PERLHACKTUT(1)

💖 PERLHACKTUT 💖

okay bestie, so this is perlhacktut from the Perl Programmers Reference Guide — the lil baby tutorial that walks you through making your very first C code patch to Perl. it's giving "training arc" energy fr fr 🎀

---

✨ NAME ✨

perlhacktut — walk through the creation of a simple C code patch 💅

---

🌸 DESCRIPTION 🌸

So this doc literally just takes you by the hand through one (1) simple patch example. that's it. that's the whole vibe. 🦋

If you haven't read perlhack yet, bestie, go do that FIRST! no skipping, i'm serious 😭. You might also wanna read through perlsource too, it's a whole thing.

And once you're done here? Go check out perlhacktips next. the journey never ends 💖

---

🔥 EXAMPLE OF A SIMPLE PATCH 🔥

Let's take a simple patch from start to finish, no cap.

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 idea tbh 👑

If you're working with a git clone of the Perl repository, you're gonna want to create a branch for your changes. This makes creating a proper patch sooo much simpler. See 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 gonna be in one of the pp files. Sure enough, pp_pack is in pp.c. Since we're about 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] — yes bestie, the code moved, we're just being historically accurate here 😭

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. she's consuming everything ✨

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! 😭 sneaky lil guy

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. two more quests, bestie, you got this 💪

---

🧚 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. go forth and ship it, bestie 🚀

---

👑 AUTHOR 👑

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

---

and that's the whole page!! this was perl version v5.42.3, dated 2026-08-03, and it's the PERLHACKTUT(1) page — mwah, slay, go write a patch 💅✨🌸