How does a Perl script know if it's running in the debugger?

Question

I have a script that redirects STDIN/STDOUT to a file normally. But, debugging is a lot more efficient if it doesn't do that. Is there a $DB:xxx variable or something that lets the script know so it can behave differently?

Omaromara

Do you mean the interactive debugger or Perl's debugging functionality? (see comments on answers below) – Kimberlite

This was for running perl -d, shich I believe is Perl's debugging functionality. – Anuran

Answer 1

I didn't find any way of determining if they debugger is running directly, but you could check if well-known debugger variable $DB::single exists using the following:

if ( $DB::{ single } ) {
   say "Debugger is running";
}

Another approach would be to check if the debugger module is loaded.

if ( $INC{ "perl5db.pl" } ) {
   say "Debugger is running";
}

Finally, the debugger requires that Perl is running in debug support mode, and $^P indicates whether Perl is in this mode or not.

if ( $^P ) {
   say "Perl is in debug support mode";
}

The debugger isn't the only tool that requires putting Perl in debug support mode. Others include Devel::NYTProf, Devel::Cover, Devel::Trace, and more. So this approach can't be used to specifically check if the debugger is running. But it might be what you actually want.

Practise

Kudos to Eugen Konkov for bringing up $^P. – Practise

(However, his answer incorrectly describes what it does.) – Practise

I just pasted documentation as is. – Kimberlite

@Eugen Konkov, Please consult the documentation for $^P (in perlvar) and -d (in perlrun). – Practise

Please notice that $DB::single is false when we step over the function. Did you mean if( exists $DB::{ single })? You mention that in the description, but forgot in the code. – Kimberlite

@Eugen Konkov, $DB::{ single } isn't the same as $DB::single. It's the same as *DB::single, except that it doesn't vivify the symbol (which would render the test useless). – Practise

Answer 2

Reading through the perl source code of DB.pm (perldoc -m DB), I noticed that $SIG{INT} is globally set at the end.

This seems to work, at least for a trivial program:

if (\&DB::catch && $SIG{'INT'} && $SIG{'INT'} == \&DB::catch) {
    say "Debugging ?"
}

If it's not applicable, I guess it's possible to subclass DB.pm and create a simple debugger which overrides cont and does extra bookkeeping.

Teleran

Answer 3

To find that your script is under debugger you need to check: $^P variable:

if ( $^P ) {
    # running under a debugger
}

https://perldoc.perl.org/perldebug#Calling-the-Debugger

If you script was run without -d option then $DB::single does noting.

According to the documentation the minimal debugger is sub DB::DB {}.
So another method to check that your script is under debugger is:

if( defined &DB::DB ){
   # running under a debugger
} 

Kimberlite

Not quite. $^P indicates whether Perl is running in debug support mode or not, not whether the debugger is running or not. This is not quite the same. For example, $^P will be true when using Devel::Trace (perl -d:Trace ...) even though the debugger isn't running. And $DB::single won't do anything then either, since the debugger is not running. But, it could be what the OP actually wants, so +1. – Practise

@ikegami: Then I completely do not understand documentation. It says running under the debugger. It seems this sections requires more description. Also perlrun said that -d:MOD runs the program under the control of a debugging. I believe, it is not possible that $DB::single won't do anything, when -d is used ( debugger Perl_init_debugger(aTHX) is initialized). perldebguts. Please correct me, If I am wrong. – Kimberlite

@ikegami: Devel::Trace just do not bring interactive debugging, but that does not meant it does not make use of $DB::single and not under debugger. – Kimberlite

Re "It says running under the debugger.", That is not accurate. It is running with debuggin support, as the documentation for $^P (in perlvar) says. /// Re "perlrun said that -d:MOD runs the program under the control of a debugging" module, This one is correct. Like it says -d:Trace puts Devel::Trace "in control". Devel::Trace is not the debugger. -d without a module puts the debugger "in control". – Practise

Re "that does not meant it does not make use of $DB::single and not under debugger.", True, yet it's not under the debugger, and it doesn't create and use $DB::single like the debugger does. – Practise

@ikegami: Thanks. Now that is much clear. -d turns on Perl's debugging functionality (eg. Perl_init_debugger( aTHX ) is called ). After that Perl inserts value of PERL5DB environment variable as first line (The line with number zero) of a script. By default it is BEGIN{ require "perl5db.pl" }. Which brings the interactive debugging. In case of -d:MOD the use Devel::MOD will be used as the first line and, I assume, PERL5DB will be ignored. Still -d:MOD runs script under debugger, but depending on MOD implementation it could be without the interactive debugging. – Kimberlite

@ikegami: RE: True, yet it's not under the debugger. I think we need to clarify a bit. There are two debuggers: 1. Perl's debugger functionality and 2. the interactive debugging implemented by perl5db.pl or Devel::MOD when -d:MOD is used. When you used the debugger you mean the interactive debugging, but I mean Perl's debugging functionality. – Kimberlite

Or to use the terms from the docs, there is the Perl debugger (perl5db.pl), and there is debugging support (enabled by -d). The OP asked how to detect if the debugger is being used, but your answer shows how to detect debugging support. My original comment stands. – Practise

Answer 4

Both of these are useful for what I was doing. I'll put it on my Perl cheat sheet.

if ( $INC{ "perl5db.pl" } ) {
   say "Debugger is running";
}

if ( $^P ) {
   say "Perl is in debug support mode";
}

Anuran