FLOCK(3B)                                                            FLOCK(3B)


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

SYNOPSIS
     #include <sys/file.h>

     int flock(int fd, int operation);

DESCRIPTION
     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.

NOTES
     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


RETURN VALUE
     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.

ERRORS
     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.

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

BUGS
     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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