PERLHACKTUT(1perl)

💖 PERLHACKTUT 💖

ok so bestie, this is perlhacktut from the Perl Programmers Reference Guide, section 1perl — and it is literally the "walk through the creation of a simple C code patch" glow-up tutorial. no cap. we're going from "what even is this bug" to a fully documented, tested, submitted patch. main character energy the whole way through. 🦋

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🌸 DESCRIPTION 🌸

This doc takes you through a simple patch example, step by step, hand in hand, bestie style. 🫶

If you haven't read perlhack yet, go do that first! Like, actually. It's the prerequisites. You probs also wanna read through perlsource too.

And once you're done here? Check out perlhacktips next. It's the sequel. The encore. The bonus track. ✨

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🔥 EXAMPLE OF A SIMPLE PATCH 🔥

Let's take a simple patch from start to finish. 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. iconic request, honestly.

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. branching = self care. 🌿

✍️ 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~. (backup before you wreck up, that's the rule 🎀)

[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: the way that 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. it's giving very methodical, very demure 🧚

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! 😭 the classic off-by-one jumpscare.

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. receipts and proof, bestie, always. 🧾

🧪 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! 🎉 slay. absolutely ate.

📝 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. go forth and ship it, queen 👑

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👑 AUTHOR 👑

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

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and that's the whole tutorial, bestie!! this page was perl v5.44.0, dated 2026-10-17, from the Perl Programmers Reference Guide — talk to you in the next patch, mwah 💖✨🦋