which will be private. All functions whose names begin C<S_> are private
(think "S" for "secret" or "static"). All other functions begin with
"Perl_", but just because a function begins with "Perl_" does not mean it is
-part of the API. The easiest way to be B<sure> a function is part of the API
-is to find its entry in L<perlapi>. If it exists in L<perlapi>, it's part
-of the API. If it doesn't, and you think it should be (i.e., you need it for
-your extension), send mail via L<perlbug> explaining why you think it
-should be.
-
-(L<perlapi> itself is generated by embed.pl, a Perl script that generates
-significant portions of the Perl source code. It has a list of almost
-all the functions defined by the Perl interpreter along with their calling
-characteristics and some flags. Functions that are part of the public API
-are marked with an 'A' in its flags.)
+part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
+function is part of the API is to find its entry in L<perlapi>.
+If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
+think it should be (i.e., you need it for your extension), send mail via
+L<perlbug> explaining why you think it should be.
Second problem: there must be a syntax so that the same subroutine
declarations and calls can pass a structure as their first argument,
There could be one or more interpreters in a process, and one or
more "hosts", with free association between them.
+=head1 Internal Functions
+
+All of Perl's internal functions which will be exposed to the outside
+world are be prefixed by C<Perl_> so that they will not conflict with XS
+functions or functions used in a program in which Perl is embedded.
+Similarly, all global variables begin with C<PL_>. (By convention,
+static functions start with C<S_>)
+
+Inside the Perl core, you can get at the functions either with or
+without the C<Perl_> prefix, thanks to a bunch of defines that live in
+F<embed.h>. This header file is generated automatically from
+F<embed.pl>. F<embed.pl> also creates the prototyping header files for
+the internal functions, generates the documentation and a lot of other
+bits and pieces. It's important that when you add a new function to the
+core or change an existing one, you change the data in the table at the
+end of F<embed.pl> as well. Here's a sample entry from that table:
+
+ Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
+
+The second column is the return type, the third column the name. Columns
+after that are the arguments. The first column is a set of flags:
+
+=over 3
+
+=item A
+
+This function is a part of the public API.
+
+=item p
+
+This function has a C<Perl_> prefix; ie, it is defined as C<Perl_av_fetch>
+
+=item d
+
+This function has documentation using the C<apidoc> feature which we'll
+look at in a second.
+
+=back
+
+Other available flags are:
+
+=over 3
+
+=item s
+
+This is a static function and is defined as C<S_whatever>.
+
+=item n
+
+This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See
+L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
+
+=item r
+
+This function never returns; C<croak>, C<exit> and friends.
+
+=item f
+
+This function takes a variable number of arguments, C<printf> style.
+The argument list should end with C<...>, like this:
+
+ Afprd |void |croak |const char* pat|...
+
+=item m
+
+This function is part of the experimental development API, and may change
+or disappear without notice.
+
+=item o
+
+This function should not have a compatibility macro to define, say,
+C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
+
+=item j
+
+This function is not a member of C<CPerlObj>. If you don't know
+what this means, don't use it.
+
+=item x
+
+This function isn't exported out of the Perl core.
+
+=back
+
+If you edit F<embed.pl>, you will need to run C<make regen_headers> to
+force a rebuild of F<embed.h> and other auto-generated files.
+
+=head2 Source Documentation
+
+There's an effort going on to document the internal functions and
+automatically produce reference manuals from them - L<perlapi> is one
+such manual which details all the functions which are available to XS
+writers. L<perlintern> is the autogenerated manual for the functions
+which are not part of the API and are supposedly for internal use only.
+
+Source documentation is created by putting POD comments into the C
+source, like this:
+
+ /*
+ =for apidoc sv_setiv
+
+ Copies an integer into the given SV. Does not handle 'set' magic. See
+ C<sv_setiv_mg>.
+
+ =cut
+ */
+
+Please try and supply some documentation if you add functions to the
+Perl core.
+
+=head1 Unicode Support
+
+Perl 5.6.0 introduced Unicode support. It's important for porters and XS
+writers to understand this support and make sure that the code they
+write does not corrupt Unicode data.
+
+=head2 What B<is> Unicode, anyway?
+
+In the olden, less enlightened times, we all used to use ASCII. Most of
+us did, anyway. The big problem with ASCII is that it's American. Well,
+no, that's not actually the problem; the problem is that it's not
+particularly useful for people who don't use the Roman alphabet. What
+used to happen was that particular languages would stick their own
+alphabet in the upper range of the sequence, between 128 and 255. Of
+course, we then ended up with plenty of variants that weren't quite
+ASCII, and the whole point of it being a standard was lost.
+
+Worse still, if you've got a language like Chinese or
+Japanese that has hundreds or thousands of characters, then you really
+can't fit them into a mere 256, so they had to forget about ASCII
+altogether, and build their own systems using pairs of numbers to refer
+to one character.
+
+To fix this, some people formed Unicode, Inc. and
+produced a new character set containing all the characters you can
+possibly think of and more. There are several ways of representing these
+characters, and the one Perl uses is called UTF8. UTF8 uses
+a variable number of bytes to represent a character, instead of just
+one. You can learn more about Unicode at
+L<http://www.unicode.org/|http://www.unicode.org/>
+
+=head2 How can I recognise a UTF8 string?
+
+You can't. This is because UTF8 data is stored in bytes just like
+non-UTF8 data. The Unicode character 200, (C<0xC8> for you hex types)
+capital E with a grave accent, is represented by the two bytes
+C<v196.172>. Unfortunately, the non-Unicode string C<chr(196).chr(172)>
+has that byte sequence as well. So you can't tell just by looking - this
+is what makes Unicode input an interesting problem.
+
+The API function C<is_utf8_string> can help; it'll tell you if a string
+contains only valid UTF8 characters. However, it can't do the work for
+you. On a character-by-character basis, C<is_utf8_char> will tell you
+whether the current character in a string is valid UTF8.
+
+=head2 How does UTF8 represent Unicode characters?
+
+As mentioned above, UTF8 uses a variable number of bytes to store a
+character. Characters with values 1...128 are stored in one byte, just
+like good ol' ASCII. Character 129 is stored as C<v194.129>; this
+contines up to character 191, which is C<v194.191>. Now we've run out of
+bits (191 is binary C<10111111>) so we move on; 192 is C<v195.128>. And
+so it goes on, moving to three bytes at character 2048.
+
+Assuming you know you're dealing with a UTF8 string, you can find out
+how long the first character in it is with the C<UTF8SKIP> macro:
+
+ char *utf = "\305\233\340\240\201";
+ I32 len;
+
+ len = UTF8SKIP(utf); /* len is 2 here */
+ utf += len;
+ len = UTF8SKIP(utf); /* len is 3 here */
+
+Another way to skip over characters in a UTF8 string is to use
+C<utf8_hop>, which takes a string and a number of characters to skip
+over. You're on your own about bounds checking, though, so don't use it
+lightly.
+
+All bytes in a multi-byte UTF8 character will have the high bit set, so
+you can test if you need to do something special with this character
+like this:
+
+ UV uv;
+
+ if (utf & 0x80)
+ /* Must treat this as UTF8 */
+ uv = utf8_to_uv(utf);
+ else
+ /* OK to treat this character as a byte */
+ uv = *utf;
+
+You can also see in that example that we use C<utf8_to_uv> to get the
+value of the character; the inverse function C<uv_to_utf8> is available
+for putting a UV into UTF8:
+
+ if (uv > 0x80)
+ /* Must treat this as UTF8 */
+ utf8 = uv_to_utf8(utf8, uv);
+ else
+ /* OK to treat this character as a byte */
+ *utf8++ = uv;
+
+You B<must> convert characters to UVs using the above functions if
+you're ever in a situation where you have to match UTF8 and non-UTF8
+characters. You may not skip over UTF8 characters in this case. If you
+do this, you'll lose the ability to match hi-bit non-UTF8 characters;
+for instance, if your UTF8 string contains C<v196.172>, and you skip
+that character, you can never match a C<chr(200)> in a non-UTF8 string.
+So don't do that!
+
+=head2 How does Perl store UTF8 strings?
+
+Currently, Perl deals with Unicode strings and non-Unicode strings
+slightly differently. If a string has been identified as being UTF-8
+encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
+manipulate this flag with the following macros:
+
+ SvUTF8(sv)
+ SvUTF8_on(sv)
+ SvUTF8_off(sv)
+
+This flag has an important effect on Perl's treatment of the string: if
+Unicode data is not properly distinguished, regular expressions,
+C<length>, C<substr> and other string handling operations will have
+undesirable results.
+
+The problem comes when you have, for instance, a string that isn't
+flagged is UTF8, and contains a byte sequence that could be UTF8 -
+especially when combining non-UTF8 and UTF8 strings.
+
+Never forget that the C<SVf_UTF8> flag is separate to the PV value; you
+need be sure you don't accidentally knock it off while you're
+manipulating SVs. More specifically, you cannot expect to do this:
+
+ SV *sv;
+ SV *nsv;
+ STRLEN len;
+ char *p;
+
+ p = SvPV(sv, len);
+ frobnicate(p);
+ nsv = newSVpvn(p, len);
+
+The C<char*> string does not tell you the whole story, and you can't
+copy or reconstruct an SV just by copying the string value. Check if the
+old SV has the UTF8 flag set, and act accordingly:
+
+ p = SvPV(sv, len);
+ frobnicate(p);
+ nsv = newSVpvn(p, len);
+ if (SvUTF8(sv))
+ SvUTF8_on(nsv);
+
+In fact, your C<frobnicate> function should be made aware of whether or
+not it's dealing with UTF8 data, so that it can handle the string
+appropriately.
+
+=head2 How do I convert a string to UTF8?
+
+If you're mixing UTF8 and non-UTF8 strings, you might find it necessary
+to upgrade one of the strings to UTF8. If you've got an SV, the easiest
+way to do this is:
+
+ sv_utf8_upgrade(sv);
+
+However, you must not do this, for example:
+
+ if (!SvUTF8(left))
+ sv_utf8_upgrade(left);
+
+If you do this in a binary operator, you will actually change one of the
+strings that came into the operator, and, while it shouldn't be noticable
+by the end user, it can cause problems.
+
+Instead, C<bytes_to_utf8> will give you a UTF8-encoded B<copy> of its
+string argument. This is useful for having the data available for
+comparisons and so on, without harming the orginal SV. There's also
+C<utf8_to_bytes> to go the other way, but naturally, this will fail if
+the string contains any characters above 255 that can't be represented
+in a single byte.
+
+=head2 Is there anything else I need to know?
+
+Not really. Just remember these things:
+
+=over 3
+
+=item *
+
+There's no way to tell if a string is UTF8 or not. You can tell if an SV
+is UTF8 by looking at is C<SvUTF8> flag. Don't forget to set the flag if
+something should be UTF8. Treat the flag as part of the PV, even though
+it's not - if you pass on the PV to somewhere, pass on the flag too.
+
+=item *
+
+If a string is UTF8, B<always> use C<utf8_to_uv> to get at the value,
+unless C<!(*s & 0x80)> in which case you can use C<*s>.
+
+=item *
+
+When writing to a UTF8 string, B<always> use C<uv_to_utf8>, unless
+C<uv < 0x80> in which case you can use C<*s = uv>.
+
+=item *
+
+Mixing UTF8 and non-UTF8 strings is tricky. Use C<bytes_to_utf8> to get
+a new string which is UTF8 encoded. There are tricks you can use to
+delay deciding whether you need to use a UTF8 string until you get to a
+high character - C<HALF_UPGRADE> is one of those.
+
+=back
+
=head1 AUTHORS
Until May 1997, this document was maintained by Jeff Okamoto
platforms and gives feedback to the CPAN testers mailing list. Both
efforts welcome volunteers.
-To become an active and patching Perl porter, you'll need to learn how
-Perl works on the inside. Chip Salzenberg, a pumpking, has written
-articles on Perl internals for The Perl Journal
-(I<http://www.tpj.com/>) which explain how various parts of the Perl
-interpreter work. The C<perlguts> manpage explains the internal data
-structures. And, of course, the C source code (sometimes sparsely
-commented, sometimes commented well) is a great place to start (begin
-with C<perl.c> and see where it goes from there). A lot of the style
-of the Perl source is explained in the I<Porting/pumpkin.pod> file in
-the source distribution.
-
-It is essential that you be comfortable using a good debugger
-(e.g. gdb, dbx) before you can patch perl. Stepping through perl
-as it executes a script is perhaps the best (if sometimes tedious)
-way to gain a precise understanding of the overall architecture of
-the language.
-
-If you build a version of the Perl interpreter with C<-DDEBUGGING>,
-Perl's B<-D> command line flag will cause copious debugging information
-to be emitted (see the C<perlrun> manpage). If you build a version of
-Perl with compiler debugging information (e.g. with the C compiler's
-C<-g> option instead of C<-O>) then you can step through the execution
-of the interpreter with your favourite C symbolic debugger, setting
-breakpoints on particular functions.
-
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
mailing list, send mail to I<perl5-porters-subscribe@perl.org>. To
unsubscribe, send mail to I<perl5-porters-unsubscribe@perl.org>.
+To hack on the Perl guts, you'll need to read the following things:
+
+=over 3
+
+=item L<perlguts>
+
+This is of paramount importance, since it's the documentation of what
+goes where in the Perl source. Read it over a couple of times and it
+might start to make sense - don't worry if it doesn't yet, because the
+best way to study it is to read it in conjunction with poking at Perl
+source, and we'll do that later on.
+
+You might also want to look at Gisle Aas's illustrated perlguts -
+there's no guarantee that this will be absolutely up-to-date with the
+latest documentation in the Perl core, but the fundamentals will be
+right. (http://gisle.aas.no/perl/illguts/)
+
+=item L<perlxstut> and L<perlxs>
+
+A working knowledge of XSUB programming is incredibly useful for core
+hacking; XSUBs use techniques drawn from the PP code, the portion of the
+guts that actually executes a Perl program. It's a lot gentler to learn
+those techniques from simple examples and explanation than from the core
+itself.
+
+=item L<perlapi>
+
+The documentation for the Perl API explains what some of the internal
+functions do, as well as the many macros used in the source.
+
+=item F<Porting/pumpkin.pod>
+
+This is a collection of words of wisdom for a Perl porter; some of it is
+only useful to the pumpkin holder, but most of it applies to anyone
+wanting to go about Perl development.
+
+=item The perl5-porters FAQ
+
+This is posted to perl5-porters at the beginning on every month, and
+should be available from http://perlhacker.org/p5p-faq; alternatively,
+you can get the FAQ emailed to you by sending mail to
+C<perl5-porters-faq@perl.org>. It contains hints on reading
+perl5-porters, information on how perl5-porters works and how Perl
+development in general works.
+
+=back
+
+=head2 Finding Your Way Around
+
+Perl 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:
+
+=over 3
+
+=item Core modules
+
+Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
+subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
+contains the core XS modules.
+
+=item Documentation
+
+Documentation maintenance includes looking after everything in the
+F<pod/> directory, (as well as contributing new documentation) and
+the documentation to the modules in core.
+
+=item Configure
+
+The configure process is the way we make Perl portable across the
+myriad of operating systems it supports. Responsibility for the
+configure, build and installation process, as well as the overall
+portability of the core code rests with the configure pumpkin - others
+help out with individual operating systems.
+
+The files involved are the operating system directories, (F<win32/>,
+F<os2/>, F<vms/> and so on) the shell scripts which generate F<config.h>
+and F<Makefile>, as well as the metaconfig files which generate
+F<Configure>. (metaconfig isn't included in the core distribution.)
+
+=item Interpreter
+
+And of course, there's the core of the Perl interpreter itself. Let's
+have a look at that in a little more detail.
+
+=back
+
+Before we leave looking at the layout, though, don't forget that
+F<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
+
+=head2 Elements of the interpreter
+
+The work of the interpreter has two main stages: compiling the code
+into the internal representation, or bytecode, and then executing it.
+L<perlguts/Compiled code> explains exactly how the compilation stage
+happens.
+
+Here is a short breakdown of perl's operation:
+
+=over 3
+
+=item Startup
+
+The action begins in F<perlmain.c>. (or F<miniperlmain.c> for miniperl)
+This is very high-level code, enough to fit on a single screen, and it
+resembles the code found in L<perlembed>; most of the real action takes
+place in F<perl.c>
+
+First, F<perlmain.c> allocates some memory and constructs a Perl
+interpreter:
+
+ 1 PERL_SYS_INIT3(&argc,&argv,&env);
+ 2
+ 3 if (!PL_do_undump) {
+ 4 my_perl = perl_alloc();
+ 5 if (!my_perl)
+ 6 exit(1);
+ 7 perl_construct(my_perl);
+ 8 PL_perl_destruct_level = 0;
+ 9 }
+
+Line 1 is a macro, and its definition is dependent on your operating
+system. Line 3 references C<PL_do_undump>, a global variable - all
+global variables in Perl start with C<PL_>. This tells you whether the
+current running program was created with the C<-u> flag to perl and then
+F<undump>, which means it's going to be false in any sane context.
+
+Line 4 calls a function in F<perl.c> to allocate memory for a Perl
+interpreter. It's quite a simple function, and the guts of it looks like
+this:
+
+ my_perl = (PerlInterpreter*)PerlMem_malloc(sizeof(PerlInterpreter));
+
+Here you see an example of Perl's system abstraction, which we'll see
+later: C<PerlMem_malloc> is either your system's C<malloc>, or Perl's
+own C<malloc> as defined in F<malloc.c> if you selected that option at
+configure time.
+
+Next, in line 7, we construct the interpreter; this sets up all the
+special variables that Perl needs, the stacks, and so on.
+
+Now we pass Perl the command line options, and tell it to go:
+
+ exitstatus = perl_parse(my_perl, xs_init, argc, argv, (char **)NULL);
+ if (!exitstatus) {
+ exitstatus = perl_run(my_perl);
+ }
+
+
+C<perl_parse> is actually a wrapper around C<S_parse_body>, as defined
+in F<perl.c>, which processes the command line options, sets up any
+statically linked XS modules, opens the program and calls C<yyparse> to
+parse it.
+
+=item Parsing
+
+The aim of this stage is to take the Perl source, and turn it into an op
+tree. We'll see what one of those looks like later. Strictly speaking,
+there's three things going on here.
+
+C<yyparse>, the parser, lives in F<perly.c>, although you're better off
+reading the original YACC input in F<perly.y>. (Yes, Virginia, there
+B<is> a YACC grammar for Perl!) The job of the parser is to take your
+code and `understand' it, splitting it into sentences, deciding which
+operands go with which operators and so on.
+
+The parser is nobly assisted by the lexer, which chunks up your input
+into tokens, and decides what type of thing each token is: a variable
+name, an operator, a bareword, a subroutine, a core function, and so on.
+The main point of entry to the lexer is C<yylex>, and that and its
+associated routines can be found in F<toke.c>. Perl isn't much like
+other computer languages; it's highly context sensitive at times, it can
+be tricky to work out what sort of token something is, or where a token
+ends. As such, there's a lot of interplay between the tokeniser and the
+parser, which can get pretty frightening if you're not used to it.
+
+As the parser understands a Perl program, it builds up a tree of
+operations for the interpreter to perform during execution. The routines
+which construct and link together the various operations are to be found
+in F<op.c>, and will be examined later.
+
+=item Optimization
+
+Now the parsing stage is complete, and the finished tree represents
+the operations that the Perl interpreter needs to perform to execute our
+program. Next, Perl does a dry run over the tree looking for
+optimisations: constant expressions such as C<3 + 4> will be computed
+now, and the optimizer will also see if any multiple operations can be
+replaced with a single one. For instance, to fetch the variable C<$foo>,
+instead of grabbing the glob C<*foo> and looking at the scalar
+component, the optimizer fiddles the op tree to use a function which
+directly looks up the scalar in question. The main optimizer is C<peep>
+in F<op.c>, and many ops have their own optimizing functions.
+
+=item Running
+
+Now we're finally ready to go: we have compiled Perl byte code, and all
+that's left to do is run it. The actual execution is done by the
+C<runops_standard> function in F<run.c>; more specifically, it's done by
+these three innocent looking lines:
+
+ while ((PL_op = CALL_FPTR(PL_op->op_ppaddr)(aTHX))) {
+ PERL_ASYNC_CHECK();
+ }
+
+You may be more comfortable with the Perl version of that:
+
+ PERL_ASYNC_CHECK() while $Perl::op = &{$Perl::op->{function}};
+
+Well, maybe not. Anyway, each op contains a function pointer, which
+stipulates the function which will actually carry out the operation.
+This function will return the next op in the sequence - this allows for
+things like C<if> which choose the next op dynamically at run time.
+The C<PERL_ASYNC_CHECK> makes sure that things like signals interrupt
+execution if required.
+
+The actual functions called are known as PP code, and they're spread
+between four files: F<pp_hot.c> contains the `hot' code, which is most
+often used and highly optimized, F<pp_sys.c> contains all the
+system-specific functions, F<pp_ctl.c> contains the functions which
+implement control structures (C<if>, C<while> and the like) and F<pp.c>
+contains everything else. These are, if you like, the C code for Perl's
+built-in functions and operators.
+
+=back
+
+=head2 Internal Variable Types
+
+You should by now have had a look at L<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 L<Devel::Peek|Devel::Peek> module lets us examine SVs from a
+Perl program. Let's see, for instance, how Perl treats the constant
+C<"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 = 6
+
+Reading C<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 C<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 C<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
+C<0xa0484e0>.
+
+Line 5 gives us the current length of the string - note that this does
+B<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 C<SvGROW>.
+
+You can get at any of these quantities from C very easily; just add
+C<Sv> to the name of the field shown in the snippet, and you've got a
+macro which will return the value: C<SvCUR(sv)> returns the current
+length of the string, C<SvREFCOUNT(sv)> returns the reference count,
+C<SvPV(sv, len)> returns the string itself with its length, and so on.
+More macros to manipulate these properties can be found in L<perlguts>.
+
+Let's take an example of manipulating a PV, from C<sv_catpvn>, in F<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, C<ptr>, of length C<len> onto
+the end of the PV stored in C<sv>. The first thing we do in line 6 is
+make sure that the SV B<has> a valid PV, by calling the C<SvPV_force>
+macro to force a PV. As a side effect, C<tlen> gets set to the current
+value of the PV, and the PV itself is returned to C<junk>.
+
+In line 7, we make sure that the SV will have enough room to accomodate
+the old string, the new string and the null terminator. If C<LEN> isn't
+big enough, C<SvGROW> will reallocate space for us.
+
+Now, if C<junk> is the same as the string we're trying to add, we can
+grab the string directly from the SV; C<SvPVX> is the address of the PV
+in the SV.
+
+Line 10 does the actual catenation: the C<Move> macro moves a chunk of
+memory around: we move the string C<ptr> to the end of the PV - that's
+the start of the PV plus its current length. We're moving C<len> bytes
+of type C<char>. After doing so, we need to tell Perl we've extended the
+string, by altering C<CUR> to reflect the new length. C<SvEND> is a
+macro which gives us the end of the string, so that needs to be a
+C<"\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 C<$a=10; $a.="6";> we don't
+want to use the old IV of 10. C<SvPOK_only_utf8> is a special UTF8-aware
+version of C<SvPOK_only>, a macro which turns off the IOK and NOK flags
+and turns on POK. The final C<SvTAINT> is a macro which launders tainted
+data if taint mode is turned on.
+
+AVs and HVs are more complicated, but SVs are by far the most common
+variable type being thrown around. Having seen something of how we
+manipulate these, let's go on and look at how the op tree is
+constructed.
+
+=head2 Op Trees
+
+First, what is the op tree, anyway? The op tree is the parsed
+representation of your program, as we saw in our section on parsing, and
+it's the sequence of operations that Perl goes through to execute your
+program, as we saw in L</Running>.
+
+An op is a fundamental operation that Perl can perform: all the built-in
+functions and operators are ops, and there are a series of ops which
+deal with concepts the interpreter needs internally - entering and
+leaving a block, ending a statement, fetching a variable, and so on.
+
+The op tree is connected in two ways: you can imagine that there are two
+"routes" through it, two orders in which you can traverse the tree.
+First, parse order reflects how the parser understood the code, and
+secondly, execution order tells perl what order to perform the
+operations in.
+
+The easiest way to examine the op tree is to stop Perl after it has
+finished parsing, and get it to dump out the tree. This is exactly what
+the compiler backends L<B::Terse|B::Terse> and L<B::Debug|B::Debug> do.
+
+Let's have a look at how Perl sees C<$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) *a
+
+Let's start in the middle, at line 4. This is a BINOP, a binary
+operator, which is at location C<0x8179828>. The specific operator in
+question is C<sassign> - scalar assignment - and you can find the code
+which implements it in the function C<pp_sassign> in F<pp_hot.c>. As a
+binary operator, it has two children: the add operator, providing the
+result of C<$b+$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 L</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) *a
+
+That is, fetch the C<a> entry from the main symbol table, and then look
+at the scalar component of it: C<gvsv> (C<pp_gvsv> into F<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 C<add> op (C<pp_add> also in F<pp_hot.c>) add together
+two C<gvsv>s.
+
+Now, what's this about?
+
+ 1 LISTOP (0x8179888) leave
+ 2 OP (0x81798b0) enter
+ 3 COP (0x8179850) nextstate
+
+C<enter> and C<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: C<leave> is a list, and its
+children are all the statements in the block. Statements are delimited
+by C<nextstate>, so a block is a collection of C<nextstate> ops, with
+the ops to be performed for each statement being the children of
+C<nextstate>. C<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 $c
+
+However, it's impossible to B<perform> the operations in this order:
+you have to find the values of C<$b> and C<$c> before you add them
+together, for instance. So, the other thread that runs through the op
+tree is the execution order: each op has a field C<op_next> which points
+to the next op to be run, so following these pointers tells us how perl
+executes the code. We can traverse the tree in this order using
+the C<exec> option to C<B::Terse>:
+
+ % perl -MO=Terse,exec -e '$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) leave
+
+This probably makes more sense for a human: enter a block, start a
+statement. Get the values of C<$b> and C<$c>, and add them together.
+Find C<$a>, and assign one to the other. Then leave.
+
+The way Perl builds up these op trees in the parsing process can be
+unravelled by examining F<perly.y>, the YACC grammar. Let's take the
+piece we need to construct the tree for C<$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, C<ADDOP>, is provided when the tokeniser sees C<+> in your
+code. C<ASSIGNOP> is provided when C<=> 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. C<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, C<term> followed by C<=>
+followed by C<term> makes a C<term>, and C<term> followed by C<+>
+followed by C<term> can also make a C<term>.
+
+So, if you see two terms with an C<=> or C<+>, 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 C<=>, you'll do the code
+in line 2. If you see C<+>, you'll do the code in line 4. It's this code
+which contributes to the op tree.
+
+ | term ADDOP term
+ { $$ = newBINOP($2, 0, scalar($1), scalar($3)); }
+
+What this does is creates a new binary op, and feeds it a number of
+variables. The variables refer to the tokens: C<$1> is the first token in
+the input, C<$2> the second, and so on - think regular expression
+backreferences. C<$$> is the op returned from this reduction. So, we
+call C<newBINOP> to create a new binary operator. The first parameter to
+C<newBINOP>, a function in F<op.c>, is the op type. It's an addition
+operator, so we want the type to be C<ADDOP>. We could specify this
+directly, but it's right there as the second token in the input, so we
+use C<$2>. The second parameter is the op's flags: 0 means `nothing
+special'. Then the things to add: the left and right hand side of our
+expression, in scalar context.
+
+=head2 Stacks
+
+When perl executes something like C<addop>, how does it pass on its
+results to the next op? The answer is, through the use of stacks. Perl
+has a number of stacks to store things it's currently working on, and
+we'll look at the three most important ones here.
+
+=over 3
+
+=item Argument stack
+
+Arguments are passed to PP code and returned from PP code using the
+argument stack, C<ST>. The typical way to handle arguments is to pop
+them off the stack, deal with them how you wish, and then push the result
+back onto the stack. This is how, for instance, the cosine operator
+works:
+
+ NV value;
+ value = POPn;
+ value = Perl_cos(value);
+ XPUSHn(value);
+
+We'll see a more tricky example of this when we consider Perl's macros
+below. C<POPn> gives you the NV (floating point value) of the top SV on
+the stack: the C<$x> in C<cos($x)>. Then we compute the cosine, and push
+the result back as an NV. The C<X> in C<XPUSHn> means that the stack
+should be extended if necessary - it can't be necessary here, because we
+know there's room for one more item on the stack, since we've just
+removed one! The C<XPUSH*> macros at least guarantee safety.
+
+Alternatively, you can fiddle with the stack directly: C<SP> gives you
+the first element in your portion of the stack, and C<TOP*> gives you
+the top SV/IV/NV/etc. on the stack. So, for instance, to do unary
+negation of an integer:
+
+ SETi(-TOPi);
+
+Just set the integer value of the top stack entry to its negation.
+
+Argument stack manipulation in the core is exactly the same as it is in
+XSUBs - see L<perlxstut>, L<perlxs> and L<perlguts> for a longer
+description of the macros used in stack manipulation.
+
+=item Mark stack
+
+I say `your portion of the stack' above because PP code doesn't
+necessarily get the whole stack to itself: if your function calls
+another function, you'll only want to expose the arguments aimed for the
+called function, and not (necessarily) let it get at your own data. The
+way we do this is to have a `virtual' bottom-of-stack, exposed to each
+function. The mark stack keeps bookmarks to locations in the argument
+stack usable by each function. For instance, when dealing with a tied
+variable, (internally, something with `P' magic) Perl has to call
+methods for accesses to the tied variables. However, we need to separate
+the arguments exposed to the method to the argument exposed to the
+original function - the store or fetch or whatever it may be. Here's how
+the tied C<push> is implemented; see C<av_push> in F<av.c>:
+
+ 1 PUSHMARK(SP);
+ 2 EXTEND(SP,2);
+ 3 PUSHs(SvTIED_obj((SV*)av, mg));
+ 4 PUSHs(val);
+ 5 PUTBACK;
+ 6 ENTER;
+ 7 call_method("PUSH", G_SCALAR|G_DISCARD);
+ 8 LEAVE;
+ 9 POPSTACK;
+
+The lines which concern the mark stack are the first, fifth and last
+lines: they save away, restore and remove the current position of the
+argument stack.
+
+Let's examine the whole implementation, for practice:
+
+ 1 PUSHMARK(SP);
+
+Push the current state of the stack pointer onto the mark stack. This is
+so that when we've finished adding items to the argument stack, Perl
+knows how many things we've added recently.
+
+ 2 EXTEND(SP,2);
+ 3 PUSHs(SvTIED_obj((SV*)av, mg));
+ 4 PUSHs(val);
+
+We're going to add two more items onto the argument stack: when you have
+a tied array, the C<PUSH> subroutine receives the object and the value
+to be pushed, and that's exactly what we have here - the tied object,
+retrieved with C<SvTIED_obj>, and the value, the SV C<val>.
+
+ 5 PUTBACK;
+
+Next we tell Perl to make the change to the global stack pointer: C<dSP>
+only gave us a local copy, not a reference to the global.
+
+ 6 ENTER;
+ 7 call_method("PUSH", G_SCALAR|G_DISCARD);
+ 8 LEAVE;
+
+C<ENTER> and C<LEAVE> localise a block of code - they make sure that all
+variables are tidied up, everything that has been localised gets
+its previous value returned, and so on. Think of them as the C<{> and
+C<}> of a Perl block.
+
+To actually do the magic method call, we have to call a subroutine in
+Perl space: C<call_method> takes care of that, and it's described in
+L<perlcall>. We call the C<PUSH> method in scalar context, and we're
+going to discard its return value.
+
+ 9 POPSTACK;
+
+Finally, we remove the value we placed on the mark stack, since we
+don't need it any more.
+
+=item Save stack
+
+C doesn't have a concept of local scope, so perl provides one. We've
+seen that C<ENTER> and C<LEAVE> are used as scoping braces; the save
+stack implements the C equivalent of, for example:
+
+ {
+ local $foo = 42;
+ ...
+ }
+
+See L<perlguts/Localising Changes> for how to use the save stack.
+
+=back
+
+=head2 Millions of Macros
+
+One 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 djSP; 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 C<d> - which pops from the top of the argument stack two NVs (hence
+C<nn>) and puts them into the variables C<right> and C<left>, hence the
+C<rl>. These are the two operands to the addition operator. Next, we
+call C<SETn> to set the NV of the return value to the result of adding
+the two values. This done, we return - the C<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 L<perlapi>, and some of the more
+important ones are explained in L<perlxs> as well. Pay special attention
+to L<perlguts/Background and PERL_IMPLICIT_CONTEXT> for information on
+the C<[pad]THX_?> macros.
+
+
+=head2 Poking at Perl
+
+To really poke around with Perl, you'll probably want to build Perl for
+debugging, like this:
+
+ ./Configure -d -D optimize=-g
+ make
+
+C<-g> is a flag to the C compiler to have it produce debugging
+information which will allow us to step through a running program.
+F<Configure> will also turn on the C<DEBUGGING> compilation symbol which
+enables all the internal debugging code in Perl. There are a whole bunch
+of things you can debug with this: L<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 conversions
+
+Some of the functionality of the debugging code can be achieved using XS
+modules.
+
+ -Dr => use re 'debug'
+ -Dx => use O 'Debug'
+
+=head2 Using a source-level debugger
+
+If the debugging output of C<-D> doesn't help you, it's time to step
+through perl's execution with a source-level debugger.
+
+=over 3
+
+=item *
+
+We'll use C<gdb> for our examples here; the principles will apply to any
+debugger, but check the manual of the one you're using.
+
+=back
+
+To fire up the debugger, type
+
+ gdb ./perl
+
+You'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)
+
+C<help> will get you into the documentation, but here are the most
+useful commands:
+
+=over 3
+
+=item run [args]
+
+Run the program with the given arguments.
+
+=item break function_name
+
+=item break source.c:xxx
+
+Tells the debugger that we'll want to pause execution when we reach
+either the named function (but see L</Function names>!) or the given
+line in the named source file.
+
+=item step
+
+Steps through the program a line at a time.
+
+=item next
+
+Steps through the program a line at a time, without descending into
+functions.
+
+=item continue
+
+Run until the next breakpoint.
+
+=item finish
+
+Run until the end of the current function, then stop again.
+
+=item
+
+Just pressing Enter will do the most recent operation again - it's a
+blessing when stepping through miles of source code.
+
+=item print
+
+Execute the given C code and print its results. B<WARNING>: Perl makes
+heavy use of macros, and F<gdb> is not aware of macros. You'll have to
+substitute them yourself. So, for instance, you can't say
+
+ print SvPV_nolen(sv)
+
+but you have to say
+
+ print Perl_sv_2pv_nolen(sv)
+
+You may find it helpful to have a "macro dictionary", which you can
+produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
+recursively apply the macros for you.
+
+=back
+
+=head2 Dumping Perl Data Structures
+
+One way to get around this macro hell is to use the dumping functions in
+F<dump.c>; these work a little like an internal
+L<Devel::Peek|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
+C<$a = $b + $c> we used before, but give it a bit of context:
+C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and poke around?
+
+What about C<pp_add>, the function we examined earlier to implement the
+C<+> operator:
+
+ (gdb) break Perl_pp_add
+ Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
+
+Notice we use C<Perl_pp_add> and not C<pp_add> - see L<perlguts/Function Names>.
+With the breakpoint in place, we can run our program:
+
+ (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
+
+Lots of junk will go past as gdb reads in the relevant source files and
+libraries, and then:
+
+ Breakpoint 1, Perl_pp_add () at pp_hot.c:309
+ 309 djSP; dATARGET; tryAMAGICbin(add,opASSIGN);
+ (gdb) step
+ 311 dPOPTOPnnrl_ul;
+ (gdb)
+
+We looked at this bit of code before, and we said that C<dPOPTOPnnrl_ul>
+arranges for two C<NV>s to be placed into C<left> and C<right> - let's
+slightly expand it:
+
+ #define dPOPTOPnnrl_ul NV right = POPn; \
+ SV *leftsv = TOPs; \
+ NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
+
+C<POPn> takes the SV from the top of the stack and obtains its NV either
+directly (if C<SvNOK> is set) or by calling the C<sv_2nv> function.
+C<TOPs> takes the next SV from the top of the stack - yes, C<POPn> uses
+C<TOPs> - but doesn't remove it. We then use C<SvNV> to get the NV from
+C<leftsv> in the same way as before - yes, C<POPn> uses C<SvNV>.
+
+Since we don't have an NV for C<$b>, we'll have to use C<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 C<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 = void
+
+We know we're going to get C<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
+C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
+similar output to L<B::Debug|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 >
+
+=head2 Patching
+
+All 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
+C<U> is the first active format during a C<pack>, (for example,
+C<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 C<pack> happens at runtime, so it's going to be in one of the F<pp>
+files. Sure enough, C<pp_pack> is in F<pp.c>. Since we're going to be
+altering this file, let's copy it to F<pp.c~>.
+
+Now let's look over C<pp_pack>: we take a pattern into C<pat>, and then
+loop over the pattern, taking each format character in turn into
+C<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 C<cat>.
+
+How do we know if the C<U> is the first format in the C<pat>? Well, if
+we have a pointer to the start of C<pat> then, if we see a C<U> we can
+test whether we're still at the start of the string. So, here's where
+C<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 C<patcopy> to be the start
+of C<pat>:
+
+ items = SP - MARK;
+ MARK++;
+ sv_setpvn(cat, "", 0);
+ + patcopy = pat;
+ while (pat < patend) {
+
+Now if we see a C<U> which was at the start of the string, we turn on
+the UTF8 flag for the output SV, C<cat>:
+
+ + if (datumtype == 'U' && pat==patcopy+1)
+ + SvUTF8_on(cat);
+ if (datumtype == '#') {
+ while (pat < patend && *pat != '\n')
+ pat++;
+
+Remember that it has to be C<patcopy+1> because the first character of
+the string is the C<U> which has been swallowed into C<datumtype!>
+
+Oops, we forgot one thing: what if there are spaces at the start of the
+pattern? C<pack(" U*", @stuff)> will have C<U> as the first active
+character, even though it's not the first thing in the pattern. In this
+case, we have to advance C<patcopy> along with C<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 F<t/op/>, and so we make
+a copy of F<t/op/pack.t> to F<t/op/pack.t~>. Now we can add our tests
+to the end. First, we'll test that the C<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 C<U> is B<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++;
+
+Musn'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 F<pod/perlfunc.pod>; again, we make a copy, and then we'll insert
+this text in the description of C<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. F<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 > patch
+
+However, 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 >> patch
+
+We 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!
+
+=head2 CONCLUSION
+
+We've had a brief look around the Perl source, an overview of the stages
+F<perl> goes through when it's running your code, and how to use a
+debugger to poke at the Perl guts. Finally, 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.
+
+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:
+
+=over 3
+
+=item *
+
+Subscribe to perl5-porters, follow the patches and try and understand
+them; don't be afraid to ask if there's a portion you're not clear on -
+who knows, you may unearth a bug in the patch...
+
+=item *
+
+Keep up to date with the bleeding edge Perl distributions and get
+familiar with the changes. Try and get an idea of what areas people are
+working on and the changes they're making.
+
+=item *
+
+Find an area of Perl that seems interesting to you, and see if you can
+work out how it works. Scan through the source, and step over it in the
+debugger. Play, poke, investigate, fiddle! You'll probably get to
+understand not just your chosen area but a much wider range of F<perl>'s
+activity as well, and probably sooner than you'd think.
+
+=back
+
+=over 3
+
+=item I<The Road goes ever on and on, down from the door where it began.>
+
+=back
+
+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!
+
=head1 AUTHOR
This document was written by Nathan Torkington, and is maintained by
projects / p5sagit/p5-mst-13.2.git / commitdiff