FLOCK(2)                             BSD                              FLOCK(2)



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

SYNOPSIS
     #include <sys/file.h>

     #define   LOCK_SH   1    /* shared lock */
     #define   LOCK_EX   2    /* exclusive lock */
     #define   LOCK_NB   4    /* don't block when locking */
     #define   LOCK_UN   8    /* unlock */

     flock(fd, operation)
     int fd, operation;

DESCRIPTION
     flock applies or removes an "advisory" lock on the file associated with
     the file descriptor fd.  A lock is applied by specifying an operation
     parameter that is the inclusive or of LOCK_SH or LOCK_EX and, possibly,
     LOCK_NB.  To unlock an existing lock, operation should be LOCK_UN.

     Advisory locks allow cooperating processes to perform consistent
     operations on files, but they do not guarantee consistency (that is,
     processes can 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 can 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 can 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 can 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.

ERRORS
     The flock call fails if any of the following are true:

     [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(2), execve(2), fork(2)

DIAGNOSTICS
     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.

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.

     Processes blocked awaiting a lock can be awakened by signals.