perlhacktut(1)


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 also want
   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 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.

   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~.

   [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": 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++;
       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 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.

   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.

AUTHOR

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





Opportunity


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.





Free Software


Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.


Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.





Free Books


The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.


Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.





Education


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.


Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.