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.\" Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>
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.\" $FreeBSD: releng/12.2/lib/libc/sys/semctl.2 363245 2020-07-16 13:38:20Z markj $
.\"
.Dd July 9, 2020
.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
The
.Fn semctl
system call
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
argument,
.Fa "union semun"
must be defined as follows:
.Bd -literal
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
Non-portable software may define
.Dv _WANT_SEMUN
before including
.Pa sys/sem.h
to use the system definition of
.Fa "union semun" .
.Pp
Commands are performed as follows:
.\"
.\" This section based on Stevens, _Advanced Programming in the UNIX
.\" Environment_.
.\"
.Bl -tag -width IPC_RMIDXXX
.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 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 .
Outstanding adjust on exit values for this semaphore in any process
are cleared.
.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 .
Outstanding adjust on exit values for all semaphores in this set,
in any process are cleared.
.El
.Pp
The
.Vt "struct semid_ds"
is defined as follows:
.\"
.\" Taken straight from <sys/sem.h>.
.\"
.Bd -literal
struct semid_ds {
        struct  ipc_perm sem_perm;      /* operation permission struct */
        u_short sem_nsems;      /* number of sems in set */
        time_t  sem_otime;      /* last operation time */
        time_t  sem_ctime;      /* last change time */
                                /* Times measured in secs since */
                                /* 00:00:00 GMT, Jan. 1, 1970 */
};
.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 ERRORS
The
.Fn semctl
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
No semaphore set corresponds to
.Fa semid .
.It Bq Er EINVAL
The
.Fa semnum
argument
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.
.It Bq Er ERANGE
.Dv SETVAL
or
.Dv SETALL
attempted to set a semaphore outside the allowable range
.Bq 0 .. Dv SEMVMX .
.El
.Sh SEE ALSO
.Xr semget 2 ,
.Xr semop 2
.Sh BUGS
.Dv SETALL
may update some semaphore elements before returning an error.