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fileevent(3tcl)


NAME

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

SYNOPSIS

   fileevent channelId readable ?script?

   fileevent channelId writable ?script?
______________________________________________________________________________

DESCRIPTION

   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
   will not 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 again.

   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 command registered with interp bgerror 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.

EXAMPLE

   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]

CREDITS

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

SEE ALSO

   fconfigure(3tcl),  gets(3tcl),  interp(3tcl),  puts(3tcl),  read(3tcl),
   Tcl_StandardChannels(3tcl)

KEYWORDS

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



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