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

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

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

NNAAMMEE #

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

DDEESSCCRRIIPPTTIIOONN #

 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.

EEXXAAMMPPLLEE OOFF AA SSIIMMPPLLEE PPAATTCCHH #

 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.

WWrriittiinngg tthhee ppaattcchh 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 _p_p files. Sure enough, “pp_pack” is in _p_p_._c. Since we’re going to be altering this file, let’s copy it to _p_p_._c_~.

 [Well, it was in _p_p_._c when this tutorial was written. It has now been
 split off with "pp_unpack" to its own file, _p_p___p_a_c_k_._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.

TTeessttiinngg tthhee ppaattcchh The regression tests for each operator live in _t_/_o_p_/, and so we make a copy of _t_/_o_p_/_p_a_c_k_._t to _t_/_o_p_/_p_a_c_k_._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 ookk(()) 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
 iiss(()) 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 nnoott
 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!

DDooccuummeennttiinngg tthhee ppaattcchh 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 _p_o_d_/_p_e_r_l_f_u_n_c_._p_o_d; 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.

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

AAUUTTHHOORR #

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

perl v5.36.3 2019-02-13 PERLHACKTUT(1)