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CentOS 7.0 - man page for tcl_createfilehandler (centos section 3)

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Tcl_CreateFileHandler(3)	      Tcl Library Procedures		 Tcl_CreateFileHandler(3)


       Tcl_CreateFileHandler, Tcl_DeleteFileHandler - associate procedure callbacks with files or
       devices (Unix only)

       #include <tcl.h>

       Tcl_CreateFileHandler(fd, mask, proc, clientData)


       int fd (in)			     Unix file descriptor for an open file or device.

       int mask (in)			     Conditions under which proc should be called:  OR-ed
					     combination   of	TCL_READABLE,  TCL_WRITABLE,  and
					     TCL_EXCEPTION.  May be set to 0 to temporarily  dis-
					     able a handler.

       Tcl_FileProc *proc (in)		     Procedure	to  invoke  whenever  the  file or device
					     indicated by file meets the conditions specified  by

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

       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  fd,	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 callback to proc is made
       by  Tcl_DoOneEvent,  so	Tcl_CreateFileHandler  is  only  useful in programs that dispatch
       events through Tcl_DoOneEvent or through Tcl commands such as vwait.

       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 clientData argument given to Tcl_Create-
       FileHandler  when the callback was created.  Typically, clientData points to a data struc-
       ture 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 con-
       tain 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_CreateFileHan-
       dler  is  called  when a handler already exists for fd, then the new callback replaces the
       information that was previously recorded.

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

       The  purpose of file handlers is to enable an application to respond to events while wait-
       ing for files to become ready for I/O.  For this to work correctly,  the  application  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  will  not  be	able to service other events. Use
       Tcl_SetChannelOption with -blocking to set the channel into blocking or	nonblocking  mode
       as required.

       Note  that these interfaces are only supported by the Unix implementation of the Tcl noti-

       callback, file, handler

Tcl					       8.0			 Tcl_CreateFileHandler(3)
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