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Practical Programming in Tcl & Tk, Third Edition
By Brent B. Welch

Table of Contents
Chapter 19.  Multiple Interpreters and Safe-Tcl


Hidden Commands

The commands listed in Table 19-2 are hidden instead of being completely removed. A hidden command can be invoked in a slave by its master. For example, a master can load Tcl scripts into a slave by using its hidden source command:

interp create -safe slave
interp invokehidden slave source filename

Without hidden commands, the master has to do a bit more work to achieve the same thing. It must open and read the file and eval the contents of the file in the slave. File operations are described in Chapter 9.

interp create -safe slave
set in [open filename]
interp eval slave [read $in]
close $in

Hidden commands were added in Tcl 7.7 in order to better support the Tcl/Tk browser plug-in described in Chapter 20. In some cases, hidden commands are strictly necessary; it is not possible to simulate them any other way. The best examples are in the context of Safe-Tk, where the master creates widgets or does potentially dangerous things on behalf of the slave. These will be discussed in more detail later.

A master can hide and expose commands using the interp hide and interp expose operations, respectively. You can even hide Tcl procedures. However, the commands inside the procedure run with the same privilege as that of the slave. For example, if you are really paranoid, you might not want an untrusted interpreter to read the clock or get timing information. You can hide the clock and time commands:

interp create -safe slave
interp hide slave clock
interp hide slave time

You can remove commands from the slave entirely like this:

interp eval slave [list rename clock {}]
interp eval slave [list rename time {}]

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