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PERLHACKTUT(1)	       Perl Programmers	Reference Guide		PERLHACKTUT(1)

NAME
       perlhacktut - Walk through the creation of a simple C code patch

DESCRIPTION
       This document takes you through a simple	patch example.

       If you haven't read perlhack yet, go do that first! You might also want
       to read through perlsource too.

       Once you're done	here, check out	perlhacktips next.

EXAMPLE	OF A SIMPLE PATCH
       Let's take a simple patch from start to finish.

       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 UTF-8 encoded.

       If you are working with a git clone of the Perl repository, you will
       want to create a	branch for your	changes. This will make	creating a
       proper patch much simpler. See the perlgit for details on how to	do
       this.

   Writing the patch
       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~.

       [Well, it was in	pp.c when this tutorial	was written. It	has now	been
       split off with "pp_unpack" to its own file, pp_pack.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;
	   char	*pat = SvPVx(*++MARK, fromlen);
	   char	*patend	= pat +	fromlen;
	   I32 len;
	   I32 datumtype;
	   SV *fromstr;

       We'll have another string pointer in there:

	   STRLEN fromlen;
	   char	*pat = SvPVx(*++MARK, fromlen);
	   char	*patend	= pat +	fromlen;
	+  char	*patcopy;
	   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++;
	   SvPVCLEAR(cat);
	+  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.

   Testing the patch
       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.

       t/op/pack.t has a sensible ok() function, but if	it didn't we could use
       the one from t/test.pl.

	require	'./test.pl';
	plan( tests => 159 );

       so instead of this:

	print 'not ' unless "1.20.300.4000" eq sprintf "%vd",
						      pack("U*",1,20,300,4000);
	print "ok $test\n"; $test++;

       we can write the	more sensible (see Test::More for a full explanation
       of is() and other testing functions).

	is( "1.20.300.4000", sprintf "%vd", pack("U*",1,20,300,4000),
					      "U* produces Unicode" );

       Now we'll test that we got that space-at-the-beginning business right:

	is( "1.20.300.4000", sprintf "%vd", pack("  U*",1,20,300,4000),
					    "  with spaces at the beginning" );

       And finally we'll test that we don't make Unicode strings if "U"	is not
       the first active	format:

	isnt( v1.20.300.4000, sprintf "%vd", pack("C0U*",1,20,300,4000),
					      "U* not first isn't Unicode" );

       Mustn't forget to change	the number of tests which appears at the top,
       or else the automated tester will get confused. This will either	look
       like this:

	print "1..156\n";

       or this:

	plan( tests => 156 );

       We now compile up Perl, and run it through the test suite. Our new
       tests pass, hooray!

   Documenting the patch
       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 UTF-8-encoded Unicode. You can force	UTF-8 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	UTF-8
	encode your string, and	then follow this with a	C<U*> somewhere	in your
	pattern.

   Submit
       See perlhack for	details	on how to submit this patch.

AUTHOR
       This document was originally written by Nathan Torkington, and is
       maintained by the perl5-porters mailing list.

perl v5.32.0			  2020-06-14			PERLHACKTUT(1)

NAME | DESCRIPTION | EXAMPLE OF A SIMPLE PATCH | AUTHOR

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