freebsd man page for flock

FLOCK(2)						      BSD System Calls Manual							  FLOCK(2)

NAME
     flock -- apply or remove an advisory lock on an open file

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/file.h>
     #define   LOCK_SH	      0x01	/* shared file lock */
     #define   LOCK_EX	      0x02	/* exclusive file lock */
     #define   LOCK_NB	      0x04	/* do not block when locking */
     #define   LOCK_UN	      0x08	/* 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 fd.  A lock is applied by speci-
     fying an 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 pre-
     vious 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.

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
     The flock() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
     the error.

ERRORS
     The flock() system 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.

     [EOPNOTSUPP]	The argument fd refers to an object that does not support file locking.

     [ENOLCK]		A lock was requested, but no locks are available.

SEE ALSO
     close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), flopen(3), lockf(3)

HISTORY
     The flock() system call appeared in 4.2BSD.

BSD
								 November 9, 2011							       BSD