| 1 | =head1 NAME |
| 2 | |
| 3 | perlhack - How to hack at the Perl internals |
| 4 | |
| 5 | =head1 DESCRIPTION |
| 6 | |
| 7 | This document attempts to explain how Perl development takes place, |
| 8 | and ends with some suggestions for people wanting to become bona fide |
| 9 | porters. |
| 10 | |
| 11 | The perl5-porters mailing list is where the Perl standard distribution |
| 12 | is maintained and developed. The list can get anywhere from 10 to 150 |
| 13 | messages a day, depending on the heatedness of the debate. Most days |
| 14 | there are two or three patches, extensions, features, or bugs being |
| 15 | discussed at a time. |
| 16 | |
| 17 | A searchable archive of the list is at either: |
| 18 | |
| 19 | http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/ |
| 20 | |
| 21 | or |
| 22 | |
| 23 | http://archive.develooper.com/perl5-porters@perl.org/ |
| 24 | |
| 25 | List subscribers (the porters themselves) come in several flavours. |
| 26 | Some are quiet curious lurkers, who rarely pitch in and instead watch |
| 27 | the ongoing development to ensure they're forewarned of new changes or |
| 28 | features in Perl. Some are representatives of vendors, who are there |
| 29 | to make sure that Perl continues to compile and work on their |
| 30 | platforms. Some patch any reported bug that they know how to fix, |
| 31 | some are actively patching their pet area (threads, Win32, the regexp |
| 32 | engine), while others seem to do nothing but complain. In other |
| 33 | words, it's your usual mix of technical people. |
| 34 | |
| 35 | Over this group of porters presides Larry Wall. He has the final word |
| 36 | in what does and does not change in the Perl language. Various |
| 37 | releases of Perl are shepherded by a ``pumpking'', a porter |
| 38 | responsible for gathering patches, deciding on a patch-by-patch |
| 39 | feature-by-feature basis what will and will not go into the release. |
| 40 | For instance, Gurusamy Sarathy was the pumpking for the 5.6 release of |
| 41 | Perl, and Jarkko Hietaniemi was the pumpking for the 5.8 release, and |
| 42 | Hugo van der Sanden and Rafael Garcia-Suarez share the pumpking for |
| 43 | the 5.10 release. |
| 44 | |
| 45 | In addition, various people are pumpkings for different things. For |
| 46 | instance, Andy Dougherty and Jarkko Hietaniemi did a grand job as the |
| 47 | I<Configure> pumpkin up till the 5.8 release. For the 5.10 release |
| 48 | H.Merijn Brand took over. |
| 49 | |
| 50 | Larry sees Perl development along the lines of the US government: |
| 51 | there's the Legislature (the porters), the Executive branch (the |
| 52 | pumpkings), and the Supreme Court (Larry). The legislature can |
| 53 | discuss and submit patches to the executive branch all they like, but |
| 54 | the executive branch is free to veto them. Rarely, the Supreme Court |
| 55 | will side with the executive branch over the legislature, or the |
| 56 | legislature over the executive branch. Mostly, however, the |
| 57 | legislature and the executive branch are supposed to get along and |
| 58 | work out their differences without impeachment or court cases. |
| 59 | |
| 60 | You might sometimes see reference to Rule 1 and Rule 2. Larry's power |
| 61 | as Supreme Court is expressed in The Rules: |
| 62 | |
| 63 | =over 4 |
| 64 | |
| 65 | =item 1 |
| 66 | |
| 67 | Larry is always by definition right about how Perl should behave. |
| 68 | This means he has final veto power on the core functionality. |
| 69 | |
| 70 | =item 2 |
| 71 | |
| 72 | Larry is allowed to change his mind about any matter at a later date, |
| 73 | regardless of whether he previously invoked Rule 1. |
| 74 | |
| 75 | =back |
| 76 | |
| 77 | Got that? Larry is always right, even when he was wrong. It's rare |
| 78 | to see either Rule exercised, but they are often alluded to. |
| 79 | |
| 80 | New features and extensions to the language are contentious, because |
| 81 | the criteria used by the pumpkings, Larry, and other porters to decide |
| 82 | which features should be implemented and incorporated are not codified |
| 83 | in a few small design goals as with some other languages. Instead, |
| 84 | the heuristics are flexible and often difficult to fathom. Here is |
| 85 | one person's list, roughly in decreasing order of importance, of |
| 86 | heuristics that new features have to be weighed against: |
| 87 | |
| 88 | =over 4 |
| 89 | |
| 90 | =item Does concept match the general goals of Perl? |
| 91 | |
| 92 | These haven't been written anywhere in stone, but one approximation |
| 93 | is: |
| 94 | |
| 95 | 1. Keep it fast, simple, and useful. |
| 96 | 2. Keep features/concepts as orthogonal as possible. |
| 97 | 3. No arbitrary limits (platforms, data sizes, cultures). |
| 98 | 4. Keep it open and exciting to use/patch/advocate Perl everywhere. |
| 99 | 5. Either assimilate new technologies, or build bridges to them. |
| 100 | |
| 101 | =item Where is the implementation? |
| 102 | |
| 103 | All the talk in the world is useless without an implementation. In |
| 104 | almost every case, the person or people who argue for a new feature |
| 105 | will be expected to be the ones who implement it. Porters capable |
| 106 | of coding new features have their own agendas, and are not available |
| 107 | to implement your (possibly good) idea. |
| 108 | |
| 109 | =item Backwards compatibility |
| 110 | |
| 111 | It's a cardinal sin to break existing Perl programs. New warnings are |
| 112 | contentious--some say that a program that emits warnings is not |
| 113 | broken, while others say it is. Adding keywords has the potential to |
| 114 | break programs, changing the meaning of existing token sequences or |
| 115 | functions might break programs. |
| 116 | |
| 117 | =item Could it be a module instead? |
| 118 | |
| 119 | Perl 5 has extension mechanisms, modules and XS, specifically to avoid |
| 120 | the need to keep changing the Perl interpreter. You can write modules |
| 121 | that export functions, you can give those functions prototypes so they |
| 122 | can be called like built-in functions, you can even write XS code to |
| 123 | mess with the runtime data structures of the Perl interpreter if you |
| 124 | want to implement really complicated things. If it can be done in a |
| 125 | module instead of in the core, it's highly unlikely to be added. |
| 126 | |
| 127 | =item Is the feature generic enough? |
| 128 | |
| 129 | Is this something that only the submitter wants added to the language, |
| 130 | or would it be broadly useful? Sometimes, instead of adding a feature |
| 131 | with a tight focus, the porters might decide to wait until someone |
| 132 | implements the more generalized feature. For instance, instead of |
| 133 | implementing a ``delayed evaluation'' feature, the porters are waiting |
| 134 | for a macro system that would permit delayed evaluation and much more. |
| 135 | |
| 136 | =item Does it potentially introduce new bugs? |
| 137 | |
| 138 | Radical rewrites of large chunks of the Perl interpreter have the |
| 139 | potential to introduce new bugs. The smaller and more localized the |
| 140 | change, the better. |
| 141 | |
| 142 | =item Does it preclude other desirable features? |
| 143 | |
| 144 | A patch is likely to be rejected if it closes off future avenues of |
| 145 | development. For instance, a patch that placed a true and final |
| 146 | interpretation on prototypes is likely to be rejected because there |
| 147 | are still options for the future of prototypes that haven't been |
| 148 | addressed. |
| 149 | |
| 150 | =item Is the implementation robust? |
| 151 | |
| 152 | Good patches (tight code, complete, correct) stand more chance of |
| 153 | going in. Sloppy or incorrect patches might be placed on the back |
| 154 | burner until the pumpking has time to fix, or might be discarded |
| 155 | altogether without further notice. |
| 156 | |
| 157 | =item Is the implementation generic enough to be portable? |
| 158 | |
| 159 | The worst patches make use of a system-specific features. It's highly |
| 160 | unlikely that nonportable additions to the Perl language will be |
| 161 | accepted. |
| 162 | |
| 163 | =item Is the implementation tested? |
| 164 | |
| 165 | Patches which change behaviour (fixing bugs or introducing new features) |
| 166 | must include regression tests to verify that everything works as expected. |
| 167 | Without tests provided by the original author, how can anyone else changing |
| 168 | perl in the future be sure that they haven't unwittingly broken the behaviour |
| 169 | the patch implements? And without tests, how can the patch's author be |
| 170 | confident that his/her hard work put into the patch won't be accidentally |
| 171 | thrown away by someone in the future? |
| 172 | |
| 173 | =item Is there enough documentation? |
| 174 | |
| 175 | Patches without documentation are probably ill-thought out or |
| 176 | incomplete. Nothing can be added without documentation, so submitting |
| 177 | a patch for the appropriate manpages as well as the source code is |
| 178 | always a good idea. |
| 179 | |
| 180 | =item Is there another way to do it? |
| 181 | |
| 182 | Larry said ``Although the Perl Slogan is I<There's More Than One Way |
| 183 | to Do It>, I hesitate to make 10 ways to do something''. This is a |
| 184 | tricky heuristic to navigate, though--one man's essential addition is |
| 185 | another man's pointless cruft. |
| 186 | |
| 187 | =item Does it create too much work? |
| 188 | |
| 189 | Work for the pumpking, work for Perl programmers, work for module |
| 190 | authors, ... Perl is supposed to be easy. |
| 191 | |
| 192 | =item Patches speak louder than words |
| 193 | |
| 194 | Working code is always preferred to pie-in-the-sky ideas. A patch to |
| 195 | add a feature stands a much higher chance of making it to the language |
| 196 | than does a random feature request, no matter how fervently argued the |
| 197 | request might be. This ties into ``Will it be useful?'', as the fact |
| 198 | that someone took the time to make the patch demonstrates a strong |
| 199 | desire for the feature. |
| 200 | |
| 201 | =back |
| 202 | |
| 203 | If you're on the list, you might hear the word ``core'' bandied |
| 204 | around. It refers to the standard distribution. ``Hacking on the |
| 205 | core'' means you're changing the C source code to the Perl |
| 206 | interpreter. ``A core module'' is one that ships with Perl. |
| 207 | |
| 208 | =head2 Keeping in sync |
| 209 | |
| 210 | The source code to the Perl interpreter, in its different versions, is |
| 211 | kept in a repository managed by a revision control system ( which is |
| 212 | currently the Perforce program, see http://perforce.com/ ). The |
| 213 | pumpkings and a few others have access to the repository to check in |
| 214 | changes. Periodically the pumpking for the development version of Perl |
| 215 | will release a new version, so the rest of the porters can see what's |
| 216 | changed. The current state of the main trunk of repository, and patches |
| 217 | that describe the individual changes that have happened since the last |
| 218 | public release are available at this location: |
| 219 | |
| 220 | http://public.activestate.com/gsar/APC/ |
| 221 | ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/ |
| 222 | |
| 223 | If you're looking for a particular change, or a change that affected |
| 224 | a particular set of files, you may find the B<Perl Repository Browser> |
| 225 | useful: |
| 226 | |
| 227 | http://public.activestate.com/cgi-bin/perlbrowse |
| 228 | |
| 229 | You may also want to subscribe to the perl5-changes mailing list to |
| 230 | receive a copy of each patch that gets submitted to the maintenance |
| 231 | and development "branches" of the perl repository. See |
| 232 | http://lists.perl.org/ for subscription information. |
| 233 | |
| 234 | If you are a member of the perl5-porters mailing list, it is a good |
| 235 | thing to keep in touch with the most recent changes. If not only to |
| 236 | verify if what you would have posted as a bug report isn't already |
| 237 | solved in the most recent available perl development branch, also |
| 238 | known as perl-current, bleading edge perl, bleedperl or bleadperl. |
| 239 | |
| 240 | Needless to say, the source code in perl-current is usually in a perpetual |
| 241 | state of evolution. You should expect it to be very buggy. Do B<not> use |
| 242 | it for any purpose other than testing and development. |
| 243 | |
| 244 | Keeping in sync with the most recent branch can be done in several ways, |
| 245 | but the most convenient and reliable way is using B<rsync>, available at |
| 246 | ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent |
| 247 | branch by FTP.) |
| 248 | |
| 249 | If you choose to keep in sync using rsync, there are two approaches |
| 250 | to doing so: |
| 251 | |
| 252 | =over 4 |
| 253 | |
| 254 | =item rsync'ing the source tree |
| 255 | |
| 256 | Presuming you are in the directory where your perl source resides |
| 257 | and you have rsync installed and available, you can `upgrade' to |
| 258 | the bleadperl using: |
| 259 | |
| 260 | # rsync -avz rsync://ftp.linux.activestate.com/perl-current/ . |
| 261 | |
| 262 | This takes care of updating every single item in the source tree to |
| 263 | the latest applied patch level, creating files that are new (to your |
| 264 | distribution) and setting date/time stamps of existing files to |
| 265 | reflect the bleadperl status. |
| 266 | |
| 267 | Note that this will not delete any files that were in '.' before |
| 268 | the rsync. Once you are sure that the rsync is running correctly, |
| 269 | run it with the --delete and the --dry-run options like this: |
| 270 | |
| 271 | # rsync -avz --delete --dry-run rsync://ftp.linux.activestate.com/perl-current/ . |
| 272 | |
| 273 | This will I<simulate> an rsync run that also deletes files not |
| 274 | present in the bleadperl master copy. Observe the results from |
| 275 | this run closely. If you are sure that the actual run would delete |
| 276 | no files precious to you, you could remove the '--dry-run' option. |
| 277 | |
| 278 | You can than check what patch was the latest that was applied by |
| 279 | looking in the file B<.patch>, which will show the number of the |
| 280 | latest patch. |
| 281 | |
| 282 | If you have more than one machine to keep in sync, and not all of |
| 283 | them have access to the WAN (so you are not able to rsync all the |
| 284 | source trees to the real source), there are some ways to get around |
| 285 | this problem. |
| 286 | |
| 287 | =over 4 |
| 288 | |
| 289 | =item Using rsync over the LAN |
| 290 | |
| 291 | Set up a local rsync server which makes the rsynced source tree |
| 292 | available to the LAN and sync the other machines against this |
| 293 | directory. |
| 294 | |
| 295 | From http://rsync.samba.org/README.html : |
| 296 | |
| 297 | "Rsync uses rsh or ssh for communication. It does not need to be |
| 298 | setuid and requires no special privileges for installation. It |
| 299 | does not require an inetd entry or a daemon. You must, however, |
| 300 | have a working rsh or ssh system. Using ssh is recommended for |
| 301 | its security features." |
| 302 | |
| 303 | =item Using pushing over the NFS |
| 304 | |
| 305 | Having the other systems mounted over the NFS, you can take an |
| 306 | active pushing approach by checking the just updated tree against |
| 307 | the other not-yet synced trees. An example would be |
| 308 | |
| 309 | #!/usr/bin/perl -w |
| 310 | |
| 311 | use strict; |
| 312 | use File::Copy; |
| 313 | |
| 314 | my %MF = map { |
| 315 | m/(\S+)/; |
| 316 | $1 => [ (stat $1)[2, 7, 9] ]; # mode, size, mtime |
| 317 | } `cat MANIFEST`; |
| 318 | |
| 319 | my %remote = map { $_ => "/$_/pro/3gl/CPAN/perl-5.7.1" } qw(host1 host2); |
| 320 | |
| 321 | foreach my $host (keys %remote) { |
| 322 | unless (-d $remote{$host}) { |
| 323 | print STDERR "Cannot Xsync for host $host\n"; |
| 324 | next; |
| 325 | } |
| 326 | foreach my $file (keys %MF) { |
| 327 | my $rfile = "$remote{$host}/$file"; |
| 328 | my ($mode, $size, $mtime) = (stat $rfile)[2, 7, 9]; |
| 329 | defined $size or ($mode, $size, $mtime) = (0, 0, 0); |
| 330 | $size == $MF{$file}[1] && $mtime == $MF{$file}[2] and next; |
| 331 | printf "%4s %-34s %8d %9d %8d %9d\n", |
| 332 | $host, $file, $MF{$file}[1], $MF{$file}[2], $size, $mtime; |
| 333 | unlink $rfile; |
| 334 | copy ($file, $rfile); |
| 335 | utime time, $MF{$file}[2], $rfile; |
| 336 | chmod $MF{$file}[0], $rfile; |
| 337 | } |
| 338 | } |
| 339 | |
| 340 | though this is not perfect. It could be improved with checking |
| 341 | file checksums before updating. Not all NFS systems support |
| 342 | reliable utime support (when used over the NFS). |
| 343 | |
| 344 | =back |
| 345 | |
| 346 | =item rsync'ing the patches |
| 347 | |
| 348 | The source tree is maintained by the pumpking who applies patches to |
| 349 | the files in the tree. These patches are either created by the |
| 350 | pumpking himself using C<diff -c> after updating the file manually or |
| 351 | by applying patches sent in by posters on the perl5-porters list. |
| 352 | These patches are also saved and rsync'able, so you can apply them |
| 353 | yourself to the source files. |
| 354 | |
| 355 | Presuming you are in a directory where your patches reside, you can |
| 356 | get them in sync with |
| 357 | |
| 358 | # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ . |
| 359 | |
| 360 | This makes sure the latest available patch is downloaded to your |
| 361 | patch directory. |
| 362 | |
| 363 | It's then up to you to apply these patches, using something like |
| 364 | |
| 365 | # last=`ls -t *.gz | sed q` |
| 366 | # rsync -avz rsync://ftp.linux.activestate.com/perl-current-diffs/ . |
| 367 | # find . -name '*.gz' -newer $last -exec gzcat {} \; >blead.patch |
| 368 | # cd ../perl-current |
| 369 | # patch -p1 -N <../perl-current-diffs/blead.patch |
| 370 | |
| 371 | or, since this is only a hint towards how it works, use CPAN-patchaperl |
| 372 | from Andreas König to have better control over the patching process. |
| 373 | |
| 374 | =back |
| 375 | |
| 376 | =head2 Why rsync the source tree |
| 377 | |
| 378 | =over 4 |
| 379 | |
| 380 | =item It's easier to rsync the source tree |
| 381 | |
| 382 | Since you don't have to apply the patches yourself, you are sure all |
| 383 | files in the source tree are in the right state. |
| 384 | |
| 385 | =item It's more reliable |
| 386 | |
| 387 | While both the rsync-able source and patch areas are automatically |
| 388 | updated every few minutes, keep in mind that applying patches may |
| 389 | sometimes mean careful hand-holding, especially if your version of |
| 390 | the C<patch> program does not understand how to deal with new files, |
| 391 | files with 8-bit characters, or files without trailing newlines. |
| 392 | |
| 393 | =back |
| 394 | |
| 395 | =head2 Why rsync the patches |
| 396 | |
| 397 | =over 4 |
| 398 | |
| 399 | =item It's easier to rsync the patches |
| 400 | |
| 401 | If you have more than one machine that you want to keep in track with |
| 402 | bleadperl, it's easier to rsync the patches only once and then apply |
| 403 | them to all the source trees on the different machines. |
| 404 | |
| 405 | In case you try to keep in pace on 5 different machines, for which |
| 406 | only one of them has access to the WAN, rsync'ing all the source |
| 407 | trees should than be done 5 times over the NFS. Having |
| 408 | rsync'ed the patches only once, I can apply them to all the source |
| 409 | trees automatically. Need you say more ;-) |
| 410 | |
| 411 | =item It's a good reference |
| 412 | |
| 413 | If you do not only like to have the most recent development branch, |
| 414 | but also like to B<fix> bugs, or extend features, you want to dive |
| 415 | into the sources. If you are a seasoned perl core diver, you don't |
| 416 | need no manuals, tips, roadmaps, perlguts.pod or other aids to find |
| 417 | your way around. But if you are a starter, the patches may help you |
| 418 | in finding where you should start and how to change the bits that |
| 419 | bug you. |
| 420 | |
| 421 | The file B<Changes> is updated on occasions the pumpking sees as his |
| 422 | own little sync points. On those occasions, he releases a tar-ball of |
| 423 | the current source tree (i.e. perl@7582.tar.gz), which will be an |
| 424 | excellent point to start with when choosing to use the 'rsync the |
| 425 | patches' scheme. Starting with perl@7582, which means a set of source |
| 426 | files on which the latest applied patch is number 7582, you apply all |
| 427 | succeeding patches available from then on (7583, 7584, ...). |
| 428 | |
| 429 | You can use the patches later as a kind of search archive. |
| 430 | |
| 431 | =over 4 |
| 432 | |
| 433 | =item Finding a start point |
| 434 | |
| 435 | If you want to fix/change the behaviour of function/feature Foo, just |
| 436 | scan the patches for patches that mention Foo either in the subject, |
| 437 | the comments, or the body of the fix. A good chance the patch shows |
| 438 | you the files that are affected by that patch which are very likely |
| 439 | to be the starting point of your journey into the guts of perl. |
| 440 | |
| 441 | =item Finding how to fix a bug |
| 442 | |
| 443 | If you've found I<where> the function/feature Foo misbehaves, but you |
| 444 | don't know how to fix it (but you do know the change you want to |
| 445 | make), you can, again, peruse the patches for similar changes and |
| 446 | look how others apply the fix. |
| 447 | |
| 448 | =item Finding the source of misbehaviour |
| 449 | |
| 450 | When you keep in sync with bleadperl, the pumpking would love to |
| 451 | I<see> that the community efforts really work. So after each of his |
| 452 | sync points, you are to 'make test' to check if everything is still |
| 453 | in working order. If it is, you do 'make ok', which will send an OK |
| 454 | report to perlbug@perl.org. (If you do not have access to a mailer |
| 455 | from the system you just finished successfully 'make test', you can |
| 456 | do 'make okfile', which creates the file C<perl.ok>, which you can |
| 457 | than take to your favourite mailer and mail yourself). |
| 458 | |
| 459 | But of course, as always, things will not always lead to a success |
| 460 | path, and one or more test do not pass the 'make test'. Before |
| 461 | sending in a bug report (using 'make nok' or 'make nokfile'), check |
| 462 | the mailing list if someone else has reported the bug already and if |
| 463 | so, confirm it by replying to that message. If not, you might want to |
| 464 | trace the source of that misbehaviour B<before> sending in the bug, |
| 465 | which will help all the other porters in finding the solution. |
| 466 | |
| 467 | Here the saved patches come in very handy. You can check the list of |
| 468 | patches to see which patch changed what file and what change caused |
| 469 | the misbehaviour. If you note that in the bug report, it saves the |
| 470 | one trying to solve it, looking for that point. |
| 471 | |
| 472 | =back |
| 473 | |
| 474 | If searching the patches is too bothersome, you might consider using |
| 475 | perl's bugtron to find more information about discussions and |
| 476 | ramblings on posted bugs. |
| 477 | |
| 478 | If you want to get the best of both worlds, rsync both the source |
| 479 | tree for convenience, reliability and ease and rsync the patches |
| 480 | for reference. |
| 481 | |
| 482 | =back |
| 483 | |
| 484 | =head2 Working with the source |
| 485 | |
| 486 | Because you cannot use the Perforce client, you cannot easily generate |
| 487 | diffs against the repository, nor will merges occur when you update |
| 488 | via rsync. If you edit a file locally and then rsync against the |
| 489 | latest source, changes made in the remote copy will I<overwrite> your |
| 490 | local versions! |
| 491 | |
| 492 | The best way to deal with this is to maintain a tree of symlinks to |
| 493 | the rsync'd source. Then, when you want to edit a file, you remove |
| 494 | the symlink, copy the real file into the other tree, and edit it. You |
| 495 | can then diff your edited file against the original to generate a |
| 496 | patch, and you can safely update the original tree. |
| 497 | |
| 498 | Perl's F<Configure> script can generate this tree of symlinks for you. |
| 499 | The following example assumes that you have used rsync to pull a copy |
| 500 | of the Perl source into the F<perl-rsync> directory. In the directory |
| 501 | above that one, you can execute the following commands: |
| 502 | |
| 503 | mkdir perl-dev |
| 504 | cd perl-dev |
| 505 | ../perl-rsync/Configure -Dmksymlinks -Dusedevel -D"optimize=-g" |
| 506 | |
| 507 | This will start the Perl configuration process. After a few prompts, |
| 508 | you should see something like this: |
| 509 | |
| 510 | Symbolic links are supported. |
| 511 | |
| 512 | Checking how to test for symbolic links... |
| 513 | Your builtin 'test -h' may be broken. |
| 514 | Trying external '/usr/bin/test -h'. |
| 515 | You can test for symbolic links with '/usr/bin/test -h'. |
| 516 | |
| 517 | Creating the symbolic links... |
| 518 | (First creating the subdirectories...) |
| 519 | (Then creating the symlinks...) |
| 520 | |
| 521 | The specifics may vary based on your operating system, of course. |
| 522 | After you see this, you can abort the F<Configure> script, and you |
| 523 | will see that the directory you are in has a tree of symlinks to the |
| 524 | F<perl-rsync> directories and files. |
| 525 | |
| 526 | If you plan to do a lot of work with the Perl source, here are some |
| 527 | Bourne shell script functions that can make your life easier: |
| 528 | |
| 529 | function edit { |
| 530 | if [ -L $1 ]; then |
| 531 | mv $1 $1.orig |
| 532 | cp $1.orig $1 |
| 533 | vi $1 |
| 534 | else |
| 535 | /bin/vi $1 |
| 536 | fi |
| 537 | } |
| 538 | |
| 539 | function unedit { |
| 540 | if [ -L $1.orig ]; then |
| 541 | rm $1 |
| 542 | mv $1.orig $1 |
| 543 | fi |
| 544 | } |
| 545 | |
| 546 | Replace "vi" with your favorite flavor of editor. |
| 547 | |
| 548 | Here is another function which will quickly generate a patch for the |
| 549 | files which have been edited in your symlink tree: |
| 550 | |
| 551 | mkpatchorig() { |
| 552 | local diffopts |
| 553 | for f in `find . -name '*.orig' | sed s,^\./,,` |
| 554 | do |
| 555 | case `echo $f | sed 's,.orig$,,;s,.*\.,,'` in |
| 556 | c) diffopts=-p ;; |
| 557 | pod) diffopts='-F^=' ;; |
| 558 | *) diffopts= ;; |
| 559 | esac |
| 560 | diff -du $diffopts $f `echo $f | sed 's,.orig$,,'` |
| 561 | done |
| 562 | } |
| 563 | |
| 564 | This function produces patches which include enough context to make |
| 565 | your changes obvious. This makes it easier for the Perl pumpking(s) |
| 566 | to review them when you send them to the perl5-porters list, and that |
| 567 | means they're more likely to get applied. |
| 568 | |
| 569 | This function assumed a GNU diff, and may require some tweaking for |
| 570 | other diff variants. |
| 571 | |
| 572 | =head2 Perlbug administration |
| 573 | |
| 574 | There is a single remote administrative interface for modifying bug status, |
| 575 | category, open issues etc. using the B<RT> I<bugtracker> system, maintained |
| 576 | by I<Robert Spier>. Become an administrator, and close any bugs you can get |
| 577 | your sticky mitts on: |
| 578 | |
| 579 | http://rt.perl.org |
| 580 | |
| 581 | The bugtracker mechanism for B<perl5> bugs in particular is at: |
| 582 | |
| 583 | http://bugs6.perl.org/perlbug |
| 584 | |
| 585 | To email the bug system administrators: |
| 586 | |
| 587 | "perlbug-admin" <perlbug-admin@perl.org> |
| 588 | |
| 589 | |
| 590 | =head2 Submitting patches |
| 591 | |
| 592 | Always submit patches to I<perl5-porters@perl.org>. If you're |
| 593 | patching a core module and there's an author listed, send the author a |
| 594 | copy (see L<Patching a core module>). This lets other porters review |
| 595 | your patch, which catches a surprising number of errors in patches. |
| 596 | Either use the diff program (available in source code form from |
| 597 | ftp://ftp.gnu.org/pub/gnu/ , or use Johan Vromans' I<makepatch> |
| 598 | (available from I<CPAN/authors/id/JV/>). Unified diffs are preferred, |
| 599 | but context diffs are accepted. Do not send RCS-style diffs or diffs |
| 600 | without context lines. More information is given in the |
| 601 | I<Porting/patching.pod> file in the Perl source distribution. Please |
| 602 | patch against the latest B<development> version (e.g., if you're |
| 603 | fixing a bug in the 5.005 track, patch against the latest 5.005_5x |
| 604 | version). Only patches that survive the heat of the development |
| 605 | branch get applied to maintenance versions. |
| 606 | |
| 607 | Your patch should update the documentation and test suite. See |
| 608 | L<Writing a test>. |
| 609 | |
| 610 | To report a bug in Perl, use the program I<perlbug> which comes with |
| 611 | Perl (if you can't get Perl to work, send mail to the address |
| 612 | I<perlbug@perl.org> or I<perlbug@perl.com>). Reporting bugs through |
| 613 | I<perlbug> feeds into the automated bug-tracking system, access to |
| 614 | which is provided through the web at http://bugs.perl.org/ . It |
| 615 | often pays to check the archives of the perl5-porters mailing list to |
| 616 | see whether the bug you're reporting has been reported before, and if |
| 617 | so whether it was considered a bug. See above for the location of |
| 618 | the searchable archives. |
| 619 | |
| 620 | The CPAN testers ( http://testers.cpan.org/ ) are a group of |
| 621 | volunteers who test CPAN modules on a variety of platforms. Perl |
| 622 | Smokers ( http://archives.develooper.com/daily-build@perl.org/ ) |
| 623 | automatically tests Perl source releases on platforms with various |
| 624 | configurations. Both efforts welcome volunteers. |
| 625 | |
| 626 | It's a good idea to read and lurk for a while before chipping in. |
| 627 | That way you'll get to see the dynamic of the conversations, learn the |
| 628 | personalities of the players, and hopefully be better prepared to make |
| 629 | a useful contribution when do you speak up. |
| 630 | |
| 631 | If after all this you still think you want to join the perl5-porters |
| 632 | mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To |
| 633 | unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>. |
| 634 | |
| 635 | To hack on the Perl guts, you'll need to read the following things: |
| 636 | |
| 637 | =over 3 |
| 638 | |
| 639 | =item L<perlguts> |
| 640 | |
| 641 | This is of paramount importance, since it's the documentation of what |
| 642 | goes where in the Perl source. Read it over a couple of times and it |
| 643 | might start to make sense - don't worry if it doesn't yet, because the |
| 644 | best way to study it is to read it in conjunction with poking at Perl |
| 645 | source, and we'll do that later on. |
| 646 | |
| 647 | You might also want to look at Gisle Aas's illustrated perlguts - |
| 648 | there's no guarantee that this will be absolutely up-to-date with the |
| 649 | latest documentation in the Perl core, but the fundamentals will be |
| 650 | right. ( http://gisle.aas.no/perl/illguts/ ) |
| 651 | |
| 652 | =item L<perlxstut> and L<perlxs> |
| 653 | |
| 654 | A working knowledge of XSUB programming is incredibly useful for core |
| 655 | hacking; XSUBs use techniques drawn from the PP code, the portion of the |
| 656 | guts that actually executes a Perl program. It's a lot gentler to learn |
| 657 | those techniques from simple examples and explanation than from the core |
| 658 | itself. |
| 659 | |
| 660 | =item L<perlapi> |
| 661 | |
| 662 | The documentation for the Perl API explains what some of the internal |
| 663 | functions do, as well as the many macros used in the source. |
| 664 | |
| 665 | =item F<Porting/pumpkin.pod> |
| 666 | |
| 667 | This is a collection of words of wisdom for a Perl porter; some of it is |
| 668 | only useful to the pumpkin holder, but most of it applies to anyone |
| 669 | wanting to go about Perl development. |
| 670 | |
| 671 | =item The perl5-porters FAQ |
| 672 | |
| 673 | This should be available from http://simon-cozens.org/writings/p5p-faq ; |
| 674 | alternatively, you can get the FAQ emailed to you by sending mail to |
| 675 | C<perl5-porters-faq@perl.org>. It contains hints on reading perl5-porters, |
| 676 | information on how perl5-porters works and how Perl development in general |
| 677 | works. |
| 678 | |
| 679 | =back |
| 680 | |
| 681 | =head2 Finding Your Way Around |
| 682 | |
| 683 | Perl maintenance can be split into a number of areas, and certain people |
| 684 | (pumpkins) will have responsibility for each area. These areas sometimes |
| 685 | correspond to files or directories in the source kit. Among the areas are: |
| 686 | |
| 687 | =over 3 |
| 688 | |
| 689 | =item Core modules |
| 690 | |
| 691 | Modules shipped as part of the Perl core live in the F<lib/> and F<ext/> |
| 692 | subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/> |
| 693 | contains the core XS modules. |
| 694 | |
| 695 | =item Tests |
| 696 | |
| 697 | There are tests for nearly all the modules, built-ins and major bits |
| 698 | of functionality. Test files all have a .t suffix. Module tests live |
| 699 | in the F<lib/> and F<ext/> directories next to the module being |
| 700 | tested. Others live in F<t/>. See L<Writing a test> |
| 701 | |
| 702 | =item Documentation |
| 703 | |
| 704 | Documentation maintenance includes looking after everything in the |
| 705 | F<pod/> directory, (as well as contributing new documentation) and |
| 706 | the documentation to the modules in core. |
| 707 | |
| 708 | =item Configure |
| 709 | |
| 710 | The configure process is the way we make Perl portable across the |
| 711 | myriad of operating systems it supports. Responsibility for the |
| 712 | configure, build and installation process, as well as the overall |
| 713 | portability of the core code rests with the configure pumpkin - others |
| 714 | help out with individual operating systems. |
| 715 | |
| 716 | The files involved are the operating system directories, (F<win32/>, |
| 717 | F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h> |
| 718 | and F<Makefile>, as well as the metaconfig files which generate |
| 719 | F<Configure>. (metaconfig isn't included in the core distribution.) |
| 720 | |
| 721 | =item Interpreter |
| 722 | |
| 723 | And of course, there's the core of the Perl interpreter itself. Let's |
| 724 | have a look at that in a little more detail. |
| 725 | |
| 726 | =back |
| 727 | |
| 728 | Before we leave looking at the layout, though, don't forget that |
| 729 | F<MANIFEST> contains not only the file names in the Perl distribution, |
| 730 | but short descriptions of what's in them, too. For an overview of the |
| 731 | important files, try this: |
| 732 | |
| 733 | perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST |
| 734 | |
| 735 | =head2 Elements of the interpreter |
| 736 | |
| 737 | The work of the interpreter has two main stages: compiling the code |
| 738 | into the internal representation, or bytecode, and then executing it. |
| 739 | L<perlguts/Compiled code> explains exactly how the compilation stage |
| 740 | happens. |
| 741 | |
| 742 | Here is a short breakdown of perl's operation: |
| 743 | |
| 744 | =over 3 |
| 745 | |
| 746 | =item Startup |
| 747 | |
| 748 | The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl) |
| 749 | This is very high-level code, enough to fit on a single screen, and it |
| 750 | resembles the code found in L<perlembed>; most of the real action takes |
| 751 | place in F<perl.c> |
| 752 | |
| 753 | First, F<perlmain.c> allocates some memory and constructs a Perl |
| 754 | interpreter: |
| 755 | |
| 756 | 1 PERL_SYS_INIT3(&argc,&argv,&env); |
| 757 | 2 |
| 758 | 3 if (!PL_do_undump) { |
| 759 | 4 my_perl = perl_alloc(); |
| 760 | 5 if (!my_perl) |
| 761 | 6 exit(1); |
| 762 | 7 perl_construct(my_perl); |
| 763 | 8 PL_perl_destruct_level = 0; |
| 764 | 9 } |
| 765 | |
| 766 | Line 1 is a macro, and its definition is dependent on your operating |
| 767 | system. Line 3 references C<PL_do_undump>, a global variable - all |
| 768 | global variables in Perl start with C<PL_>. This tells you whether the |
| 769 | current running program was created with the C<-u> flag to perl and then |
| 770 | F<undump>, which means it's going to be false in any sane context. |
| 771 | |
| 772 | Line 4 calls a function in F<perl.c> to allocate memory for a Perl |
| 773 | interpreter. It's quite a simple function, and the guts of it looks like |
| 774 | this: |
| 775 | |
| 776 | my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter)); |
| 777 | |
| 778 | Here you see an example of Perl's system abstraction, which we'll see |
| 779 | later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's |
| 780 | own C<malloc> as defined in F<malloc.c> if you selected that option at |
| 781 | configure time. |
| 782 | |
| 783 | Next, in line 7, we construct the interpreter; this sets up all the |
| 784 | special variables that Perl needs, the stacks, and so on. |
| 785 | |
| 786 | Now we pass Perl the command line options, and tell it to go: |
| 787 | |
| 788 | exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL); |
| 789 | if (!exitstatus) { |
| 790 | exitstatus = perl_run(my_perl); |
| 791 | } |
| 792 | |
| 793 | |
| 794 | C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined |
| 795 | in F<perl.c>, which processes the command line options, sets up any |
| 796 | statically linked XS modules, opens the program and calls C<yyparse> to |
| 797 | parse it. |
| 798 | |
| 799 | =item Parsing |
| 800 | |
| 801 | The aim of this stage is to take the Perl source, and turn it into an op |
| 802 | tree. We'll see what one of those looks like later. Strictly speaking, |
| 803 | there's three things going on here. |
| 804 | |
| 805 | C<yyparse>, the parser, lives in F<perly.c>, although you're better off |
| 806 | reading the original YACC input in F<perly.y>. (Yes, Virginia, there |
| 807 | B<is> a YACC grammar for Perl!) The job of the parser is to take your |
| 808 | code and `understand' it, splitting it into sentences, deciding which |
| 809 | operands go with which operators and so on. |
| 810 | |
| 811 | The parser is nobly assisted by the lexer, which chunks up your input |
| 812 | into tokens, and decides what type of thing each token is: a variable |
| 813 | name, an operator, a bareword, a subroutine, a core function, and so on. |
| 814 | The main point of entry to the lexer is C<yylex>, and that and its |
| 815 | associated routines can be found in F<toke.c>. Perl isn't much like |
| 816 | other computer languages; it's highly context sensitive at times, it can |
| 817 | be tricky to work out what sort of token something is, or where a token |
| 818 | ends. As such, there's a lot of interplay between the tokeniser and the |
| 819 | parser, which can get pretty frightening if you're not used to it. |
| 820 | |
| 821 | As the parser understands a Perl program, it builds up a tree of |
| 822 | operations for the interpreter to perform during execution. The routines |
| 823 | which construct and link together the various operations are to be found |
| 824 | in F<op.c>, and will be examined later. |
| 825 | |
| 826 | =item Optimization |
| 827 | |
| 828 | Now the parsing stage is complete, and the finished tree represents |
| 829 | the operations that the Perl interpreter needs to perform to execute our |
| 830 | program. Next, Perl does a dry run over the tree looking for |
| 831 | optimisations: constant expressions such as C<3 + 4> will be computed |
| 832 | now, and the optimizer will also see if any multiple operations can be |
| 833 | replaced with a single one. For instance, to fetch the variable C<$foo>, |
| 834 | instead of grabbing the glob C<*foo> and looking at the scalar |
| 835 | component, the optimizer fiddles the op tree to use a function which |
| 836 | directly looks up the scalar in question. The main optimizer is C<peep> |
| 837 | in F<op.c>, and many ops have their own optimizing functions. |
| 838 | |
| 839 | =item Running |
| 840 | |
| 841 | Now we're finally ready to go: we have compiled Perl byte code, and all |
| 842 | that's left to do is run it. The actual execution is done by the |
| 843 | C<runops_standard> function in F<run.c>; more specifically, it's done by |
| 844 | these three innocent looking lines: |
| 845 | |
| 846 | while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) { |
| 847 | PERL_ASYNC_CHECK(); |
| 848 | } |
| 849 | |
| 850 | You may be more comfortable with the Perl version of that: |
| 851 | |
| 852 | PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}}; |
| 853 | |
| 854 | Well, maybe not. Anyway, each op contains a function pointer, which |
| 855 | stipulates the function which will actually carry out the operation. |
| 856 | This function will return the next op in the sequence - this allows for |
| 857 | things like C<if> which choose the next op dynamically at run time. |
| 858 | The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt |
| 859 | execution if required. |
| 860 | |
| 861 | The actual functions called are known as PP code, and they're spread |
| 862 | between four files: F<pp_hot.c> contains the `hot' code, which is most |
| 863 | often used and highly optimized, F<pp_sys.c> contains all the |
| 864 | system-specific functions, F<pp_ctl.c> contains the functions which |
| 865 | implement control structures (C<if>, C<while> and the like) and F<pp.c> |
| 866 | contains everything else. These are, if you like, the C code for Perl's |
| 867 | built-in functions and operators. |
| 868 | |
| 869 | =back |
| 870 | |
| 871 | =head2 Internal Variable Types |
| 872 | |
| 873 | You should by now have had a look at L<perlguts>, which tells you about |
| 874 | Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do |
| 875 | that now. |
| 876 | |
| 877 | These variables are used not only to represent Perl-space variables, but |
| 878 | also any constants in the code, as well as some structures completely |
| 879 | internal to Perl. The symbol table, for instance, is an ordinary Perl |
| 880 | hash. Your code is represented by an SV as it's read into the parser; |
| 881 | any program files you call are opened via ordinary Perl filehandles, and |
| 882 | so on. |
| 883 | |
| 884 | The core L<Devel::Peek|Devel::Peek> module lets us examine SVs from a |
| 885 | Perl program. Let's see, for instance, how Perl treats the constant |
| 886 | C<"hello">. |
| 887 | |
| 888 | % perl -MDevel::Peek -e 'Dump("hello")' |
| 889 | 1 SV = PV(0xa041450) at 0xa04ecbc |
| 890 | 2 REFCNT = 1 |
| 891 | 3 FLAGS = (POK,READONLY,pPOK) |
| 892 | 4 PV = 0xa0484e0 "hello"\0 |
| 893 | 5 CUR = 5 |
| 894 | 6 LEN = 6 |
| 895 | |
| 896 | Reading C<Devel::Peek> output takes a bit of practise, so let's go |
| 897 | through it line by line. |
| 898 | |
| 899 | Line 1 tells us we're looking at an SV which lives at C<0xa04ecbc> in |
| 900 | memory. SVs themselves are very simple structures, but they contain a |
| 901 | pointer to a more complex structure. In this case, it's a PV, a |
| 902 | structure which holds a string value, at location C<0xa041450>. Line 2 |
| 903 | is the reference count; there are no other references to this data, so |
| 904 | it's 1. |
| 905 | |
| 906 | Line 3 are the flags for this SV - it's OK to use it as a PV, it's a |
| 907 | read-only SV (because it's a constant) and the data is a PV internally. |
| 908 | Next we've got the contents of the string, starting at location |
| 909 | C<0xa0484e0>. |
| 910 | |
| 911 | Line 5 gives us the current length of the string - note that this does |
| 912 | B<not> include the null terminator. Line 6 is not the length of the |
| 913 | string, but the length of the currently allocated buffer; as the string |
| 914 | grows, Perl automatically extends the available storage via a routine |
| 915 | called C<SvGROW>. |
| 916 | |
| 917 | You can get at any of these quantities from C very easily; just add |
| 918 | C<Sv> to the name of the field shown in the snippet, and you've got a |
| 919 | macro which will return the value: C<SvCUR(sv)> returns the current |
| 920 | length of the string, C<SvREFCOUNT(sv)> returns the reference count, |
| 921 | C<SvPV(sv, len)> returns the string itself with its length, and so on. |
| 922 | More macros to manipulate these properties can be found in L<perlguts>. |
| 923 | |
| 924 | Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<sv.c> |
| 925 | |
| 926 | 1 void |
| 927 | 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len) |
| 928 | 3 { |
| 929 | 4 STRLEN tlen; |
| 930 | 5 char *junk; |
| 931 | |
| 932 | 6 junk = SvPV_force(sv, tlen); |
| 933 | 7 SvGROW(sv, tlen + len + 1); |
| 934 | 8 if (ptr == junk) |
| 935 | 9 ptr = SvPVX(sv); |
| 936 | 10 Move(ptr,SvPVX(sv)+tlen,len,char); |
| 937 | 11 SvCUR(sv) += len; |
| 938 | 12 *SvEND(sv) = '\0'; |
| 939 | 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */ |
| 940 | 14 SvTAINT(sv); |
| 941 | 15 } |
| 942 | |
| 943 | This is a function which adds a string, C<ptr>, of length C<len> onto |
| 944 | the end of the PV stored in C<sv>. The first thing we do in line 6 is |
| 945 | make sure that the SV B<has> a valid PV, by calling the C<SvPV_force> |
| 946 | macro to force a PV. As a side effect, C<tlen> gets set to the current |
| 947 | value of the PV, and the PV itself is returned to C<junk>. |
| 948 | |
| 949 | In line 7, we make sure that the SV will have enough room to accommodate |
| 950 | the old string, the new string and the null terminator. If C<LEN> isn't |
| 951 | big enough, C<SvGROW> will reallocate space for us. |
| 952 | |
| 953 | Now, if C<junk> is the same as the string we're trying to add, we can |
| 954 | grab the string directly from the SV; C<SvPVX> is the address of the PV |
| 955 | in the SV. |
| 956 | |
| 957 | Line 10 does the actual catenation: the C<Move> macro moves a chunk of |
| 958 | memory around: we move the string C<ptr> to the end of the PV - that's |
| 959 | the start of the PV plus its current length. We're moving C<len> bytes |
| 960 | of type C<char>. After doing so, we need to tell Perl we've extended the |
| 961 | string, by altering C<CUR> to reflect the new length. C<SvEND> is a |
| 962 | macro which gives us the end of the string, so that needs to be a |
| 963 | C<"\0">. |
| 964 | |
| 965 | Line 13 manipulates the flags; since we've changed the PV, any IV or NV |
| 966 | values will no longer be valid: if we have C<$a=10; $a.="6";> we don't |
| 967 | want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF-8-aware |
| 968 | version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags |
| 969 | and turns on POK. The final C<SvTAINT> is a macro which launders tainted |
| 970 | data if taint mode is turned on. |
| 971 | |
| 972 | AVs and HVs are more complicated, but SVs are by far the most common |
| 973 | variable type being thrown around. Having seen something of how we |
| 974 | manipulate these, let's go on and look at how the op tree is |
| 975 | constructed. |
| 976 | |
| 977 | =head2 Op Trees |
| 978 | |
| 979 | First, what is the op tree, anyway? The op tree is the parsed |
| 980 | representation of your program, as we saw in our section on parsing, and |
| 981 | it's the sequence of operations that Perl goes through to execute your |
| 982 | program, as we saw in L</Running>. |
| 983 | |
| 984 | An op is a fundamental operation that Perl can perform: all the built-in |
| 985 | functions and operators are ops, and there are a series of ops which |
| 986 | deal with concepts the interpreter needs internally - entering and |
| 987 | leaving a block, ending a statement, fetching a variable, and so on. |
| 988 | |
| 989 | The op tree is connected in two ways: you can imagine that there are two |
| 990 | "routes" through it, two orders in which you can traverse the tree. |
| 991 | First, parse order reflects how the parser understood the code, and |
| 992 | secondly, execution order tells perl what order to perform the |
| 993 | operations in. |
| 994 | |
| 995 | The easiest way to examine the op tree is to stop Perl after it has |
| 996 | finished parsing, and get it to dump out the tree. This is exactly what |
| 997 | the compiler backends L<B::Terse|B::Terse>, L<B::Concise|B::Concise> |
| 998 | and L<B::Debug|B::Debug> do. |
| 999 | |
| 1000 | Let's have a look at how Perl sees C<$a = $b + $c>: |
| 1001 | |
| 1002 | % perl -MO=Terse -e '$a=$b+$c' |
| 1003 | 1 LISTOP (0x8179888) leave |
| 1004 | 2 OP (0x81798b0) enter |
| 1005 | 3 COP (0x8179850) nextstate |
| 1006 | 4 BINOP (0x8179828) sassign |
| 1007 | 5 BINOP (0x8179800) add [1] |
| 1008 | 6 UNOP (0x81796e0) null [15] |
| 1009 | 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b |
| 1010 | 8 UNOP (0x81797e0) null [15] |
| 1011 | 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c |
| 1012 | 10 UNOP (0x816b4f0) null [15] |
| 1013 | 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *a |
| 1014 | |
| 1015 | Let's start in the middle, at line 4. This is a BINOP, a binary |
| 1016 | operator, which is at location C<0x8179828>. The specific operator in |
| 1017 | question is C<sassign> - scalar assignment - and you can find the code |
| 1018 | which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a |
| 1019 | binary operator, it has two children: the add operator, providing the |
| 1020 | result of C<$b+$c>, is uppermost on line 5, and the left hand side is on |
| 1021 | line 10. |
| 1022 | |
| 1023 | Line 10 is the null op: this does exactly nothing. What is that doing |
| 1024 | there? If you see the null op, it's a sign that something has been |
| 1025 | optimized away after parsing. As we mentioned in L</Optimization>, |
| 1026 | the optimization stage sometimes converts two operations into one, for |
| 1027 | example when fetching a scalar variable. When this happens, instead of |
| 1028 | rewriting the op tree and cleaning up the dangling pointers, it's easier |
| 1029 | just to replace the redundant operation with the null op. Originally, |
| 1030 | the tree would have looked like this: |
| 1031 | |
| 1032 | 10 SVOP (0x816b4f0) rv2sv [15] |
| 1033 | 11 SVOP (0x816dcf0) gv GV (0x80fa460) *a |
| 1034 | |
| 1035 | That is, fetch the C<a> entry from the main symbol table, and then look |
| 1036 | at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<pp_hot.c>) |
| 1037 | happens to do both these things. |
| 1038 | |
| 1039 | The right hand side, starting at line 5 is similar to what we've just |
| 1040 | seen: we have the C<add> op (C<pp_add> also in F<pp_hot.c>) add together |
| 1041 | two C<gvsv>s. |
| 1042 | |
| 1043 | Now, what's this about? |
| 1044 | |
| 1045 | 1 LISTOP (0x8179888) leave |
| 1046 | 2 OP (0x81798b0) enter |
| 1047 | 3 COP (0x8179850) nextstate |
| 1048 | |
| 1049 | C<enter> and C<leave> are scoping ops, and their job is to perform any |
| 1050 | housekeeping every time you enter and leave a block: lexical variables |
| 1051 | are tidied up, unreferenced variables are destroyed, and so on. Every |
| 1052 | program will have those first three lines: C<leave> is a list, and its |
| 1053 | children are all the statements in the block. Statements are delimited |
| 1054 | by C<nextstate>, so a block is a collection of C<nextstate> ops, with |
| 1055 | the ops to be performed for each statement being the children of |
| 1056 | C<nextstate>. C<enter> is a single op which functions as a marker. |
| 1057 | |
| 1058 | That's how Perl parsed the program, from top to bottom: |
| 1059 | |
| 1060 | Program |
| 1061 | | |
| 1062 | Statement |
| 1063 | | |
| 1064 | = |
| 1065 | / \ |
| 1066 | / \ |
| 1067 | $a + |
| 1068 | / \ |
| 1069 | $b $c |
| 1070 | |
| 1071 | However, it's impossible to B<perform> the operations in this order: |
| 1072 | you have to find the values of C<$b> and C<$c> before you add them |
| 1073 | together, for instance. So, the other thread that runs through the op |
| 1074 | tree is the execution order: each op has a field C<op_next> which points |
| 1075 | to the next op to be run, so following these pointers tells us how perl |
| 1076 | executes the code. We can traverse the tree in this order using |
| 1077 | the C<exec> option to C<B::Terse>: |
| 1078 | |
| 1079 | % perl -MO=Terse,exec -e '$a=$b+$c' |
| 1080 | 1 OP (0x8179928) enter |
| 1081 | 2 COP (0x81798c8) nextstate |
| 1082 | 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b |
| 1083 | 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c |
| 1084 | 5 BINOP (0x8179878) add [1] |
| 1085 | 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a |
| 1086 | 7 BINOP (0x81798a0) sassign |
| 1087 | 8 LISTOP (0x8179900) leave |
| 1088 | |
| 1089 | This probably makes more sense for a human: enter a block, start a |
| 1090 | statement. Get the values of C<$b> and C<$c>, and add them together. |
| 1091 | Find C<$a>, and assign one to the other. Then leave. |
| 1092 | |
| 1093 | The way Perl builds up these op trees in the parsing process can be |
| 1094 | unravelled by examining F<perly.y>, the YACC grammar. Let's take the |
| 1095 | piece we need to construct the tree for C<$a = $b + $c> |
| 1096 | |
| 1097 | 1 term : term ASSIGNOP term |
| 1098 | 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); } |
| 1099 | 3 | term ADDOP term |
| 1100 | 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); } |
| 1101 | |
| 1102 | If you're not used to reading BNF grammars, this is how it works: You're |
| 1103 | fed certain things by the tokeniser, which generally end up in upper |
| 1104 | case. Here, C<ADDOP>, is provided when the tokeniser sees C<+> in your |
| 1105 | code. C<ASSIGNOP> is provided when C<=> is used for assigning. These are |
| 1106 | `terminal symbols', because you can't get any simpler than them. |
| 1107 | |
| 1108 | The grammar, lines one and three of the snippet above, tells you how to |
| 1109 | build up more complex forms. These complex forms, `non-terminal symbols' |
| 1110 | are generally placed in lower case. C<term> here is a non-terminal |
| 1111 | symbol, representing a single expression. |
| 1112 | |
| 1113 | The grammar gives you the following rule: you can make the thing on the |
| 1114 | left of the colon if you see all the things on the right in sequence. |
| 1115 | This is called a "reduction", and the aim of parsing is to completely |
| 1116 | reduce the input. There are several different ways you can perform a |
| 1117 | reduction, separated by vertical bars: so, C<term> followed by C<=> |
| 1118 | followed by C<term> makes a C<term>, and C<term> followed by C<+> |
| 1119 | followed by C<term> can also make a C<term>. |
| 1120 | |
| 1121 | So, if you see two terms with an C<=> or C<+>, between them, you can |
| 1122 | turn them into a single expression. When you do this, you execute the |
| 1123 | code in the block on the next line: if you see C<=>, you'll do the code |
| 1124 | in line 2. If you see C<+>, you'll do the code in line 4. It's this code |
| 1125 | which contributes to the op tree. |
| 1126 | |
| 1127 | | term ADDOP term |
| 1128 | { $$ = newBINOP($2, 0, scalar($1), scalar($3)); } |
| 1129 | |
| 1130 | What this does is creates a new binary op, and feeds it a number of |
| 1131 | variables. The variables refer to the tokens: C<$1> is the first token in |
| 1132 | the input, C<$2> the second, and so on - think regular expression |
| 1133 | backreferences. C<$$> is the op returned from this reduction. So, we |
| 1134 | call C<newBINOP> to create a new binary operator. The first parameter to |
| 1135 | C<newBINOP>, a function in F<op.c>, is the op type. It's an addition |
| 1136 | operator, so we want the type to be C<ADDOP>. We could specify this |
| 1137 | directly, but it's right there as the second token in the input, so we |
| 1138 | use C<$2>. The second parameter is the op's flags: 0 means `nothing |
| 1139 | special'. Then the things to add: the left and right hand side of our |
| 1140 | expression, in scalar context. |
| 1141 | |
| 1142 | =head2 Stacks |
| 1143 | |
| 1144 | When perl executes something like C<addop>, how does it pass on its |
| 1145 | results to the next op? The answer is, through the use of stacks. Perl |
| 1146 | has a number of stacks to store things it's currently working on, and |
| 1147 | we'll look at the three most important ones here. |
| 1148 | |
| 1149 | =over 3 |
| 1150 | |
| 1151 | =item Argument stack |
| 1152 | |
| 1153 | Arguments are passed to PP code and returned from PP code using the |
| 1154 | argument stack, C<ST>. The typical way to handle arguments is to pop |
| 1155 | them off the stack, deal with them how you wish, and then push the result |
| 1156 | back onto the stack. This is how, for instance, the cosine operator |
| 1157 | works: |
| 1158 | |
| 1159 | NV value; |
| 1160 | value = POPn; |
| 1161 | value = Perl_cos(value); |
| 1162 | XPUSHn(value); |
| 1163 | |
| 1164 | We'll see a more tricky example of this when we consider Perl's macros |
| 1165 | below. C<POPn> gives you the NV (floating point value) of the top SV on |
| 1166 | the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push |
| 1167 | the result back as an NV. The C<X> in C<XPUSHn> means that the stack |
| 1168 | should be extended if necessary - it can't be necessary here, because we |
| 1169 | know there's room for one more item on the stack, since we've just |
| 1170 | removed one! The C<XPUSH*> macros at least guarantee safety. |
| 1171 | |
| 1172 | Alternatively, you can fiddle with the stack directly: C<SP> gives you |
| 1173 | the first element in your portion of the stack, and C<TOP*> gives you |
| 1174 | the top SV/IV/NV/etc. on the stack. So, for instance, to do unary |
| 1175 | negation of an integer: |
| 1176 | |
| 1177 | SETi(-TOPi); |
| 1178 | |
| 1179 | Just set the integer value of the top stack entry to its negation. |
| 1180 | |
| 1181 | Argument stack manipulation in the core is exactly the same as it is in |
| 1182 | XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer |
| 1183 | description of the macros used in stack manipulation. |
| 1184 | |
| 1185 | =item Mark stack |
| 1186 | |
| 1187 | I say `your portion of the stack' above because PP code doesn't |
| 1188 | necessarily get the whole stack to itself: if your function calls |
| 1189 | another function, you'll only want to expose the arguments aimed for the |
| 1190 | called function, and not (necessarily) let it get at your own data. The |
| 1191 | way we do this is to have a `virtual' bottom-of-stack, exposed to each |
| 1192 | function. The mark stack keeps bookmarks to locations in the argument |
| 1193 | stack usable by each function. For instance, when dealing with a tied |
| 1194 | variable, (internally, something with `P' magic) Perl has to call |
| 1195 | methods for accesses to the tied variables. However, we need to separate |
| 1196 | the arguments exposed to the method to the argument exposed to the |
| 1197 | original function - the store or fetch or whatever it may be. Here's how |
| 1198 | the tied C<push> is implemented; see C<av_push> in F<av.c>: |
| 1199 | |
| 1200 | 1 PUSHMARK(SP); |
| 1201 | 2 EXTEND(SP,2); |
| 1202 | 3 PUSHs(SvTIED_obj((SV*)av, mg)); |
| 1203 | 4 PUSHs(val); |
| 1204 | 5 PUTBACK; |
| 1205 | 6 ENTER; |
| 1206 | 7 call_method("PUSH", G_SCALAR|G_DISCARD); |
| 1207 | 8 LEAVE; |
| 1208 | 9 POPSTACK; |
| 1209 | |
| 1210 | The lines which concern the mark stack are the first, fifth and last |
| 1211 | lines: they save away, restore and remove the current position of the |
| 1212 | argument stack. |
| 1213 | |
| 1214 | Let's examine the whole implementation, for practice: |
| 1215 | |
| 1216 | 1 PUSHMARK(SP); |
| 1217 | |
| 1218 | Push the current state of the stack pointer onto the mark stack. This is |
| 1219 | so that when we've finished adding items to the argument stack, Perl |
| 1220 | knows how many things we've added recently. |
| 1221 | |
| 1222 | 2 EXTEND(SP,2); |
| 1223 | 3 PUSHs(SvTIED_obj((SV*)av, mg)); |
| 1224 | 4 PUSHs(val); |
| 1225 | |
| 1226 | We're going to add two more items onto the argument stack: when you have |
| 1227 | a tied array, the C<PUSH> subroutine receives the object and the value |
| 1228 | to be pushed, and that's exactly what we have here - the tied object, |
| 1229 | retrieved with C<SvTIED_obj>, and the value, the SV C<val>. |
| 1230 | |
| 1231 | 5 PUTBACK; |
| 1232 | |
| 1233 | Next we tell Perl to make the change to the global stack pointer: C<dSP> |
| 1234 | only gave us a local copy, not a reference to the global. |
| 1235 | |
| 1236 | 6 ENTER; |
| 1237 | 7 call_method("PUSH", G_SCALAR|G_DISCARD); |
| 1238 | 8 LEAVE; |
| 1239 | |
| 1240 | C<ENTER> and C<LEAVE> localise a block of code - they make sure that all |
| 1241 | variables are tidied up, everything that has been localised gets |
| 1242 | its previous value returned, and so on. Think of them as the C<{> and |
| 1243 | C<}> of a Perl block. |
| 1244 | |
| 1245 | To actually do the magic method call, we have to call a subroutine in |
| 1246 | Perl space: C<call_method> takes care of that, and it's described in |
| 1247 | L<perlcall>. We call the C<PUSH> method in scalar context, and we're |
| 1248 | going to discard its return value. |
| 1249 | |
| 1250 | 9 POPSTACK; |
| 1251 | |
| 1252 | Finally, we remove the value we placed on the mark stack, since we |
| 1253 | don't need it any more. |
| 1254 | |
| 1255 | =item Save stack |
| 1256 | |
| 1257 | C doesn't have a concept of local scope, so perl provides one. We've |
| 1258 | seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save |
| 1259 | stack implements the C equivalent of, for example: |
| 1260 | |
| 1261 | { |
| 1262 | local $foo = 42; |
| 1263 | ... |
| 1264 | } |
| 1265 | |
| 1266 | See L<perlguts/Localising Changes> for how to use the save stack. |
| 1267 | |
| 1268 | =back |
| 1269 | |
| 1270 | =head2 Millions of Macros |
| 1271 | |
| 1272 | One thing you'll notice about the Perl source is that it's full of |
| 1273 | macros. Some have called the pervasive use of macros the hardest thing |
| 1274 | to understand, others find it adds to clarity. Let's take an example, |
| 1275 | the code which implements the addition operator: |
| 1276 | |
| 1277 | 1 PP(pp_add) |
| 1278 | 2 { |
| 1279 | 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN); |
| 1280 | 4 { |
| 1281 | 5 dPOPTOPnnrl_ul; |
| 1282 | 6 SETn( left + right ); |
| 1283 | 7 RETURN; |
| 1284 | 8 } |
| 1285 | 9 } |
| 1286 | |
| 1287 | Every line here (apart from the braces, of course) contains a macro. The |
| 1288 | first line sets up the function declaration as Perl expects for PP code; |
| 1289 | line 3 sets up variable declarations for the argument stack and the |
| 1290 | target, the return value of the operation. Finally, it tries to see if |
| 1291 | the addition operation is overloaded; if so, the appropriate subroutine |
| 1292 | is called. |
| 1293 | |
| 1294 | Line 5 is another variable declaration - all variable declarations start |
| 1295 | with C<d> - which pops from the top of the argument stack two NVs (hence |
| 1296 | C<nn>) and puts them into the variables C<right> and C<left>, hence the |
| 1297 | C<rl>. These are the two operands to the addition operator. Next, we |
| 1298 | call C<SETn> to set the NV of the return value to the result of adding |
| 1299 | the two values. This done, we return - the C<RETURN> macro makes sure |
| 1300 | that our return value is properly handled, and we pass the next operator |
| 1301 | to run back to the main run loop. |
| 1302 | |
| 1303 | Most of these macros are explained in L<perlapi>, and some of the more |
| 1304 | important ones are explained in L<perlxs> as well. Pay special attention |
| 1305 | to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on |
| 1306 | the C<[pad]THX_?> macros. |
| 1307 | |
| 1308 | =head2 The .i Targets |
| 1309 | |
| 1310 | You can expand the macros in a F<foo.c> file by saying |
| 1311 | |
| 1312 | make foo.i |
| 1313 | |
| 1314 | which will expand the macros using cpp. Don't be scared by the results. |
| 1315 | |
| 1316 | =head2 Poking at Perl |
| 1317 | |
| 1318 | To really poke around with Perl, you'll probably want to build Perl for |
| 1319 | debugging, like this: |
| 1320 | |
| 1321 | ./Configure -d -D optimize=-g |
| 1322 | make |
| 1323 | |
| 1324 | C<-g> is a flag to the C compiler to have it produce debugging |
| 1325 | information which will allow us to step through a running program. |
| 1326 | F<Configure> will also turn on the C<DEBUGGING> compilation symbol which |
| 1327 | enables all the internal debugging code in Perl. There are a whole bunch |
| 1328 | of things you can debug with this: L<perlrun> lists them all, and the |
| 1329 | best way to find out about them is to play about with them. The most |
| 1330 | useful options are probably |
| 1331 | |
| 1332 | l Context (loop) stack processing |
| 1333 | t Trace execution |
| 1334 | o Method and overloading resolution |
| 1335 | c String/numeric conversions |
| 1336 | |
| 1337 | Some of the functionality of the debugging code can be achieved using XS |
| 1338 | modules. |
| 1339 | |
| 1340 | -Dr => use re 'debug' |
| 1341 | -Dx => use O 'Debug' |
| 1342 | |
| 1343 | =head2 Using a source-level debugger |
| 1344 | |
| 1345 | If the debugging output of C<-D> doesn't help you, it's time to step |
| 1346 | through perl's execution with a source-level debugger. |
| 1347 | |
| 1348 | =over 3 |
| 1349 | |
| 1350 | =item * |
| 1351 | |
| 1352 | We'll use C<gdb> for our examples here; the principles will apply to any |
| 1353 | debugger, but check the manual of the one you're using. |
| 1354 | |
| 1355 | =back |
| 1356 | |
| 1357 | To fire up the debugger, type |
| 1358 | |
| 1359 | gdb ./perl |
| 1360 | |
| 1361 | You'll want to do that in your Perl source tree so the debugger can read |
| 1362 | the source code. You should see the copyright message, followed by the |
| 1363 | prompt. |
| 1364 | |
| 1365 | (gdb) |
| 1366 | |
| 1367 | C<help> will get you into the documentation, but here are the most |
| 1368 | useful commands: |
| 1369 | |
| 1370 | =over 3 |
| 1371 | |
| 1372 | =item run [args] |
| 1373 | |
| 1374 | Run the program with the given arguments. |
| 1375 | |
| 1376 | =item break function_name |
| 1377 | |
| 1378 | =item break source.c:xxx |
| 1379 | |
| 1380 | Tells the debugger that we'll want to pause execution when we reach |
| 1381 | either the named function (but see L<perlguts/Internal Functions>!) or the given |
| 1382 | line in the named source file. |
| 1383 | |
| 1384 | =item step |
| 1385 | |
| 1386 | Steps through the program a line at a time. |
| 1387 | |
| 1388 | =item next |
| 1389 | |
| 1390 | Steps through the program a line at a time, without descending into |
| 1391 | functions. |
| 1392 | |
| 1393 | =item continue |
| 1394 | |
| 1395 | Run until the next breakpoint. |
| 1396 | |
| 1397 | =item finish |
| 1398 | |
| 1399 | Run until the end of the current function, then stop again. |
| 1400 | |
| 1401 | =item 'enter' |
| 1402 | |
| 1403 | Just pressing Enter will do the most recent operation again - it's a |
| 1404 | blessing when stepping through miles of source code. |
| 1405 | |
| 1406 | =item print |
| 1407 | |
| 1408 | Execute the given C code and print its results. B<WARNING>: Perl makes |
| 1409 | heavy use of macros, and F<gdb> does not necessarily support macros |
| 1410 | (see later L</"gdb macro support">). You'll have to substitute them |
| 1411 | yourself, or to invoke cpp on the source code files |
| 1412 | (see L</"The .i Targets">) |
| 1413 | So, for instance, you can't say |
| 1414 | |
| 1415 | print SvPV_nolen(sv) |
| 1416 | |
| 1417 | but you have to say |
| 1418 | |
| 1419 | print Perl_sv_2pv_nolen(sv) |
| 1420 | |
| 1421 | =back |
| 1422 | |
| 1423 | You may find it helpful to have a "macro dictionary", which you can |
| 1424 | produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't |
| 1425 | recursively apply those macros for you. |
| 1426 | |
| 1427 | =head2 gdb macro support |
| 1428 | |
| 1429 | Recent versions of F<gdb> have fairly good macro support, but |
| 1430 | in order to use it you'll need to compile perl with macro definitions |
| 1431 | included in the debugging information. Using F<gcc> version 3.1, this |
| 1432 | means configuring with C<-Doptimize=-g3>. Other compilers might use a |
| 1433 | different switch (if they support debugging macros at all). |
| 1434 | |
| 1435 | =head2 Dumping Perl Data Structures |
| 1436 | |
| 1437 | One way to get around this macro hell is to use the dumping functions in |
| 1438 | F<dump.c>; these work a little like an internal |
| 1439 | L<Devel::Peek|Devel::Peek>, but they also cover OPs and other structures |
| 1440 | that you can't get at from Perl. Let's take an example. We'll use the |
| 1441 | C<$a = $b + $c> we used before, but give it a bit of context: |
| 1442 | C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around? |
| 1443 | |
| 1444 | What about C<pp_add>, the function we examined earlier to implement the |
| 1445 | C<+> operator: |
| 1446 | |
| 1447 | (gdb) break Perl_pp_add |
| 1448 | Breakpoint 1 at 0x46249f: file pp_hot.c, line 309. |
| 1449 | |
| 1450 | Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Internal Functions>. |
| 1451 | With the breakpoint in place, we can run our program: |
| 1452 | |
| 1453 | (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c' |
| 1454 | |
| 1455 | Lots of junk will go past as gdb reads in the relevant source files and |
| 1456 | libraries, and then: |
| 1457 | |
| 1458 | Breakpoint 1, Perl_pp_add () at pp_hot.c:309 |
| 1459 | 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN); |
| 1460 | (gdb) step |
| 1461 | 311 dPOPTOPnnrl_ul; |
| 1462 | (gdb) |
| 1463 | |
| 1464 | We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul> |
| 1465 | arranges for two C<NV>s to be placed into C<left> and C<right> - let's |
| 1466 | slightly expand it: |
| 1467 | |
| 1468 | #define dPOPTOPnnrl_ul NV right = POPn; \ |
| 1469 | SV *leftsv = TOPs; \ |
| 1470 | NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0 |
| 1471 | |
| 1472 | C<POPn> takes the SV from the top of the stack and obtains its NV either |
| 1473 | directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function. |
| 1474 | C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses |
| 1475 | C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from |
| 1476 | C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>. |
| 1477 | |
| 1478 | Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to |
| 1479 | convert it. If we step again, we'll find ourselves there: |
| 1480 | |
| 1481 | Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669 |
| 1482 | 1669 if (!sv) |
| 1483 | (gdb) |
| 1484 | |
| 1485 | We can now use C<Perl_sv_dump> to investigate the SV: |
| 1486 | |
| 1487 | SV = PV(0xa057cc0) at 0xa0675d0 |
| 1488 | REFCNT = 1 |
| 1489 | FLAGS = (POK,pPOK) |
| 1490 | PV = 0xa06a510 "6XXXX"\0 |
| 1491 | CUR = 5 |
| 1492 | LEN = 6 |
| 1493 | $1 = void |
| 1494 | |
| 1495 | We know we're going to get C<6> from this, so let's finish the |
| 1496 | subroutine: |
| 1497 | |
| 1498 | (gdb) finish |
| 1499 | Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671 |
| 1500 | 0x462669 in Perl_pp_add () at pp_hot.c:311 |
| 1501 | 311 dPOPTOPnnrl_ul; |
| 1502 | |
| 1503 | We can also dump out this op: the current op is always stored in |
| 1504 | C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us |
| 1505 | similar output to L<B::Debug|B::Debug>. |
| 1506 | |
| 1507 | { |
| 1508 | 13 TYPE = add ===> 14 |
| 1509 | TARG = 1 |
| 1510 | FLAGS = (SCALAR,KIDS) |
| 1511 | { |
| 1512 | TYPE = null ===> (12) |
| 1513 | (was rv2sv) |
| 1514 | FLAGS = (SCALAR,KIDS) |
| 1515 | { |
| 1516 | 11 TYPE = gvsv ===> 12 |
| 1517 | FLAGS = (SCALAR) |
| 1518 | GV = main::b |
| 1519 | } |
| 1520 | } |
| 1521 | |
| 1522 | # finish this later # |
| 1523 | |
| 1524 | =head2 Patching |
| 1525 | |
| 1526 | All right, we've now had a look at how to navigate the Perl sources and |
| 1527 | some things you'll need to know when fiddling with them. Let's now get |
| 1528 | on and create a simple patch. Here's something Larry suggested: if a |
| 1529 | C<U> is the first active format during a C<pack>, (for example, |
| 1530 | C<pack "U3C8", @stuff>) then the resulting string should be treated as |
| 1531 | UTF-8 encoded. |
| 1532 | |
| 1533 | How do we prepare to fix this up? First we locate the code in question - |
| 1534 | the C<pack> happens at runtime, so it's going to be in one of the F<pp> |
| 1535 | files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be |
| 1536 | altering this file, let's copy it to F<pp.c~>. |
| 1537 | |
| 1538 | [Well, it was in F<pp.c> when this tutorial was written. It has now been |
| 1539 | split off with C<pp_unpack> to its own file, F<pp_pack.c>] |
| 1540 | |
| 1541 | Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then |
| 1542 | loop over the pattern, taking each format character in turn into |
| 1543 | C<datum_type>. Then for each possible format character, we swallow up |
| 1544 | the other arguments in the pattern (a field width, an asterisk, and so |
| 1545 | on) and convert the next chunk input into the specified format, adding |
| 1546 | it onto the output SV C<cat>. |
| 1547 | |
| 1548 | How do we know if the C<U> is the first format in the C<pat>? Well, if |
| 1549 | we have a pointer to the start of C<pat> then, if we see a C<U> we can |
| 1550 | test whether we're still at the start of the string. So, here's where |
| 1551 | C<pat> is set up: |
| 1552 | |
| 1553 | STRLEN fromlen; |
| 1554 | register char *pat = SvPVx(*++MARK, fromlen); |
| 1555 | register char *patend = pat + fromlen; |
| 1556 | register I32 len; |
| 1557 | I32 datumtype; |
| 1558 | SV *fromstr; |
| 1559 | |
| 1560 | We'll have another string pointer in there: |
| 1561 | |
| 1562 | STRLEN fromlen; |
| 1563 | register char *pat = SvPVx(*++MARK, fromlen); |
| 1564 | register char *patend = pat + fromlen; |
| 1565 | + char *patcopy; |
| 1566 | register I32 len; |
| 1567 | I32 datumtype; |
| 1568 | SV *fromstr; |
| 1569 | |
| 1570 | And just before we start the loop, we'll set C<patcopy> to be the start |
| 1571 | of C<pat>: |
| 1572 | |
| 1573 | items = SP - MARK; |
| 1574 | MARK++; |
| 1575 | sv_setpvn(cat, "", 0); |
| 1576 | + patcopy = pat; |
| 1577 | while (pat < patend) { |
| 1578 | |
| 1579 | Now if we see a C<U> which was at the start of the string, we turn on |
| 1580 | the C<UTF8> flag for the output SV, C<cat>: |
| 1581 | |
| 1582 | + if (datumtype == 'U' && pat==patcopy+1) |
| 1583 | + SvUTF8_on(cat); |
| 1584 | if (datumtype == '#') { |
| 1585 | while (pat < patend && *pat != '\n') |
| 1586 | pat++; |
| 1587 | |
| 1588 | Remember that it has to be C<patcopy+1> because the first character of |
| 1589 | the string is the C<U> which has been swallowed into C<datumtype!> |
| 1590 | |
| 1591 | Oops, we forgot one thing: what if there are spaces at the start of the |
| 1592 | pattern? C<pack(" U*", @stuff)> will have C<U> as the first active |
| 1593 | character, even though it's not the first thing in the pattern. In this |
| 1594 | case, we have to advance C<patcopy> along with C<pat> when we see spaces: |
| 1595 | |
| 1596 | if (isSPACE(datumtype)) |
| 1597 | continue; |
| 1598 | |
| 1599 | needs to become |
| 1600 | |
| 1601 | if (isSPACE(datumtype)) { |
| 1602 | patcopy++; |
| 1603 | continue; |
| 1604 | } |
| 1605 | |
| 1606 | OK. That's the C part done. Now we must do two additional things before |
| 1607 | this patch is ready to go: we've changed the behaviour of Perl, and so |
| 1608 | we must document that change. We must also provide some more regression |
| 1609 | tests to make sure our patch works and doesn't create a bug somewhere |
| 1610 | else along the line. |
| 1611 | |
| 1612 | The regression tests for each operator live in F<t/op/>, and so we |
| 1613 | make a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our |
| 1614 | tests to the end. First, we'll test that the C<U> does indeed create |
| 1615 | Unicode strings. |
| 1616 | |
| 1617 | t/op/pack.t has a sensible ok() function, but if it didn't we could |
| 1618 | use the one from t/test.pl. |
| 1619 | |
| 1620 | require './test.pl'; |
| 1621 | plan( tests => 159 ); |
| 1622 | |
| 1623 | so instead of this: |
| 1624 | |
| 1625 | print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000); |
| 1626 | print "ok $test\n"; $test++; |
| 1627 | |
| 1628 | we can write the more sensible (see L<Test::More> for a full |
| 1629 | explanation of is() and other testing functions). |
| 1630 | |
| 1631 | is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000), |
| 1632 | "U* produces unicode" ); |
| 1633 | |
| 1634 | Now we'll test that we got that space-at-the-beginning business right: |
| 1635 | |
| 1636 | is( "1.20.300.4000", sprintf "%vd", pack(" U*",1,20,300,4000), |
| 1637 | " with spaces at the beginning" ); |
| 1638 | |
| 1639 | And finally we'll test that we don't make Unicode strings if C<U> is B<not> |
| 1640 | the first active format: |
| 1641 | |
| 1642 | isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000), |
| 1643 | "U* not first isn't unicode" ); |
| 1644 | |
| 1645 | Mustn't forget to change the number of tests which appears at the top, |
| 1646 | or else the automated tester will get confused. This will either look |
| 1647 | like this: |
| 1648 | |
| 1649 | print "1..156\n"; |
| 1650 | |
| 1651 | or this: |
| 1652 | |
| 1653 | plan( tests => 156 ); |
| 1654 | |
| 1655 | We now compile up Perl, and run it through the test suite. Our new |
| 1656 | tests pass, hooray! |
| 1657 | |
| 1658 | Finally, the documentation. The job is never done until the paperwork is |
| 1659 | over, so let's describe the change we've just made. The relevant place |
| 1660 | is F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert |
| 1661 | this text in the description of C<pack>: |
| 1662 | |
| 1663 | =item * |
| 1664 | |
| 1665 | If the pattern begins with a C<U>, the resulting string will be treated |
| 1666 | as UTF-8-encoded Unicode. You can force UTF-8 encoding on in a string |
| 1667 | with an initial C<U0>, and the bytes that follow will be interpreted as |
| 1668 | Unicode characters. If you don't want this to happen, you can begin your |
| 1669 | pattern with C<C0> (or anything else) to force Perl not to UTF-8 encode your |
| 1670 | string, and then follow this with a C<U*> somewhere in your pattern. |
| 1671 | |
| 1672 | All done. Now let's create the patch. F<Porting/patching.pod> tells us |
| 1673 | that if we're making major changes, we should copy the entire directory |
| 1674 | to somewhere safe before we begin fiddling, and then do |
| 1675 | |
| 1676 | diff -ruN old new > patch |
| 1677 | |
| 1678 | However, we know which files we've changed, and we can simply do this: |
| 1679 | |
| 1680 | diff -u pp.c~ pp.c > patch |
| 1681 | diff -u t/op/pack.t~ t/op/pack.t >> patch |
| 1682 | diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patch |
| 1683 | |
| 1684 | We end up with a patch looking a little like this: |
| 1685 | |
| 1686 | --- pp.c~ Fri Jun 02 04:34:10 2000 |
| 1687 | +++ pp.c Fri Jun 16 11:37:25 2000 |
| 1688 | @@ -4375,6 +4375,7 @@ |
| 1689 | register I32 items; |
| 1690 | STRLEN fromlen; |
| 1691 | register char *pat = SvPVx(*++MARK, fromlen); |
| 1692 | + char *patcopy; |
| 1693 | register char *patend = pat + fromlen; |
| 1694 | register I32 len; |
| 1695 | I32 datumtype; |
| 1696 | @@ -4405,6 +4406,7 @@ |
| 1697 | ... |
| 1698 | |
| 1699 | And finally, we submit it, with our rationale, to perl5-porters. Job |
| 1700 | done! |
| 1701 | |
| 1702 | =head2 Patching a core module |
| 1703 | |
| 1704 | This works just like patching anything else, with an extra |
| 1705 | consideration. Many core modules also live on CPAN. If this is so, |
| 1706 | patch the CPAN version instead of the core and send the patch off to |
| 1707 | the module maintainer (with a copy to p5p). This will help the module |
| 1708 | maintainer keep the CPAN version in sync with the core version without |
| 1709 | constantly scanning p5p. |
| 1710 | |
| 1711 | =head2 Adding a new function to the core |
| 1712 | |
| 1713 | If, as part of a patch to fix a bug, or just because you have an |
| 1714 | especially good idea, you decide to add a new function to the core, |
| 1715 | discuss your ideas on p5p well before you start work. It may be that |
| 1716 | someone else has already attempted to do what you are considering and |
| 1717 | can give lots of good advice or even provide you with bits of code |
| 1718 | that they already started (but never finished). |
| 1719 | |
| 1720 | You have to follow all of the advice given above for patching. It is |
| 1721 | extremely important to test any addition thoroughly and add new tests |
| 1722 | to explore all boundary conditions that your new function is expected |
| 1723 | to handle. If your new function is used only by one module (e.g. toke), |
| 1724 | then it should probably be named S_your_function (for static); on the |
| 1725 | other hand, if you expect it to accessible from other functions in |
| 1726 | Perl, you should name it Perl_your_function. See L<perlguts/Internal Functions> |
| 1727 | for more details. |
| 1728 | |
| 1729 | The location of any new code is also an important consideration. Don't |
| 1730 | just create a new top level .c file and put your code there; you would |
| 1731 | have to make changes to Configure (so the Makefile is created properly), |
| 1732 | as well as possibly lots of include files. This is strictly pumpking |
| 1733 | business. |
| 1734 | |
| 1735 | It is better to add your function to one of the existing top level |
| 1736 | source code files, but your choice is complicated by the nature of |
| 1737 | the Perl distribution. Only the files that are marked as compiled |
| 1738 | static are located in the perl executable. Everything else is located |
| 1739 | in the shared library (or DLL if you are running under WIN32). So, |
| 1740 | for example, if a function was only used by functions located in |
| 1741 | toke.c, then your code can go in toke.c. If, however, you want to call |
| 1742 | the function from universal.c, then you should put your code in another |
| 1743 | location, for example util.c. |
| 1744 | |
| 1745 | In addition to writing your c-code, you will need to create an |
| 1746 | appropriate entry in embed.pl describing your function, then run |
| 1747 | 'make regen_headers' to create the entries in the numerous header |
| 1748 | files that perl needs to compile correctly. See L<perlguts/Internal Functions> |
| 1749 | for information on the various options that you can set in embed.pl. |
| 1750 | You will forget to do this a few (or many) times and you will get |
| 1751 | warnings during the compilation phase. Make sure that you mention |
| 1752 | this when you post your patch to P5P; the pumpking needs to know this. |
| 1753 | |
| 1754 | When you write your new code, please be conscious of existing code |
| 1755 | conventions used in the perl source files. See L<perlstyle> for |
| 1756 | details. Although most of the guidelines discussed seem to focus on |
| 1757 | Perl code, rather than c, they all apply (except when they don't ;). |
| 1758 | See also I<Porting/patching.pod> file in the Perl source distribution |
| 1759 | for lots of details about both formatting and submitting patches of |
| 1760 | your changes. |
| 1761 | |
| 1762 | Lastly, TEST TEST TEST TEST TEST any code before posting to p5p. |
| 1763 | Test on as many platforms as you can find. Test as many perl |
| 1764 | Configure options as you can (e.g. MULTIPLICITY). If you have |
| 1765 | profiling or memory tools, see L<EXTERNAL TOOLS FOR DEBUGGING PERL> |
| 1766 | below for how to use them to further test your code. Remember that |
| 1767 | most of the people on P5P are doing this on their own time and |
| 1768 | don't have the time to debug your code. |
| 1769 | |
| 1770 | =head2 Writing a test |
| 1771 | |
| 1772 | Every module and built-in function has an associated test file (or |
| 1773 | should...). If you add or change functionality, you have to write a |
| 1774 | test. If you fix a bug, you have to write a test so that bug never |
| 1775 | comes back. If you alter the docs, it would be nice to test what the |
| 1776 | new documentation says. |
| 1777 | |
| 1778 | In short, if you submit a patch you probably also have to patch the |
| 1779 | tests. |
| 1780 | |
| 1781 | For modules, the test file is right next to the module itself. |
| 1782 | F<lib/strict.t> tests F<lib/strict.pm>. This is a recent innovation, |
| 1783 | so there are some snags (and it would be wonderful for you to brush |
| 1784 | them out), but it basically works that way. Everything else lives in |
| 1785 | F<t/>. |
| 1786 | |
| 1787 | =over 3 |
| 1788 | |
| 1789 | =item F<t/base/> |
| 1790 | |
| 1791 | Testing of the absolute basic functionality of Perl. Things like |
| 1792 | C<if>, basic file reads and writes, simple regexes, etc. These are |
| 1793 | run first in the test suite and if any of them fail, something is |
| 1794 | I<really> broken. |
| 1795 | |
| 1796 | =item F<t/cmd/> |
| 1797 | |
| 1798 | These test the basic control structures, C<if/else>, C<while>, |
| 1799 | subroutines, etc. |
| 1800 | |
| 1801 | =item F<t/comp/> |
| 1802 | |
| 1803 | Tests basic issues of how Perl parses and compiles itself. |
| 1804 | |
| 1805 | =item F<t/io/> |
| 1806 | |
| 1807 | Tests for built-in IO functions, including command line arguments. |
| 1808 | |
| 1809 | =item F<t/lib/> |
| 1810 | |
| 1811 | The old home for the module tests, you shouldn't put anything new in |
| 1812 | here. There are still some bits and pieces hanging around in here |
| 1813 | that need to be moved. Perhaps you could move them? Thanks! |
| 1814 | |
| 1815 | =item F<t/op/> |
| 1816 | |
| 1817 | Tests for perl's built in functions that don't fit into any of the |
| 1818 | other directories. |
| 1819 | |
| 1820 | =item F<t/pod/> |
| 1821 | |
| 1822 | Tests for POD directives. There are still some tests for the Pod |
| 1823 | modules hanging around in here that need to be moved out into F<lib/>. |
| 1824 | |
| 1825 | =item F<t/run/> |
| 1826 | |
| 1827 | Testing features of how perl actually runs, including exit codes and |
| 1828 | handling of PERL* environment variables. |
| 1829 | |
| 1830 | =item F<t/uni/> |
| 1831 | |
| 1832 | Tests for the core support of Unicode. |
| 1833 | |
| 1834 | =item F<t/win32/> |
| 1835 | |
| 1836 | Windows-specific tests. |
| 1837 | |
| 1838 | =item F<t/x2p> |
| 1839 | |
| 1840 | A test suite for the s2p converter. |
| 1841 | |
| 1842 | =back |
| 1843 | |
| 1844 | The core uses the same testing style as the rest of Perl, a simple |
| 1845 | "ok/not ok" run through Test::Harness, but there are a few special |
| 1846 | considerations. |
| 1847 | |
| 1848 | There are three ways to write a test in the core. Test::More, |
| 1849 | t/test.pl and ad hoc C<print $test ? "ok 42\n" : "not ok 42\n">. The |
| 1850 | decision of which to use depends on what part of the test suite you're |
| 1851 | working on. This is a measure to prevent a high-level failure (such |
| 1852 | as Config.pm breaking) from causing basic functionality tests to fail. |
| 1853 | |
| 1854 | =over 4 |
| 1855 | |
| 1856 | =item t/base t/comp |
| 1857 | |
| 1858 | Since we don't know if require works, or even subroutines, use ad hoc |
| 1859 | tests for these two. Step carefully to avoid using the feature being |
| 1860 | tested. |
| 1861 | |
| 1862 | =item t/cmd t/run t/io t/op |
| 1863 | |
| 1864 | Now that basic require() and subroutines are tested, you can use the |
| 1865 | t/test.pl library which emulates the important features of Test::More |
| 1866 | while using a minimum of core features. |
| 1867 | |
| 1868 | You can also conditionally use certain libraries like Config, but be |
| 1869 | sure to skip the test gracefully if it's not there. |
| 1870 | |
| 1871 | =item t/lib ext lib |
| 1872 | |
| 1873 | Now that the core of Perl is tested, Test::More can be used. You can |
| 1874 | also use the full suite of core modules in the tests. |
| 1875 | |
| 1876 | =back |
| 1877 | |
| 1878 | When you say "make test" Perl uses the F<t/TEST> program to run the |
| 1879 | test suite. All tests are run from the F<t/> directory, B<not> the |
| 1880 | directory which contains the test. This causes some problems with the |
| 1881 | tests in F<lib/>, so here's some opportunity for some patching. |
| 1882 | |
| 1883 | You must be triply conscious of cross-platform concerns. This usually |
| 1884 | boils down to using File::Spec and avoiding things like C<fork()> and |
| 1885 | C<system()> unless absolutely necessary. |
| 1886 | |
| 1887 | =head2 Special Make Test Targets |
| 1888 | |
| 1889 | There are various special make targets that can be used to test Perl |
| 1890 | slightly differently than the standard "test" target. Not all them |
| 1891 | are expected to give a 100% success rate. Many of them have several |
| 1892 | aliases. |
| 1893 | |
| 1894 | =over 4 |
| 1895 | |
| 1896 | =item coretest |
| 1897 | |
| 1898 | Run F<perl> on all core tests (F<t/*> and F<lib/[a-z]*> pragma tests). |
| 1899 | |
| 1900 | =item test.deparse |
| 1901 | |
| 1902 | Run all the tests through B::Deparse. Not all tests will succeed. |
| 1903 | |
| 1904 | =item test.taintwarn |
| 1905 | |
| 1906 | Run all tests with the B<-t> command-line switch. Not all tests |
| 1907 | are expected to succeed (until they're specifically fixed, of course). |
| 1908 | |
| 1909 | =item minitest |
| 1910 | |
| 1911 | Run F<miniperl> on F<t/base>, F<t/comp>, F<t/cmd>, F<t/run>, F<t/io>, |
| 1912 | F<t/op>, and F<t/uni> tests. |
| 1913 | |
| 1914 | =item test.valgrind check.valgrind utest.valgrind ucheck.valgrind |
| 1915 | |
| 1916 | (Only in Linux) Run all the tests using the memory leak + naughty |
| 1917 | memory access tool "valgrind". The log files will be named |
| 1918 | F<testname.valgrind>. |
| 1919 | |
| 1920 | =item test.third check.third utest.third ucheck.third |
| 1921 | |
| 1922 | (Only in Tru64) Run all the tests using the memory leak + naughty |
| 1923 | memory access tool "Third Degree". The log files will be named |
| 1924 | F<perl3.log.testname>. |
| 1925 | |
| 1926 | =item test.torture torturetest |
| 1927 | |
| 1928 | Run all the usual tests and some extra tests. As of Perl 5.8.0 the |
| 1929 | only extra tests are Abigail's JAPHs, F<t/japh/abigail.t>. |
| 1930 | |
| 1931 | You can also run the torture test with F<t/harness> by giving |
| 1932 | C<-torture> argument to F<t/harness>. |
| 1933 | |
| 1934 | =item utest ucheck test.utf8 check.utf8 |
| 1935 | |
| 1936 | Run all the tests with -Mutf8. Not all tests will succeed. |
| 1937 | |
| 1938 | =item minitest.utf16 test.utf16 |
| 1939 | |
| 1940 | Runs the tests with UTF-16 encoded scripts, encoded with different |
| 1941 | versions of this encoding. |
| 1942 | |
| 1943 | C<make utest.utf16> runs the test suite with a combination of C<-utf8> and |
| 1944 | C<-utf16> arguments to F<t/TEST>. |
| 1945 | |
| 1946 | =item test_harness |
| 1947 | |
| 1948 | Run the test suite with the F<t/harness> controlling program, instead of |
| 1949 | F<t/TEST>. F<t/harness> is more sophisticated, and uses the |
| 1950 | L<Test::Harness> module, thus using this test target supposes that perl |
| 1951 | mostly works. The main advantage for our purposes is that it prints a |
| 1952 | detailed summary of failed tests at the end. Also, unlike F<t/TEST>, it |
| 1953 | doesn't redirect stderr to stdout. |
| 1954 | |
| 1955 | =back |
| 1956 | |
| 1957 | =head2 Running tests by hand |
| 1958 | |
| 1959 | You can run part of the test suite by hand by using one the following |
| 1960 | commands from the F<t/> directory : |
| 1961 | |
| 1962 | ./perl -I../lib TEST list-of-.t-files |
| 1963 | |
| 1964 | or |
| 1965 | |
| 1966 | ./perl -I../lib harness list-of-.t-files |
| 1967 | |
| 1968 | (if you don't specify test scripts, the whole test suite will be run.) |
| 1969 | |
| 1970 | You can run an individual test by a command similar to |
| 1971 | |
| 1972 | ./perl -I../lib patho/to/foo.t |
| 1973 | |
| 1974 | except that the harnesses set up some environment variables that may |
| 1975 | affect the execution of the test : |
| 1976 | |
| 1977 | =over 4 |
| 1978 | |
| 1979 | =item PERL_CORE=1 |
| 1980 | |
| 1981 | indicates that we're running this test part of the perl core test suite. |
| 1982 | This is useful for modules that have a dual life on CPAN. |
| 1983 | |
| 1984 | =item PERL_DESTRUCT_LEVEL=2 |
| 1985 | |
| 1986 | is set to 2 if it isn't set already (see L</PERL_DESTRUCT_LEVEL>) |
| 1987 | |
| 1988 | =item PERL |
| 1989 | |
| 1990 | (used only by F<t/TEST>) if set, overrides the path to the perl executable |
| 1991 | that should be used to run the tests (the default being F<./perl>). |
| 1992 | |
| 1993 | =item PERL_SKIP_TTY_TEST |
| 1994 | |
| 1995 | if set, tells to skip the tests that need a terminal. It's actually set |
| 1996 | automatically by the Makefile, but can also be forced artificially by |
| 1997 | running 'make test_notty'. |
| 1998 | |
| 1999 | =back |
| 2000 | |
| 2001 | =head1 EXTERNAL TOOLS FOR DEBUGGING PERL |
| 2002 | |
| 2003 | Sometimes it helps to use external tools while debugging and |
| 2004 | testing Perl. This section tries to guide you through using |
| 2005 | some common testing and debugging tools with Perl. This is |
| 2006 | meant as a guide to interfacing these tools with Perl, not |
| 2007 | as any kind of guide to the use of the tools themselves. |
| 2008 | |
| 2009 | B<NOTE 1>: Running under memory debuggers such as Purify, valgrind, or |
| 2010 | Third Degree greatly slows down the execution: seconds become minutes, |
| 2011 | minutes become hours. For example as of Perl 5.8.1, the |
| 2012 | ext/Encode/t/Unicode.t takes extraordinarily long to complete under |
| 2013 | e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more |
| 2014 | than six hours, even on a snappy computer-- the said test must be |
| 2015 | doing something that is quite unfriendly for memory debuggers. If you |
| 2016 | don't feel like waiting, that you can simply kill away the perl |
| 2017 | process. |
| 2018 | |
| 2019 | B<NOTE 2>: To minimize the number of memory leak false alarms (see |
| 2020 | L</PERL_DESTRUCT_LEVEL> for more information), you have to have |
| 2021 | environment variable PERL_DESTRUCT_LEVEL set to 2. The F<TEST> |
| 2022 | and harness scripts do that automatically. But if you are running |
| 2023 | some of the tests manually-- for csh-like shells: |
| 2024 | |
| 2025 | setenv PERL_DESTRUCT_LEVEL 2 |
| 2026 | |
| 2027 | and for Bourne-type shells: |
| 2028 | |
| 2029 | PERL_DESTRUCT_LEVEL=2 |
| 2030 | export PERL_DESTRUCT_LEVEL |
| 2031 | |
| 2032 | or in UNIXy environments you can also use the C<env> command: |
| 2033 | |
| 2034 | env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ... |
| 2035 | |
| 2036 | B<NOTE 3>: There are known memory leaks when there are compile-time |
| 2037 | errors within eval or require, seeing C<S_doeval> in the call stack |
| 2038 | is a good sign of these. Fixing these leaks is non-trivial, |
| 2039 | unfortunately, but they must be fixed eventually. |
| 2040 | |
| 2041 | =head2 Rational Software's Purify |
| 2042 | |
| 2043 | Purify is a commercial tool that is helpful in identifying |
| 2044 | memory overruns, wild pointers, memory leaks and other such |
| 2045 | badness. Perl must be compiled in a specific way for |
| 2046 | optimal testing with Purify. Purify is available under |
| 2047 | Windows NT, Solaris, HP-UX, SGI, and Siemens Unix. |
| 2048 | |
| 2049 | =head2 Purify on Unix |
| 2050 | |
| 2051 | On Unix, Purify creates a new Perl binary. To get the most |
| 2052 | benefit out of Purify, you should create the perl to Purify |
| 2053 | using: |
| 2054 | |
| 2055 | sh Configure -Accflags=-DPURIFY -Doptimize='-g' \ |
| 2056 | -Uusemymalloc -Dusemultiplicity |
| 2057 | |
| 2058 | where these arguments mean: |
| 2059 | |
| 2060 | =over 4 |
| 2061 | |
| 2062 | =item -Accflags=-DPURIFY |
| 2063 | |
| 2064 | Disables Perl's arena memory allocation functions, as well as |
| 2065 | forcing use of memory allocation functions derived from the |
| 2066 | system malloc. |
| 2067 | |
| 2068 | =item -Doptimize='-g' |
| 2069 | |
| 2070 | Adds debugging information so that you see the exact source |
| 2071 | statements where the problem occurs. Without this flag, all |
| 2072 | you will see is the source filename of where the error occurred. |
| 2073 | |
| 2074 | =item -Uusemymalloc |
| 2075 | |
| 2076 | Disable Perl's malloc so that Purify can more closely monitor |
| 2077 | allocations and leaks. Using Perl's malloc will make Purify |
| 2078 | report most leaks in the "potential" leaks category. |
| 2079 | |
| 2080 | =item -Dusemultiplicity |
| 2081 | |
| 2082 | Enabling the multiplicity option allows perl to clean up |
| 2083 | thoroughly when the interpreter shuts down, which reduces the |
| 2084 | number of bogus leak reports from Purify. |
| 2085 | |
| 2086 | =back |
| 2087 | |
| 2088 | Once you've compiled a perl suitable for Purify'ing, then you |
| 2089 | can just: |
| 2090 | |
| 2091 | make pureperl |
| 2092 | |
| 2093 | which creates a binary named 'pureperl' that has been Purify'ed. |
| 2094 | This binary is used in place of the standard 'perl' binary |
| 2095 | when you want to debug Perl memory problems. |
| 2096 | |
| 2097 | As an example, to show any memory leaks produced during the |
| 2098 | standard Perl testset you would create and run the Purify'ed |
| 2099 | perl as: |
| 2100 | |
| 2101 | make pureperl |
| 2102 | cd t |
| 2103 | ../pureperl -I../lib harness |
| 2104 | |
| 2105 | which would run Perl on test.pl and report any memory problems. |
| 2106 | |
| 2107 | Purify outputs messages in "Viewer" windows by default. If |
| 2108 | you don't have a windowing environment or if you simply |
| 2109 | want the Purify output to unobtrusively go to a log file |
| 2110 | instead of to the interactive window, use these following |
| 2111 | options to output to the log file "perl.log": |
| 2112 | |
| 2113 | setenv PURIFYOPTIONS "-chain-length=25 -windows=no \ |
| 2114 | -log-file=perl.log -append-logfile=yes" |
| 2115 | |
| 2116 | If you plan to use the "Viewer" windows, then you only need this option: |
| 2117 | |
| 2118 | setenv PURIFYOPTIONS "-chain-length=25" |
| 2119 | |
| 2120 | In Bourne-type shells: |
| 2121 | |
| 2122 | PURIFYOPTIONS="..." |
| 2123 | export PURIFYOPTIONS |
| 2124 | |
| 2125 | or if you have the "env" utility: |
| 2126 | |
| 2127 | env PURIFYOPTIONS="..." ../pureperl ... |
| 2128 | |
| 2129 | =head2 Purify on NT |
| 2130 | |
| 2131 | Purify on Windows NT instruments the Perl binary 'perl.exe' |
| 2132 | on the fly. There are several options in the makefile you |
| 2133 | should change to get the most use out of Purify: |
| 2134 | |
| 2135 | =over 4 |
| 2136 | |
| 2137 | =item DEFINES |
| 2138 | |
| 2139 | You should add -DPURIFY to the DEFINES line so the DEFINES |
| 2140 | line looks something like: |
| 2141 | |
| 2142 | DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1 |
| 2143 | |
| 2144 | to disable Perl's arena memory allocation functions, as |
| 2145 | well as to force use of memory allocation functions derived |
| 2146 | from the system malloc. |
| 2147 | |
| 2148 | =item USE_MULTI = define |
| 2149 | |
| 2150 | Enabling the multiplicity option allows perl to clean up |
| 2151 | thoroughly when the interpreter shuts down, which reduces the |
| 2152 | number of bogus leak reports from Purify. |
| 2153 | |
| 2154 | =item #PERL_MALLOC = define |
| 2155 | |
| 2156 | Disable Perl's malloc so that Purify can more closely monitor |
| 2157 | allocations and leaks. Using Perl's malloc will make Purify |
| 2158 | report most leaks in the "potential" leaks category. |
| 2159 | |
| 2160 | =item CFG = Debug |
| 2161 | |
| 2162 | Adds debugging information so that you see the exact source |
| 2163 | statements where the problem occurs. Without this flag, all |
| 2164 | you will see is the source filename of where the error occurred. |
| 2165 | |
| 2166 | =back |
| 2167 | |
| 2168 | As an example, to show any memory leaks produced during the |
| 2169 | standard Perl testset you would create and run Purify as: |
| 2170 | |
| 2171 | cd win32 |
| 2172 | make |
| 2173 | cd ../t |
| 2174 | purify ../perl -I../lib harness |
| 2175 | |
| 2176 | which would instrument Perl in memory, run Perl on test.pl, |
| 2177 | then finally report any memory problems. |
| 2178 | |
| 2179 | =head2 valgrind |
| 2180 | |
| 2181 | The excellent valgrind tool can be used to find out both memory leaks |
| 2182 | and illegal memory accesses. As of August 2003 it unfortunately works |
| 2183 | only on x86 (ELF) Linux. The special "test.valgrind" target can be used |
| 2184 | to run the tests under valgrind. Found errors and memory leaks are |
| 2185 | logged in files named F<test.valgrind>. |
| 2186 | |
| 2187 | As system libraries (most notably glibc) are also triggering errors, |
| 2188 | valgrind allows to suppress such errors using suppression files. The |
| 2189 | default suppression file that comes with valgrind already catches a lot |
| 2190 | of them. Some additional suppressions are defined in F<t/perl.supp>. |
| 2191 | |
| 2192 | To get valgrind and for more information see |
| 2193 | |
| 2194 | http://developer.kde.org/~sewardj/ |
| 2195 | |
| 2196 | =head2 Compaq's/Digital's/HP's Third Degree |
| 2197 | |
| 2198 | Third Degree is a tool for memory leak detection and memory access checks. |
| 2199 | It is one of the many tools in the ATOM toolkit. The toolkit is only |
| 2200 | available on Tru64 (formerly known as Digital UNIX formerly known as |
| 2201 | DEC OSF/1). |
| 2202 | |
| 2203 | When building Perl, you must first run Configure with -Doptimize=-g |
| 2204 | and -Uusemymalloc flags, after that you can use the make targets |
| 2205 | "perl.third" and "test.third". (What is required is that Perl must be |
| 2206 | compiled using the C<-g> flag, you may need to re-Configure.) |
| 2207 | |
| 2208 | The short story is that with "atom" you can instrument the Perl |
| 2209 | executable to create a new executable called F<perl.third>. When the |
| 2210 | instrumented executable is run, it creates a log of dubious memory |
| 2211 | traffic in file called F<perl.3log>. See the manual pages of atom and |
| 2212 | third for more information. The most extensive Third Degree |
| 2213 | documentation is available in the Compaq "Tru64 UNIX Programmer's |
| 2214 | Guide", chapter "Debugging Programs with Third Degree". |
| 2215 | |
| 2216 | The "test.third" leaves a lot of files named F<foo_bar.3log> in the t/ |
| 2217 | subdirectory. There is a problem with these files: Third Degree is so |
| 2218 | effective that it finds problems also in the system libraries. |
| 2219 | Therefore you should used the Porting/thirdclean script to cleanup |
| 2220 | the F<*.3log> files. |
| 2221 | |
| 2222 | There are also leaks that for given certain definition of a leak, |
| 2223 | aren't. See L</PERL_DESTRUCT_LEVEL> for more information. |
| 2224 | |
| 2225 | =head2 PERL_DESTRUCT_LEVEL |
| 2226 | |
| 2227 | If you want to run any of the tests yourself manually using e.g. |
| 2228 | valgrind, or the pureperl or perl.third executables, please note that |
| 2229 | by default perl B<does not> explicitly cleanup all the memory it has |
| 2230 | allocated (such as global memory arenas) but instead lets the exit() |
| 2231 | of the whole program "take care" of such allocations, also known as |
| 2232 | "global destruction of objects". |
| 2233 | |
| 2234 | There is a way to tell perl to do complete cleanup: set the |
| 2235 | environment variable PERL_DESTRUCT_LEVEL to a non-zero value. |
| 2236 | The t/TEST wrapper does set this to 2, and this is what you |
| 2237 | need to do too, if you don't want to see the "global leaks": |
| 2238 | For example, for "third-degreed" Perl: |
| 2239 | |
| 2240 | env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t |
| 2241 | |
| 2242 | (Note: the mod_perl apache module uses also this environment variable |
| 2243 | for its own purposes and extended its semantics. Refer to the mod_perl |
| 2244 | documentation for more information. Also, spawned threads do the |
| 2245 | equivalent of setting this variable to the value 1.) |
| 2246 | |
| 2247 | If, at the end of a run you get the message I<N scalars leaked>, you can |
| 2248 | recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause |
| 2249 | the addresses of all those leaked SVs to be dumped; it also converts |
| 2250 | C<new_SV()> from a macro into a real function, so you can use your |
| 2251 | favourite debugger to discover where those pesky SVs were allocated. |
| 2252 | |
| 2253 | =head2 Profiling |
| 2254 | |
| 2255 | Depending on your platform there are various of profiling Perl. |
| 2256 | |
| 2257 | There are two commonly used techniques of profiling executables: |
| 2258 | I<statistical time-sampling> and I<basic-block counting>. |
| 2259 | |
| 2260 | The first method takes periodically samples of the CPU program |
| 2261 | counter, and since the program counter can be correlated with the code |
| 2262 | generated for functions, we get a statistical view of in which |
| 2263 | functions the program is spending its time. The caveats are that very |
| 2264 | small/fast functions have lower probability of showing up in the |
| 2265 | profile, and that periodically interrupting the program (this is |
| 2266 | usually done rather frequently, in the scale of milliseconds) imposes |
| 2267 | an additional overhead that may skew the results. The first problem |
| 2268 | can be alleviated by running the code for longer (in general this is a |
| 2269 | good idea for profiling), the second problem is usually kept in guard |
| 2270 | by the profiling tools themselves. |
| 2271 | |
| 2272 | The second method divides up the generated code into I<basic blocks>. |
| 2273 | Basic blocks are sections of code that are entered only in the |
| 2274 | beginning and exited only at the end. For example, a conditional jump |
| 2275 | starts a basic block. Basic block profiling usually works by |
| 2276 | I<instrumenting> the code by adding I<enter basic block #nnnn> |
| 2277 | book-keeping code to the generated code. During the execution of the |
| 2278 | code the basic block counters are then updated appropriately. The |
| 2279 | caveat is that the added extra code can skew the results: again, the |
| 2280 | profiling tools usually try to factor their own effects out of the |
| 2281 | results. |
| 2282 | |
| 2283 | =head2 Gprof Profiling |
| 2284 | |
| 2285 | gprof is a profiling tool available in many UNIX platforms, |
| 2286 | it uses F<statistical time-sampling>. |
| 2287 | |
| 2288 | You can build a profiled version of perl called "perl.gprof" by |
| 2289 | invoking the make target "perl.gprof" (What is required is that Perl |
| 2290 | must be compiled using the C<-pg> flag, you may need to re-Configure). |
| 2291 | Running the profiled version of Perl will create an output file called |
| 2292 | F<gmon.out> is created which contains the profiling data collected |
| 2293 | during the execution. |
| 2294 | |
| 2295 | The gprof tool can then display the collected data in various ways. |
| 2296 | Usually gprof understands the following options: |
| 2297 | |
| 2298 | =over 4 |
| 2299 | |
| 2300 | =item -a |
| 2301 | |
| 2302 | Suppress statically defined functions from the profile. |
| 2303 | |
| 2304 | =item -b |
| 2305 | |
| 2306 | Suppress the verbose descriptions in the profile. |
| 2307 | |
| 2308 | =item -e routine |
| 2309 | |
| 2310 | Exclude the given routine and its descendants from the profile. |
| 2311 | |
| 2312 | =item -f routine |
| 2313 | |
| 2314 | Display only the given routine and its descendants in the profile. |
| 2315 | |
| 2316 | =item -s |
| 2317 | |
| 2318 | Generate a summary file called F<gmon.sum> which then may be given |
| 2319 | to subsequent gprof runs to accumulate data over several runs. |
| 2320 | |
| 2321 | =item -z |
| 2322 | |
| 2323 | Display routines that have zero usage. |
| 2324 | |
| 2325 | =back |
| 2326 | |
| 2327 | For more detailed explanation of the available commands and output |
| 2328 | formats, see your own local documentation of gprof. |
| 2329 | |
| 2330 | =head2 GCC gcov Profiling |
| 2331 | |
| 2332 | Starting from GCC 3.0 I<basic block profiling> is officially available |
| 2333 | for the GNU CC. |
| 2334 | |
| 2335 | You can build a profiled version of perl called F<perl.gcov> by |
| 2336 | invoking the make target "perl.gcov" (what is required that Perl must |
| 2337 | be compiled using gcc with the flags C<-fprofile-arcs |
| 2338 | -ftest-coverage>, you may need to re-Configure). |
| 2339 | |
| 2340 | Running the profiled version of Perl will cause profile output to be |
| 2341 | generated. For each source file an accompanying ".da" file will be |
| 2342 | created. |
| 2343 | |
| 2344 | To display the results you use the "gcov" utility (which should |
| 2345 | be installed if you have gcc 3.0 or newer installed). F<gcov> is |
| 2346 | run on source code files, like this |
| 2347 | |
| 2348 | gcov sv.c |
| 2349 | |
| 2350 | which will cause F<sv.c.gcov> to be created. The F<.gcov> files |
| 2351 | contain the source code annotated with relative frequencies of |
| 2352 | execution indicated by "#" markers. |
| 2353 | |
| 2354 | Useful options of F<gcov> include C<-b> which will summarise the |
| 2355 | basic block, branch, and function call coverage, and C<-c> which |
| 2356 | instead of relative frequencies will use the actual counts. For |
| 2357 | more information on the use of F<gcov> and basic block profiling |
| 2358 | with gcc, see the latest GNU CC manual, as of GCC 3.0 see |
| 2359 | |
| 2360 | http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html |
| 2361 | |
| 2362 | and its section titled "8. gcov: a Test Coverage Program" |
| 2363 | |
| 2364 | http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132 |
| 2365 | |
| 2366 | =head2 Pixie Profiling |
| 2367 | |
| 2368 | Pixie is a profiling tool available on IRIX and Tru64 (aka Digital |
| 2369 | UNIX aka DEC OSF/1) platforms. Pixie does its profiling using |
| 2370 | I<basic-block counting>. |
| 2371 | |
| 2372 | You can build a profiled version of perl called F<perl.pixie> by |
| 2373 | invoking the make target "perl.pixie" (what is required is that Perl |
| 2374 | must be compiled using the C<-g> flag, you may need to re-Configure). |
| 2375 | |
| 2376 | In Tru64 a file called F<perl.Addrs> will also be silently created, |
| 2377 | this file contains the addresses of the basic blocks. Running the |
| 2378 | profiled version of Perl will create a new file called "perl.Counts" |
| 2379 | which contains the counts for the basic block for that particular |
| 2380 | program execution. |
| 2381 | |
| 2382 | To display the results you use the F<prof> utility. The exact |
| 2383 | incantation depends on your operating system, "prof perl.Counts" in |
| 2384 | IRIX, and "prof -pixie -all -L. perl" in Tru64. |
| 2385 | |
| 2386 | In IRIX the following prof options are available: |
| 2387 | |
| 2388 | =over 4 |
| 2389 | |
| 2390 | =item -h |
| 2391 | |
| 2392 | Reports the most heavily used lines in descending order of use. |
| 2393 | Useful for finding the hotspot lines. |
| 2394 | |
| 2395 | =item -l |
| 2396 | |
| 2397 | Groups lines by procedure, with procedures sorted in descending order of use. |
| 2398 | Within a procedure, lines are listed in source order. |
| 2399 | Useful for finding the hotspots of procedures. |
| 2400 | |
| 2401 | =back |
| 2402 | |
| 2403 | In Tru64 the following options are available: |
| 2404 | |
| 2405 | =over 4 |
| 2406 | |
| 2407 | =item -p[rocedures] |
| 2408 | |
| 2409 | Procedures sorted in descending order by the number of cycles executed |
| 2410 | in each procedure. Useful for finding the hotspot procedures. |
| 2411 | (This is the default option.) |
| 2412 | |
| 2413 | =item -h[eavy] |
| 2414 | |
| 2415 | Lines sorted in descending order by the number of cycles executed in |
| 2416 | each line. Useful for finding the hotspot lines. |
| 2417 | |
| 2418 | =item -i[nvocations] |
| 2419 | |
| 2420 | The called procedures are sorted in descending order by number of calls |
| 2421 | made to the procedures. Useful for finding the most used procedures. |
| 2422 | |
| 2423 | =item -l[ines] |
| 2424 | |
| 2425 | Grouped by procedure, sorted by cycles executed per procedure. |
| 2426 | Useful for finding the hotspots of procedures. |
| 2427 | |
| 2428 | =item -testcoverage |
| 2429 | |
| 2430 | The compiler emitted code for these lines, but the code was unexecuted. |
| 2431 | |
| 2432 | =item -z[ero] |
| 2433 | |
| 2434 | Unexecuted procedures. |
| 2435 | |
| 2436 | =back |
| 2437 | |
| 2438 | For further information, see your system's manual pages for pixie and prof. |
| 2439 | |
| 2440 | =head2 Miscellaneous tricks |
| 2441 | |
| 2442 | =over 4 |
| 2443 | |
| 2444 | =item * |
| 2445 | |
| 2446 | Those debugging perl with the DDD frontend over gdb may find the |
| 2447 | following useful: |
| 2448 | |
| 2449 | You can extend the data conversion shortcuts menu, so for example you |
| 2450 | can display an SV's IV value with one click, without doing any typing. |
| 2451 | To do that simply edit ~/.ddd/init file and add after: |
| 2452 | |
| 2453 | ! Display shortcuts. |
| 2454 | Ddd*gdbDisplayShortcuts: \ |
| 2455 | /t () // Convert to Bin\n\ |
| 2456 | /d () // Convert to Dec\n\ |
| 2457 | /x () // Convert to Hex\n\ |
| 2458 | /o () // Convert to Oct(\n\ |
| 2459 | |
| 2460 | the following two lines: |
| 2461 | |
| 2462 | ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\ |
| 2463 | ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx |
| 2464 | |
| 2465 | so now you can do ivx and pvx lookups or you can plug there the |
| 2466 | sv_peek "conversion": |
| 2467 | |
| 2468 | Perl_sv_peek(my_perl, (SV*)()) // sv_peek |
| 2469 | |
| 2470 | (The my_perl is for threaded builds.) |
| 2471 | Just remember that every line, but the last one, should end with \n\ |
| 2472 | |
| 2473 | Alternatively edit the init file interactively via: |
| 2474 | 3rd mouse button -> New Display -> Edit Menu |
| 2475 | |
| 2476 | Note: you can define up to 20 conversion shortcuts in the gdb |
| 2477 | section. |
| 2478 | |
| 2479 | =item * |
| 2480 | |
| 2481 | If you see in a debugger a memory area mysteriously full of 0xabababab, |
| 2482 | you may be seeing the effect of the Poison() macro, see L<perlclib>. |
| 2483 | |
| 2484 | =back |
| 2485 | |
| 2486 | =head2 CONCLUSION |
| 2487 | |
| 2488 | We've had a brief look around the Perl source, an overview of the stages |
| 2489 | F<perl> goes through when it's running your code, and how to use a |
| 2490 | debugger to poke at the Perl guts. We took a very simple problem and |
| 2491 | demonstrated how to solve it fully - with documentation, regression |
| 2492 | tests, and finally a patch for submission to p5p. Finally, we talked |
| 2493 | about how to use external tools to debug and test Perl. |
| 2494 | |
| 2495 | I'd now suggest you read over those references again, and then, as soon |
| 2496 | as possible, get your hands dirty. The best way to learn is by doing, |
| 2497 | so: |
| 2498 | |
| 2499 | =over 3 |
| 2500 | |
| 2501 | =item * |
| 2502 | |
| 2503 | Subscribe to perl5-porters, follow the patches and try and understand |
| 2504 | them; don't be afraid to ask if there's a portion you're not clear on - |
| 2505 | who knows, you may unearth a bug in the patch... |
| 2506 | |
| 2507 | =item * |
| 2508 | |
| 2509 | Keep up to date with the bleeding edge Perl distributions and get |
| 2510 | familiar with the changes. Try and get an idea of what areas people are |
| 2511 | working on and the changes they're making. |
| 2512 | |
| 2513 | =item * |
| 2514 | |
| 2515 | Do read the README associated with your operating system, e.g. README.aix |
| 2516 | on the IBM AIX OS. Don't hesitate to supply patches to that README if |
| 2517 | you find anything missing or changed over a new OS release. |
| 2518 | |
| 2519 | =item * |
| 2520 | |
| 2521 | Find an area of Perl that seems interesting to you, and see if you can |
| 2522 | work out how it works. Scan through the source, and step over it in the |
| 2523 | debugger. Play, poke, investigate, fiddle! You'll probably get to |
| 2524 | understand not just your chosen area but a much wider range of F<perl>'s |
| 2525 | activity as well, and probably sooner than you'd think. |
| 2526 | |
| 2527 | =back |
| 2528 | |
| 2529 | =over 3 |
| 2530 | |
| 2531 | =item I<The Road goes ever on and on, down from the door where it began.> |
| 2532 | |
| 2533 | =back |
| 2534 | |
| 2535 | If you can do these things, you've started on the long road to Perl porting. |
| 2536 | Thanks for wanting to help make Perl better - and happy hacking! |
| 2537 | |
| 2538 | =head1 AUTHOR |
| 2539 | |
| 2540 | This document was written by Nathan Torkington, and is maintained by |
| 2541 | the perl5-porters mailing list. |
| 2542 | |