perlhacktut - Walk through the creation of a simple C code patch
This document takes you through a simple patch example.
If you haven't readperlhack yet, go do that first! You might also want to read throughperlsource too.
Once you're done here, check outperlhacktips next.
Let's take a simple patch from start to finish.
Here's something Larry suggested: if aU is the first active format during apack, (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 theperlgit for details on how to do this.
How do we prepare to fix this up? First we locate the code in question - thepack happens at runtime, so it's going to be in one of thepp files. Sure enough,pp_pack is inpp.c. Since we're going to be altering this file, let's copy it topp.c~.
[Well, it was inpp.c when this tutorial was written. It has now been split off withpp_unpack to its own file,pp_pack.c]
Now let's look overpp_pack: we take a pattern intopat, and then loop over the pattern, taking each format character in turn intodatum_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 SVcat.
How do we know if theU is the first format in thepat? Well, if we have a pointer to the start ofpat then, if we see aU we can test whether we're still at the start of the string. So, here's wherepat 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 setpatcopy to be the start ofpat:
items = SP - MARK; MARK++; SvPVCLEAR(cat);+ patcopy = pat; while (pat < patend) {Now if we see aU which was at the start of the string, we turn on theUTF8 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 bepatcopy+1 because the first character of the string is theU which has been swallowed intodatumtype!
Oops, we forgot one thing: what if there are spaces at the start of the pattern?pack(" U*", @stuff) will haveU as the first active character, even though it's not the first thing in the pattern. In this case, we have to advancepatcopy along withpat 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 int/op/, and so we make a copy oft/op/pack.t tot/op/pack.t~. Now we can add our tests to the end. First, we'll test that theU 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 (seeTest::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 ifU 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 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!
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 ispod/perlfunc.pod; again, we make a copy, and then we'll insert this text in the description ofpack:
=item *If the pattern begins with a C<U>, the resulting string will be treatedas UTF-8-encoded Unicode. You can force UTF-8 encoding on in a stringwith an initial C<U0>, and the bytes that follow will be interpreted asUnicode characters. If you don't want this to happen, you can beginyour pattern with C<C0> (or anything else) to force Perl not to UTF-8encode your string, and then follow this with a C<U*> somewhere in yourpattern.Seeperlhack for details on how to submit this patch.
This document was originally written by Nathan Torkington, and is maintained by the perl5-porters mailing list.
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