Provided by: tcl8.4-doc_8.4.19-4ubuntu3_all bug


       fileevent - Execute a script when a channel becomes readable or writable


       fileevent channelId readable ?script?

       fileevent channelId writable ?script?


       This  command  is  used  to create file event handlers.  A file event handler is a binding
       between a channel and a script, such that the script is  evaluated  whenever  the  channel
       becomes readable or writable.  File event handlers are most commonly used to allow data to
       be received from another process on an  event-driven  basis,  so  that  the  receiver  can
       continue  to  interact  with  the  user  while  waiting  for  the  data  to arrive.  If an
       application invokes gets or read on a  blocking  channel  when  there  is  no  input  data
       available,  the  process  will block; until the input data arrives, it will not be able to
       service other events, so it will appear to the user to ``freeze up''.  With fileevent, the
       process can tell when data is present and only invoke gets or read when they won't block.

       The  channelId  argument  to  fileevent  refers  to an open channel such as a Tcl standard │
       channel (stdin, stdout, or stderr), the return value from an invocation of open or socket, │
       or the result of a channel creation command provided by a Tcl extension.

       If  the  script argument is specified, then fileevent creates a new event handler:  script
       will be evaluated whenever the channel becomes readable  or  writable  (depending  on  the
       second  argument  to  fileevent).   In  this  case fileevent returns an empty string.  The
       readable and writable event handlers for a file are independent, and may  be  created  and
       deleted  separately.   However, there may be at most one readable and one writable handler
       for a file at a given time in a given  interpreter.   If  fileevent  is  called  when  the
       specified  handler already exists in the invoking interpreter, the new script replaces the
       old one.

       If the script argument  is  not  specified,  fileevent  returns  the  current  script  for
       channelId, or an empty string if there is none.  If the script argument is specified as an
       empty string then the event handler is deleted, so that no script will be invoked.  A file
       event  handler  is  also  deleted  automatically  whenever  its  channel  is closed or its
       interpreter is deleted.

       A channel is considered to be readable if there is unread data available on the underlying
       device.   A  channel is also considered to be readable if there is unread data in an input
       buffer, except in the special case where the most recent attempt to read from the  channel
       was  a  gets  call  that could not find a complete line in the input buffer.  This feature
       allows a file to be read a line at a time in nonblocking mode using events.  A channel  is
       also  considered  to  be  readable  if an end of file or error condition is present on the
       underlying file or device.  It is important for script to check for these  conditions  and
       handle  them appropriately;  for example, if there is no special check for end of file, an
       infinite loop may occur where script reads no data, returns, and  is  immediately  invoked

       A  channel is considered to be writable if at least one byte of data can be written to the
       underlying file or device without blocking, or if an error condition  is  present  on  the
       underlying file or device.

       Event-driven  I/O works best for channels that have been placed into nonblocking mode with
       the fconfigure command.  In blocking mode, a puts command may block if you  give  it  more
       data  than the underlying file or device can accept, and a gets or read command will block
       if you attempt to read more data than is ready;  no events will  be  processed  while  the
       commands  block.   In  nonblocking  mode  puts,  read,  and  gets  never  block.   See the
       documentation for the individual commands for information on how they handle blocking  and
       nonblocking channels.

       The  script  for  a file event is executed at global level (outside the context of any Tcl
       procedure) in the interpreter in which the fileevent command was  invoked.   If  an  error
       occurs  while executing the script then the bgerror mechanism is used to report the error.
       In addition, the file event handler is deleted if it ever returns an error;  this is  done
       in order to prevent infinite loops due to buggy handlers.


       In  this  setup  GetData  will  be  called  with the channel as an argument whenever $chan
       becomes readable.
              proc GetData {chan} {
                  if {![eof $chan]} {
                      puts [gets $chan]

              fileevent $chan readable [list GetData $chan]


       fileevent is based on the addinput command created by Mark Diekhans.


       bgerror(3tcl),      fconfigure(3tcl),      gets(3tcl),       puts(3tcl),       read(3tcl),


       asynchronous  I/O,  blocking,  channel,  event  handler,  nonblocking,  readable,  script,