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.\"     @(#)flock.2	8.2 (Berkeley) 12/11/93
.\"
.Dd $Mdocdate: June 25 2019 $
.Dt FLOCK 2
.Os
.Sh NAME
.Nm flock
.Nd apply or remove an advisory lock on an open file
.Sh SYNOPSIS
.In fcntl.h
.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 .
The
.Fa operation
argument is one of:
.Pp
.Bl -tag -width LOCK_SH -offset indent -compact
.It Dv LOCK_SH
Apply a shared lock.
.It Dv LOCK_EX
Apply an exclusive lock.
.It Dv LOCK_UN
Remove an existing lock.
.El
.Pp
.Dv LOCK_SH
and
.Dv LOCK_EX
may be combined with the optional
.Dv LOCK_NB
for nonblocking mode.
.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
.Fa operation
is the bitwise OR of
.Dv LOCK_NB
and
.Dv LOCK_SH
or
.Dv LOCK_EX ,
then this will not happen; instead the call will fail and the error
.Er EWOULDBLOCK
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
.Rv -std
.Sh ERRORS
The
.Fn flock
call fails if:
.Bl -tag -width Er
.It Bq Er EWOULDBLOCK
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
has an invalid value.
.It Bq Er EOPNOTSUPP
The argument
.Fa fd
refers to a file that does not support locking.
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr dup 2 ,
.Xr execve 2 ,
.Xr fcntl 2 ,
.Xr fork 2 ,
.Xr open 2
.Sh HISTORY
The
.Fn flock
system call first appeared in
.Bx 4.1c .