man-pages/man2/semctl.2

.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\" and Copyright 2004, 2005 Michael Kerrisk <mtk-manpages@gmx.net>
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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@gmx.net>
.\" Modified 21 Dec 2001, aeb
.\" Modified 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added notes on CAP_IPC_OWNER requirement
.\" Modified 17 Jun 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"	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 2004-11-10 "Linux" "Linux Programmer's Manual"
.SH NAME
semctl \- semaphore control operations
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/ipc.h>
.B #include <sys/sem.h>
.sp
.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
.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:

.nf
.in +4n
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) */
};
.in -4n
.fi
.PP
The
.I semid_ds
data structure is defined in <sys/sem.h> as follows:
.nf
.in +4n

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 */
};
.in -4n
.fi
.PP
The
.I ipc_perm
structure is defined in <sys/ipc.h> as follows
(the highlighted fields are settable using
.BR IPC_SET ):
.PP
.nf
.in +4n
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 */
};
.in -4n
.fi
.PP
Valid values for
.I cmd
are:
.TP 12
.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)"
Returns 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

struct  seminfo {
    int semmap;  /* # of entries in semaphore map;
                    unused */
    int semmni;  /* Max. # of semaphore sets */
    int semmns;  /* Max. # 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(2) */
    int semume;  /* Max. # of undo entries per
                    process; unused */
    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) */
};

.in -2n
.fi
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)"
Returns 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)"
Returns 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
.IB 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
.B semncnt
(i.e., the number of processes waiting for the value of
this semaphore to increase) 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
The system call returns 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).
The calling process must have read permission on the semaphore set.
.TP
.B GETVAL
The system call returns 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
The system call returns the value of
.B semzcnt
(i.e., the number of processes waiting for the value of
this semaphore to become zero) 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
.IB 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
.IB 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.

Otherwise the system call returns a non-negative value depending on
.I cmd
as follows:
.TP 11
.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 .
.LP
All other
.I cmd
values return 0 on success.
.SH ERRORS
On failure,
.I errno
will be set to one of the following:
.TP 11
.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.
.TP
.B EFAULT
The address pointed to by
.IB arg.buf
or
.IB 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"
SVr4, POSIX.1-2001.
.\" SVr4 documents more error conditions EINVAL and EOVERFLOW.
.SH NOTES
The
.BR IPC_INFO ,
.BR SEM_STAT
and
.B SEM_INFO
operations are used by the
.BR ipcs (8)
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,
a recompilation under glibc-2.1.91 or later should suffice.
(The kernel distinguishes old and new calls by an 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
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>.
.PP
The following system limit on semaphore sets affects a
.BR semctl ()
call:
.TP 11
.B SEMVMX
Maximum value for
.BR semval :
implementation dependent (32767).
.LP
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 "SEE ALSO"
.BR ipc (2),
.BR semget (2),
.BR semop (2),
.BR capabilities (7),
.BR sem_overview (7),
.BR svipc (7)