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fileevent(3)          User Contributed Perl Documentation         fileevent(3)




NAME

       Tk::fileevent - Execute a callback when a filehandle becomes readable
       or writable


SYNOPSIS

       $widget->fileevent(fileHandle,readable?,callback?)

       $widget->fileevent(fileHandle,writable?,callback?)


DESCRIPTION

       This command is used to create file event handlers.  A file event
       handler is a binding between a filehandle and a callback, such that the
       callback is evaluated whenever the filehandle 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 "<>", "sysread" or "read" on
       a blocking filehandle 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 fileHandle argument to fileevent refers to an open filehandle, such
       as the return value from a previous open or socket command.  If the
       callback argument is specified, then fileevent creates a new event
       handler:  callback will be evaluated whenever the filehandle becomes
       readable or writable (depending on the 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 callback replaces the old one.

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

       A filehandle is considered to be readable if there is unread data
       available on the underlying device.  A filehandle 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 callback 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
       callback reads no data, returns, and is immediately invoked again.

       A filehandle 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 filehandles that have been placed into
       nonblocking mode.  In blocking mode, a "print" command may block if you
       give it more data than the underlying file or device can accept, and a
       "<>", "sysread" 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 "print", "<>", "sysread" and
       "read" never block.  See the documentation for the individual commands
       for information on how they handle blocking and nonblocking
       filehandles.

       The callback for a file event is executed in the context of $widget
       with which fileevent was invoked.  If an error occurs while executing
       the callback then the Tk::Error 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.


BUGS

       On windows platforms fileevent is limited in the types of filehandles
       that behave correctly. Making filefhandles non-blocking is only
       implemented on a subset of UNIX platforms (see Tk::IO).


CREDITS

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


SEE ALSO

       Tk::IO Tk::callbacks


KEYWORDS

       asynchronous I/O, blocking, filehandle, event handler, nonblocking,
       readable, callback, writable.



perl v5.18.0                      2010-05-29                      fileevent(3)

perl-Tk 804.030_502 - Generated Sat Aug 17 14:32:37 CDT 2013
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