flock(2) System Calls Manual flock(2)
NAME
flock() - apply or remove an advisory or enforced lock on an open file
SYNOPSIS
Parameters
filedes Specifies a file descriptor returned by a successful or function, identifying the file on which the locking operation is to be
performed.
operation Specifies one of the following constants for defined in the file:
Apply a shared lock.
Apply an exclusive lock.
Remove a lock.
In addition to the preceding constants, the following constant can be bitwise ORed with either or
LOCK_NB Do not block when locking.
DESCRIPTION
The function applies or removes an advisory or enforced lock on the whole file associated with the filedes file descriptor. Advisory locks
allow cooperating processes to perform consistent operations on files, but do not guarantee consistency (that is, processes may still
access files without using advisory locks, possibly resulting in inconsistencies).Enforced locks are in effect if the enforcement bit in
the file mode is set (see chmod(2)). This might affect and other system calls performing file operations.
You can use the function to coordinate a file's lock status on local and NFS file systems.
The locking mechanism allows two types of locks: and At any time multiple shared locks may be applied to a file, but at no time are multi-
ple exclusive locks 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 is
included in operation, this will not happen; instead, the call will fail and will be set to
APPLICATION USAGE
Locks are on files, not file descriptors. This means that:
o Locks are not inherited by a child process resulting from a call.
o All locks associated with a file for a given process are removed when any thread in the process closes any file descriptor for that
file.
Processes that are blocked awaiting a lock may be awakened by signals. An application needs to check the return value to make sure that
the lock is really acquired.
The interface is not part of any UNIX standard. Therefore, if you are designing and writing applications to be portable across platforms,
you should use the file locking interface instead of
NETWORKING FEATURES
NFS
The advisory file locking capabilities of are implemented throughout the network by the network lock daemon If the file server crashes and
is rebooted, the lock daemon attempts to recover all locks associated with the crashed server. If a lock cannot be reclaimed, the process
that held the lock is issued a signal. See lockd(1M).
File locking as implemented for NFS files is only advisory.
RETURN VALUES
flock() returns:
for successful completion
for unsuccessful completion;
is set to indicate the error
ERRORS
If the function fails, may be set to one of the following values:
The filedes parameter is not a valid open file descriptor.
A shared lock was requested and filedes was not opened for reading, or an exclusive lock was requested and filedes was not opened
for writing.
The lock is blocked by some lock from another process.
Putting the calling process to sleep while waiting for that lock to become free would cause a deadlock.
A signal interrupted the
call.
The filedes parameter refers to a file that does not support locking.
The operation parameter is not valid.
The lock table is full.
Too many files are already locked.
The filedes parameter refers to an NFS file with the access bits set for enforcement mode.
The filedes parameter refers to an NFS file, and a system error occurred on the remote node.
The file is locked and the
option was specified.
The file is mapped into virtual memory via the system call (see mmap(2)).
SEE ALSO
lockd(1M) chmod(2), close(2), creat(2), dup(2), dup2(2), exec(2), fcntl(2), fork(2), lockf(2), open(2), read(2), truncate(2), write(2).
STANDARDS CONFORMANCE
flock(2)