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Manpages PERLHACKSection: Perl Programmers Reference Guide (1)Updated: 2005-03-28 Index Return to Main Contents NAMEperlhack - How to hack at the Perl internalsDESCRIPTIONThis document attempts to explain how Perl development takes place, and ends with some suggestions for people wanting to become bona fide porters.The perl5-porters mailing list is where the Perl standard distribution is maintained and developed. The list can get anywhere from 10 to 150 messages a day, depending on the heatedness of the debate. Most days there are two or three patches, extensions, features, or bugs being discussed at a time. A searchable archive of the list is at:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/The list is also archived under the usenet group name "perl.porters-gw" at:
http://www.deja.com/List subscribers (the porters themselves) come in several flavours. Some are quiet curious lurkers, who rarely pitch in and instead watch the ongoing development to ensure they're forewarned of new changes or features in Perl. Some are representatives of vendors, who are there to make sure that Perl continues to compile and work on their platforms. Some patch any reported bug that they know how to fix, some are actively patching their pet area (threads, Win32, the regexp engine), while others seem to do nothing but complain. In other words, it's your usual mix of technical people. Over this group of porters presides Larry Wall. He has the final word in what does and does not change in the Perl language. Various releases of Perl are shepherded by a ``pumpking'', a porter responsible for gathering patches, deciding on a patch-by-patch feature-by-feature basis what will and will not go into the release. For instance, Gurusamy Sarathy is the pumpking for the 5.6 release of Perl. In addition, various people are pumpkings for different things. For instance, Andy Dougherty and Jarkko Hietaniemi share the Configure pumpkin, and Tom Christiansen is the documentation pumpking. Larry sees Perl development along the lines of the US government: there's the Legislature (the porters), the Executive branch (the pumpkings), and the Supreme Court (Larry). The legislature can discuss and submit patches to the executive branch all they like, but the executive branch is free to veto them. Rarely, the Supreme Court will side with the executive branch over the legislature, or the legislature over the executive branch. Mostly, however, the legislature and the executive branch are supposed to get along and work out their differences without impeachment or court cases. You might sometimes see reference to Rule 1 and Rule 2. Larry's power as Supreme Court is expressed in The Rules:
Got that? Larry is always right, even when he was wrong. It's rare to see either Rule exercised, but they are often alluded to. New features and extensions to the language are contentious, because the criteria used by the pumpkings, Larry, and other porters to decide which features should be implemented and incorporated are not codified in a few small design goals as with some other languages. Instead, the heuristics are flexible and often difficult to fathom. Here is one person's list, roughly in decreasing order of importance, of heuristics that new features have to be weighed against:
If you're on the list, you might hear the word ``core'' bandied around. It refers to the standard distribution. ``Hacking on the core'' means you're changing the C source code to the Perl interpreter. ``A core module'' is one that ships with Perl. Keeping in syncThe source code to the Perl interpreter, in its different versions, is kept in a repository managed by a revision control system (which is currently the Perforce program, see http://perforce.com/). The pumpkings and a few others have access to the repository to check in changes. Periodically the pumpking for the development version of Perl will release a new version, so the rest of the porters can see what's changed. The current state of the main trunk of repository, and patches that describe the individual changes that have happened since the last public release are available at this location:
ftp://ftp.linux.activestate.com/pub/staff/gsar/APC/If you are a member of the perl5-porters mailing list, it is a good thing to keep in touch with the most recent changes. If not only to verify if what you would have posted as a bug report isn't already solved in the most recent available perl development branch, also known as perl-current, bleading edge perl, bleedperl or bleadperl. Needless to say, the source code in perl-current is usually in a perpetual state of evolution. You should expect it to be very buggy. Do not use it for any purpose other than testing and development. Keeping in sync with the most recent branch can be done in several ways, but the most convenient and reliable way is using rsync, available at ftp://rsync.samba.org/pub/rsync/ . (You can also get the most recent branch by FTP.) If you choose to keep in sync using rsync, there are two approaches to doing so:
Why rsync the source tree
Why rsync the patches
If you want to get the best of both worlds, rsync both the source tree for convenience, reliability and ease and rsync the patches for reference. Submitting patchesAlways submit patches to perl5-porters@perl.org. This lets other porters review your patch, which catches a surprising number of errors in patches. Either use the diff program (available in source code form from ftp://ftp.gnu.org/pub/gnu/), or use Johan Vromans' makepatch (available from CPAN/authors/id/JV/). Unified diffs are preferred, but context diffs are accepted. Do not send RCS-style diffs or diffs without context lines. More information is given in the Porting/patching.pod file in the Perl source distribution. Please patch against the latest development version (e.g., if you're fixing a bug in the 5.005 track, patch against the latest 5.005_5x version). Only patches that survive the heat of the development branch get applied to maintenance versions.Your patch should update the documentation and test suite. To report a bug in Perl, use the program perlbug which comes with Perl (if you can't get Perl to work, send mail to the address perlbug@perl.org or perlbug@perl.com). Reporting bugs through perlbug feeds into the automated bug-tracking system, access to which is provided through the web at http://bugs.perl.org/. It often pays to check the archives of the perl5-porters mailing list to see whether the bug you're reporting has been reported before, and if so whether it was considered a bug. See above for the location of the searchable archives. The CPAN testers (http://testers.cpan.org/) are a group of volunteers who test CPAN modules on a variety of platforms. Perl Labs (http://labs.perl.org/) automatically tests Perl source releases on platforms and gives feedback to the CPAN testers mailing list. Both efforts welcome volunteers. It's a good idea to read and lurk for a while before chipping in. That way you'll get to see the dynamic of the conversations, learn the personalities of the players, and hopefully be better prepared to make a useful contribution when do you speak up. If after all this you still think you want to join the perl5-porters mailing list, send mail to perl5-porters-subscribe@perl.org. To unsubscribe, send mail to perl5-porters-unsubscribe@perl.org. To hack on the Perl guts, you'll need to read the following things:
Finding Your Way AroundPerl maintenance can be split into a number of areas, and certain people (pumpkins) will have responsibility for each area. These areas sometimes correspond to files or directories in the source kit. Among the areas are:
Before we leave looking at the layout, though, don't forget that MANIFEST contains not only the file names in the Perl distribution, but short descriptions of what's in them, too. For an overview of the important files, try this:
perl -lne 'print if /^[^\/]+\.[ch]\s+/' MANIFEST Elements of the interpreterThe work of the interpreter has two main stages: compiling the code into the internal representation, or bytecode, and then executing it. ``Compiled code'' in perlguts explains exactly how the compilation stage happens.Here is a short breakdown of perl's operation:
Internal Variable TypesYou should by now have had a look at perlguts, which tells you about Perl's internal variable types: SVs, HVs, AVs and the rest. If not, do that now.These variables are used not only to represent Perl-space variables, but also any constants in the code, as well as some structures completely internal to Perl. The symbol table, for instance, is an ordinary Perl hash. Your code is represented by an SV as it's read into the parser; any program files you call are opened via ordinary Perl filehandles, and so on. The core Devel::Peek module lets us examine SVs from a Perl program. Let's see, for instance, how Perl treats the constant "hello".
% perl -MDevel::Peek -e 'Dump("hello")' 1 SV = PV(0xa041450) at 0xa04ecbc 2 REFCNT = 1 3 FLAGS = (POK,READONLY,pPOK) 4 PV = 0xa0484e0 "hello"\0 5 CUR = 5 6 LEN = 6Reading "Devel::Peek" output takes a bit of practise, so let's go through it line by line. Line 1 tells us we're looking at an SV which lives at 0xa04ecbc in memory. SVs themselves are very simple structures, but they contain a pointer to a more complex structure. In this case, it's a PV, a structure which holds a string value, at location 0xa041450. Line 2 is the reference count; there are no other references to this data, so it's 1. Line 3 are the flags for this SV - it's OK to use it as a PV, it's a read-only SV (because it's a constant) and the data is a PV internally. Next we've got the contents of the string, starting at location 0xa0484e0. Line 5 gives us the current length of the string - note that this does not include the null terminator. Line 6 is not the length of the string, but the length of the currently allocated buffer; as the string grows, Perl automatically extends the available storage via a routine called "SvGROW". You can get at any of these quantities from C very easily; just add "Sv" to the name of the field shown in the snippet, and you've got a macro which will return the value: "SvCUR(sv)" returns the current length of the string, "SvREFCOUNT(sv)" returns the reference count, "SvPV(sv, len)" returns the string itself with its length, and so on. More macros to manipulate these properties can be found in perlguts. Let's take an example of manipulating a PV, from "sv_catpvn", in sv.c
1 void 2 Perl_sv_catpvn(pTHX_ register SV *sv, register const char *ptr, register STRLEN len) 3 { 4 STRLEN tlen; 5 char *junk; 6 junk = SvPV_force(sv, tlen); 7 SvGROW(sv, tlen + len + 1); 8 if (ptr == junk) 9 ptr = SvPVX(sv); 10 Move(ptr,SvPVX(sv)+tlen,len,char); 11 SvCUR(sv) += len; 12 *SvEND(sv) = '\0'; 13 (void)SvPOK_only_UTF8(sv); /* validate pointer */ 14 SvTAINT(sv); 15 }This is a function which adds a string, "ptr", of length "len" onto the end of the PV stored in "sv". The first thing we do in line 6 is make sure that the SV has a valid PV, by calling the "SvPV_force" macro to force a PV. As a side effect, "tlen" gets set to the current value of the PV, and the PV itself is returned to "junk". In line 7, we make sure that the SV will have enough room to accommodate the old string, the new string and the null terminator. If "LEN" isn't big enough, "SvGROW" will reallocate space for us. Now, if "junk" is the same as the string we're trying to add, we can grab the string directly from the SV; "SvPVX" is the address of the PV in the SV. Line 10 does the actual catenation: the "Move" macro moves a chunk of memory around: we move the string "ptr" to the end of the PV - that's the start of the PV plus its current length. We're moving "len" bytes of type "char". After doing so, we need to tell Perl we've extended the string, by altering "CUR" to reflect the new length. "SvEND" is a macro which gives us the end of the string, so that needs to be a "\0". Line 13 manipulates the flags; since we've changed the PV, any IV or NV values will no longer be valid: if we have "$a=10; $a.="6";" we don't want to use the old IV of 10. "SvPOK_only_utf8" is a special UTF8-aware version of "SvPOK_only", a macro which turns off the IOK and NOK flags and turns on POK. The final "SvTAINT" is a macro which launders tainted data if taint mode is turned on. AVs and HVs are more complicated, but SVs are by far the most common variable type being thrown around. Having seen something of how we manipulate these, let's go on and look at how the op tree is constructed. [Trees]First, what is the op tree, anyway? The op tree is the parsed representation of your program, as we saw in our section on parsing, and it's the sequence of operations that Perl goes through to execute your program, as we saw in ``Running''.An op is a fundamental operation that Perl can perform: all the built-in functions and operators are ops, and there are a series of ops which deal with concepts the interpreter needs internally - entering and leaving a block, ending a statement, fetching a variable, and so on. The op tree is connected in two ways: you can imagine that there are two ``routes'' through it, two orders in which you can traverse the tree. First, parse order reflects how the parser understood the code, and secondly, execution order tells perl what order to perform the operations in. The easiest way to examine the op tree is to stop Perl after it has finished parsing, and get it to dump out the tree. This is exactly what the compiler backends B::Terse and B::Debug do. Let's have a look at how Perl sees "$a = $b + $c":
% perl -MO=Terse -e '$a=$b+$c' 1 LISTOP (0x8179888) leave 2 OP (0x81798b0) enter 3 COP (0x8179850) nextstate 4 BINOP (0x8179828) sassign 5 BINOP (0x8179800) add [1] 6 UNOP (0x81796e0) null [15] 7 SVOP (0x80fafe0) gvsv GV (0x80fa4cc) *b 8 UNOP (0x81797e0) null [15] 9 SVOP (0x8179700) gvsv GV (0x80efeb0) *c 10 UNOP (0x816b4f0) null [15] 11 SVOP (0x816dcf0) gvsv GV (0x80fa460) *aLet's start in the middle, at line 4. This is a BINOP, a binary operator, which is at location 0x8179828. The specific operator in question is "sassign" - scalar assignment - and you can find the code which implements it in the function "pp_sassign" in pp_hot.c. As a binary operator, it has two children: the add operator, providing the result of "$b+$c", is uppermost on line 5, and the left hand side is on line 10. Line 10 is the null op: this does exactly nothing. What is that doing there? If you see the null op, it's a sign that something has been optimized away after parsing. As we mentioned in ``Optimization'', the optimization stage sometimes converts two operations into one, for example when fetching a scalar variable. When this happens, instead of rewriting the op tree and cleaning up the dangling pointers, it's easier just to replace the redundant operation with the null op. Originally, the tree would have looked like this:
10 SVOP (0x816b4f0) rv2sv [15] 11 SVOP (0x816dcf0) gv GV (0x80fa460) *aThat is, fetch the "a" entry from the main symbol table, and then look at the scalar component of it: "gvsv" ("pp_gvsv" into pp_hot.c) happens to do both these things. The right hand side, starting at line 5 is similar to what we've just seen: we have the "add" op ("pp_add" also in pp_hot.c) add together two "gvsv"s. Now, what's this about?
1 LISTOP (0x8179888) leave 2 OP (0x81798b0) enter 3 COP (0x8179850) nextstate"enter" and "leave" are scoping ops, and their job is to perform any housekeeping every time you enter and leave a block: lexical variables are tidied up, unreferenced variables are destroyed, and so on. Every program will have those first three lines: "leave" is a list, and its children are all the statements in the block. Statements are delimited by "nextstate", so a block is a collection of "nextstate" ops, with the ops to be performed for each statement being the children of "nextstate". "enter" is a single op which functions as a marker. That's how Perl parsed the program, from top to bottom:
Program | Statement | = / \ / \ $a + / \ $b $cHowever, it's impossible to perform the operations in this order: you have to find the values of $b and $c before you add them together, for instance. So, the other thread that runs through the op tree is the execution order: each op has a field "op_next" which points to the next op to be run, so following these pointers tells us how perl executes the code. We can traverse the tree in this order using the "exec" option to "B::Terse":
% perl -MO=Terse,exec -e '$a=$b+$c' 1 OP (0x8179928) enter 2 COP (0x81798c8) nextstate 3 SVOP (0x81796c8) gvsv GV (0x80fa4d4) *b 4 SVOP (0x8179798) gvsv GV (0x80efeb0) *c 5 BINOP (0x8179878) add [1] 6 SVOP (0x816dd38) gvsv GV (0x80fa468) *a 7 BINOP (0x81798a0) sassign 8 LISTOP (0x8179900) leaveThis probably makes more sense for a human: enter a block, start a statement. Get the values of $b and $c, and add them together. Find $a, and assign one to the other. Then leave. The way Perl builds up these op trees in the parsing process can be unravelled by examining perly.y, the YACC grammar. Let's take the piece we need to construct the tree for "$a = $b + $c"
1 term : term ASSIGNOP term 2 { $$ = newASSIGNOP(OPf_STACKED, $1, $2, $3); } 3 | term ADDOP term 4 { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }If you're not used to reading BNF grammars, this is how it works: You're fed certain things by the tokeniser, which generally end up in upper case. Here, "ADDOP", is provided when the tokeniser sees "+" in your code. "ASSIGNOP" is provided when "=" is used for assigning. These are `terminal symbols', because you can't get any simpler than them. The grammar, lines one and three of the snippet above, tells you how to build up more complex forms. These complex forms, `non-terminal symbols' are generally placed in lower case. "term" here is a non-terminal symbol, representing a single expression. The grammar gives you the following rule: you can make the thing on the left of the colon if you see all the things on the right in sequence. This is called a ``reduction'', and the aim of parsing is to completely reduce the input. There are several different ways you can perform a reduction, separated by vertical bars: so, "term" followed by "=" followed by "term" makes a "term", and "term" followed by "+" followed by "term" can also make a "term". So, if you see two terms with an "=" or "+", between them, you can turn them into a single expression. When you do this, you execute the code in the block on the next line: if you see "=", you'll do the code in line 2. If you see "+", you'll do the code in line 4. It's this code which contributes to the op tree.
| term ADDOP term { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }What this does is creates a new binary op, and feeds it a number of variables. The variables refer to the tokens: $1 is the first token in the input, $2 the second, and so on - think regular expression backreferences. $$ is the op returned from this reduction. So, we call "newBINOP" to create a new binary operator. The first parameter to "newBINOP", a function in op.c, is the op type. It's an addition operator, so we want the type to be "ADDOP". We could specify this directly, but it's right there as the second token in the input, so we use $2. The second parameter is the op's flags: 0 means `nothing special'. Then the things to add: the left and right hand side of our expression, in scalar context. StacksWhen perl executes something like "addop", how does it pass on its results to the next op? The answer is, through the use of stacks. Perl has a number of stacks to store things it's currently working on, and we'll look at the three most important ones here.
Millions of MacrosOne thing you'll notice about the Perl source is that it's full of macros. Some have called the pervasive use of macros the hardest thing to understand, others find it adds to clarity. Let's take an example, the code which implements the addition operator:
1 PP(pp_add) 2 { 3 dSP; dATARGET; tryAMAGICbin(add,opASSIGN); 4 { 5 dPOPTOPnnrl_ul; 6 SETn( left + right ); 7 RETURN; 8 } 9 }Every line here (apart from the braces, of course) contains a macro. The first line sets up the function declaration as Perl expects for PP code; line 3 sets up variable declarations for the argument stack and the target, the return value of the operation. Finally, it tries to see if the addition operation is overloaded; if so, the appropriate subroutine is called. Line 5 is another variable declaration - all variable declarations start with "d" - which pops from the top of the argument stack two NVs (hence "nn") and puts them into the variables "right" and "left", hence the "rl". These are the two operands to the addition operator. Next, we call "SETn" to set the NV of the return value to the result of adding the two values. This done, we return - the "RETURN" macro makes sure that our return value is properly handled, and we pass the next operator to run back to the main run loop. Most of these macros are explained in perlapi, and some of the more important ones are explained in perlxs as well. Pay special attention to ``Background and PERL_IMPLICIT_CONTEXT'' in perlguts for information on the "[pad]THX_?" macros. Poking at PerlTo really poke around with Perl, you'll probably want to build Perl for debugging, like this:
./Configure -d -D optimize=-g make"-g" is a flag to the C compiler to have it produce debugging information which will allow us to step through a running program. Configure will also turn on the "DEBUGGING" compilation symbol which enables all the internal debugging code in Perl. There are a whole bunch of things you can debug with this: perlrun lists them all, and the best way to find out about them is to play about with them. The most useful options are probably
l Context (loop) stack processing t Trace execution o Method and overloading resolution c String/numeric conversionsSome of the functionality of the debugging code can be achieved using XS modules.
-Dr => use re 'debug' -Dx => use O 'Debug' Using a source-level debuggerIf the debugging output of "-D" doesn't help you, it's time to step through perl's execution with a source-level debugger.
To fire up the debugger, type
gdb ./perlYou'll want to do that in your Perl source tree so the debugger can read the source code. You should see the copyright message, followed by the prompt.
(gdb)"help" will get you into the documentation, but here are the most useful commands:
Dumping Perl Data StructuresOne way to get around this macro hell is to use the dumping functions in dump.c; these work a little like an internal Devel::Peek, but they also cover OPs and other structures that you can't get at from Perl. Let's take an example. We'll use the "$a = $b + $c" we used before, but give it a bit of context: "$b = "6XXXX"; $c = 2.3;". Where's a good place to stop and poke around?What about "pp_add", the function we examined earlier to implement the "+" operator:
(gdb) break Perl_pp_add Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.Notice we use "Perl_pp_add" and not "pp_add" - see ``Internal Functions'' in perlguts. With the breakpoint in place, we can run our program:
(gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'Lots of junk will go past as gdb reads in the relevant source files and libraries, and then:
Breakpoint 1, Perl_pp_add () at pp_hot.c:309 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN); (gdb) step 311 dPOPTOPnnrl_ul; (gdb)We looked at this bit of code before, and we said that "dPOPTOPnnrl_ul" arranges for two "NV"s to be placed into "left" and "right" - let's slightly expand it:
#define dPOPTOPnnrl_ul NV right = POPn; \ SV *leftsv = TOPs; \ NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0"POPn" takes the SV from the top of the stack and obtains its NV either directly (if "SvNOK" is set) or by calling the "sv_2nv" function. "TOPs" takes the next SV from the top of the stack - yes, "POPn" uses "TOPs" - but doesn't remove it. We then use "SvNV" to get the NV from "leftsv" in the same way as before - yes, "POPn" uses "SvNV". Since we don't have an NV for $b, we'll have to use "sv_2nv" to convert it. If we step again, we'll find ourselves there:
Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669 1669 if (!sv) (gdb)We can now use "Perl_sv_dump" to investigate the SV:
SV = PV(0xa057cc0) at 0xa0675d0 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0xa06a510 "6XXXX"\0 CUR = 5 LEN = 6 $1 = voidWe know we're going to get 6 from this, so let's finish the subroutine:
(gdb) finish Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671 0x462669 in Perl_pp_add () at pp_hot.c:311 311 dPOPTOPnnrl_ul;We can also dump out this op: the current op is always stored in "PL_op", and we can dump it with "Perl_op_dump". This'll give us similar output to B::Debug.
{ 13 TYPE = add ===> 14 TARG = 1 FLAGS = (SCALAR,KIDS) { TYPE = null ===> (12) (was rv2sv) FLAGS = (SCALAR,KIDS) { 11 TYPE = gvsv ===> 12 FLAGS = (SCALAR) GV = main::b } }< finish this later > PatchingAll right, we've now had a look at how to navigate the Perl sources and some things you'll need to know when fiddling with them. Let's now get on and create a simple patch. Here's something Larry suggested: if a "U" is the first active format during a "pack", (for example, "pack "U3C8", @stuff") then the resulting string should be treated as UTF8 encoded.How do we prepare to fix this up? First we locate the code in question - the "pack" happens at runtime, so it's going to be in one of the pp files. Sure enough, "pp_pack" is in pp.c. Since we're going to be altering this file, let's copy it to pp.c~. Now let's look over "pp_pack": we take a pattern into "pat", and then loop over the pattern, taking each format character in turn into "datum_type". Then for each possible format character, we swallow up the other arguments in the pattern (a field width, an asterisk, and so on) and convert the next chunk input into the specified format, adding it onto the output SV "cat". How do we know if the "U" is the first format in the "pat"? Well, if we have a pointer to the start of "pat" then, if we see a "U" we can test whether we're still at the start of the string. So, here's where "pat" is set up:
STRLEN fromlen; register char *pat = SvPVx(*++MARK, fromlen); register char *patend = pat + fromlen; register I32 len; I32 datumtype; SV *fromstr;We'll have another string pointer in there:
STRLEN fromlen; register char *pat = SvPVx(*++MARK, fromlen); register char *patend = pat + fromlen; + char *patcopy; register I32 len; I32 datumtype; SV *fromstr;And just before we start the loop, we'll set "patcopy" to be the start of "pat":
items = SP - MARK; MARK++; sv_setpvn(cat, "", 0); + patcopy = pat; while (pat < patend) {Now if we see a "U" which was at the start of the string, we turn on the UTF8 flag for the output SV, "cat":
+ if (datumtype == 'U' && pat==patcopy+1) + SvUTF8_on(cat); if (datumtype == '#') { while (pat < patend && *pat != '\n') pat++;Remember that it has to be "patcopy+1" because the first character of the string is the "U" which has been swallowed into "datumtype!" Oops, we forgot one thing: what if there are spaces at the start of the pattern? "pack(" U*", @stuff)" will have "U" as the first active character, even though it's not the first thing in the pattern. In this case, we have to advance "patcopy" along with "pat" when we see spaces:
if (isSPACE(datumtype)) continue;needs to become
if (isSPACE(datumtype)) { patcopy++; continue; }OK. That's the C part done. Now we must do two additional things before this patch is ready to go: we've changed the behaviour of Perl, and so we must document that change. We must also provide some more regression tests to make sure our patch works and doesn't create a bug somewhere else along the line. The regression tests for each operator live in t/op/, and so we make a copy of t/op/pack.t to t/op/pack.t~. Now we can add our tests to the end. First, we'll test that the "U" does indeed create Unicode strings:
print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack("U*",1,20,300,4000); print "ok $test\n"; $test++;Now we'll test that we got that space-at-the-beginning business right:
print 'not ' unless "1.20.300.4000" eq sprintf "%vd", pack(" U*",1,20,300,4000); print "ok $test\n"; $test++;And finally we'll test that we don't make Unicode strings if "U" is not the first active format:
print 'not ' unless v1.20.300.4000 ne sprintf "%vd", pack("C0U*",1,20,300,4000); print "ok $test\n"; $test++;Mustn't forget to change the number of tests which appears at the top, or else the automated tester will get confused:
-print "1..156\n"; +print "1..159\n";We now compile up Perl, and run it through the test suite. Our new tests pass, hooray! Finally, the documentation. The job is never done until the paperwork is over, so let's describe the change we've just made. The relevant place is pod/perlfunc.pod; again, we make a copy, and then we'll insert this text in the description of "pack":
=item * If the pattern begins with a C<U>, the resulting string will be treated as Unicode-encoded. You can force UTF8 encoding on in a string with an initial C<U0>, and the bytes that follow will be interpreted as Unicode characters. If you don't want this to happen, you can begin your pattern with C<C0> (or anything else) to force Perl not to UTF8 encode your string, and then follow this with a C<U*> somewhere in your pattern.All done. Now let's create the patch. Porting/patching.pod tells us that if we're making major changes, we should copy the entire directory to somewhere safe before we begin fiddling, and then do
diff -ruN old new > patchHowever, we know which files we've changed, and we can simply do this:
diff -u pp.c~ pp.c > patch diff -u t/op/pack.t~ t/op/pack.t >> patch diff -u pod/perlfunc.pod~ pod/perlfunc.pod >> patchWe end up with a patch looking a little like this:
--- pp.c~ Fri Jun 02 04:34:10 2000 +++ pp.c Fri Jun 16 11:37:25 2000 @@ -4375,6 +4375,7 @@ register I32 items; STRLEN fromlen; register char *pat = SvPVx(*++MARK, fromlen); + char *patcopy; register char *patend = pat + fromlen; register I32 len; I32 datumtype; @@ -4405,6 +4406,7 @@ ...And finally, we submit it, with our rationale, to perl5-porters. Job done! EXTERNAL TOOLS FOR DEBUGGING PERLSometimes it helps to use external tools while debugging and testing Perl. This section tries to guide you through using some common testing and debugging tools with Perl. This is meant as a guide to interfacing these tools with Perl, not as any kind of guide to the use of the tools themselves.Rational Software's PurifyPurify is a commercial tool that is helpful in identifying memory overruns, wild pointers, memory leaks and other such badness. Perl must be compiled in a specific way for optimal testing with Purify. Purify is available under Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.The only currently known leaks happen when there are compile-time errors within eval or require. (Fixing these is non-trivial, unfortunately, but they must be fixed eventually.) Purify on UnixOn Unix, Purify creates a new Perl binary. To get the most benefit out of Purify, you should create the perl to Purify using:
sh Configure -Accflags=-DPURIFY -Doptimize='-g' \ -Uusemymalloc -Dusemultiplicitywhere these arguments mean:
Once you've compiled a perl suitable for Purify'ing, then you can just:
make pureperlwhich creates a binary named 'pureperl' that has been Purify'ed. This binary is used in place of the standard 'perl' binary when you want to debug Perl memory problems. As an example, to show any memory leaks produced during the standard Perl testset you would create and run the Purify'ed perl as:
make pureperl cd t ../pureperl -I../lib harnesswhich would run Perl on test.pl and report any memory problems. Purify outputs messages in ``Viewer'' windows by default. If you don't have a windowing environment or if you simply want the Purify output to unobtrusively go to a log file instead of to the interactive window, use these following options to output to the log file ``perl.log'':
setenv PURIFYOPTIONS "-chain-length=25 -windows=no \ -log-file=perl.log -append-logfile=yes"If you plan to use the ``Viewer'' windows, then you only need this option:
setenv PURIFYOPTIONS "-chain-length=25" Purify on NTPurify on Windows NT instruments the Perl binary 'perl.exe' on the fly. There are several options in the makefile you should change to get the most use out of Purify:
As an example, to show any memory leaks produced during the standard Perl testset you would create and run Purify as:
cd win32 make cd ../t purify ../perl -I../lib harnesswhich would instrument Perl in memory, run Perl on test.pl, then finally report any memory problems. CONCLUSIONWe've had a brief look around the Perl source, an overview of the stages perl goes through when it's running your code, and how to use a debugger to poke at the Perl guts. We took a very simple problem and demonstrated how to solve it fully - with documentation, regression tests, and finally a patch for submission to p5p. Finally, we talked about how to use external tools to debug and test Perl.I'd now suggest you read over those references again, and then, as soon as possible, get your hands dirty. The best way to learn is by doing, so:
If you can do these things, you've started on the long road to Perl porting. Thanks for wanting to help make Perl better - and happy hacking! AUTHORThis document was written by Nathan Torkington, and is maintained by the perl5-porters mailing list.
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