SunOS/BSD Compatibility Library Functions             flock(3UCB)



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

SYNOPSIS
     /usr/ucb/cc[ flag ... ] file ...
     #include 

     int flock( fd,  operation)

     int fd, operation;


DESCRIPTION
     flock() applies or removes an  advisory  lock  on  the  file
     associated  with  the  file descriptor fd. The compatibility
     version of flock() has been implemented on top  of  fcntl(2)
     locking.  It does not provide complete binary compatibility.


     Advisory locks allow cooperating processes to  perform  con-
     sistent  operations on files, but do not guarantee exclusive
     access (that is, processes may still  access  files  without
     using  advisory  locks,  possibly  resulting in inconsisten-
     cies).


     The locking mechanism allows  two  types  of  locks:  shared
     locks  and exclusive locks. More than one process may hold a
     shared lock for a file  at  any  given  time,  but  multiple
     exclusive, or both shared and exclusive, locks may not exist
     simultaneously on a file.


     A lock is  applied  by  specifying  an  operation  parameter
     LOCKSH  for a shared lock or LOCKEX for an exclusive lock.
     The operation paramerer may be ORed with LOCKNB to make the
     operation  non-blocking.  To  unlock  an  existing lock, the
     operation should be LOCKUN.


     Read permission is required on a file  to  obtain  a  shared
     lock,   and  write  permission  is  required  to  obtain  an
     exclusive lock. Locking a segment that is already locked  by
     the  calling  process causes the old lock type to be removed
     and the new lock type to take effect.


     Requesting a lock on an object that is already  locked  nor-
     mally  causes  the  caller  to  block  until the lock may be
     acquired.  If LOCKNB is included in  operation,  then  this
     will  not  happen; instead, the call will fail and the error
     EWOULDBLOCK will be returned.



SunOS 5.11          Last change: 30 Oct 2007                    1






SunOS/BSD Compatibility Library Functions             flock(3UCB)



RETURN VALUES
     flock() returns:

     0        on success.


     -1       on failure and sets errno to indicate the error.


ERORS
     EBADF          The argument fd is an invalid descriptor.


     EINVAL         operation is not a valid argument.


     EOPNOTSUP     The argument fd refers  to  an  object  other
                    than a file.


     EWOULDBLOCK    The file is locked and  the   LOCKNB  option
                    was specified.


SEE ALSO
     cc(1B),  lockd(1M),  chmod(2),  close(2),  dup(2),  exec(2),
     fcntl(2), fork(2), open(2), lockf(3C)

NOTES
     Use of these interfaces should be restricted to only  appli-
     cations  written  on BSD platforms.  Use of these interfaces
     with any of the system libraries or in multi-thread applica-
     tions is unsupported.


     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.  Locks are not inherited by a
     child process.


     Processes blocked awaiting a lock may be  awakened  by  sig-
     nals.


     Mandatory locking may occur, depending on the mode  bits  of
     the file.  See chmod(2).





SunOS 5.11          Last change: 30 Oct 2007                    2






SunOS/BSD Compatibility Library Functions             flock(3UCB)



     Locks obtained through the flock() mechanism under SunOS 4.1
     were known only within the system on which they were placed.
     This is no longer true.




















































SunOS 5.11          Last change: 30 Oct 2007                    3