Go to main content

man pages section 1: User Commands

Exit Print View

Updated: Wednesday, February 9, 2022

fileevent (1t)


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


fileevent channelId readable ?script?

fileevent channelId writable ?script?


fileevent(1t)                Tcl Built-In Commands               fileevent(1t)


       fileevent  -  Execute  a  script  when  a  channel  becomes readable or

       fileevent channelId readable ?script?

       fileevent channelId writable ?script?

       This command is used to create file event handlers.  A file event  han-
       dler  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 con-
       tinue  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 read-
       able 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 inter-
       preter is deleted.

       A  channel  is considered to be readable if there is unread data avail-
       able on the underlying device.  A channel  is  also  considered  to  be
       readable if there is unread data in an input buffer, except in the spe-
       cial 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; a readable underlying file  or  device
       may  not even guarantee that a blocking [read 1] will succeed (counter-
       examples being multi-byte encodings, compression or  encryption  trans-
       forms  ). In all such cases, no events will be processed while the com-
       mands block.

       In nonblocking mode puts, read, and gets never block.  See the documen-
       tation  for  the individual commands for information on how they handle
       blocking and nonblocking channels.

       Testing for the end of file condition should be done after any attempts
       read  the channel data. The eof flag is set once an attempt to read the
       end of data has occurred and testing before this read will  require  an
       additional event to be fired.

       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.

       In  this  setup  GetData will be called with the channel as an argument
       whenever $chan becomes readable.  The  read  call  will  read  whatever
       binary  data is currently available without blocking.  Here the channel
       has the fileevent removed when an end of file  occurs  to  avoid  being
       continually called (see above). Alternatively the channel may be closed
       on this condition.

              proc GetData {chan} {
                  set data [read $chan]
                  puts "[string length $data] $data"
                  if {[eof $chan]} {
                      fileevent $chan readable {}

              fconfigure $chan -blocking 0 -encoding binary
              fileevent $chan readable [list GetData $chan]

       The next example demonstrates use of gets to read line-oriented data.

              proc GetData {chan} {
                  if {[gets $chan line] >= 0} {
                      puts $line
                  if {[eof $chan]} {
                      close $chan

              fconfigure $chan -blocking 0 -buffering line -translation crlf
              fileevent $chan readable [list GetData $chan]

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

       See attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |

       fconfigure(n), gets(n), interp(n), puts(n), read(n),  Tcl_StandardChan-

       asynchronous  I/O, blocking, channel, event handler, nonblocking, read-
       able, script, writable.

       Source code for open source software components in Oracle  Solaris  can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                   7.5                        fileevent(1t)