flock(2)flock(2)NAMEflock - Apply or remove an advisory lock on an open file
SYNOPSIS
#include <sys/fcntl.h>
int flock(
int filedes,
int operation );
PARAMETERS
Specifies a file descriptor returned by a successful open() or fcntl()
function, identifying the file to which the lock is to be applied or
removed. Specifies one of the following constants for flock(), defined
in the <sys/fcntl.h> file: Apply a shared lock. Apply an exclusive
lock. Do not block when locking. This value can be logically ORed
with either LOCK_SH or LOCK_EX. Remove a lock.
DESCRIPTION
The flock() function applies or removes an advisory lock on the 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).
You can use the flock() function to coordinate a file's lock status on
local, CFS, and NFS file systems.
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 exclu‐
sive, 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 operation, then this will not happen; instead, the call
will fail and errno will be set to [EWOULDBLOCK].
NOTES
Locks are on files, not file descriptors. This means that: Locks are
not inherited by a child process resulting from a fork() call. All
locks associated with a file for a given process are removed when the
process closes any file descriptor for that file.
Processes that are blocked awaiting a lock may be awakened by signals.
The flock() interface is not part of any UNIX standard. Therefore, if
you are designing and writing applications to be portable across plat‐
forms, you should use the fcntl() file locking interface instead of
flock().
RETURN VALUES
Upon successful completion, 0 (zero) is returned. Otherwise, -1 is
returned and errno is set to indicate the error.
ERRORS
If the flock() function fails, errno may be set to one of the following
values: The file is locked and the LOCK_NB option was specified. The
filedes argument is not a valid open file descriptor. A signal inter‐
rupted the flock() call. The operator is not valid. The lock table is
full. Too many regions are already locked. 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.
SEE ALSO
Functions: close(2), exec(2), fcntl(2), fork(2), open(2), lockf(3)flock(2)
