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[thirdparty/man-pages.git] / 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.
.\"
.TH SEMCTL 2 2012-05-31 "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
.fi
.PP
The
.I semid_ds
data structure is defined in \fI<sys/sem.h>\fP 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
.fi
.PP
The
.I ipc_perm
structure is defined 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
.fi
.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)"
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
.B _GNU_SOURCE
feature test macro is defined:
.nf
.in +4n

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) */
};

.in
.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
.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
.B semncnt