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.

     [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.