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1 =head1 NAME 2 3 perldebtut - Perl debugging tutorial 4 5 =head1 DESCRIPTION 6 7 A (very) lightweight introduction in the use of the perl debugger, and a 8 pointer to existing, deeper sources of information on the subject of debugging 9 perl programs. 10 11 There's an extraordinary number of people out there who don't appear to know 12 anything about using the perl debugger, though they use the language every 13 day. 14 This is for them. 15 16 17 =head1 use strict 18 19 First of all, there's a few things you can do to make your life a lot more 20 straightforward when it comes to debugging perl programs, without using the 21 debugger at all. To demonstrate, here's a simple script, named "hello", with 22 a problem: 23 24 #!/usr/bin/perl 25 26 $var1 = 'Hello World'; # always wanted to do that :-) 27 $var2 = "$varl\n"; 28 29 print $var2; 30 exit; 31 32 While this compiles and runs happily, it probably won't do what's expected, 33 namely it doesn't print "Hello World\n" at all; It will on the other hand do 34 exactly what it was told to do, computers being a bit that way inclined. That 35 is, it will print out a newline character, and you'll get what looks like a 36 blank line. It looks like there's 2 variables when (because of the typo) 37 there's really 3: 38 39 $var1 = 'Hello World'; 40 $varl = undef; 41 $var2 = "\n"; 42 43 To catch this kind of problem, we can force each variable to be declared 44 before use by pulling in the strict module, by putting 'use strict;' after the 45 first line of the script. 46 47 Now when you run it, perl complains about the 3 undeclared variables and we 48 get four error messages because one variable is referenced twice: 49 50 Global symbol "$var1" requires explicit package name at ./t1 line 4. 51 Global symbol "$var2" requires explicit package name at ./t1 line 5. 52 Global symbol "$varl" requires explicit package name at ./t1 line 5. 53 Global symbol "$var2" requires explicit package name at ./t1 line 7. 54 Execution of ./hello aborted due to compilation errors. 55 56 Luvverly! and to fix this we declare all variables explicitly and now our 57 script looks like this: 58 59 #!/usr/bin/perl 60 use strict; 61 62 my $var1 = 'Hello World'; 63 my $varl = undef; 64 my $var2 = "$varl\n"; 65 66 print $var2; 67 exit; 68 69 We then do (always a good idea) a syntax check before we try to run it again: 70 71 > perl -c hello 72 hello syntax OK 73 74 And now when we run it, we get "\n" still, but at least we know why. Just 75 getting this script to compile has exposed the '$varl' (with the letter 'l') 76 variable, and simply changing $varl to $var1 solves the problem. 77 78 79 =head1 Looking at data and -w and v 80 81 Ok, but how about when you want to really see your data, what's in that 82 dynamic variable, just before using it? 83 84 #!/usr/bin/perl 85 use strict; 86 87 my $key = 'welcome'; 88 my %data = ( 89 'this' => qw(that), 90 'tom' => qw(and jerry), 91 'welcome' => q(Hello World), 92 'zip' => q(welcome), 93 ); 94 my @data = keys %data; 95 96 print "$data{$key}\n"; 97 exit; 98 99 Looks OK, after it's been through the syntax check (perl -c scriptname), we 100 run it and all we get is a blank line again! Hmmmm. 101 102 One common debugging approach here, would be to liberally sprinkle a few print 103 statements, to add a check just before we print out our data, and another just 104 after: 105 106 print "All OK\n" if grep($key, keys %data); 107 print "$data{$key}\n"; 108 print "done: '$data{$key}'\n"; 109 110 And try again: 111 112 > perl data 113 All OK 114 115 done: '' 116 117 After much staring at the same piece of code and not seeing the wood for the 118 trees for some time, we get a cup of coffee and try another approach. That 119 is, we bring in the cavalry by giving perl the 'B<-d>' switch on the command 120 line: 121 122 > perl -d data 123 Default die handler restored. 124 125 Loading DB routines from perl5db.pl version 1.07 126 Editor support available. 127 128 Enter h or `h h' for help, or `man perldebug' for more help. 129 130 main::(./data:4): my $key = 'welcome'; 131 132 Now, what we've done here is to launch the built-in perl debugger on our 133 script. It's stopped at the first line of executable code and is waiting for 134 input. 135 136 Before we go any further, you'll want to know how to quit the debugger: use 137 just the letter 'B<q>', not the words 'quit' or 'exit': 138 139 DB<1> q 140 > 141 142 That's it, you're back on home turf again. 143 144 145 =head1 help 146 147 Fire the debugger up again on your script and we'll look at the help menu. 148 There's a couple of ways of calling help: a simple 'B<h>' will get the summary 149 help list, 'B<|h>' (pipe-h) will pipe the help through your pager (which is 150 (probably 'more' or 'less'), and finally, 'B<h h>' (h-space-h) will give you 151 the entire help screen. Here is the summary page: 152 153 DB<1>h 154 155 List/search source lines: Control script execution: 156 l [ln|sub] List source code T Stack trace 157 - or . List previous/current line s [expr] Single step [in expr] 158 v [line] View around line n [expr] Next, steps over subs 159 f filename View source in file <CR/Enter> Repeat last n or s 160 /pattern/ ?patt? Search forw/backw r Return from subroutine 161 M Show module versions c [ln|sub] Continue until position 162 Debugger controls: L List break/watch/actions 163 o [...] Set debugger options t [expr] Toggle trace [trace expr] 164 <[<]|{[{]|>[>] [cmd] Do pre/post-prompt b [ln|event|sub] [cnd] Set breakpoint 165 ! [N|pat] Redo a previous command B ln|* Delete a/all breakpoints 166 H [-num] Display last num commands a [ln] cmd Do cmd before line 167 = [a val] Define/list an alias A ln|* Delete a/all actions 168 h [db_cmd] Get help on command w expr Add a watch expression 169 h h Complete help page W expr|* Delete a/all watch exprs 170 |[|]db_cmd Send output to pager ![!] syscmd Run cmd in a subprocess 171 q or ^D Quit R Attempt a restart 172 Data Examination: expr Execute perl code, also see: s,n,t expr 173 x|m expr Evals expr in list context, dumps the result or lists methods. 174 p expr Print expression (uses script's current package). 175 S [[!]pat] List subroutine names [not] matching pattern 176 V [Pk [Vars]] List Variables in Package. Vars can be ~pattern or !pattern. 177 X [Vars] Same as "V current_package [Vars]". 178 y [n [Vars]] List lexicals in higher scope <n>. Vars same as V. 179 For more help, type h cmd_letter, or run man perldebug for all docs. 180 181 More confusing options than you can shake a big stick at! It's not as bad as 182 it looks and it's very useful to know more about all of it, and fun too! 183 184 There's a couple of useful ones to know about straight away. You wouldn't 185 think we're using any libraries at all at the moment, but 'B<M>' will show 186 which modules are currently loaded, and their version number, while 'B<m>' 187 will show the methods, and 'B<S>' shows all subroutines (by pattern) as 188 shown below. 'B<V>' and 'B<X>' show variables in the program by package 189 scope and can be constrained by pattern. 190 191 DB<2>S str 192 dumpvar::stringify 193 strict::bits 194 strict::import 195 strict::unimport 196 197 Using 'X' and cousins requires you not to use the type identifiers ($@%), just 198 the 'name': 199 200 DM<3>X ~err 201 FileHandle(stderr) => fileno(2) 202 203 Remember we're in our tiny program with a problem, we should have a look at 204 where we are, and what our data looks like. First of all let's view some code 205 at our present position (the first line of code in this case), via 'B<v>': 206 207 DB<4> v 208 1 #!/usr/bin/perl 209 2: use strict; 210 3 211 4==> my $key = 'welcome'; 212 5: my %data = ( 213 6 'this' => qw(that), 214 7 'tom' => qw(and jerry), 215 8 'welcome' => q(Hello World), 216 9 'zip' => q(welcome), 217 10 ); 218 219 At line number 4 is a helpful pointer, that tells you where you are now. To 220 see more code, type 'v' again: 221 222 DB<4> v 223 8 'welcome' => q(Hello World), 224 9 'zip' => q(welcome), 225 10 ); 226 11: my @data = keys %data; 227 12: print "All OK\n" if grep($key, keys %data); 228 13: print "$data{$key}\n"; 229 14: print "done: '$data{$key}'\n"; 230 15: exit; 231 232 And if you wanted to list line 5 again, type 'l 5', (note the space): 233 234 DB<4> l 5 235 5: my %data = ( 236 237 In this case, there's not much to see, but of course normally there's pages of 238 stuff to wade through, and 'l' can be very useful. To reset your view to the 239 line we're about to execute, type a lone period '.': 240 241 DB<5> . 242 main::(./data_a:4): my $key = 'welcome'; 243 244 The line shown is the one that is about to be executed B<next>, it hasn't 245 happened yet. So while we can print a variable with the letter 'B<p>', at 246 this point all we'd get is an empty (undefined) value back. What we need to 247 do is to step through the next executable statement with an 'B<s>': 248 249 DB<6> s 250 main::(./data_a:5): my %data = ( 251 main::(./data_a:6): 'this' => qw(that), 252 main::(./data_a:7): 'tom' => qw(and jerry), 253 main::(./data_a:8): 'welcome' => q(Hello World), 254 main::(./data_a:9): 'zip' => q(welcome), 255 main::(./data_a:10): ); 256 257 Now we can have a look at that first ($key) variable: 258 259 DB<7> p $key 260 welcome 261 262 line 13 is where the action is, so let's continue down to there via the letter 263 'B<c>', which by the way, inserts a 'one-time-only' breakpoint at the given 264 line or sub routine: 265 266 DB<8> c 13 267 All OK 268 main::(./data_a:13): print "$data{$key}\n"; 269 270 We've gone past our check (where 'All OK' was printed) and have stopped just 271 before the meat of our task. We could try to print out a couple of variables 272 to see what is happening: 273 274 DB<9> p $data{$key} 275 276 Not much in there, lets have a look at our hash: 277 278 DB<10> p %data 279 Hello Worldziptomandwelcomejerrywelcomethisthat 280 281 DB<11> p keys %data 282 Hello Worldtomwelcomejerrythis 283 284 Well, this isn't very easy to read, and using the helpful manual (B<h h>), the 285 'B<x>' command looks promising: 286 287 DB<12> x %data 288 0 'Hello World' 289 1 'zip' 290 2 'tom' 291 3 'and' 292 4 'welcome' 293 5 undef 294 6 'jerry' 295 7 'welcome' 296 8 'this' 297 9 'that' 298 299 That's not much help, a couple of welcomes in there, but no indication of 300 which are keys, and which are values, it's just a listed array dump and, in 301 this case, not particularly helpful. The trick here, is to use a B<reference> 302 to the data structure: 303 304 DB<13> x \%data 305 0 HASH(0x8194bc4) 306 'Hello World' => 'zip' 307 'jerry' => 'welcome' 308 'this' => 'that' 309 'tom' => 'and' 310 'welcome' => undef 311 312 The reference is truly dumped and we can finally see what we're dealing with. 313 Our quoting was perfectly valid but wrong for our purposes, with 'and jerry' 314 being treated as 2 separate words rather than a phrase, thus throwing the 315 evenly paired hash structure out of alignment. 316 317 The 'B<-w>' switch would have told us about this, had we used it at the start, 318 and saved us a lot of trouble: 319 320 > perl -w data 321 Odd number of elements in hash assignment at ./data line 5. 322 323 We fix our quoting: 'tom' => q(and jerry), and run it again, this time we get 324 our expected output: 325 326 > perl -w data 327 Hello World 328 329 330 While we're here, take a closer look at the 'B<x>' command, it's really useful 331 and will merrily dump out nested references, complete objects, partial objects 332 - just about whatever you throw at it: 333 334 Let's make a quick object and x-plode it, first we'll start the debugger: 335 it wants some form of input from STDIN, so we give it something non-committal, 336 a zero: 337 338 > perl -de 0 339 Default die handler restored. 340 341 Loading DB routines from perl5db.pl version 1.07 342 Editor support available. 343 344 Enter h or `h h' for help, or `man perldebug' for more help. 345 346 main::(-e:1): 0 347 348 Now build an on-the-fly object over a couple of lines (note the backslash): 349 350 DB<1> $obj = bless({'unique_id'=>'123', 'attr'=> \ 351 cont: {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class') 352 353 And let's have a look at it: 354 355 DB<2> x $obj 356 0 MY_class=HASH(0x828ad98) 357 'attr' => HASH(0x828ad68) 358 'col' => 'black' 359 'things' => ARRAY(0x828abb8) 360 0 'this' 361 1 'that' 362 2 'etc' 363 'unique_id' => 123 364 DB<3> 365 366 Useful, huh? You can eval nearly anything in there, and experiment with bits 367 of code or regexes until the cows come home: 368 369 DB<3> @data = qw(this that the other atheism leather theory scythe) 370 371 DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data)) 372 atheism 373 leather 374 other 375 scythe 376 the 377 theory 378 saw -> 6 379 380 If you want to see the command History, type an 'B<H>': 381 382 DB<5> H 383 4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data)) 384 3: @data = qw(this that the other atheism leather theory scythe) 385 2: x $obj 386 1: $obj = bless({'unique_id'=>'123', 'attr'=> 387 {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class') 388 DB<5> 389 390 And if you want to repeat any previous command, use the exclamation: 'B<!>': 391 392 DB<5> !4 393 p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data)) 394 atheism 395 leather 396 other 397 scythe 398 the 399 theory 400 saw -> 12 401 402 For more on references see L<perlref> and L<perlreftut> 403 404 405 =head1 Stepping through code 406 407 Here's a simple program which converts between Celsius and Fahrenheit, it too 408 has a problem: 409 410 #!/usr/bin/perl -w 411 use strict; 412 413 my $arg = $ARGV[0] || '-c20'; 414 415 if ($arg =~ /^\-(c|f)((\-|\+)*\d+(\.\d+)*)$/) { 416 my ($deg, $num) = ($1, $2); 417 my ($in, $out) = ($num, $num); 418 if ($deg eq 'c') { 419 $deg = 'f'; 420 $out = &c2f($num); 421 } else { 422 $deg = 'c'; 423 $out = &f2c($num); 424 } 425 $out = sprintf('%0.2f', $out); 426 $out =~ s/^((\-|\+)*\d+)\.0+$/$1/; 427 print "$out $deg\n"; 428 } else { 429 print "Usage: $0 -[c|f] num\n"; 430 } 431 exit; 432 433 sub f2c { 434 my $f = shift; 435 my $c = 5 * $f - 32 / 9; 436 return $c; 437 } 438 439 sub c2f { 440 my $c = shift; 441 my $f = 9 * $c / 5 + 32; 442 return $f; 443 } 444 445 446 For some reason, the Fahrenheit to Celsius conversion fails to return the 447 expected output. This is what it does: 448 449 > temp -c0.72 450 33.30 f 451 452 > temp -f33.3 453 162.94 c 454 455 Not very consistent! We'll set a breakpoint in the code manually and run it 456 under the debugger to see what's going on. A breakpoint is a flag, to which 457 the debugger will run without interruption, when it reaches the breakpoint, it 458 will stop execution and offer a prompt for further interaction. In normal 459 use, these debugger commands are completely ignored, and they are safe - if a 460 little messy, to leave in production code. 461 462 my ($in, $out) = ($num, $num); 463 $DB::single=2; # insert at line 9! 464 if ($deg eq 'c') 465 ... 466 467 > perl -d temp -f33.3 468 Default die handler restored. 469 470 Loading DB routines from perl5db.pl version 1.07 471 Editor support available. 472 473 Enter h or `h h' for help, or `man perldebug' for more help. 474 475 main::(temp:4): my $arg = $ARGV[0] || '-c100'; 476 477 We'll simply continue down to our pre-set breakpoint with a 'B<c>': 478 479 DB<1> c 480 main::(temp:10): if ($deg eq 'c') { 481 482 Followed by a view command to see where we are: 483 484 DB<1> v 485 7: my ($deg, $num) = ($1, $2); 486 8: my ($in, $out) = ($num, $num); 487 9: $DB::single=2; 488 10==> if ($deg eq 'c') { 489 11: $deg = 'f'; 490 12: $out = &c2f($num); 491 13 } else { 492 14: $deg = 'c'; 493 15: $out = &f2c($num); 494 16 } 495 496 And a print to show what values we're currently using: 497 498 DB<1> p $deg, $num 499 f33.3 500 501 We can put another break point on any line beginning with a colon, we'll use 502 line 17 as that's just as we come out of the subroutine, and we'd like to 503 pause there later on: 504 505 DB<2> b 17 506 507 There's no feedback from this, but you can see what breakpoints are set by 508 using the list 'L' command: 509 510 DB<3> L 511 temp: 512 17: print "$out $deg\n"; 513 break if (1) 514 515 Note that to delete a breakpoint you use 'd' or 'D'. 516 517 Now we'll continue down into our subroutine, this time rather than by line 518 number, we'll use the subroutine name, followed by the now familiar 'v': 519 520 DB<3> c f2c 521 main::f2c(temp:30): my $f = shift; 522 523 DB<4> v 524 24: exit; 525 25 526 26 sub f2c { 527 27==> my $f = shift; 528 28: my $c = 5 * $f - 32 / 9; 529 29: return $c; 530 30 } 531 31 532 32 sub c2f { 533 33: my $c = shift; 534 535 536 Note that if there was a subroutine call between us and line 29, and we wanted 537 to B<single-step> through it, we could use the 'B<s>' command, and to step 538 over it we would use 'B<n>' which would execute the sub, but not descend into 539 it for inspection. In this case though, we simply continue down to line 29: 540 541 DB<4> c 29 542 main::f2c(temp:29): return $c; 543 544 And have a look at the return value: 545 546 DB<5> p $c 547 162.944444444444 548 549 This is not the right answer at all, but the sum looks correct. I wonder if 550 it's anything to do with operator precedence? We'll try a couple of other 551 possibilities with our sum: 552 553 DB<6> p (5 * $f - 32 / 9) 554 162.944444444444 555 556 DB<7> p 5 * $f - (32 / 9) 557 162.944444444444 558 559 DB<8> p (5 * $f) - 32 / 9 560 162.944444444444 561 562 DB<9> p 5 * ($f - 32) / 9 563 0.722222222222221 564 565 :-) that's more like it! Ok, now we can set our return variable and we'll 566 return out of the sub with an 'r': 567 568 DB<10> $c = 5 * ($f - 32) / 9 569 570 DB<11> r 571 scalar context return from main::f2c: 0.722222222222221 572 573 Looks good, let's just continue off the end of the script: 574 575 DB<12> c 576 0.72 c 577 Debugged program terminated. Use q to quit or R to restart, 578 use O inhibit_exit to avoid stopping after program termination, 579 h q, h R or h O to get additional info. 580 581 A quick fix to the offending line (insert the missing parentheses) in the 582 actual program and we're finished. 583 584 585 =head1 Placeholder for a, w, t, T 586 587 Actions, watch variables, stack traces etc.: on the TODO list. 588 589 a 590 591 w 592 593 t 594 595 T 596 597 598 =head1 REGULAR EXPRESSIONS 599 600 Ever wanted to know what a regex looked like? You'll need perl compiled with 601 the DEBUGGING flag for this one: 602 603 > perl -Dr -e '/^pe(a)*rl$/i' 604 Compiling REx `^pe(a)*rl$' 605 size 17 first at 2 606 rarest char 607 at 0 608 1: BOL(2) 609 2: EXACTF <pe>(4) 610 4: CURLYN[1] {0,32767}(14) 611 6: NOTHING(8) 612 8: EXACTF <a>(0) 613 12: WHILEM(0) 614 13: NOTHING(14) 615 14: EXACTF <rl>(16) 616 16: EOL(17) 617 17: END(0) 618 floating `'$ at 4..2147483647 (checking floating) stclass `EXACTF <pe>' 619 anchored(BOL) minlen 4 620 Omitting $` $& $' support. 621 622 EXECUTING... 623 624 Freeing REx: `^pe(a)*rl$' 625 626 Did you really want to know? :-) 627 For more gory details on getting regular expressions to work, have a look at 628 L<perlre>, L<perlretut>, and to decode the mysterious labels (BOL and CURLYN, 629 etc. above), see L<perldebguts>. 630 631 632 =head1 OUTPUT TIPS 633 634 To get all the output from your error log, and not miss any messages via 635 helpful operating system buffering, insert a line like this, at the start of 636 your script: 637 638 $|=1; 639 640 To watch the tail of a dynamically growing logfile, (from the command line): 641 642 tail -f $error_log 643 644 Wrapping all die calls in a handler routine can be useful to see how, and from 645 where, they're being called, L<perlvar> has more information: 646 647 BEGIN { $SIG{__DIE__} = sub { require Carp; Carp::confess(@_) } } 648 649 Various useful techniques for the redirection of STDOUT and STDERR filehandles 650 are explained in L<perlopentut> and L<perlfaq8>. 651 652 653 =head1 CGI 654 655 Just a quick hint here for all those CGI programmers who can't figure out how 656 on earth to get past that 'waiting for input' prompt, when running their CGI 657 script from the command-line, try something like this: 658 659 > perl -d my_cgi.pl -nodebug 660 661 Of course L<CGI> and L<perlfaq9> will tell you more. 662 663 664 =head1 GUIs 665 666 The command line interface is tightly integrated with an B<emacs> extension 667 and there's a B<vi> interface too. 668 669 You don't have to do this all on the command line, though, there are a few GUI 670 options out there. The nice thing about these is you can wave a mouse over a 671 variable and a dump of its data will appear in an appropriate window, or in a 672 popup balloon, no more tiresome typing of 'x $varname' :-) 673 674 In particular have a hunt around for the following: 675 676 B<ptkdb> perlTK based wrapper for the built-in debugger 677 678 B<ddd> data display debugger 679 680 B<PerlDevKit> and B<PerlBuilder> are NT specific 681 682 NB. (more info on these and others would be appreciated). 683 684 685 =head1 SUMMARY 686 687 We've seen how to encourage good coding practices with B<use strict> and 688 B<-w>. We can run the perl debugger B<perl -d scriptname> to inspect your 689 data from within the perl debugger with the B<p> and B<x> commands. You can 690 walk through your code, set breakpoints with B<b> and step through that code 691 with B<s> or B<n>, continue with B<c> and return from a sub with B<r>. Fairly 692 intuitive stuff when you get down to it. 693 694 There is of course lots more to find out about, this has just scratched the 695 surface. The best way to learn more is to use perldoc to find out more about 696 the language, to read the on-line help (L<perldebug> is probably the next 697 place to go), and of course, experiment. 698 699 700 =head1 SEE ALSO 701 702 L<perldebug>, 703 L<perldebguts>, 704 L<perldiag>, 705 L<dprofpp>, 706 L<perlrun> 707 708 709 =head1 AUTHOR 710 711 Richard Foley <richard@rfi.net> Copyright (c) 2000 712 713 714 =head1 CONTRIBUTORS 715 716 Various people have made helpful suggestions and contributions, in particular: 717 718 Ronald J Kimball <rjk@linguist.dartmouth.edu> 719 720 Hugo van der Sanden <hv@crypt0.demon.co.uk> 721 722 Peter Scott <Peter@PSDT.com> 723
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