4.3BSD - /usr/man/cat2/flock.0

FLOCK(2)                                                              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 opera‐
       tions  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 exclu‐
       sive, 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.

       Processes blocked awaiting a lock may be awakened by signals.

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 spec‐
                           ified.

       [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)


4.2 Berkeley Distribution        May 22, 1986                         FLOCK(2)