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


       #include <tcl.h>

       Tcl_CreateFileHandler(fd, mask, proc, clientData)                       |

       Tcl_DeleteFileHandler(fd)                                               |


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

       int            mask         (in)      Conditions   under   which   proc
                                             should  be called: OR-ed combina-
                                             tion       of       TCL_READABLE,
                                             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 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_READ-
       ABLE,  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_CreateFileHandler when the callback  was  created.   Typi-
       cally,  clientData  points  to a data structure containing application-
       specific information about the file.  Mask is an integer mask  indicat-
       ing which of the requested conditions actually exists for the file;  it
       will contain a subset of the bits in the mask argument  to  Tcl_Create-

       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  fd,
       then  the  new  callback  replaces  the information that was previously

       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

       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 application may need to use non-blocking I/O opera-
       tions  on  the  files  for  which handlers are declared.  Otherwise the
       application may block if it reads or writes too much data; while  wait-
       ing  for  the  I/O to complete the application won't be able to service
       other events. Use Tcl_SetChannelOption with -blocking to set the  chan-
       nel into blocking or nonblocking mode as required.

       Note  that  these interfaces are only supported by the Unix implementa- |
       tion of the Tcl notifier.


       callback, file, handler

Tcl                                   8.0             Tcl_CreateFileHandler(3)

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