Version 13 of A minimal debugger

Updated 2011-10-23 11:27:49 by RLE

Richard Suchenwirth 2001-01-16 -- Of course, Tcl's most minimal debugger is puts. But here is a cute little piece of code that offers some more debugging functionality (if you have stdin and stdout available - so not for wish on Windows):

 proc bp {{s {}}} {
        if ![info exists ::bp_skip] {
           set ::bp_skip [list]
        } elseif {[lsearch -exact $::bp_skip $s]>=0} return
        if [catch {info level -1} who] {set who ::}
        while 1 {
                puts -nonewline "$who/$s> "; flush stdout
                gets stdin line
                if {$line=="c"} {puts "continuing.."; break}
                if {$line=="i"} {set line "info locals"}
                catch {uplevel 1 $line} res
                puts $res

The idea is that you insert breakpoints, calls to bp, in critical code with an optional string argument (that may be used for distinguishing), like this:

 proc foo {args} {
        set x 1
        bp 1
        string toupper $args
 foo bar and grill

When execution reaches bp, you get a prompt on stdout, giving the calling context and the bp string, like this:

 foo bar and grill/1> pwd
 foo bar and grill/1> i
 args x
 foo bar and grill/1> set args
 bar and grill
 foo bar and grill/1> set args lowercase
 foo bar and grill/1> c

on which you can issue any Tcl command (especially getting or setting variable values in the scope of foo), shorthand commands ("i" for "info locals"; add which others you need), and exit this micro-debugger with "c"(ontinue). Because you modified a local variable, foo's result will be


To turn off all breakpoints for an application, just say (maybe from inside bp):

 proc bp args {}

You can disable single breakpoints labeled e.g. x with the command

 lappend ::bp_skip x

Stepping is made easy with the new execution traces from 8.4 - see Steppin' out, but in some situations a micro-debugger like this is good enough. See also What debugging tools are available to a Tcl programmer

Bits and pieces.. Here's a minimal variable watcher, that logs every change to the specified variables to stdout:

 proc watch {args} {
     foreach arg $args {
        uplevel 1 "trace var $arg w {puts $arg:\[set $arg\] ;#}"

[Incidental remark: Expect builds in lots of interesting debugging capabilities.]

Tkcon has a couple of debugging features like those described above. idebug is like bp above but with a few more bells and whistles, and observe is like watch above. [kpv]

AM It is very easy to extend this minimal debugger to become a mini debugger. To keep the intrusion as small as possible, just handle all debugging commands that you want to implement in a switch statement (rather than the cascading "if") for readability. This way you introduce but a single new command, bp, instead of a handful (but it may be useful to define a few, like watch, as a procedure, so that you can log the variables automatically.

One problem remains (one that I would like to solve or see solved!): how to get this to work under Windows - perhaps a solution via a pipe? The reason I am concerned is that I use (want to use) Tcl in an "embedded" mode. I can not use Tk in there, so a solution via Tkcon or console show becomes difficult... - RS: What is the problem? The little one above works only with stdin and stdout, no Tk required; and I've made good use of it debugging our app where a Tcl interpreter is embedded.

See also An error experiment with an even minimaller debugger...