flock
- apply or remove an advisory lock on an open file
LIBRARY
Lb libc
SYNOPSIS
#include <sys/file.h>
Fd #defineLOCK_SH0x01/* shared file lock */
Fd #defineLOCK_EX0x02/* exclusive file lock */
Fd #defineLOCK_NB0x04/* do not block when locking */
Fd #defineLOCK_UN0x08/* unlock file */
int
flock (int fd int operation);
DESCRIPTION
The
flock ();
system call applies or removes an
advisory
lock on the file associated with the file descriptor
Fa fd .
A lock is applied by specifying an
Fa operation
argument that is one of
LOCK_SH
or
LOCK_EX
with the optional addition of
LOCK_NB
To unlock
an existing lock
operation
should be
LOCK_UN
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).
The locking mechanism allows two types of locks:
shared
locks and
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.
A shared lock may be
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).
Requesting a lock on an object that is already locked
normally causes the caller to be blocked until the lock may be
acquired.
If
LOCK_NB
is included in
Fa operation ,
then this will not happen; instead the call will fail and
the error
Er EWOULDBLOCK
will be returned.
NOTES
Locks are on files, not file descriptors.
That is, file descriptors
duplicated through
dup(2)
or
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.
The
flock (,); fcntl(2),
and
lockf(3)
locks are compatible.
Processes using different locking interfaces can cooperate
over the same file safely.
However, only one of such interfaces should be used within
the same process.
If a file is locked by a process through
flock (,);
any record within the file will be seen as locked
from the viewpoint of another process using
fcntl(2)
or
lockf(3),
and vice versa.
Processes blocked awaiting a lock may be awakened by signals.
RETURN VALUES
Rv -std flock
ERRORS
The
flock ();
system call fails if:
Bq Er EWOULDBLOCK
The file is locked and the
LOCK_NB
option was specified.
Bq Er EBADF
The argument
Fa fd
is an invalid descriptor.
Bq Er EINVAL
The argument
Fa fd
refers to an object other than a file.
Bq Er EOPNOTSUPP
The argument
Fa fd
refers to an object that does not support file locking.
