Programming Perl, 4th Edition

Writing Your Own Debugger

The minimal working debugger consists of one line:

sub DB::DB {}

which, since it does nothing whatsoever, can easily be defined via the PERL5DB environment variable:

$ PERL5DB="sub DB::DB {}" perl –d your–program

Another tiny, slightly more useful debugger could be created like this:

sub DB::DB {print ++$i; scalar <STDIN>}

This little debugger would print the sequential number of each encountered statement and would wait for you to hit a newline before continuing.

The following debugger, small though it may appear, is really quite functional:

{
    package DB;
    sub DB  {}
    sub sub {print ++$i, " $sub\n"; &$sub}
}

It prints the sequential number of the subroutine call and the name of the called subroutine. Note that &DB::sub must be compiled from the package DB, as we’ve done here.

If you base your new debugger on the current debugger, there are some hooks that can help you customize it. At startup, the debugger reads your init file from the current directory or your home directory. After the file is read, the debugger reads the PERLDB_OPTS environment variable and parses this as the remainder of an O ... line such as you might enter at the debugger prompt.

The debugger also maintains magical internal variables, such as @DB::dbline and %DB::dbline, which are aliases for @{"::_<current_file"} %{"::_<current_file"}. Here, current_file is the currently selected file, either explicitly chosen with the debugger’s f command or implicitly by flow of execution.

Some functions can ...