FLOCK(3B) FLOCK(3B)
flock - apply or remove an advisory lock on an open file
#include <sys/file.h>
int flock(int fd, int operation);
Flock applies or removes an advisory lock on the file associated with the
file descriptor fd. A lock is applied by specifying one of the following
as an operation parameter: LOCK_SH, LOCK_EX, LOCK_SH|LOCK_NB or
LOCK_EX|LOCK_NB. Note that LOCK_NB, if used, must appear in an inclusive
OR expression with either LOCK_SH or LOCK_EX. 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 operation, then this will not happen; instead the call will
fail and the error EWOULDBLOCK will be returned.
Locks are on files, not file descriptors. That is, file descriptors
duplicated through dup(3C) (but not through fork(2), see the BUGS section
below) do not result in multiple instances of a lock, but rather multiple
references to a single lock. Thus if any of the descriptors associated
with the same file are closed, the lock associated with the file is lost.
Processes blocked awaiting a lock may be awakened by signals.
In C++, the function name flock collides with the structure name flock
(which is declared in <sys/fcntl.h> and included in <sys/file.h> ). When
using flock() in C++, one must define _BSD_COMPAT before including
sys/file.h
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FLOCK(3B) FLOCK(3B)
Zero is returned if the operation was successful; on an error a -1 is
returned and an error code is left in the global location errno.
The flock call fails if:
[EWOULDBLOCK] The file is locked and the LOCK_NB option was
specified.
[EBADF] The argument fd is an invalid descriptor.
[EINVAL] The argument fd refers to an object other than a
file.
open(2), close(2), dup(3C), execve(2), fcntl(2), fork(2), lockf(3)
Unlike BSD, child processes created by fork(2) do not inherit references
to locks acquired by their parents through flock(3B) calls. This bug
results from flock's implementation atop System V file and record locks.
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