3 perlXStut - Tutorial for writing XSUBs
7 This tutorial will educate the reader on the steps involved in creating
8 a Perl extension. The reader is assumed to have access to L<perlguts> and
11 This tutorial starts with very simple examples and becomes more complex,
12 with each new example adding new features. Certain concepts may not be
13 completely explained until later in the tutorial in order to slowly ease
14 the reader into building extensions.
16 This tutorial was written from a Unix point of view. Where I know them
17 to be otherwise different for other platforms (e.g. Win32), I will list
18 them. If you find something that was missed, please let me know.
24 This tutorial assumes that the make program that Perl is configured to
25 use is called C<make>. Instead of running "make" in the examples that
26 follow, you may have to substitute whatever make program Perl has been
27 configured to use. Running "perl -V:make" should tell you what it is.
31 This tutorial tries hard to keep up with the latest development versions
32 of Perl. This often means that it is sometimes in advance of the latest
33 released version of Perl, and that certain features described here might
34 not work on earlier versions. See the section on "Troubleshooting
35 these Examples" for more information.
37 =head2 Dynamic Loading versus Static Loading
39 It is commonly thought that if a system does not have the capability to
40 dynamically load a library, you cannot build XSUBs. This is incorrect.
41 You I<can> build them, but you must link the XSUBs subroutines with the
42 rest of Perl, creating a new executable. This situation is similar to
45 This tutorial can still be used on such a system. The XSUB build mechanism
46 will check the system and build a dynamically-loadable library if possible,
47 or else a static library and then, optionally, a new statically-linked
48 executable with that static library linked in.
50 Should you wish to build a statically-linked executable on a system which
51 can dynamically load libraries, you may, in all the following examples,
52 where the command "C<make>" with no arguments is executed, run the command
53 "C<make perl>" instead.
55 If you have generated such a statically-linked executable by choice, then
56 instead of saying "C<make test>", you should say "C<make test_static>".
57 On systems that cannot build dynamically-loadable libraries at all, simply
58 saying "C<make test>" is sufficient.
62 Now let's go on with the show!
66 Our first extension will be very simple. When we call the routine in the
67 extension, it will print out a well-known message and return.
69 Run "C<h2xs -A -n Mytest>". This creates a directory named Mytest,
70 possibly under ext/ if that directory exists in the current working
71 directory. Several files will be created in the Mytest dir, including
72 MANIFEST, Makefile.PL, Mytest.pm, Mytest.xs, test.pl, and Changes.
74 The MANIFEST file contains the names of all the files just created in the
77 The file Makefile.PL should look something like this:
79 use ExtUtils::MakeMaker;
80 # See lib/ExtUtils/MakeMaker.pm for details of how to influence
81 # the contents of the Makefile that is written.
84 VERSION_FROM => 'Mytest.pm', # finds $VERSION
85 LIBS => [''], # e.g., '-lm'
86 DEFINE => '', # e.g., '-DHAVE_SOMETHING'
87 INC => '', # e.g., '-I/usr/include/other'
90 The file Mytest.pm should start with something like this:
99 our @ISA = qw(Exporter DynaLoader);
100 # Items to export into callers namespace by default. Note: do not export
101 # names by default without a very good reason. Use EXPORT_OK instead.
102 # Do not simply export all your public functions/methods/constants.
106 our $VERSION = '0.01';
108 bootstrap Mytest $VERSION;
110 # Preloaded methods go here.
112 # Autoload methods go after __END__, and are processed by the autosplit program.
116 # Below is the stub of documentation for your module. You better edit it!
118 The rest of the .pm file contains sample code for providing documentation for
121 Finally, the Mytest.xs file should look something like this:
127 MODULE = Mytest PACKAGE = Mytest
129 Let's edit the .xs file by adding this to the end of the file:
134 printf("Hello, world!\n");
136 It is okay for the lines starting at the "CODE:" line to not be indented.
137 However, for readability purposes, it is suggested that you indent CODE:
138 one level and the lines following one more level.
140 Now we'll run "C<perl Makefile.PL>". This will create a real Makefile,
141 which make needs. Its output looks something like:
144 Checking if your kit is complete...
146 Writing Makefile for Mytest
149 Now, running make will produce output that looks something like this (some
150 long lines have been shortened for clarity and some extraneous lines have
154 umask 0 && cp Mytest.pm ./blib/Mytest.pm
155 perl xsubpp -typemap typemap Mytest.xs >Mytest.tc && mv Mytest.tc Mytest.c
156 Please specify prototyping behavior for Mytest.xs (see perlxs manual)
158 Running Mkbootstrap for Mytest ()
160 LD_RUN_PATH="" ld -o ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl -b Mytest.o
161 chmod 755 ./blib/PA-RISC1.1/auto/Mytest/Mytest.sl
162 cp Mytest.bs ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
163 chmod 644 ./blib/PA-RISC1.1/auto/Mytest/Mytest.bs
164 Manifying ./blib/man3/Mytest.3
167 You can safely ignore the line about "prototyping behavior".
169 If you are on a Win32 system, and the build process fails with linker
170 errors for functions in the C library, check if your Perl is configured
171 to use PerlCRT (running "perl -V:libc" should show you if this is the
172 case). If Perl is configured to use PerlCRT, you have to make sure
173 PerlCRT.lib is copied to the same location that msvcrt.lib lives in,
174 so that the compiler can find it on its own. msvcrt.lib is usually
175 found in the Visual C compiler's lib directory (e.g. C:/DevStudio/VC/lib).
177 Perl has its own special way of easily writing test scripts, but for this
178 example only, we'll create our own test script. Create a file called hello
179 that looks like this:
181 #! /opt/perl5/bin/perl
183 use ExtUtils::testlib;
189 Now we make the script executable (C<chmod -x hello>), run the script
190 and we should see the following output:
198 Now let's add to our extension a subroutine that will take a single numeric
199 argument as input and return 0 if the number is even or 1 if the number
202 Add the following to the end of Mytest.xs:
208 RETVAL = (input % 2 == 0);
212 There does not need to be white space at the start of the "C<int input>"
213 line, but it is useful for improving readability. Placing a semi-colon at
214 the end of that line is also optional. Any amount and kind of white space
215 may be placed between the "C<int>" and "C<input>".
217 Now re-run make to rebuild our new shared library.
219 Now perform the same steps as before, generating a Makefile from the
220 Makefile.PL file, and running make.
222 In order to test that our extension works, we now need to look at the
223 file test.pl. This file is set up to imitate the same kind of testing
224 structure that Perl itself has. Within the test script, you perform a
225 number of tests to confirm the behavior of the extension, printing "ok"
226 when the test is correct, "not ok" when it is not. Change the print
227 statement in the BEGIN block to print "1..4", and add the following code
228 to the end of the file:
230 print &Mytest::is_even(0) == 1 ? "ok 2" : "not ok 2", "\n";
231 print &Mytest::is_even(1) == 0 ? "ok 3" : "not ok 3", "\n";
232 print &Mytest::is_even(2) == 1 ? "ok 4" : "not ok 4", "\n";
234 We will be calling the test script through the command "C<make test>". You
235 should see output that looks something like this:
238 PERL_DL_NONLAZY=1 /opt/perl5.004/bin/perl (lots of -I arguments) test.pl
246 =head2 What has gone on?
248 The program h2xs is the starting point for creating extensions. In later
249 examples we'll see how we can use h2xs to read header files and generate
250 templates to connect to C routines.
252 h2xs creates a number of files in the extension directory. The file
253 Makefile.PL is a perl script which will generate a true Makefile to build
254 the extension. We'll take a closer look at it later.
256 The .pm and .xs files contain the meat of the extension. The .xs file holds
257 the C routines that make up the extension. The .pm file contains routines
258 that tell Perl how to load your extension.
260 Generating the Makefile and running C<make> created a directory called blib
261 (which stands for "build library") in the current working directory. This
262 directory will contain the shared library that we will build. Once we have
263 tested it, we can install it into its final location.
265 Invoking the test script via "C<make test>" did something very important.
266 It invoked perl with all those C<-I> arguments so that it could find the
267 various files that are part of the extension. It is I<very> important that
268 while you are still testing extensions that you use "C<make test>". If you
269 try to run the test script all by itself, you will get a fatal error.
270 Another reason it is important to use "C<make test>" to run your test
271 script is that if you are testing an upgrade to an already-existing version,
272 using "C<make test>" insures that you will test your new extension, not the
273 already-existing version.
275 When Perl sees a C<use extension;>, it searches for a file with the same name
276 as the C<use>'d extension that has a .pm suffix. If that file cannot be found,
277 Perl dies with a fatal error. The default search path is contained in the
280 In our case, Mytest.pm tells perl that it will need the Exporter and Dynamic
281 Loader extensions. It then sets the C<@ISA> and C<@EXPORT> arrays and the
282 C<$VERSION> scalar; finally it tells perl to bootstrap the module. Perl
283 will call its dynamic loader routine (if there is one) and load the shared
286 The two arrays C<@ISA> and C<@EXPORT> are very important. The C<@ISA>
287 array contains a list of other packages in which to search for methods (or
288 subroutines) that do not exist in the current package. This is usually
289 only important for object-oriented extensions (which we will talk about
290 much later), and so usually doesn't need to be modified.
292 The C<@EXPORT> array tells Perl which of the extension's variables and
293 subroutines should be placed into the calling package's namespace. Because
294 you don't know if the user has already used your variable and subroutine
295 names, it's vitally important to carefully select what to export. Do I<not>
296 export method or variable names I<by default> without a good reason.
298 As a general rule, if the module is trying to be object-oriented then don't
299 export anything. If it's just a collection of functions and variables, then
300 you can export them via another array, called C<@EXPORT_OK>. This array
301 does not automatically place its subroutine and variable names into the
302 namespace unless the user specifically requests that this be done.
304 See L<perlmod> for more information.
306 The C<$VERSION> variable is used to ensure that the .pm file and the shared
307 library are "in sync" with each other. Any time you make changes to
308 the .pm or .xs files, you should increment the value of this variable.
310 =head2 Writing good test scripts
312 The importance of writing good test scripts cannot be overemphasized. You
313 should closely follow the "ok/not ok" style that Perl itself uses, so that
314 it is very easy and unambiguous to determine the outcome of each test case.
315 When you find and fix a bug, make sure you add a test case for it.
317 By running "C<make test>", you ensure that your test.pl script runs and uses
318 the correct version of your extension. If you have many test cases, you
319 might want to copy Perl's test style. Create a directory named "t" in the
320 extension's directory and append the suffix ".t" to the names of your test
321 files. When you run "C<make test>", all of these test files will be executed.
325 Our third extension will take one argument as its input, round off that
326 value, and set the I<argument> to the rounded value.
328 Add the following to the end of Mytest.xs:
335 arg = floor(arg + 0.5);
336 } else if (arg < 0.0) {
337 arg = ceil(arg - 0.5);
344 Edit the Makefile.PL file so that the corresponding line looks like this:
346 'LIBS' => ['-lm'], # e.g., '-lm'
348 Generate the Makefile and run make. Change the BEGIN block to print
349 "1..9" and add the following to test.pl:
351 $i = -1.5; &Mytest::round($i); print $i == -2.0 ? "ok 5" : "not ok 5", "\n";
352 $i = -1.1; &Mytest::round($i); print $i == -1.0 ? "ok 6" : "not ok 6", "\n";
353 $i = 0.0; &Mytest::round($i); print $i == 0.0 ? "ok 7" : "not ok 7", "\n";
354 $i = 0.5; &Mytest::round($i); print $i == 1.0 ? "ok 8" : "not ok 8", "\n";
355 $i = 1.2; &Mytest::round($i); print $i == 1.0 ? "ok 9" : "not ok 9", "\n";
357 Running "C<make test>" should now print out that all nine tests are okay.
359 Notice that in these new test cases, the argument passed to round was a
360 scalar variable. You might be wondering if you can round a constant or
361 literal. To see what happens, temporarily add the following line to test.pl:
365 Run "C<make test>" and notice that Perl dies with a fatal error. Perl won't
366 let you change the value of constants!
368 =head2 What's new here?
374 We've made some changes to Makefile.PL. In this case, we've specified an
375 extra library to be linked into the extension's shared library, the math
376 library libm in this case. We'll talk later about how to write XSUBs that
377 can call every routine in a library.
381 The value of the function is not being passed back as the function's return
382 value, but by changing the value of the variable that was passed into the
383 function. You might have guessed that when you saw that the return value
384 of round is of type "void".
388 =head2 Input and Output Parameters
390 You specify the parameters that will be passed into the XSUB on the line(s)
391 after you declare the function's return value and name. Each input parameter
392 line starts with optional white space, and may have an optional terminating
395 The list of output parameters occurs at the very end of the function, just
396 before after the OUTPUT: directive. The use of RETVAL tells Perl that you
397 wish to send this value back as the return value of the XSUB function. In
398 Example 3, we wanted the "return value" placed in the original variable
399 which we passed in, so we listed it (and not RETVAL) in the OUTPUT: section.
401 =head2 The XSUBPP Program
403 The xsubpp program takes the XS code in the .xs file and translates it into
404 C code, placing it in a file whose suffix is .c. The C code created makes
405 heavy use of the C functions within Perl.
407 =head2 The TYPEMAP file
409 The xsubpp program uses rules to convert from Perl's data types (scalar,
410 array, etc.) to C's data types (int, char, etc.). These rules are stored
411 in the typemap file ($PERLLIB/ExtUtils/typemap). This file is split into
414 The first section maps various C data types to a name, which corresponds
415 somewhat with the various Perl types. The second section contains C code
416 which xsubpp uses to handle input parameters. The third section contains
417 C code which xsubpp uses to handle output parameters.
419 Let's take a look at a portion of the .c file created for our extension.
420 The file name is Mytest.c:
426 croak("Usage: Mytest::round(arg)");
428 double arg = (double)SvNV(ST(0)); /* XXXXX */
430 arg = floor(arg + 0.5);
431 } else if (arg < 0.0) {
432 arg = ceil(arg - 0.5);
436 sv_setnv(ST(0), (double)arg); /* XXXXX */
441 Notice the two lines commented with "XXXXX". If you check the first section
442 of the typemap file, you'll see that doubles are of type T_DOUBLE. In the
443 INPUT section, an argument that is T_DOUBLE is assigned to the variable
444 arg by calling the routine SvNV on something, then casting it to double,
445 then assigned to the variable arg. Similarly, in the OUTPUT section,
446 once arg has its final value, it is passed to the sv_setnv function to
447 be passed back to the calling subroutine. These two functions are explained
448 in L<perlguts>; we'll talk more later about what that "ST(0)" means in the
449 section on the argument stack.
451 =head2 Warning about Output Arguments
453 In general, it's not a good idea to write extensions that modify their input
454 parameters, as in Example 3. Instead, you should probably return multiple
455 values in an array and let the caller handle them (we'll do this in a later
456 example). However, in order to better accomodate calling pre-existing C
457 routines, which often do modify their input parameters, this behavior is
462 In this example, we'll now begin to write XSUBs that will interact with
463 pre-defined C libraries. To begin with, we will build a small library of
464 our own, then let h2xs write our .pm and .xs files for us.
466 Create a new directory called Mytest2 at the same level as the directory
467 Mytest. In the Mytest2 directory, create another directory called mylib,
468 and cd into that directory.
470 Here we'll create some files that will generate a test library. These will
471 include a C source file and a header file. We'll also create a Makefile.PL
472 in this directory. Then we'll make sure that running make at the Mytest2
473 level will automatically run this Makefile.PL file and the resulting Makefile.
475 In the mylib directory, create a file mylib.h that looks like this:
479 extern double foo(int, long, const char*);
481 Also create a file mylib.c that looks like this:
487 foo(int a, long b, const char *c)
489 return (a + b + atof(c) + TESTVAL);
492 And finally create a file Makefile.PL that looks like this:
494 use ExtUtils::MakeMaker;
497 NAME => 'Mytest2::mylib',
498 SKIP => [qw(all static static_lib dynamic dynamic_lib)],
499 clean => {'FILES' => 'libmylib$(LIBEEXT)'},
503 sub MY::top_targets {
509 static :: libmylib$(LIB_EXT)
511 libmylib$(LIB_EXT): $(O_FILES)
512 $(AR) cr libmylib$(LIB_EXT) $(O_FILES)
513 $(RANLIB) libmylib$(LIB_EXT)
518 Make sure you use a tab and not spaces on the lines beginning with "$(AR)"
519 and "$(RANLIB)". Make will not function properly if you use spaces.
520 It has also been reported that the "cr" argument to $(AR) is unnecessary
523 We will now create the main top-level Mytest2 files. Change to the directory
524 above Mytest2 and run the following command:
526 % h2xs -O -n Mytest2 ./Mytest2/mylib/mylib.h
528 This will print out a warning about overwriting Mytest2, but that's okay.
529 Our files are stored in Mytest2/mylib, and will be untouched.
531 The normal Makefile.PL that h2xs generates doesn't know about the mylib
532 directory. We need to tell it that there is a subdirectory and that we
533 will be generating a library in it. Let's add the argument MYEXTLIB to
534 the WriteMakefile call so that it looks like this:
538 'VERSION_FROM' => 'Mytest2.pm', # finds $VERSION
539 'LIBS' => [''], # e.g., '-lm'
540 'DEFINE' => '', # e.g., '-DHAVE_SOMETHING'
541 'INC' => '', # e.g., '-I/usr/include/other'
542 'MYEXTLIB' => 'mylib/libmylib$(LIB_EXT)',
545 and then at the end add a subroutine (which will override the pre-existing
546 subroutine). Remember to use a tab character to indent the line beginning
551 $(MYEXTLIB): mylib/Makefile
552 cd mylib && $(MAKE) $(PASSTHRU)
556 Let's also fix the MANIFEST file so that it accurately reflects the contents
557 of our extension. The single line that says "mylib" should be replaced by
558 the following three lines:
564 To keep our namespace nice and unpolluted, edit the .pm file and change
565 the variable C<@EXPORT> to C<@EXPORT_OK>. Finally, in the
566 .xs file, edit the #include line to read:
568 #include "mylib/mylib.h"
570 And also add the following function definition to the end of the .xs file:
580 Now we also need to create a typemap file because the default Perl doesn't
581 currently support the const char * type. Create a file called typemap in
582 the Mytest2 directory and place the following in it:
586 Now run perl on the top-level Makefile.PL. Notice that it also created a
587 Makefile in the mylib directory. Run make and watch that it does cd into
588 the mylib directory and run make in there as well.
590 Now edit the test.pl script and change the BEGIN block to print "1..4",
591 and add the following lines to the end of the script:
593 print &Mytest2::foo(1, 2, "Hello, world!") == 7 ? "ok 2\n" : "not ok 2\n";
594 print &Mytest2::foo(1, 2, "0.0") == 7 ? "ok 3\n" : "not ok 3\n";
595 print abs(&Mytest2::foo(0, 0, "-3.4") - 0.6) <= 0.01 ? "ok 4\n" : "not ok 4\n";
597 (When dealing with floating-point comparisons, it is best to not check for
598 equality, but rather that the difference between the expected and actual
599 result is below a certain amount (called epsilon) which is 0.01 in this case)
601 Run "C<make test>" and all should be well.
603 =head2 What has happened here?
605 Unlike previous examples, we've now run h2xs on a real include file. This
606 has caused some extra goodies to appear in both the .pm and .xs files.
612 In the .xs file, there's now a #include directive with the absolute path to
613 the mylib.h header file. We changed this to a relative path so that we
614 could move the extension directory if we wanted to.
618 There's now some new C code that's been added to the .xs file. The purpose
619 of the C<constant> routine is to make the values that are #define'd in the
620 header file accessible by the Perl script (by calling either C<TESTVAL> or
621 C<&Mytest2::TESTVAL>). There's also some XS code to allow calls to the
626 The .pm file originally exported the name C<TESTVAL> in the C<@EXPORT> array.
627 This could lead to name clashes. A good rule of thumb is that if the #define
628 is only going to be used by the C routines themselves, and not by the user,
629 they should be removed from the C<@EXPORT> array. Alternately, if you don't
630 mind using the "fully qualified name" of a variable, you could move most
631 or all of the items from the C<@EXPORT> array into the C<@EXPORT_OK> array.
635 If our include file had contained #include directives, these would not have
636 been processed by h2xs. There is no good solution to this right now.
640 We've also told Perl about the library that we built in the mylib
641 subdirectory. That required only the addition of the C<MYEXTLIB> variable
642 to the WriteMakefile call and the replacement of the postamble subroutine
643 to cd into the subdirectory and run make. The Makefile.PL for the
644 library is a bit more complicated, but not excessively so. Again we
645 replaced the postamble subroutine to insert our own code. This code
646 simply specified that the library to be created here was a static archive
647 library (as opposed to a dynamically loadable library) and provided the
648 commands to build it.
652 =head2 More about XSUBPP
654 With the completion of Example 4, we now have an easy way to simulate some
655 real-life libraries whose interfaces may not be the cleanest in the world.
656 We shall now continue with a discussion of the arguments passed to the
659 When you specify arguments to routines in the .xs file, you are really
660 passing three pieces of information for each argument listed. The first
661 piece is the order of that argument relative to the others (first, second,
662 etc). The second is the type of argument, and consists of the type
663 declaration of the argument (e.g., int, char*, etc). The third piece is
664 the exact way in which the argument should be used in the call to the
665 library function from this XSUB. This would mean whether or not to place
666 a "&" before the argument or not, meaning the argument expects to be
667 passed the address of the specified data type.
669 There is a difference between the two arguments in this hypothetical function:
676 The first argument to this function would be treated as a char and assigned
677 to the variable a, and its address would be passed into the function foo.
678 The second argument would be treated as a string pointer and assigned to the
679 variable b. The I<value> of b would be passed into the function foo. The
680 actual call to the function foo that xsubpp generates would look like this:
684 Xsubpp will parse the following function argument lists identically:
690 However, to help ease understanding, it is suggested that you place a "&"
691 next to the variable name and away from the variable type), and place a
692 "*" near the variable type, but away from the variable name (as in the
693 call to foo above). By doing so, it is easy to understand exactly what
694 will be passed to the C function -- it will be whatever is in the "last
697 You should take great pains to try to pass the function the type of variable
698 it wants, when possible. It will save you a lot of trouble in the long run.
700 =head2 The Argument Stack
702 If we look at any of the C code generated by any of the examples except
703 example 1, you will notice a number of references to ST(n), where n is
704 usually 0. "ST" is actually a macro that points to the n'th argument
705 on the argument stack. ST(0) is thus the first argument on the stack and
706 therefore the first argument passed to the XSUB, ST(1) is the second
709 When you list the arguments to the XSUB in the .xs file, that tells xsubpp
710 which argument corresponds to which of the argument stack (i.e., the first
711 one listed is the first argument, and so on). You invite disaster if you
712 do not list them in the same order as the function expects them.
714 The actual values on the argument stack are pointers to the values passed
715 in. When an argument is listed as being an OUTPUT value, its corresponding
716 value on the stack (i.e., ST(0) if it was the first argument) is changed.
717 You can verify this by looking at the C code generated for Example 3.
718 The code for the round() XSUB routine contains lines that look like this:
720 double arg = (double)SvNV(ST(0));
721 /* Round the contents of the variable arg */
722 sv_setnv(ST(0), (double)arg);
724 The arg variable is initially set by taking the value from ST(0), then is
725 stored back into ST(0) at the end of the routine.
727 =head2 Extending your Extension
729 Sometimes you might want to provide some extra methods or subroutines
730 to assist in making the interface between Perl and your extension simpler
731 or easier to understand. These routines should live in the .pm file.
732 Whether they are automatically loaded when the extension itself is loaded
733 or only loaded when called depends on where in the .pm file the subroutine
734 definition is placed. You can also consult L<Autoloader> for an alternate
735 way to store and load your extra subroutines.
737 =head2 Documenting your Extension
739 There is absolutely no excuse for not documenting your extension.
740 Documentation belongs in the .pm file. This file will be fed to pod2man,
741 and the embedded documentation will be converted to the man page format,
742 then placed in the blib directory. It will be copied to Perl's man
743 page directory when the extension is installed.
745 You may intersperse documentation and Perl code within the .pm file.
746 In fact, if you want to use method autoloading, you must do this,
747 as the comment inside the .pm file explains.
749 See L<perlpod> for more information about the pod format.
751 =head2 Installing your Extension
753 Once your extension is complete and passes all its tests, installing it
754 is quite simple: you simply run "make install". You will either need
755 to have write permission into the directories where Perl is installed,
756 or ask your system administrator to run the make for you.
758 Alternately, you can specify the exact directory to place the extension's
759 files by placing a "PREFIX=/destination/directory" after the make install.
760 (or in between the make and install if you have a brain-dead version of make).
761 This can be very useful if you are building an extension that will eventually
762 be distributed to multiple systems. You can then just archive the files in
763 the destination directory and distribute them to your destination systems.
767 In this example, we'll do some more work with the argument stack. The
768 previous examples have all returned only a single value. We'll now
769 create an extension that returns an array.
771 This extension is very Unix-oriented (struct statfs and the statfs system
772 call). If you are not running on a Unix system, you can substitute for
773 statfs any other function that returns multiple values, you can hard-code
774 values to be returned to the caller (although this will be a bit harder
775 to test the error case), or you can simply not do this example. If you
776 change the XSUB, be sure to fix the test cases to match the changes.
778 Return to the Mytest directory and add the following code to the end of
789 i = statfs(path, &buf);
791 XPUSHs(sv_2mortal(newSVnv(buf.f_bavail)));
792 XPUSHs(sv_2mortal(newSVnv(buf.f_bfree)));
793 XPUSHs(sv_2mortal(newSVnv(buf.f_blocks)));
794 XPUSHs(sv_2mortal(newSVnv(buf.f_bsize)));
795 XPUSHs(sv_2mortal(newSVnv(buf.f_ffree)));
796 XPUSHs(sv_2mortal(newSVnv(buf.f_files)));
797 XPUSHs(sv_2mortal(newSVnv(buf.f_type)));
798 XPUSHs(sv_2mortal(newSVnv(buf.f_fsid[0])));
799 XPUSHs(sv_2mortal(newSVnv(buf.f_fsid[1])));
801 XPUSHs(sv_2mortal(newSVnv(errno)));
804 You'll also need to add the following code to the top of the .xs file, just
805 after the include of "XSUB.h":
809 Also add the following code segment to test.pl while incrementing the "1..9"
810 string in the BEGIN block to "1..11":
812 @a = &Mytest::statfs("/blech");
813 print ((scalar(@a) == 1 && $a[0] == 2) ? "ok 10\n" : "not ok 10\n");
814 @a = &Mytest::statfs("/");
815 print scalar(@a) == 9 ? "ok 11\n" : "not ok 11\n";
817 =head2 New Things in this Example
819 This example added quite a few new concepts. We'll take them one at a time.
825 The PREINIT: directive contains code that will be placed immediately after
826 variable declaration and before the argument stack is decoded. Some compilers
827 cannot handle variable declarations at arbitrary locations inside a function,
828 so this is usually the best way to declare local variables needed by the XSUB.
832 This routine also returns a different number of arguments depending on the
833 success or failure of the call to statfs. If there is an error, the error
834 number is returned as a single-element array. If the call is successful,
835 then a 9-element array is returned. Since only one argument is passed into
836 this function, we need room on the stack to hold the 9 values which may be
839 We do this by using the PPCODE: directive, rather than the CODE: directive.
840 This tells xsubpp that we will be managing the return values that will be
841 put on the argument stack by ourselves.
845 When we want to place values to be returned to the caller onto the stack,
846 we use the series of macros that begin with "XPUSH". There are five
847 different versions, for placing integers, unsigned integers, doubles,
848 strings, and Perl scalars on the stack. In our example, we placed a
849 Perl scalar onto the stack.
851 The XPUSH* macros will automatically extend the return stack to prevent
852 it from being overrun. You push values onto the stack in the order you
853 want them seen by the calling program.
857 The values pushed onto the return stack of the XSUB are actually mortal SV's.
858 They are made mortal so that once the values are copied by the calling
859 program, the SV's that held the returned values can be deallocated.
860 If they were not mortal, then they would continue to exist after the XSUB
861 routine returned, but would not be accessible. This is a memory leak.
865 =head2 EXAMPLE 6 (Coming Soon)
867 Passing in and returning references to arrays and/or hashes
869 =head2 EXAMPLE 7 (Coming Soon)
871 XPUSH args AND set RETVAL AND assign return value to array
873 =head2 EXAMPLE 8 (Coming Soon)
877 =head2 EXAMPLE 9 (Coming Soon)
879 Getting fd's from filehandles
881 =head2 Troubleshooting these Examples
883 As mentioned at the top of this document, if you are having problems with
884 these example extensions, you might see if any of these help you.
890 In versions of 5.002 prior to the gamma version, the test script in Example
891 1 will not function properly. You need to change the "use lib" line to
898 In versions of 5.002 prior to version 5.002b1h, the test.pl file was not
899 automatically created by h2xs. This means that you cannot say "make test"
900 to run the test script. You will need to add the following line before the
901 "use extension" statement:
907 In versions 5.000 and 5.001, instead of using the above line, you will need
908 to use the following line:
910 BEGIN { unshift(@INC, "./blib") }
914 This document assumes that the executable named "perl" is Perl version 5.
915 Some systems may have installed Perl version 5 as "perl5".
921 For more information, consult L<perlguts>, L<perlxs>, L<perlmod>,
926 Jeff Okamoto <F<okamoto@corp.hp.com>>
928 Reviewed and assisted by Dean Roehrich, Ilya Zakharevich, Andreas Koenig,