[MJ] - With the use of [namespace ensemble]s to create newer Tcl core commands (e.g. [chan]) it's very straightforward to extend core commands with your own functionallity. One way is to change the Tcl core as described in [chan mode]. 

A better way is to write an extension adding the functionallity. For instance we can extend the Tcl [chan] command to have a new subcommand [chan mode] that returns a list of the access modes of a file handle (r for readable, w for writeable) with the small extension below.

To register the new subcommand in the [namespace ensemble] the ensemble map needs to be updated (as described by [NEM] at the [chan mode] page)

'''examples'''
 % package require chanmode
 0.1
 % chan mode stderr
 w
 % chan mode stdin
 r
 % set f [open c:/temp.txt w+]
 filee1f908
 % chan mode $f
 r w

'''pkgIndex.tcl'''

 package ifneeded chanmode 0.1 \
 [list source [file dirname [info script]]/chanmode.tcl]


'''chanmode.tcl'''

 load [file dirname [info script]]/chanmode0.1[info sharedlibextension]
 set map [namespace ensemble configure ::chan -map]
 dict set map mode ::chanmode::mode
 namespace ensemble configure ::chan -map $map

'''chanmode.c'''

 #include <tcl.h>
 #include <stdio.h>
  int
  Chanmode_ModeObjCmd(
     ClientData dummy,           /* Not used. */
     Tcl_Interp *interp,         /* Current interpreter. */
     int objc,                   /* Number of arguments. */
     Tcl_Obj *const objv[])      /* Argument objects. */
  {
     Tcl_Channel chan;           /* The channel to puts on. */
     const char *channelId; /* Name of channel for puts. */
     int mode;                   /* Mode in which channel is opened. */
 
     if (objc != 2) {
         Tcl_WrongNumArgs(interp, 1, objv, "channelId");
         return TCL_ERROR;
     }
 
     channelId = Tcl_GetString(objv[1]);
 
     chan = Tcl_GetChannel(interp, channelId, &mode);
     if (chan == (Tcl_Channel) NULL) {
         return TCL_ERROR;
     }
 
     if ((mode & TCL_READABLE) != 0) {
         Tcl_AppendElement(interp, "r");
     }
 
     if ((mode & TCL_WRITABLE) != 0) {
         Tcl_AppendElement(interp, "w");
     }
 
     return TCL_OK;
 
  }
 
 int DLLEXPORT
 Chanmode_Init(Tcl_Interp *interp)
 {
   if (Tcl_InitStubs(interp, "8.5", 0) == 0L) {
     return TCL_ERROR;
   }
   Tcl_CreateObjCommand(interp, "chanmode::mode", Chanmode_ModeObjCmd, NULL, NULL);
   Tcl_PkgProvide(interp, "chanmode", "0.1");
   return TCL_OK;
 }
----
[[ [Category Example]|[Category Channel] ]]