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.\" $FreeBSD: src/lib/libc/sys/semctl.2,v 1.9.2.6 2002/03/11 08:37:47 maxim Exp $
.\"
.Dd January 4, 2014
.Dt SEMCTL 2
.Os
.Sh NAME
.Nm semctl
.Nd control operations on a semaphore set
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/ipc.h
.In sys/sem.h
.Ft int
.Fn semctl "int semid" "int semnum" "int cmd" ...
.Sh DESCRIPTION
.Fn Semctl
performs the operation indicated by
.Fa cmd
on the semaphore set indicated by
.Fa semid .
A fourth argument, a
.Fa "union semun arg" ,
is required for certain values of
.Fa cmd .
For the commands that use the
.Fa arg
parameter,
.Fa "union semun"
is defined as follows:
.Bd -literal
.\"
.\" From <sys/sem.h>:
.\"
union semun {
        int     val;            /* value for SETVAL */
        struct  semid_ds *buf;  /* buffer for IPC_STAT & IPC_SET */
        u_short *array;         /* array for GETALL & SETALL */
};
.Ed
.Pp
Commands are performed as follows:
.\"
.\" This section based on Stevens, _Advanced Programming in the UNIX
.\" Environment_.
.\"
.Bl -tag -width ".Dv IPC_RMID"
.It Dv IPC_STAT
Fetch the semaphore set's
.Fa "struct semid_ds" ,
storing it in the memory pointed to by
.Fa arg.buf .
.It Dv IPC_SET
Changes the
.Fa sem_perm.uid ,
.Fa sem_perm.gid ,
and
.Fa sem_perm.mode
members of the semaphore set's
.Fa "struct semid_ds"
to match those of the struct pointed to by
.Fa arg.buf .
The calling process's effective uid must
match either
.Fa sem_perm.uid
or
.Fa sem_perm.cuid ,
or it must have superuser privileges.
.It Dv IPC_RMID
Immediately removes the semaphore set from the system.
The calling process's effective uid must equal the semaphore set's
.Fa sem_perm.uid
or
.Fa sem_perm.cuid ,
or the process must have superuser privileges.
.It Dv GETVAL
Return the value of semaphore number
.Fa semnum .
.It Dv SETVAL
Set the value of semaphore number
.Fa semnum
to
.Fa arg.val .
.It Dv GETPID
Return the pid of the last process to perform an operation on
semaphore number
.Fa semnum .
.It Dv GETNCNT
Return the number of processes waiting for semaphore number
.Fa semnum Ns 's
value to become greater than its current value.
.It Dv GETZCNT
Return the number of processes waiting for semaphore number
.Fa semnum Ns 's
value to become 0.
.It Dv GETALL
Fetch the value of all of the semaphores in the set into the
array pointed to by
.Fa arg.array .
.It Dv SETALL
Set the values of all of the semaphores in the set to the values
in the array pointed to by
.Fa arg.array .
.El
.Pp
The
.Fa "struct semid_ds"
is defined as follows:
.Bd -literal
.\"
.\" Taken straight from <sys/sem.h>.
.\"
struct semid_ds {
        struct  ipc_perm sem_perm;      /* operation permission struct */
        struct  sem *sem_base;  /* pointer to first semaphore in set */
        u_short sem_nsems;      /* number of sems in set */
        time_t  sem_otime;      /* last operation time */
        long    sem_pad1;       /* SVABI/386 says I need this here */
        time_t  sem_ctime;      /* last change time */
                                /* Times measured in secs since */
                                /* 00:00:00 GMT, Jan. 1, 1970 */
        long    sem_pad2;       /* SVABI/386 says I need this here */
        long    sem_pad3[4];    /* SVABI/386 says I need this here */
};
.Ed
.Sh RETURN VALUES
On success, when
.Fa cmd
is one of
.Dv GETVAL , GETPID , GETNCNT
or
.Dv GETZCNT ,
.Fn semctl
returns the corresponding value; otherwise, 0 is returned.
On failure, -1 is returned, and
.Va errno
is set to indicate the error.
.Sh ENVIRONMENT
The XSI Interprocess Communication family of functions is also available
as an implementation in userspace.
To use it, the
.Xr sysvipcd 8
daemon has to be running.
.Pp
If the
.Ev USR_SYSVIPC
variable is set in a process' environment, the process and its children
will use the userspace implementation.
.Sh ERRORS
.Fn Semctl
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
No semaphore set corresponds to
.Fa semid .
.It Bq Er EINVAL
.Fa semnum
is not in the range of valid semaphores for given semaphore set.
.It Bq Er EPERM
The calling process's effective uid does not match the uid of
the semaphore set's owner or creator.
.It Bq Er EACCES
Permission denied due to mismatch between operation and mode of
semaphore set.
.El
.Sh SEE ALSO
.Xr semget 2 ,
.Xr semop 2
.Sh AUTHORS
.An -nosplit
The
.Dx
specific userspace implementation (see
.Sx ENVIRONMENT )
was written by
.An Larisa Grigore .