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.\"     @(#)flock.2	8.2 (Berkeley) 12/11/93
.\"
.Dd October 15, 2011
.Dt FLOCK 2
.Os
.Sh NAME
.Nm flock
.Nd "apply or remove an advisory lock on an open file"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In fcntl.h
.Fd #define	LOCK_SH	1	/* shared lock */
.Fd #define	LOCK_EX	2	/* exclusive lock */
.Fd #define	LOCK_NB	4	/* don't block when locking */
.Fd #define	LOCK_UN	8	/* unlock */
.Ft int
.Fn flock "int fd" "int operation"
.Sh DESCRIPTION
.Fn flock
applies or removes an
.Em advisory
lock on the file associated with the file descriptor
.Fa fd .
A lock is applied by specifying an
.Fa operation
parameter that is one of
.Dv LOCK_SH
or
.Dv LOCK_EX
with the optional addition of
.Dv LOCK_NB .
To unlock
an existing lock
.Dv operation
should be
.Dv LOCK_UN .
.Pp
Advisory locks allow cooperating processes to perform
consistent operations on files, but do not guarantee
consistency (i.e., processes may still access files
without using advisory locks possibly resulting in
inconsistencies).
.Pp
The locking mechanism allows two types of locks:
.Em shared
locks and
.Em exclusive
locks.
At any time multiple shared locks may be applied to a file,
but at no time are multiple exclusive, or both shared and exclusive,
locks allowed simultaneously on a file.
.Pp
A shared lock may be
.Em upgraded
to an exclusive lock, and vice versa, simply by specifying
the appropriate lock type; this results in the previous
lock being released and the new lock applied (possibly
after other processes have gained and released the lock).
.Pp
Requesting a lock on an object that is already locked
normally causes the caller to be blocked until the lock may be
acquired.
If
.Dv LOCK_NB
is included in
.Fa operation ,
then this will not happen; instead the call will fail and
the error
.Er EAGAIN
will be returned.
.Sh NOTES
Locks are on files, not file descriptors.
That is, file descriptors duplicated through
.Xr dup 2
or
.Xr fork 2
do not result in multiple instances of a lock, but rather multiple
references to a single lock.
If a process holding a lock on a file
forks and the child explicitly unlocks the file, the parent will
lose its lock.
.Pp
Processes blocked awaiting a lock may be awakened by signals.
.Sh RETURN VALUES
Zero is returned if the operation was successful;
on an error a \-1 is returned and an error code is left in
the global location
.Va errno .
.Sh ERRORS
The
.Fn flock
call fails if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The file is locked and the
.Dv LOCK_NB
option was specified.
.It Bq Er EBADF
The argument
.Fa fd
is an invalid descriptor.
.It Bq Er EINVAL
The argument
.Fa operation
does not include exactly one of
.Dv LOCK_EX ,
.Dv LOCK_SH ,
or
.Dv LOCK_UN .
.It Bq Eq ENOMEM
The file lock limit for the current unprivilegied user 
has been reached. It can be modifed using sysctl
.Li kern.maxfiles .
.It Bq Er EOPNOTSUPP
The argument
.Fa fd
refers to an object other than a file.
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr dup 2 ,
.Xr execve 2 ,
.Xr fork 2 ,
.Xr open 2 ,
.Xr flockfile 3 ,
.Xr lockf 3
.Sh HISTORY
The
.Fn flock
function call appeared in
.Bx 4.2 .