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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 specifying
     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
     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(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.

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.