| ActiveTcl User Guide |

## Tcl/Tk Documentation > TclLib > CrtFileHdlr

### Tcl/Tk Applications | Tcl Commands | Tk Commands | Tcl Library | Tk Library

### NAME

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

### SYNOPSIS

#include <tcl.h>
Tcl_CreateFileHandler(fd, mask, proc, clientData)
Tcl_DeleteFileHandler(fd)

### ARGUMENTS

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 disable a handler.
Tcl_FileProc *proc (in)
  Procedure to invoke whenever the file or device indicated by file meets the conditions 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 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_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 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 waiting 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 notifier.

### KEYWORDS

callback, file, handler
Copyright © 1995-1997 Roger E. Critchlow Jr. Copyright © 1990-1994 The Regents of the University of California. Copyright © 1994-1997 Sun Microsystems, Inc.