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4633a7c4 1=head1 NAME
2
360e660c 3perlXStut - Tutorial for writing XSUBs
4633a7c4 4
5=head1 DESCRIPTION
6
7This tutorial will educate the reader on the steps involved in creating
171891c7 8a Perl extension. The reader is assumed to have access to L<perlguts>,
9L<perlapi> and L<perlxs>.
4633a7c4 10
11This tutorial starts with very simple examples and becomes more complex,
c07a80fd 12with each new example adding new features. Certain concepts may not be
360e660c 13completely explained until later in the tutorial in order to slowly ease
14the reader into building extensions.
4633a7c4 15
360e660c 16This tutorial was written from a Unix point of view. Where I know them
17to be otherwise different for other platforms (e.g. Win32), I will list
18them. If you find something that was missed, please let me know.
4633a7c4 19
360e660c 20=head1 SPECIAL NOTES
c07a80fd 21
360e660c 22=head2 make
c07a80fd 23
360e660c 24This tutorial assumes that the make program that Perl is configured to
25use is called C<make>. Instead of running "make" in the examples that
26follow, you may have to substitute whatever make program Perl has been
5a3e7812 27configured to use. Running B<perl -V:make> should tell you what it is.
c07a80fd 28
360e660c 29=head2 Version caveat
c07a80fd 30
beb31b0b 31When writing a Perl extension for general consumption, one should expect that
32the extension will be used with versions of Perl different from the
33version available on your machine. Since you are reading this document,
34the version of Perl on your machine is probably 5.005 or later, but the users
35of your extension may have more ancient versions.
36
37To understand what kinds of incompatibilities one may expect, and in the rare
38case that the version of Perl on your machine is older than this document,
39see the section on "Troubleshooting these Examples" for more information.
40
41If your extension uses some features of Perl which are not available on older
42releases of Perl, your users would appreciate an early meaningful warning.
43You would probably put this information into the F<README> file, but nowadays
44installation of extensions may be performed automatically, guided by F<CPAN.pm>
45module or other tools.
46
47In MakeMaker-based installations, F<Makefile.PL> provides the earliest
48opportunity to perform version checks. One can put something like this
49in F<Makefile.PL> for this purpose:
50
51 eval { require 5.007 }
52 or die <<EOD;
53 ############
54 ### This module uses frobnication framework which is not available before
55 ### version 5.007 of Perl. Upgrade your Perl before installing Kara::Mba.
56 ############
57 EOD
c07a80fd 58
360e660c 59=head2 Dynamic Loading versus Static Loading
c07a80fd 60
61It is commonly thought that if a system does not have the capability to
360e660c 62dynamically load a library, you cannot build XSUBs. This is incorrect.
63You I<can> build them, but you must link the XSUBs subroutines with the
c07a80fd 64rest of Perl, creating a new executable. This situation is similar to
65Perl 4.
66
67This tutorial can still be used on such a system. The XSUB build mechanism
68will check the system and build a dynamically-loadable library if possible,
69or else a static library and then, optionally, a new statically-linked
70executable with that static library linked in.
71
72Should you wish to build a statically-linked executable on a system which
73can dynamically load libraries, you may, in all the following examples,
360e660c 74where the command "C<make>" with no arguments is executed, run the command
75"C<make perl>" instead.
c07a80fd 76
77If you have generated such a statically-linked executable by choice, then
360e660c 78instead of saying "C<make test>", you should say "C<make test_static>".
79On systems that cannot build dynamically-loadable libraries at all, simply
80saying "C<make test>" is sufficient.
81
82=head1 TUTORIAL
83
84Now let's go on with the show!
c07a80fd 85
86=head2 EXAMPLE 1
4633a7c4 87
88Our first extension will be very simple. When we call the routine in the
c07a80fd 89extension, it will print out a well-known message and return.
4633a7c4 90
360e660c 91Run "C<h2xs -A -n Mytest>". This creates a directory named Mytest,
92possibly under ext/ if that directory exists in the current working
93directory. Several files will be created in the Mytest dir, including
94MANIFEST, Makefile.PL, Mytest.pm, Mytest.xs, test.pl, and Changes.
4633a7c4 95
360e660c 96The MANIFEST file contains the names of all the files just created in the
97Mytest directory.
4633a7c4 98
99The file Makefile.PL should look something like this:
100
101 use ExtUtils::MakeMaker;
102 # See lib/ExtUtils/MakeMaker.pm for details of how to influence
103 # the contents of the Makefile that is written.
104 WriteMakefile(
360e660c 105 NAME => 'Mytest',
106 VERSION_FROM => 'Mytest.pm', # finds $VERSION
107 LIBS => [''], # e.g., '-lm'
108 DEFINE => '', # e.g., '-DHAVE_SOMETHING'
109 INC => '', # e.g., '-I/usr/include/other'
4633a7c4 110 );
111
791fa977 112The file Mytest.pm should start with something like this:
c07a80fd 113
791fa977 114 package Mytest;
4633a7c4 115
360e660c 116 use strict;
9f1b1f2d 117 use warnings;
360e660c 118
4633a7c4 119 require Exporter;
120 require DynaLoader;
c07a80fd 121
77ca0c92 122 our @ISA = qw(Exporter DynaLoader);
4633a7c4 123 # Items to export into callers namespace by default. Note: do not export
124 # names by default without a very good reason. Use EXPORT_OK instead.
125 # Do not simply export all your public functions/methods/constants.
77ca0c92 126 our @EXPORT = qw(
c07a80fd 127
4633a7c4 128 );
77ca0c92 129 our $VERSION = '0.01';
c07a80fd 130
791fa977 131 bootstrap Mytest $VERSION;
c07a80fd 132
4633a7c4 133 # Preloaded methods go here.
c07a80fd 134
4633a7c4 135 # Autoload methods go after __END__, and are processed by the autosplit program.
c07a80fd 136
4633a7c4 137 1;
138 __END__
c07a80fd 139 # Below is the stub of documentation for your module. You better edit it!
4633a7c4 140
360e660c 141The rest of the .pm file contains sample code for providing documentation for
142the extension.
143
144Finally, the Mytest.xs file should look something like this:
4633a7c4 145
146 #include "EXTERN.h"
147 #include "perl.h"
148 #include "XSUB.h"
791fa977 149
150 MODULE = Mytest PACKAGE = Mytest
4633a7c4 151
152Let's edit the .xs file by adding this to the end of the file:
153
154 void
155 hello()
360e660c 156 CODE:
4633a7c4 157 printf("Hello, world!\n");
158
360e660c 159It is okay for the lines starting at the "CODE:" line to not be indented.
160However, for readability purposes, it is suggested that you indent CODE:
161one level and the lines following one more level.
162
163Now we'll run "C<perl Makefile.PL>". This will create a real Makefile,
d9d2a7fb 164which make needs. Its output looks something like:
4633a7c4 165
166 % perl Makefile.PL
167 Checking if your kit is complete...
168 Looks good
791fa977 169 Writing Makefile for Mytest
4633a7c4 170 %
171
360e660c 172Now, running make will produce output that looks something like this (some
173long lines have been shortened for clarity and some extraneous lines have
174been deleted):
4633a7c4 175
176 % make
791fa977 177 umask 0 && cp Mytest.pm ./blib/Mytest.pm
178 perl xsubpp -typemap typemap Mytest.xs >Mytest.tc && mv Mytest.tc Mytest.c
360e660c 179 Please specify prototyping behavior for Mytest.xs (see perlxs manual)
791fa977 180 cc -c Mytest.c
181 Running Mkbootstrap for Mytest ()
182 chmod 644 Mytest.bs
183 LD_RUN_PATH="" ld -o ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl -b Mytest.o
184 chmod 755 ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl
185 cp Mytest.bs ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
186 chmod 644 ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
360e660c 187 Manifying ./blib/man3/Mytest.3
188 %
189
171891c7 190You can safely ignore the line about "prototyping behavior" - it is
191explained in the section "The PROTOTYPES: Keyword" in L<perlxs>.
360e660c 192
193If you are on a Win32 system, and the build process fails with linker
194errors for functions in the C library, check if your Perl is configured
5a3e7812 195to use PerlCRT (running B<perl -V:libc> should show you if this is the
360e660c 196case). If Perl is configured to use PerlCRT, you have to make sure
197PerlCRT.lib is copied to the same location that msvcrt.lib lives in,
198so that the compiler can find it on its own. msvcrt.lib is usually
199found in the Visual C compiler's lib directory (e.g. C:/DevStudio/VC/lib).
c07a80fd 200
360e660c 201Perl has its own special way of easily writing test scripts, but for this
202example only, we'll create our own test script. Create a file called hello
c07a80fd 203that looks like this:
204
c07a80fd 205 #! /opt/perl5/bin/perl
c47ff5f1 206
d9d2a7fb 207 use ExtUtils::testlib;
c47ff5f1 208
791fa977 209 use Mytest;
c47ff5f1 210
791fa977 211 Mytest::hello();
4633a7c4 212
360e660c 213Now we make the script executable (C<chmod -x hello>), run the script
214and we should see the following output:
4633a7c4 215
360e660c 216 % ./hello
4633a7c4 217 Hello, world!
218 %
219
c07a80fd 220=head2 EXAMPLE 2
4633a7c4 221
360e660c 222Now let's add to our extension a subroutine that will take a single numeric
223argument as input and return 0 if the number is even or 1 if the number
224is odd.
4633a7c4 225
791fa977 226Add the following to the end of Mytest.xs:
4633a7c4 227
228 int
229 is_even(input)
230 int input
360e660c 231 CODE:
c07a80fd 232 RETVAL = (input % 2 == 0);
360e660c 233 OUTPUT:
4633a7c4 234 RETVAL
235
360e660c 236There does not need to be white space at the start of the "C<int input>"
237line, but it is useful for improving readability. Placing a semi-colon at
238the end of that line is also optional. Any amount and kind of white space
239may be placed between the "C<int>" and "C<input>".
4633a7c4 240
360e660c 241Now re-run make to rebuild our new shared library.
4633a7c4 242
c07a80fd 243Now perform the same steps as before, generating a Makefile from the
244Makefile.PL file, and running make.
4633a7c4 245
360e660c 246In order to test that our extension works, we now need to look at the
c07a80fd 247file test.pl. This file is set up to imitate the same kind of testing
248structure that Perl itself has. Within the test script, you perform a
249number of tests to confirm the behavior of the extension, printing "ok"
d9d2a7fb 250when the test is correct, "not ok" when it is not. Change the print
251statement in the BEGIN block to print "1..4", and add the following code
252to the end of the file:
c07a80fd 253
791fa977 254 print &Mytest::is_even(0) == 1 ? "ok 2" : "not ok 2", "\n";
255 print &Mytest::is_even(1) == 0 ? "ok 3" : "not ok 3", "\n";
256 print &Mytest::is_even(2) == 1 ? "ok 4" : "not ok 4", "\n";
c07a80fd 257
360e660c 258We will be calling the test script through the command "C<make test>". You
c07a80fd 259should see output that looks something like this:
260
261 % make test
360e660c 262 PERL_DL_NONLAZY=1 /opt/perl5.004/bin/perl (lots of -I arguments) test.pl
c07a80fd 263 1..4
264 ok 1
265 ok 2
266 ok 3
267 ok 4
4633a7c4 268 %
269
360e660c 270=head2 What has gone on?
4633a7c4 271
272The program h2xs is the starting point for creating extensions. In later
c07a80fd 273examples we'll see how we can use h2xs to read header files and generate
4633a7c4 274templates to connect to C routines.
275
276h2xs creates a number of files in the extension directory. The file
277Makefile.PL is a perl script which will generate a true Makefile to build
278the extension. We'll take a closer look at it later.
279
360e660c 280The .pm and .xs files contain the meat of the extension. The .xs file holds
281the C routines that make up the extension. The .pm file contains routines
282that tell Perl how to load your extension.
283
284Generating the Makefile and running C<make> created a directory called blib
285(which stands for "build library") in the current working directory. This
286directory will contain the shared library that we will build. Once we have
287tested it, we can install it into its final location.
288
289Invoking the test script via "C<make test>" did something very important.
290It invoked perl with all those C<-I> arguments so that it could find the
291various files that are part of the extension. It is I<very> important that
292while you are still testing extensions that you use "C<make test>". If you
293try to run the test script all by itself, you will get a fatal error.
294Another reason it is important to use "C<make test>" to run your test
295script is that if you are testing an upgrade to an already-existing version,
296using "C<make test>" insures that you will test your new extension, not the
297already-existing version.
4633a7c4 298
c07a80fd 299When Perl sees a C<use extension;>, it searches for a file with the same name
360e660c 300as the C<use>'d extension that has a .pm suffix. If that file cannot be found,
c07a80fd 301Perl dies with a fatal error. The default search path is contained in the
360e660c 302C<@INC> array.
4633a7c4 303
791fa977 304In our case, Mytest.pm tells perl that it will need the Exporter and Dynamic
360e660c 305Loader extensions. It then sets the C<@ISA> and C<@EXPORT> arrays and the
306C<$VERSION> scalar; finally it tells perl to bootstrap the module. Perl
307will call its dynamic loader routine (if there is one) and load the shared
308library.
4633a7c4 309
360e660c 310The two arrays C<@ISA> and C<@EXPORT> are very important. The C<@ISA>
c07a80fd 311array contains a list of other packages in which to search for methods (or
360e660c 312subroutines) that do not exist in the current package. This is usually
313only important for object-oriented extensions (which we will talk about
314much later), and so usually doesn't need to be modified.
4633a7c4 315
360e660c 316The C<@EXPORT> array tells Perl which of the extension's variables and
317subroutines should be placed into the calling package's namespace. Because
318you don't know if the user has already used your variable and subroutine
319names, it's vitally important to carefully select what to export. Do I<not>
320export method or variable names I<by default> without a good reason.
4633a7c4 321
c07a80fd 322As a general rule, if the module is trying to be object-oriented then don't
360e660c 323export anything. If it's just a collection of functions and variables, then
324you can export them via another array, called C<@EXPORT_OK>. This array
325does not automatically place its subroutine and variable names into the
326namespace unless the user specifically requests that this be done.
4633a7c4 327
c07a80fd 328See L<perlmod> for more information.
4633a7c4 329
360e660c 330The C<$VERSION> variable is used to ensure that the .pm file and the shared
791fa977 331library are "in sync" with each other. Any time you make changes to
332the .pm or .xs files, you should increment the value of this variable.
333
360e660c 334=head2 Writing good test scripts
791fa977 335
336The importance of writing good test scripts cannot be overemphasized. You
337should closely follow the "ok/not ok" style that Perl itself uses, so that
338it is very easy and unambiguous to determine the outcome of each test case.
339When you find and fix a bug, make sure you add a test case for it.
340
360e660c 341By running "C<make test>", you ensure that your test.pl script runs and uses
791fa977 342the correct version of your extension. If you have many test cases, you
360e660c 343might want to copy Perl's test style. Create a directory named "t" in the
344extension's directory and append the suffix ".t" to the names of your test
345files. When you run "C<make test>", all of these test files will be executed.
4633a7c4 346
c07a80fd 347=head2 EXAMPLE 3
4633a7c4 348
349Our third extension will take one argument as its input, round off that
c07a80fd 350value, and set the I<argument> to the rounded value.
4633a7c4 351
791fa977 352Add the following to the end of Mytest.xs:
4633a7c4 353
354 void
355 round(arg)
356 double arg
360e660c 357 CODE:
4633a7c4 358 if (arg > 0.0) {
359 arg = floor(arg + 0.5);
360 } else if (arg < 0.0) {
361 arg = ceil(arg - 0.5);
362 } else {
363 arg = 0.0;
364 }
360e660c 365 OUTPUT:
4633a7c4 366 arg
367
c07a80fd 368Edit the Makefile.PL file so that the corresponding line looks like this:
4633a7c4 369
370 'LIBS' => ['-lm'], # e.g., '-lm'
371
360e660c 372Generate the Makefile and run make. Change the BEGIN block to print
c07a80fd 373"1..9" and add the following to test.pl:
4633a7c4 374
791fa977 375 $i = -1.5; &Mytest::round($i); print $i == -2.0 ? "ok 5" : "not ok 5", "\n";
376 $i = -1.1; &Mytest::round($i); print $i == -1.0 ? "ok 6" : "not ok 6", "\n";
377 $i = 0.0; &Mytest::round($i); print $i == 0.0 ? "ok 7" : "not ok 7", "\n";
378 $i = 0.5; &Mytest::round($i); print $i == 1.0 ? "ok 8" : "not ok 8", "\n";
379 $i = 1.2; &Mytest::round($i); print $i == 1.0 ? "ok 9" : "not ok 9", "\n";
c07a80fd 380
360e660c 381Running "C<make test>" should now print out that all nine tests are okay.
4633a7c4 382
360e660c 383Notice that in these new test cases, the argument passed to round was a
384scalar variable. You might be wondering if you can round a constant or
385literal. To see what happens, temporarily add the following line to test.pl:
4633a7c4 386
791fa977 387 &Mytest::round(3);
4633a7c4 388
360e660c 389Run "C<make test>" and notice that Perl dies with a fatal error. Perl won't
390let you change the value of constants!
4633a7c4 391
360e660c 392=head2 What's new here?
4633a7c4 393
360e660c 394=over 4
4633a7c4 395
360e660c 396=item *
4633a7c4 397
360e660c 398We've made some changes to Makefile.PL. In this case, we've specified an
399extra library to be linked into the extension's shared library, the math
400library libm in this case. We'll talk later about how to write XSUBs that
401can call every routine in a library.
4633a7c4 402
360e660c 403=item *
404
405The value of the function is not being passed back as the function's return
406value, but by changing the value of the variable that was passed into the
407function. You might have guessed that when you saw that the return value
408of round is of type "void".
409
410=back
411
412=head2 Input and Output Parameters
4633a7c4 413
360e660c 414You specify the parameters that will be passed into the XSUB on the line(s)
415after you declare the function's return value and name. Each input parameter
416line starts with optional white space, and may have an optional terminating
417semicolon.
4633a7c4 418
360e660c 419The list of output parameters occurs at the very end of the function, just
420before after the OUTPUT: directive. The use of RETVAL tells Perl that you
421wish to send this value back as the return value of the XSUB function. In
422Example 3, we wanted the "return value" placed in the original variable
423which we passed in, so we listed it (and not RETVAL) in the OUTPUT: section.
4633a7c4 424
360e660c 425=head2 The XSUBPP Program
426
beb31b0b 427The B<xsubpp> program takes the XS code in the .xs file and translates it into
4633a7c4 428C code, placing it in a file whose suffix is .c. The C code created makes
429heavy use of the C functions within Perl.
430
360e660c 431=head2 The TYPEMAP file
4633a7c4 432
beb31b0b 433The B<xsubpp> program uses rules to convert from Perl's data types (scalar,
360e660c 434array, etc.) to C's data types (int, char, etc.). These rules are stored
4633a7c4 435in the typemap file ($PERLLIB/ExtUtils/typemap). This file is split into
436three parts.
437
360e660c 438The first section maps various C data types to a name, which corresponds
439somewhat with the various Perl types. The second section contains C code
beb31b0b 440which B<xsubpp> uses to handle input parameters. The third section contains
441C code which B<xsubpp> uses to handle output parameters.
4633a7c4 442
360e660c 443Let's take a look at a portion of the .c file created for our extension.
444The file name is Mytest.c:
4633a7c4 445
791fa977 446 XS(XS_Mytest_round)
4633a7c4 447 {
448 dXSARGS;
c07a80fd 449 if (items != 1)
791fa977 450 croak("Usage: Mytest::round(arg)");
4633a7c4 451 {
c07a80fd 452 double arg = (double)SvNV(ST(0)); /* XXXXX */
4633a7c4 453 if (arg > 0.0) {
454 arg = floor(arg + 0.5);
455 } else if (arg < 0.0) {
456 arg = ceil(arg - 0.5);
c07a80fd 457 } else {
458 arg = 0.0;
4633a7c4 459 }
360e660c 460 sv_setnv(ST(0), (double)arg); /* XXXXX */
4633a7c4 461 }
462 XSRETURN(1);
463 }
4633a7c4 464
360e660c 465Notice the two lines commented with "XXXXX". If you check the first section
466of the typemap file, you'll see that doubles are of type T_DOUBLE. In the
4633a7c4 467INPUT section, an argument that is T_DOUBLE is assigned to the variable
468arg by calling the routine SvNV on something, then casting it to double,
469then assigned to the variable arg. Similarly, in the OUTPUT section,
ef50df4b 470once arg has its final value, it is passed to the sv_setnv function to
471be passed back to the calling subroutine. These two functions are explained
472in L<perlguts>; we'll talk more later about what that "ST(0)" means in the
473section on the argument stack.
4633a7c4 474
360e660c 475=head2 Warning about Output Arguments
4633a7c4 476
c07a80fd 477In general, it's not a good idea to write extensions that modify their input
360e660c 478parameters, as in Example 3. Instead, you should probably return multiple
479values in an array and let the caller handle them (we'll do this in a later
a2293a43 480example). However, in order to better accommodate calling pre-existing C
360e660c 481routines, which often do modify their input parameters, this behavior is
482tolerated.
791fa977 483
484=head2 EXAMPLE 4
485
68dc0745 486In this example, we'll now begin to write XSUBs that will interact with
360e660c 487pre-defined C libraries. To begin with, we will build a small library of
791fa977 488our own, then let h2xs write our .pm and .xs files for us.
489
490Create a new directory called Mytest2 at the same level as the directory
491Mytest. In the Mytest2 directory, create another directory called mylib,
492and cd into that directory.
493
494Here we'll create some files that will generate a test library. These will
495include a C source file and a header file. We'll also create a Makefile.PL
496in this directory. Then we'll make sure that running make at the Mytest2
497level will automatically run this Makefile.PL file and the resulting Makefile.
498
9693b09d 499In the mylib directory, create a file mylib.h that looks like this:
791fa977 500
501 #define TESTVAL 4
502
503 extern double foo(int, long, const char*);
504
505Also create a file mylib.c that looks like this:
506
507 #include <stdlib.h>
508 #include "./mylib.h"
c47ff5f1 509
791fa977 510 double
360e660c 511 foo(int a, long b, const char *c)
791fa977 512 {
513 return (a + b + atof(c) + TESTVAL);
514 }
515
516And finally create a file Makefile.PL that looks like this:
517
518 use ExtUtils::MakeMaker;
519 $Verbose = 1;
520 WriteMakefile(
360e660c 521 NAME => 'Mytest2::mylib',
522 SKIP => [qw(all static static_lib dynamic dynamic_lib)],
523 clean => {'FILES' => 'libmylib$(LIBEEXT)'},
791fa977 524 );
525
526
8227f81c 527 sub MY::top_targets {
791fa977 528 '
529 all :: static
530
360e660c 531 pure_all :: static
532
791fa977 533 static :: libmylib$(LIB_EXT)
534
535 libmylib$(LIB_EXT): $(O_FILES)
536 $(AR) cr libmylib$(LIB_EXT) $(O_FILES)
537 $(RANLIB) libmylib$(LIB_EXT)
538
539 ';
540 }
541
360e660c 542Make sure you use a tab and not spaces on the lines beginning with "$(AR)"
543and "$(RANLIB)". Make will not function properly if you use spaces.
544It has also been reported that the "cr" argument to $(AR) is unnecessary
545on Win32 systems.
546
791fa977 547We will now create the main top-level Mytest2 files. Change to the directory
548above Mytest2 and run the following command:
549
d9d2a7fb 550 % h2xs -O -n Mytest2 ./Mytest2/mylib/mylib.h
791fa977 551
552This will print out a warning about overwriting Mytest2, but that's okay.
553Our files are stored in Mytest2/mylib, and will be untouched.
554
555The normal Makefile.PL that h2xs generates doesn't know about the mylib
556directory. We need to tell it that there is a subdirectory and that we
360e660c 557will be generating a library in it. Let's add the argument MYEXTLIB to
558the WriteMakefile call so that it looks like this:
4633a7c4 559
360e660c 560 WriteMakefile(
561 'NAME' => 'Mytest2',
562 'VERSION_FROM' => 'Mytest2.pm', # finds $VERSION
563 'LIBS' => [''], # e.g., '-lm'
564 'DEFINE' => '', # e.g., '-DHAVE_SOMETHING'
565 'INC' => '', # e.g., '-I/usr/include/other'
566 'MYEXTLIB' => 'mylib/libmylib$(LIB_EXT)',
567 );
791fa977 568
360e660c 569and then at the end add a subroutine (which will override the pre-existing
570subroutine). Remember to use a tab character to indent the line beginning
571with "cd"!
791fa977 572
573 sub MY::postamble {
574 '
575 $(MYEXTLIB): mylib/Makefile
360e660c 576 cd mylib && $(MAKE) $(PASSTHRU)
791fa977 577 ';
578 }
579
791fa977 580Let's also fix the MANIFEST file so that it accurately reflects the contents
581of our extension. The single line that says "mylib" should be replaced by
582the following three lines:
583
584 mylib/Makefile.PL
585 mylib/mylib.c
586 mylib/mylib.h
587
588To keep our namespace nice and unpolluted, edit the .pm file and change
77ca0c92 589the variable C<@EXPORT> to C<@EXPORT_OK>. Finally, in the
d9d2a7fb 590.xs file, edit the #include line to read:
791fa977 591
592 #include "mylib/mylib.h"
593
594And also add the following function definition to the end of the .xs file:
595
596 double
597 foo(a,b,c)
598 int a
599 long b
600 const char * c
360e660c 601 OUTPUT:
791fa977 602 RETVAL
603
604Now we also need to create a typemap file because the default Perl doesn't
360e660c 605currently support the const char * type. Create a file called typemap in
606the Mytest2 directory and place the following in it:
791fa977 607
608 const char * T_PV
609
610Now run perl on the top-level Makefile.PL. Notice that it also created a
360e660c 611Makefile in the mylib directory. Run make and watch that it does cd into
791fa977 612the mylib directory and run make in there as well.
613
614Now edit the test.pl script and change the BEGIN block to print "1..4",
615and add the following lines to the end of the script:
616
617 print &Mytest2::foo(1, 2, "Hello, world!") == 7 ? "ok 2\n" : "not ok 2\n";
618 print &Mytest2::foo(1, 2, "0.0") == 7 ? "ok 3\n" : "not ok 3\n";
619 print abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 ? "ok 4\n" : "not ok 4\n";
620
360e660c 621(When dealing with floating-point comparisons, it is best to not check for
622equality, but rather that the difference between the expected and actual
623result is below a certain amount (called epsilon) which is 0.01 in this case)
791fa977 624
360e660c 625Run "C<make test>" and all should be well.
791fa977 626
360e660c 627=head2 What has happened here?
791fa977 628
629Unlike previous examples, we've now run h2xs on a real include file. This
630has caused some extra goodies to appear in both the .pm and .xs files.
631
84dc3c4d 632=over 4
633
791fa977 634=item *
635
360e660c 636In the .xs file, there's now a #include directive with the absolute path to
637the mylib.h header file. We changed this to a relative path so that we
638could move the extension directory if we wanted to.
791fa977 639
640=item *
641
642There's now some new C code that's been added to the .xs file. The purpose
643of the C<constant> routine is to make the values that are #define'd in the
360e660c 644header file accessible by the Perl script (by calling either C<TESTVAL> or
645C<&Mytest2::TESTVAL>). There's also some XS code to allow calls to the
791fa977 646C<constant> routine.
647
648=item *
649
360e660c 650The .pm file originally exported the name C<TESTVAL> in the C<@EXPORT> array.
651This could lead to name clashes. A good rule of thumb is that if the #define
652is only going to be used by the C routines themselves, and not by the user,
653they should be removed from the C<@EXPORT> array. Alternately, if you don't
654mind using the "fully qualified name" of a variable, you could move most
655or all of the items from the C<@EXPORT> array into the C<@EXPORT_OK> array.
791fa977 656
d9d2a7fb 657=item *
658
360e660c 659If our include file had contained #include directives, these would not have
660been processed by h2xs. There is no good solution to this right now.
d9d2a7fb 661
360e660c 662=item *
791fa977 663
664We've also told Perl about the library that we built in the mylib
360e660c 665subdirectory. That required only the addition of the C<MYEXTLIB> variable
791fa977 666to the WriteMakefile call and the replacement of the postamble subroutine
667to cd into the subdirectory and run make. The Makefile.PL for the
668library is a bit more complicated, but not excessively so. Again we
669replaced the postamble subroutine to insert our own code. This code
360e660c 670simply specified that the library to be created here was a static archive
671library (as opposed to a dynamically loadable library) and provided the
791fa977 672commands to build it.
4633a7c4 673
360e660c 674=back
675
beb31b0b 676=head2 Anatomy of .xs file
677
678The .xs file of L<"EXAMPLE 4"> contained some new elements. To understand
679the meaning of these elements, pay attention to the line which reads
680
681 MODULE = Mytest2 PACKAGE = Mytest2
682
683Anything before this line is plain C code which describes which headers
684to include, and defines some convenience functions. No translations are
7817ba4d 685performed on this part, apart from having embedded POD documentation
686skipped over (see L<perlpod>) it goes into the generated output C file as is.
beb31b0b 687
688Anything after this line is the description of XSUB functions.
689These descriptions are translated by B<xsubpp> into C code which
690implements these functions using Perl calling conventions, and which
691makes these functions visible from Perl interpreter.
692
693Pay a special attention to the function C<constant>. This name appears
694twice in the generated .xs file: once in the first part, as a static C
695function, the another time in the second part, when an XSUB interface to
696this static C function is defined.
697
698This is quite typical for .xs files: usually the .xs file provides
699an interface to an existing C function. Then this C function is defined
700somewhere (either in an external library, or in the first part of .xs file),
701and a Perl interface to this function (i.e. "Perl glue") is described in the
702second part of .xs file. The situation in L<"EXAMPLE 1">, L<"EXAMPLE 2">,
703and L<"EXAMPLE 3">, when all the work is done inside the "Perl glue", is
704somewhat of an exception rather than the rule.
705
706=head2 Getting the fat out of XSUBs
707
708In L<"EXAMPLE 4"> the second part of .xs file contained the following
709description of an XSUB:
710
711 double
712 foo(a,b,c)
713 int a
714 long b
715 const char * c
716 OUTPUT:
717 RETVAL
718
719Note that in contrast with L<"EXAMPLE 1">, L<"EXAMPLE 2"> and L<"EXAMPLE 3">,
720this description does not contain the actual I<code> for what is done
721is done during a call to Perl function foo(). To understand what is going
722on here, one can add a CODE section to this XSUB:
723
724 double
725 foo(a,b,c)
726 int a
727 long b
728 const char * c
729 CODE:
730 RETVAL = foo(a,b,c);
731 OUTPUT:
732 RETVAL
733
734However, these two XSUBs provide almost identical generated C code: B<xsubpp>
735compiler is smart enough to figure out the C<CODE:> section from the first
736two lines of the description of XSUB. What about C<OUTPUT:> section? In
737fact, that is absolutely the same! The C<OUTPUT:> section can be removed
738as well, I<as far as C<CODE:> section or C<PPCODE:> section> is not
739specified: B<xsubpp> can see that it needs to generate a function call
740section, and will autogenerate the OUTPUT section too. Thus one can
741shortcut the XSUB to become:
742
743 double
744 foo(a,b,c)
745 int a
746 long b
747 const char * c
748
749Can we do the same with an XSUB
750
751 int
752 is_even(input)
753 int input
754 CODE:
755 RETVAL = (input % 2 == 0);
756 OUTPUT:
757 RETVAL
758
759of L<"EXAMPLE 2">? To do this, one needs to define a C function C<int
760is_even(int input)>. As we saw in L<Anatomy of .xs file>, a proper place
761for this definition is in the first part of .xs file. In fact a C function
762
763 int
764 is_even(int arg)
765 {
766 return (arg % 2 == 0);
767 }
768
769is probably overkill for this. Something as simple as a C<#define> will
770do too:
771
772 #define is_even(arg) ((arg) % 2 == 0)
773
774After having this in the first part of .xs file, the "Perl glue" part becomes
775as simple as
776
777 int
778 is_even(input)
779 int input
780
781This technique of separation of the glue part from the workhorse part has
782obvious tradeoffs: if you want to change a Perl interface, you need to
783change two places in your code. However, it removes a lot of clutter,
784and makes the workhorse part independent from idiosyncrasies of Perl calling
785convention. (In fact, there is nothing Perl-specific in the above description,
786a different version of B<xsubpp> might have translated this to TCL glue or
787Python glue as well.)
788
789=head2 More about XSUB arguments
4633a7c4 790
791fa977 791With the completion of Example 4, we now have an easy way to simulate some
c07a80fd 792real-life libraries whose interfaces may not be the cleanest in the world.
793We shall now continue with a discussion of the arguments passed to the
beb31b0b 794B<xsubpp> compiler.
4633a7c4 795
360e660c 796When you specify arguments to routines in the .xs file, you are really
797passing three pieces of information for each argument listed. The first
798piece is the order of that argument relative to the others (first, second,
799etc). The second is the type of argument, and consists of the type
800declaration of the argument (e.g., int, char*, etc). The third piece is
beb31b0b 801the calling convention for the argument in the call to the library function.
802
803While Perl passes arguments to functions by reference,
804C passes arguments by value; to implement a C function which modifies data
805of one of the "arguments", the actual argument of this C function would be
806a pointer to the data. Thus two C functions with declarations
807
808 int string_length(char *s);
809 int upper_case_char(char *cp);
810
811may have completely different semantics: the first one may inspect an array
812of chars pointed by s, and the second one may immediately dereference C<cp>
813and manipulate C<*cp> only (using the return value as, say, a success
814indicator). From Perl one would use these functions in
815a completely different manner.
816
817One conveys this info to B<xsubpp> by replacing C<*> before the
818argument by C<&>. C<&> means that the argument should be passed to a library
819function by its address. The above two function may be XSUB-ified as
820
821 int
822 string_length(s)
823 char * s
824
825 int
826 upper_case_char(cp)
827 char &cp
4633a7c4 828
beb31b0b 829For example, consider:
4633a7c4 830
4633a7c4 831 int
c07a80fd 832 foo(a,b)
833 char &a
834 char * b
4633a7c4 835
beb31b0b 836The first Perl argument to this function would be treated as a char and assigned
c07a80fd 837to the variable a, and its address would be passed into the function foo.
beb31b0b 838The second Perl argument would be treated as a string pointer and assigned to the
c07a80fd 839variable b. The I<value> of b would be passed into the function foo. The
beb31b0b 840actual call to the function foo that B<xsubpp> generates would look like this:
4633a7c4 841
c07a80fd 842 foo(&a, b);
4633a7c4 843
beb31b0b 844B<xsubpp> will parse the following function argument lists identically:
791fa977 845
846 char &a
847 char&a
848 char & a
849
850However, to help ease understanding, it is suggested that you place a "&"
851next to the variable name and away from the variable type), and place a
852"*" near the variable type, but away from the variable name (as in the
360e660c 853call to foo above). By doing so, it is easy to understand exactly what
854will be passed to the C function -- it will be whatever is in the "last
855column".
4633a7c4 856
c07a80fd 857You should take great pains to try to pass the function the type of variable
858it wants, when possible. It will save you a lot of trouble in the long run.
4633a7c4 859
360e660c 860=head2 The Argument Stack
4633a7c4 861
c07a80fd 862If we look at any of the C code generated by any of the examples except
863example 1, you will notice a number of references to ST(n), where n is
360e660c 864usually 0. "ST" is actually a macro that points to the n'th argument
865on the argument stack. ST(0) is thus the first argument on the stack and
866therefore the first argument passed to the XSUB, ST(1) is the second
867argument, and so on.
4633a7c4 868
beb31b0b 869When you list the arguments to the XSUB in the .xs file, that tells B<xsubpp>
c07a80fd 870which argument corresponds to which of the argument stack (i.e., the first
871one listed is the first argument, and so on). You invite disaster if you
872do not list them in the same order as the function expects them.
4633a7c4 873
360e660c 874The actual values on the argument stack are pointers to the values passed
875in. When an argument is listed as being an OUTPUT value, its corresponding
876value on the stack (i.e., ST(0) if it was the first argument) is changed.
877You can verify this by looking at the C code generated for Example 3.
878The code for the round() XSUB routine contains lines that look like this:
879
880 double arg = (double)SvNV(ST(0));
881 /* Round the contents of the variable arg */
882 sv_setnv(ST(0), (double)arg);
883
884The arg variable is initially set by taking the value from ST(0), then is
885stored back into ST(0) at the end of the routine.
886
beb31b0b 887XSUBs are also allowed to return lists, not just scalars. This must be
888done by manipulating stack values ST(0), ST(1), etc, in a subtly
889different way. See L<perlxs> for details.
890
891XSUBs are also allowed to avoid automatic conversion of Perl function arguments
892to C function arguments. See L<perlxs> for details. Some people prefer
893manual conversion by inspecting C<ST(i)> even in the cases when automatic
894conversion will do, arguing that this makes the logic of an XSUB call clearer.
895Compare with L<"Getting the fat out of XSUBs"> for a similar tradeoff of
896a complete separation of "Perl glue" and "workhorse" parts of an XSUB.
897
898While experts may argue about these idioms, a novice to Perl guts may
899prefer a way which is as little Perl-guts-specific as possible, meaning
900automatic conversion and automatic call generation, as in
901L<"Getting the fat out of XSUBs">. This approach has the additional
902benefit of protecting the XSUB writer from future changes to the Perl API.
903
360e660c 904=head2 Extending your Extension
4633a7c4 905
c07a80fd 906Sometimes you might want to provide some extra methods or subroutines
907to assist in making the interface between Perl and your extension simpler
908or easier to understand. These routines should live in the .pm file.
909Whether they are automatically loaded when the extension itself is loaded
360e660c 910or only loaded when called depends on where in the .pm file the subroutine
4a4eefd0 911definition is placed. You can also consult L<AutoLoader> for an alternate
360e660c 912way to store and load your extra subroutines.
4633a7c4 913
360e660c 914=head2 Documenting your Extension
4633a7c4 915
c07a80fd 916There is absolutely no excuse for not documenting your extension.
917Documentation belongs in the .pm file. This file will be fed to pod2man,
3958b146 918and the embedded documentation will be converted to the manpage format,
919then placed in the blib directory. It will be copied to Perl's
920manpage directory when the extension is installed.
4633a7c4 921
c07a80fd 922You may intersperse documentation and Perl code within the .pm file.
923In fact, if you want to use method autoloading, you must do this,
924as the comment inside the .pm file explains.
4633a7c4 925
c07a80fd 926See L<perlpod> for more information about the pod format.
4633a7c4 927
360e660c 928=head2 Installing your Extension
4633a7c4 929
c07a80fd 930Once your extension is complete and passes all its tests, installing it
360e660c 931is quite simple: you simply run "make install". You will either need
c07a80fd 932to have write permission into the directories where Perl is installed,
933or ask your system administrator to run the make for you.
4633a7c4 934
360e660c 935Alternately, you can specify the exact directory to place the extension's
936files by placing a "PREFIX=/destination/directory" after the make install.
937(or in between the make and install if you have a brain-dead version of make).
938This can be very useful if you are building an extension that will eventually
939be distributed to multiple systems. You can then just archive the files in
940the destination directory and distribute them to your destination systems.
941
942=head2 EXAMPLE 5
943
944In this example, we'll do some more work with the argument stack. The
945previous examples have all returned only a single value. We'll now
946create an extension that returns an array.
947
948This extension is very Unix-oriented (struct statfs and the statfs system
949call). If you are not running on a Unix system, you can substitute for
950statfs any other function that returns multiple values, you can hard-code
951values to be returned to the caller (although this will be a bit harder
952to test the error case), or you can simply not do this example. If you
953change the XSUB, be sure to fix the test cases to match the changes.
954
955Return to the Mytest directory and add the following code to the end of
956Mytest.xs:
957
958 void
959 statfs(path)
960 char * path
beb31b0b 961 INIT:
360e660c 962 int i;
963 struct statfs buf;
964
965 PPCODE:
966 i = statfs(path, &buf);
967 if (i == 0) {
968 XPUSHs(sv_2mortal(newSVnv(buf.f_bavail)));
969 XPUSHs(sv_2mortal(newSVnv(buf.f_bfree)));
970 XPUSHs(sv_2mortal(newSVnv(buf.f_blocks)));
971 XPUSHs(sv_2mortal(newSVnv(buf.f_bsize)));
972 XPUSHs(sv_2mortal(newSVnv(buf.f_ffree)));
973 XPUSHs(sv_2mortal(newSVnv(buf.f_files)));
974 XPUSHs(sv_2mortal(newSVnv(buf.f_type)));
975 XPUSHs(sv_2mortal(newSVnv(buf.f_fsid[0])));
976 XPUSHs(sv_2mortal(newSVnv(buf.f_fsid[1])));
977 } else {
978 XPUSHs(sv_2mortal(newSVnv(errno)));
979 }
980
981You'll also need to add the following code to the top of the .xs file, just
982after the include of "XSUB.h":
983
984 #include <sys/vfs.h>
985
986Also add the following code segment to test.pl while incrementing the "1..9"
987string in the BEGIN block to "1..11":
988
989 @a = &Mytest::statfs("/blech");
990 print ((scalar(@a) == 1 && $a[0] == 2) ? "ok 10\n" : "not ok 10\n");
991 @a = &Mytest::statfs("/");
992 print scalar(@a) == 9 ? "ok 11\n" : "not ok 11\n";
993
994=head2 New Things in this Example
995
996This example added quite a few new concepts. We'll take them one at a time.
997
998=over 4
999
1000=item *
1001
beb31b0b 1002The INIT: directive contains code that will be placed immediately after
1003the argument stack is decoded. C does not allow variable declarations at
1004arbitrary locations inside a function,
360e660c 1005so this is usually the best way to declare local variables needed by the XSUB.
beb31b0b 1006(Alternatively, one could put the whole C<PPCODE:> section into braces, and
1007put these declarations on top.)
360e660c 1008
1009=item *
1010
1011This routine also returns a different number of arguments depending on the
1012success or failure of the call to statfs. If there is an error, the error
1013number is returned as a single-element array. If the call is successful,
1014then a 9-element array is returned. Since only one argument is passed into
1015this function, we need room on the stack to hold the 9 values which may be
1016returned.
1017
1018We do this by using the PPCODE: directive, rather than the CODE: directive.
beb31b0b 1019This tells B<xsubpp> that we will be managing the return values that will be
360e660c 1020put on the argument stack by ourselves.
1021
1022=item *
1023
1024When we want to place values to be returned to the caller onto the stack,
1025we use the series of macros that begin with "XPUSH". There are five
1026different versions, for placing integers, unsigned integers, doubles,
1027strings, and Perl scalars on the stack. In our example, we placed a
beb31b0b 1028Perl scalar onto the stack. (In fact this is the only macro which
1029can be used to return multiple values.)
360e660c 1030
1031The XPUSH* macros will automatically extend the return stack to prevent
1032it from being overrun. You push values onto the stack in the order you
1033want them seen by the calling program.
1034
1035=item *
1036
1037The values pushed onto the return stack of the XSUB are actually mortal SV's.
1038They are made mortal so that once the values are copied by the calling
1039program, the SV's that held the returned values can be deallocated.
1040If they were not mortal, then they would continue to exist after the XSUB
1041routine returned, but would not be accessible. This is a memory leak.
1042
beb31b0b 1043=item *
1044
1045If we were interested in performance, not in code compactness, in the success
1046branch we would not use C<XPUSHs> macros, but C<PUSHs> macros, and would
1047pre-extend the stack before pushing the return values:
1048
1049 EXTEND(SP, 9);
1050
1051The tradeoff is that one needs to calculate the number of return values
1052in advance (though overextending the stack will not typically hurt
1053anything but memory consumption).
1054
1055Similarly, in the failure branch we could use C<PUSHs> I<without> extending
1056the stack: the Perl function reference comes to an XSUB on the stack, thus
1057the stack is I<always> large enough to take one return value.
1058
360e660c 1059=back
1060
171891c7 1061=head2 EXAMPLE 6
360e660c 1062
171891c7 1063In this example, we will accept a reference to an array as an input
1064parameter, and return a reference to an array of hashes. This will
1065demonstrate manipulation of complex Perl data types from an XSUB.
1066
1067This extension is somewhat contrived. It is based on the code in
1068the previous example. It calls the statfs function multiple times,
1069accepting a reference to an array of filenames as input, and returning
1070a reference to an array of hashes containing the data for each of the
1071filesystems.
1072
1073Return to the Mytest directory and add the following code to the end of
1074Mytest.xs:
1075
1076 SV *
1077 multi_statfs(paths)
1078 SV * paths
1079 INIT:
1080 AV * results;
1081 I32 numpaths = 0;
1082 int i, n;
1083 struct statfs buf;
1084
1085 if ((!SvROK(paths))
1086 || (SvTYPE(SvRV(paths)) != SVt_PVAV)
1087 || ((numpaths = av_len((AV *)SvRV(paths))) < 0))
1088 {
1089 XSRETURN_UNDEF;
1090 }
1091 results = (AV *)sv_2mortal((SV *)newAV());
1092 CODE:
1093 for (n = 0; n <= numpaths; n++) {
1094 HV * rh;
1095 STRLEN l;
1096 char * fn = SvPV(*av_fetch((AV *)SvRV(paths), n, 0), l);
cea6626f 1097
171891c7 1098 i = statfs(fn, &buf);
1099 if (i != 0) {
1100 av_push(results, newSVnv(errno));
1101 continue;
1102 }
cea6626f 1103
171891c7 1104 rh = (HV *)sv_2mortal((SV *)newHV());
cea6626f 1105
171891c7 1106 hv_store(rh, "f_bavail", 8, newSVnv(buf.f_bavail), 0);
1107 hv_store(rh, "f_bfree", 7, newSVnv(buf.f_bfree), 0);
1108 hv_store(rh, "f_blocks", 8, newSVnv(buf.f_blocks), 0);
1109 hv_store(rh, "f_bsize", 7, newSVnv(buf.f_bsize), 0);
1110 hv_store(rh, "f_ffree", 7, newSVnv(buf.f_ffree), 0);
1111 hv_store(rh, "f_files", 7, newSVnv(buf.f_files), 0);
1112 hv_store(rh, "f_type", 6, newSVnv(buf.f_type), 0);
cea6626f 1113
171891c7 1114 av_push(results, newRV((SV *)rh));
1115 }
1116 RETVAL = newRV((SV *)results);
1117 OUTPUT:
1118 RETVAL
1119
1120And add the following code to test.pl, while incrementing the "1..11"
1121string in the BEGIN block to "1..13":
1122
1123 $results = Mytest::multi_statfs([ '/', '/blech' ]);
1124 print ((ref $results->[0]) ? "ok 12\n" : "not ok 12\n");
1125 print ((! ref $results->[1]) ? "ok 13\n" : "not ok 13\n");
1126
1127=head2 New Things in this Example
1128
1129There are a number of new concepts introduced here, described below:
1130
1131=over 4
1132
1133=item *
1134
1135This function does not use a typemap. Instead, we declare it as accepting
1136one SV* (scalar) parameter, and returning an SV* value, and we take care of
1137populating these scalars within the code. Because we are only returning
1138one value, we don't need a C<PPCODE:> directive - instead, we use C<CODE:>
1139and C<OUTPUT:> directives.
1140
1141=item *
1142
1143When dealing with references, it is important to handle them with caution.
1144The C<INIT:> block first checks that
1145C<SvROK> returns true, which indicates that paths is a valid reference. It
1146then verifies that the object referenced by paths is an array, using C<SvRV>
1147to dereference paths, and C<SvTYPE> to discover its type. As an added test,
1148it checks that the array referenced by paths is non-empty, using the C<av_len>
1149function (which returns -1 if the array is empty). The XSRETURN_UNDEF macro
1150is used to abort the XSUB and return the undefined value whenever all three of
1151these conditions are not met.
1152
1153=item *
1154
1155We manipulate several arrays in this XSUB. Note that an array is represented
1156internally by an AV* pointer. The functions and macros for manipulating
1157arrays are similar to the functions in Perl: C<av_len> returns the highest
1158index in an AV*, much like $#array; C<av_fetch> fetches a single scalar value
1159from an array, given its index; C<av_push> pushes a scalar value onto the
1160end of the array, automatically extending the array as necessary.
1161
1162Specifically, we read pathnames one at a time from the input array, and
1163store the results in an output array (results) in the same order. If
1164statfs fails, the element pushed onto the return array is the value of
1165errno after the failure. If statfs succeeds, though, the value pushed
1166onto the return array is a reference to a hash containing some of the
1167information in the statfs structure.
1168
1169As with the return stack, it would be possible (and a small performance win)
1170to pre-extend the return array before pushing data into it, since we know
1171how many elements we will return:
1172
1173 av_extend(results, numpaths);
1174
1175=item *
1176
1177We are performing only one hash operation in this function, which is storing
1178a new scalar under a key using C<hv_store>. A hash is represented by an HV*
1179pointer. Like arrays, the functions for manipulating hashes from an XSUB
1180mirror the functionality available from Perl. See L<perlguts> and L<perlapi>
1181for details.
1182
1183=item *
1184
1185To create a reference, we use the C<newRV> function. Note that you can
1186cast an AV* or an HV* to type SV* in this case (and many others). This
1187allows you to take references to arrays, hashes and scalars with the same
1188function. Conversely, the C<SvRV> function always returns an SV*, which may
da75cd15 1189need to be cast to the appropriate type if it is something other than a
171891c7 1190scalar (check with C<SvTYPE>).
1191
1192=item *
1193
1194At this point, xsubpp is doing very little work - the differences between
1195Mytest.xs and Mytest.c are minimal.
1196
1197=back
360e660c 1198
1199=head2 EXAMPLE 7 (Coming Soon)
1200
1201XPUSH args AND set RETVAL AND assign return value to array
1202
1203=head2 EXAMPLE 8 (Coming Soon)
1204
1205Setting $!
1206
1207=head2 EXAMPLE 9 (Coming Soon)
1208
1209Getting fd's from filehandles
1210
1211=head2 Troubleshooting these Examples
1212
1213As mentioned at the top of this document, if you are having problems with
1214these example extensions, you might see if any of these help you.
1215
1216=over 4
1217
1218=item *
1219
1220In versions of 5.002 prior to the gamma version, the test script in Example
12211 will not function properly. You need to change the "use lib" line to
1222read:
1223
1224 use lib './blib';
1225
1226=item *
1227
1228In versions of 5.002 prior to version 5.002b1h, the test.pl file was not
1229automatically created by h2xs. This means that you cannot say "make test"
1230to run the test script. You will need to add the following line before the
1231"use extension" statement:
1232
1233 use lib './blib';
1234
1235=item *
1236
1237In versions 5.000 and 5.001, instead of using the above line, you will need
1238to use the following line:
1239
1240 BEGIN { unshift(@INC, "./blib") }
1241
1242=item *
1243
1244This document assumes that the executable named "perl" is Perl version 5.
1245Some systems may have installed Perl version 5 as "perl5".
1246
1247=back
1248
1249=head1 See also
4633a7c4 1250
171891c7 1251For more information, consult L<perlguts>, L<perlapi>, L<perlxs>, L<perlmod>,
c07a80fd 1252and L<perlpod>.
4633a7c4 1253
360e660c 1254=head1 Author
4633a7c4 1255
9607fc9c 1256Jeff Okamoto <F<okamoto@corp.hp.com>>
4633a7c4 1257
c07a80fd 1258Reviewed and assisted by Dean Roehrich, Ilya Zakharevich, Andreas Koenig,
1259and Tim Bunce.
4633a7c4 1260
c07a80fd 1261=head2 Last Changed
4633a7c4 1262
beb31b0b 12631999/11/30