.B SETALL
Set the
.B semval
values 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 semaphore value
.BR ( 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 success,
.BR semctl ()
returns a nonnegative value depending on
.I cmd
as follows:
.TP
.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
or
.B SEM_STAT_ANY
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 .
.TP
.B SEM_STAT_ANY
as for
.BR SEM_STAT .
.PP
All other
.I cmd
values return 0 on success.
.PP
On failure,
.BR semctl ()
returns \-1 and sets
.I errno
to indicate the error.
.SH ERRORS
.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 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 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/ipc.h>
isn't required on Linux or by any version of POSIX.
However,
some old implementations required the inclusion of this header file,
and the SVID also documented its inclusion.
Applications intended to be portable to such old systems may need
to include this header file.
.\" Like Linux, the FreeBSD man pages still document
.\" the inclusion of these header files.
.PP
The
.BR IPC_INFO ,
.BR 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 Linux 4.5,
the kernel did not update
.I sempid
for
.BR SETALL
operations.
This was rectified
.\" commit a5f4db877177d2a3d7ae62a7bac3a5a27e083d7f
in Linux 4.6.
.SH EXAMPLES
See
.BR shmop (2).
.SH SEE ALSO
.BR ipc (2),
.BR semget (2),
.BR semop (2),
.BR capabilities (7),
.BR sem_overview (7),
.BR sysvipc (7)