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.

Debian                                          November 9, 2011                                        FLOCK(2)