Daftar Isi

• NAME
• DESCRIPTION
• EXAMPLE OF A SIMPLE PATCH
  • Writing the patch
  • Testing the patch
  • Documenting the patch
  • Submit
• AUTHOR

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 alsowant 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 formatduring a pack, (for example, pack "U3C8", @stuff) then theresulting string should be treated as UTF-8 encoded.

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

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 thepp files. Sure enough, pp_pack is in pp.c. Since we're goingto be altering this file, let's copy it to pp.c~.

[Well, it was in pp.c when this tutorial was written. It has nowbeen 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 thenloop over the pattern, taking each format character in turn intodatum_type. Then for each possible format character, we swallow upthe other arguments in the pattern (a field width, an asterisk, and soon) and convert the next chunk input into the specified format, addingit onto the output SV cat.

How do we know if the U is the first format in the pat? Well, ifwe have a pointer to the start of pat then, if we see a U we cantest whether we're still at the start of the string. So, here's wherepat 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 startof 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 onthe 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 ofthe string is the U which has been swallowed into datumtype!

Oops, we forgot one thing: what if there are spaces at the start of thepattern? pack(" U*", @stuff) will have U as the first activecharacter, even though it's not the first thing in the pattern. In thiscase, we have to advance patcopy along with pat when we seespaces:

 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 beforethis patch is ready to go: we've changed the behaviour of Perl, and sowe must document that change. We must also provide some more regressiontests to make sure our patch works and doesn't create a bug somewhereelse along the line.

Testing the patch

The regression tests for each operator live in t/op/, and so we makea copy of t/op/pack.t to t/op/pack.t~. Now we can add our teststo the end. First, we'll test that the U does indeed create Unicodestrings.

t/op/pack.t has a sensible ok() function, but if it didn't we could usethe 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 fullexplanation 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 isnot 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 looklike this:

 print "1..156\n";
 

or this:

 plan( tests => 156 );
 

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

Documenting the patch

Finally, the documentation. The job is never done until the paperworkis over, so let's describe the change we've just made. The relevantplace is pod/perlfunc.pod; again, we make a copy, and then we'llinsert 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 ismaintained by the perl5-porters mailing list.