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Tcl_CreateFileHandler(Tcl Library ProceduTcl_CreateFileHandler(3)

_________________________________________________________________

NAME
       Tcl_CreateFileHandler,  Tcl_DeleteFileHandler  - associate
       procedure callbacks with files or devices

SYNOPSIS
       #include <tcl.h>

       Tcl_CreateFileHandler(file, mask, proc, clientData)

       Tcl_DeleteFileHandler(file)

ARGUMENTS
       Tcl_File	      file	   (in)	     Generic file  handle
					     for  an open file or
					     device   (such    as
					     returned by Tcl_Get-
					     File call).

       int	      mask	   (in)	     Conditions	    under
					     which proc should be
					     called: OR-ed combi-
					     nation  of TCL_READ-
					     ABLE,  TCL_WRITABLE,
					     and   TCL_EXCEPTION.
					     May be set to  0  to
					     temporarily  disable
					     a handler.

       Tcl_FileProc   *proc	   (in)	     Procedure to  invoke
					     whenever the file or
					     device indicated  by
					     file  meets the con-
					     ditions specified by
					     mask.

       ClientData     clientData   (in)	     Arbitrary	 one-word
					     value  to	pass   to
					     proc.
_________________________________________________________________

DESCRIPTION
       Tcl_CreateFileHandler  arranges	for proc to be invoked in
       the future whenever I/O becomes possible on a file  or  an
       exceptional  condition  exists  for the file.  The file is
       indicated by file, and  the  conditions	of  interest  are
       indicated  by mask.  For example, if mask is TCL_READABLE,
       proc will be called when the file is readable.  The  call-
       back to proc is made by Tcl_DoOneEvent, so Tcl_CreateFile-
       Handler is only useful in programs  that	 dispatch  events
       through	Tcl_DoOneEvent	or  through  Tcl commands such as
       vwait.

Tcl			       7.5				1

Tcl_CreateFileHandler(Tcl Library ProceduTcl_CreateFileHandler(3)

       Proc should have arguments and result that match the  type
       Tcl_FileProc:
	      typedef void Tcl_FileProc(
		ClientData clientData,
		int mask);
       The  clientData parameter to proc is a copy of the client-
       Data argument  given  to	 Tcl_CreateFileHandler	when  the
       callback	 was  created.	Typically, clientData points to a
       data structure containing application-specific information
       about  the file.	 Mask is an integer mask indicating which
       of the requested conditions actually exists for the  file;
       it  will contain a subset of the bits in the mask argument
       to Tcl_CreateFileHandler.

       There may exist only one handler for a  given  file  at	a
       given  time.   If  Tcl_CreateFileHandler	 is called when a
       handler already exists for file,	 then  the  new	 callback
       replaces the information that was previously recorded.

       Tcl_DeleteFileHandler  may  be  called  to delete the file
       handler for file;  if no handler exists for the file given
       by file then the procedure has no effect.

       The  purpose  of file handlers is to enable an application
       to respond to events while waiting  for	files  to  become
       ready  for  I/O.	 For this to work correctly, the applica-
       tion may need to use non-blocking I/O  operations  on  the
       files  for  which  handlers  are	 declared.  Otherwise the
       application may block if it reads or writes too much data;
       while  waiting  for  the	 I/O  to complete the application
       won't be able to service other  events.	Use  Tcl_SetChan-
       nelOption  with -blocking to set the channel into blocking
       or nonblocking mode as required.

KEYWORDS
       callback, file, handler

Tcl			       7.5				2

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