X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperldebtut.pod;h=f9f19ac08c8cd832d4ca2030476b75e30cf47bd5;hb=bbd5c0f5ad81733b079008f34cd05cd9aef7d917;hp=ece58482697f83e2c35213a8ed3c7b8a26db10cf;hpb=0e06870bf080a38cda51c06c6612359afc2334e1;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perldebtut.pod b/pod/perldebtut.pod index ece5848..f9f19ac 100644 --- a/pod/perldebtut.pod +++ b/pod/perldebtut.pod @@ -21,10 +21,10 @@ straightforward when it comes to debugging perl programs, without using the debugger at all. To demonstrate, here's a simple script with a problem: #!/usr/bin/perl - + $var1 = 'Hello World'; # always wanted to do that :-) $var2 = "$varl\n"; - + print $var2; exit; @@ -45,7 +45,7 @@ first line of the script. Now when you run it, perl complains about the 3 undeclared variables and we get four error messages because one variable is referenced twice: - + Global symbol "$var1" requires explicit package name at ./t1 line 4. Global symbol "$var2" requires explicit package name at ./t1 line 5. Global symbol "$varl" requires explicit package name at ./t1 line 5. @@ -57,11 +57,11 @@ script looks like this: #!/usr/bin/perl use strict; - + my $var1 = 'Hello World'; my $varl = ''; my $var2 = "$varl\n"; - + print $var2; exit; @@ -97,7 +97,7 @@ dynamic variable, just before using it? Looks OK, after it's been through the syntax check (perl -c scriptname), we run it and all we get is a blank line again! Hmmmm. - + One common debugging approach here, would be to liberally sprinkle a few print statements, to add a check just before we print out our data, and another just after: @@ -107,10 +107,10 @@ after: print "done: '$data{$key}'\n"; And try again: - + > perl data All OK - + done: '' After much staring at the same piece of code and not seeing the wood for the @@ -137,7 +137,7 @@ just the letter 'B', not the words 'quit' or 'exit': DB<1> q > - + That's it, you're back on home turf again. @@ -174,7 +174,7 @@ break/watch/actions V [Pk [Vars]] List Variables in Package. Vars can be ~pattern or !pattern. X [Vars] Same as "V current_package [Vars]". For more help, type h cmd_letter, or run man perldebug for all docs. - + More confusing options than you can shake a big stick at! It's not as bad as it looks and it's very useful to know more about all of it, and fun too! @@ -196,7 +196,7 @@ the 'name': DM<3>X ~err FileHandle(stderr) => fileno(2) - + Remember we're in our tiny program with a problem, we should have a look at where we are, and what our data looks like. First of all let's have a window on our present position (the first line of code in this case), via the letter @@ -216,7 +216,7 @@ on our present position (the first line of code in this case), via the letter At line number 4 is a helpful pointer, that tells you where you are now. To see more code, type 'w' again: - + DB<4> w 8 'welcome' => q(Hello World), 9 'zip' => q(welcome), @@ -231,19 +231,19 @@ And if you wanted to list line 5 again, type 'l 5', (note the space): DB<4> l 5 5: my %data = ( - + In this case, there's not much to see, but of course normally there's pages of stuff to wade through, and 'l' can be very useful. To reset your view to the line we're about to execute, type a lone period '.': DB<5> . main::(./data_a:4): my $key = 'welcome'; - + The line shown is the one that is about to be executed B, it hasn't happened yet. So while we can print a variable with the letter 'B

', at this point all we'd get is an empty (undefined) value back. What we need to do is to step through the next executable statement with an 'B': - + DB<6> s main::(./data_a:5): my %data = ( main::(./data_a:6): 'this' => qw(that), @@ -264,21 +264,21 @@ line or sub routine: DB<8> c 13 All OK main::(./data_a:13): print "$data{$key}\n"; - + We've gone past our check (where 'All OK' was printed) and have stopped just before the meat of our task. We could try to print out a couple of variables to see what is happening: DB<9> p $data{$key} - + Not much in there, lets have a look at our hash: - + DB<10> p %data Hello Worldziptomandwelcomejerrywelcomethisthat DB<11> p keys %data Hello Worldtomwelcomejerrythis - + Well, this isn't very easy to read, and using the helpful manual (B), the 'B' command looks promising: @@ -329,7 +329,7 @@ While we're here, take a closer look at the 'B' command, it's really useful and will merrily dump out nested references, complete objects, partial objects - just about whatever you throw at it: -Let's make a quick object and x-plode it, first we'll start the the debugger: +Let's make a quick object and x-plode it, first we'll start the debugger: it wants some form of input from STDIN, so we give it something non-commital, a zero: @@ -349,7 +349,7 @@ Now build an on-the-fly object over a couple of lines (note the backslash): cont: {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class') And let's have a look at it: - + DB<2> x $obj 0 MY_class=HASH(0x828ad98) 'attr' => HASH(0x828ad68) @@ -365,7 +365,7 @@ Useful, huh? You can eval nearly anything in there, and experiment with bits of code or regexes until the cows come home: DB<3> @data = qw(this that the other atheism leather theory scythe) - + DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data)) atheism leather @@ -384,7 +384,7 @@ If you want to see the command History, type an 'B': 1: $obj = bless({'unique_id'=>'123', 'attr'=> {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class') DB<5> - + And if you want to repeat any previous command, use the exclamation: 'B': DB<5> !4 @@ -446,22 +446,22 @@ expected output. This is what it does: > temp -c0.72 33.30 f - + > temp -f33.3 162.94 c - + Not very consistent! We'll set a breakpoint in the code manually and run it under the debugger to see what's going on. A breakpoint is a flag, to which the debugger will run without interruption, when it reaches the breakpoint, it will stop execution and offer a prompt for further interaction. In normal use, these debugger commands are completely ignored, and they are safe - if a little messy, to leave in production code. - + my ($in, $out) = ($num, $num); $DB::single=2; # insert at line 9! if ($deg eq 'c') ... - + > perl -d temp -f33.3 Default die handler restored. @@ -478,7 +478,7 @@ We'll simply continue down to our pre-set breakpoint with a 'B': main::(temp:10): if ($deg eq 'c') { Followed by a window command to see where we are: - + DB<1> w 7: my ($deg, $num) = ($1, $2); 8: my ($in, $out) = ($num, $num); @@ -499,9 +499,9 @@ And a print to show what values we're currently using: We can put another break point on any line beginning with a colon, we'll use line 17 as that's just as we come out of the subroutine, and we'd like to pause there later on: - + DB<2> b 17 - + There's no feedback from this, but you can see what breakpoints are set by using the list 'L' command: @@ -550,13 +550,13 @@ possibilities with our sum: DB<6> p (5 * $f - 32 / 9) 162.944444444444 - + DB<7> p 5 * $f - (32 / 9) 162.944444444444 - + DB<8> p (5 * $f) - 32 / 9 162.944444444444 - + DB<9> p 5 * ($f - 32) / 9 0.722222222222221 @@ -564,10 +564,10 @@ possibilities with our sum: return out of the sub with an 'r': DB<10> $c = 5 * ($f - 32) / 9 - + DB<11> r scalar context return from main::f2c: 0.722222222222221 - + Looks good, let's just continue off the end of the script: DB<12> c @@ -585,11 +585,11 @@ actual program and we're finished. Actions, watch variables, stack traces etc.: on the TODO list. a - + W - + t - + T @@ -597,7 +597,7 @@ Actions, watch variables, stack traces etc.: on the TODO list. Ever wanted to know what a regex looked like? You'll need perl compiled with the DEBUGGING flag for this one: - + > perl -Dr -e '/^pe(a)*rl$/i' Compiling REx `^pe(a)*rl$' size 17 first at 2 @@ -666,7 +666,7 @@ and there's a B interface too. You don't have to do this all on the command line, though, there are a few GUI options out there. The nice thing about these is you can wave a mouse over a -variable and a dump of it's data will appear in an appropriate window, or in a +variable and a dump of its data will appear in an appropriate window, or in a popup balloon, no more tiresome typing of 'x $varname' :-) In particular have a hunt around for the following: @@ -674,7 +674,7 @@ In particular have a hunt around for the following: B perlTK based wrapper for the built-in debugger B data display debugger - + B and B are NT specific NB. (more info on these and others would be appreciated).