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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)flock.2 8.2 (Berkeley) 12/11/93 29.\" $FreeBSD$ 30.\" 31.Dd November 9, 2011 32.Dt FLOCK 2 33.Os 34.Sh NAME 35.Nm flock 36.Nd "apply or remove an advisory lock on an open file" 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In sys/file.h 41.Fd "#define LOCK_SH 0x01 /* shared file lock */" 42.Fd "#define LOCK_EX 0x02 /* exclusive file lock */" 43.Fd "#define LOCK_NB 0x04 /* do not block when locking */" 44.Fd "#define LOCK_UN 0x08 /* unlock file */" 45.Ft int 46.Fn flock "int fd" "int operation" 47.Sh DESCRIPTION 48The 49.Fn flock 50system call applies or removes an 51.Em advisory 52lock on the file associated with the file descriptor 53.Fa fd . 54A lock is applied by specifying an 55.Fa operation 56argument that is one of 57.Dv LOCK_SH 58or 59.Dv LOCK_EX 60with the optional addition of 61.Dv LOCK_NB . 62To unlock 63an existing lock 64.Dv operation 65should be 66.Dv LOCK_UN . 67.Pp 68Advisory locks allow cooperating processes to perform 69consistent operations on files, but do not guarantee 70consistency (i.e., processes may still access files 71without using advisory locks possibly resulting in 72inconsistencies). 73.Pp 74The locking mechanism allows two types of locks: 75.Em shared 76locks and 77.Em exclusive 78locks. 79At any time multiple shared locks may be applied to a file, 80but at no time are multiple exclusive, or both shared and exclusive, 81locks allowed simultaneously on a file. 82.Pp 83A shared lock may be 84.Em upgraded 85to an exclusive lock, and vice versa, simply by specifying 86the appropriate lock type; this results in the previous 87lock being released and the new lock applied (possibly 88after other processes have gained and released the lock). 89.Pp 90Requesting a lock on an object that is already locked 91normally causes the caller to be blocked until the lock may be 92acquired. 93If 94.Dv LOCK_NB 95is included in 96.Fa operation , 97then this will not happen; instead the call will fail and 98the error 99.Er EWOULDBLOCK 100will be returned. 101.Sh NOTES 102Locks are on files, not file descriptors. 103That is, file descriptors 104duplicated through 105.Xr dup 2 106or 107.Xr fork 2 108do not result in multiple instances of a lock, but rather multiple 109references to a single lock. 110If a process holding a lock on a file 111forks and the child explicitly unlocks the file, the parent will 112lose its lock. 113.Pp 114The 115.Fn flock , 116.Xr fcntl 2 , 117and 118.Xr lockf 3 119locks are compatible. 120Processes using different locking interfaces can cooperate 121over the same file safely. 122However, only one of such interfaces should be used within 123the same process. 124If a file is locked by a process through 125.Fn flock , 126any record within the file will be seen as locked 127from the viewpoint of another process using 128.Xr fcntl 2 129or 130.Xr lockf 3 , 131and vice versa. 132.Pp 133Processes blocked awaiting a lock may be awakened by signals. 134.Sh RETURN VALUES 135.Rv -std flock 136.Sh ERRORS 137The 138.Fn flock 139system call fails if: 140.Bl -tag -width Er 141.It Bq Er EWOULDBLOCK 142The file is locked and the 143.Dv LOCK_NB 144option was specified. 145.It Bq Er EBADF 146The argument 147.Fa fd 148is an invalid descriptor. 149.It Bq Er EINVAL 150The argument 151.Fa fd 152refers to an object other than a file. 153.It Bq Er EOPNOTSUPP 154The argument 155.Fa fd 156refers to an object that does not support file locking. 157.It Bq Er ENOLCK 158A lock was requested, but no locks are available. 159.El 160.Sh SEE ALSO 161.Xr close 2 , 162.Xr dup 2 , 163.Xr execve 2 , 164.Xr fcntl 2 , 165.Xr fork 2 , 166.Xr open 2 , 167.Xr flopen 3 , 168.Xr lockf 3 169.Sh HISTORY 170The 171.Fn flock 172system call appeared in 173.Bx 4.2 . 174