       flock options fileId ?start? ?length? ?origin?
              This  command  places  a lock on all or part of the
              file specified by fileId.  The lock is either advi-
              sory  or  mandatory,  depending on the mode bits of
              the file.  The lock is placed beginning at relative
              byte  offset  start  for length bytes.  If start or
              length is omitted or empty, zero  is  assumed.   If
              length is zero, then the lock always extents to end
              of file, even if the  file  grows.   If  origin  is
              "start",  then the offset is relative to the begin-
              ning of the file. If it is "current", it  is  rela-
              tive  to  the  current access position in the file.
              If it is "end", then it is relative to the  end-of-
              file  (a  negative  is  before the EOF, positive is
              after).  If origin is omitted, start is assumed.

              The following options are recognized:

              -read - Place a read lock on  the  file.   Multiple
              processes  may  be  accessing  the  file with read-
              locks.

              -write - Place a write lock on the file.  Only  one
              process may be accessing a file if there is a write
              lock.

              -nowait - If specified, then the process  will  not
              block  if  the lock can not be obtained.  With this
              option, the  command  returns  1  if  the  lock  is
              obtained and 0 if it is not.

              See  your  system's fcntl system call documentation
              for full details of the behavior of  file  locking.
              If locking is being done on ranges of a file, it is
              best to use unbuffered file access (see  the  fcntl
              command).

              The  flock  command is not available on Windows 95.
              It is available on Windows NT.


              This command is provided by Extended Tcl.
