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Jack Jansen	a630813	1996-03-18 13:38:52 +0000	[diff] [blame]	1	<HTML><HEAD><TITLE>Creating a C extension module on the Macintosh</TITLE></HEAD>
				2	<BODY>
				3	<H1>Creating a C extension module on the Macintosh</H1>
				4	<HR>
				5
				6	This document gives a step-by-step example of how to create a new C
				7	extension module on the mac. For this example, we will create a module
				8	to interface to the programmers' API of InterSLIP, a package that
				9	allows you to use MacTCP (and, hence, all internet services) over a
				10	modem connection. <p>
				11
				12	<H2>Prerequisites</H2>
				13
				14	There are a few things you need to pull this off. First and foremost,
				15	you need a C development environment. Actually, you need a specific
				16	development environment, CodeWarrior by <A
				17	HREF="http://www.metrowerks.com/">MetroWerks</A>. You will probably
				18	need the latest version. You may be able to get by with an older
				19	version of CodeWarrior or with another development environment (Up to
				20	about 1994 python was developed with THINK C, and in the dim past it
				21	was compiled with MPW C) assuming you have managed to get Python to
				22	compile under your development environment, but the step-by-step
				23	character of this document will be lost. <p>
				24
Jack Jansen	3412c5d	1997-08-27 14:08:22 +0000	[diff] [blame^]	25	Next, you need a <A
				26	HREF="http://www.python.org/python/Sources.html">python source
				27	distribution</A>. For PowerPC and cfm68k development you can actually
				28	get by without a full source distribution, using the Development
				29	distribution (if I have gotten around to putting it together by the time
				30	you read this). You'll also need a functional python interpreter, and
				31	the Modulator program (which lives in <CODE>Tools:Modulator</CODE> in
				32	the standard source distribution). You may also find that Guido's <A
Jack Jansen	a630813	1996-03-18 13:38:52 +0000	[diff] [blame]	33	HREF="http://www.python.org/doc/ext/ext.html">Extending and embedding
				34	the Python interpreter</A> is a very handy piece of documentation. I
				35	will skip lots of details that are handled there, like complete
Jack Jansen	3412c5d	1997-08-27 14:08:22 +0000	[diff] [blame^]	36	descriptions of <CODE>Py_ParseTuple</CODE> and such utility routines, or
				37	the general structure of extension modules. <p>
Jack Jansen	a630813	1996-03-18 13:38:52 +0000	[diff] [blame]	38
				39	<H2>InterSLIP and the C API to it</H2>
				40
				41	InterSLIP, the utility to which we are going to create a python
				42	interface, is a system extension that does all the work of connecting
				43	to the internet over a modem connection. InterSLIP is provided
				44	free-of-charge by <A
				45	HREF="http://www.intercon.com/">InterCon</A>. First it connects to
				46	your modem, then it goes through the whole process of dialling,
				47	logging in and possibly starting the SLIP software on the remote
				48	computer and finally it starts with the real work: packing up IP
				49	packets handed to it by MacTCP and sending them to the remote side
				50	(and, of course, the reverse action of receiving incoming packets,
				51	unpacking them and handing them to MacTCP). InterSLIP is a device
				52	driver, and you control it using a application supplied with it,
				53	InterSLIP Setup. The API that InterSLIP Setup uses to talk to the
				54	device driver is published in the documentation and, hence, also
				55	useable by other applications. <p>
				56
				57	I happened to have a C interface to the API, which is all ugly
				58	low-level device-driver calls by itself. The C interface is in <A
				59	HREF="interslip/InterslipLib.c">InterslipLib.c</A> and <A
				60	HREF="interslip/InterslipLib.h">InterslipLib.h</A>, we'll
				61	concentrate here on how to build the Python wrapper module around
				62	it. Note that this is the "normal" situation when you are writing a
				63	Python extension module: you have some sort of functionality available
				64	to C programmers and want to make a Python interface to it. <p>
				65
				66	<H2>Using Modulator</H2>
				67
				68	The method we describe in this document, using Modulator, is the best
				69	method for small interfaces. For large interfaces there is another
				70	tool, Bgen, which actually generates the complete module without you
				71	lifting a single finger. Bgen, however, has the disadvantage of having
				72	a very steep learning curve, so an example using it will have to wait
				73	until another document, when I have more time. <p>
				74
				75	First, let us look at the <A
				76	HREF="interslip/InterslipLib.h">InterslipLib.h</A> header file,
				77	and see that the whole interface consists of six routines:
				78	<CODE>is_open</CODE>, <CODE>is_connect</CODE>,
				79	<CODE>is_disconnect</CODE>, <CODE>is_status</CODE>,
				80	<CODE>is_getconfig</CODE> and <CODE>is_setconfig</CODE>. Our first
				81	step will be to create a skeleton file <A
				82	HREF="interslip/@interslipmodule.c">@interslipmodule.c</A>, a
				83	dummy module that will contain all the glue code that python expects
				84	of an extension module. Creating this glue code is a breeze with
				85	modulator, a tool that we only have to tell that we want to create a
				86	module with methods of the six names above and that will create the
				87	complete skeleton C code for us. <p>
				88
				89	Why call this dummy module <CODE>@interslipmodule.c</CODE> and not
				90	<CODE>interslipmodule.c</CODE>? Self-preservation: if ever you happen
				91	to repeat the whole process after you have actually turned the
				92	skeleton module into a real module you would overwrite your
				93	hand-written code. By calling the dummy module a different name you
				94	have to make <EM>two</EM> mistakes in a row before you do this. <p>
				95
Jack Jansen	3412c5d	1997-08-27 14:08:22 +0000	[diff] [blame^]	96	If you installed Tk support when you installed Python this is extremely
				97	simple. You start modulator and are provided with a form in which you
Jack Jansen	a630813	1996-03-18 13:38:52 +0000	[diff] [blame]	98	fill out the details of the module you are creating. <p>
				99
				100	<IMG SRC="html.icons/modulator.gif" ALIGN=CENTER><p>
				101
				102	You'll need to supply a module name (<CODE>interslip</CODE>, in our
				103	case), a module abbreviation (<CODE>pyis</CODE>, which is used as a
				104	prefix to all the routines and data structures modulator will create
				105	for you) and you enter the names of all the methods your module will
				106	export (the list above, with <CODE>is_</CODE> stripped off). Note that
				107	we use <CODE>pyis</CODE> as the prefix instead of the more logical
				108	<CODE>is</CODE>, since the latter would cause our routine names to
				109	collide with those in the API we are interfacing to! The method names
				110	are the names as seen by the python program, and the C routine names
				111	will have the prefix and an underscore prepended. Modulator can do
				112	much more, like generating code for objects and such, but that is a
				113	topic for a later example. <p>
				114
				115	Once you have told modulator all about the module you want to create
				116	you press "check", which checks that you haven't omitted any
				117	information and "Generate code". This will prompt you for a C output
				118	file and generate your module for you. <p>
				119
				120	<H2>Using Modulator without Tk</H2>
				121
				122
				123	Modulator actually uses a two-stage process to create your code: first
				124	the information you provided is turned into a number of python
				125	statements and then these statements are executed to generate your
				126	code. This is done so that you can even use modulator if you don't
				127	have Tk support in Python: you'll just have to write the modulator
				128	python statements by hand (about 10 lines, in our example) and
				129	modulator will generate the C code (about 150 lines, in our
				130	example). Here is the Python code you'll want to execute to generate
				131	our skeleton module: <p>
				132
				133	<CODE><PRE>
				134	import addpack
				135	addpack.addpack('Tools')
				136	addpack.addpack('modulator')
				137	import genmodule
				138
				139	m = genmodule.module()
				140	m.name = 'interslip'
				141	m.abbrev = 'pyis'
				142	m.methodlist = ['open', 'connect', 'disconnect', 'status', \
				143	'getconfig', 'setconfig']
				144	m.objects = []
				145
				146	fp = open('@interslipmodule.c', 'w')
				147	genmodule.write(fp, m)
				148	</PRE></CODE>
				149
				150	Drop this program on the python interpreter and out will come your
				151	skeleton module. <p>
				152
				153	Now, rename the file to interslipmodule.c and you're all set to start
				154	developing. The module is complete in the sense that it should
				155	compile, and that if you import it in a python program you will see
				156	all the methods. It is, of course, not yet complete in a functional
				157	way... <p>
				158
Jack Jansen	3412c5d	1997-08-27 14:08:22 +0000	[diff] [blame^]	159	<H2>Adding a module to Classic 68K Python</H2>
Jack Jansen	a630813	1996-03-18 13:38:52 +0000	[diff] [blame]	160
				161	What you do now depends on whether you're developing for PowerPC (or
				162	for CFM68K) or for "traditional" mac. For a traditional 68K Python,
				163	you will have to add your new module to the project file of the Python
				164	interpreter, and you have to edit "config.c" to add the module to the
				165	set of builtin modules. In config.c you will add the module at two
				166	places: near the start of the file there is a list of external
				167	declarations for all init() routines. Add a line of the form
				168	<CODE><PRE>
				169	extern void initinterslip();
				170	</PRE></CODE>
				171	here. Further down the file there is an array that is initialized with
				172	modulename/initfunction pairs. Add a line of the form
				173	<CODE><PRE>
				174	{"interslip", initinterslip},
				175	</PRE></CODE>
				176	here. You may want to bracket these two lines with
				177	<CODE><PRE>
				178	#ifdef USE_INTERSLIP
				179	#endif
				180	</PRE></CODE>
				181	lines, that way you can easily control whether the module is
				182	incorporated into python at compile time. If you decide to do the
				183	latter edit your config file (you can find the name in the "C/C++
				184	language" section of the MW preferences dialog, it will probably be
				185	"mwerks_nonshared_config.h") and add a
				186	<CODE><PRE>
				187	#define USE_INTERSLIP
				188	</PRE></CODE>
				189
				190	Make the new interpreter and check that you can import the module, see
				191	the methods (with "dir(interslip)") and call them. <p>
				192
				193	<H2>Creating a PowerPC plugin module</H2>
				194
				195	For PowerPC development you could follow the same path, but it is
				196	actually a better idea to use a dynamically loadable module. The
				197	advantage of dynamically loadable modules is that they are not loaded
				198	until a python program actually uses them (resulting in less memory
				199	usage by the interpreter) and that development is a lot simpler (since
				200	your projects will all be smaller). Moreover, you can distribute a
				201	plugin module by itself without haveing to distribute a complete
				202	python interpreter. <p>
				203
Jack Jansen	3412c5d	1997-08-27 14:08:22 +0000	[diff] [blame^]	204	Go to the "PlugIns" folder and copy the files xx.prj,
				205	and xx.prj.exp to interslipmodule.prj and
				206	interslipmodule.prj.exp, respectively. Edit
				207	interslipmodule.prj.exp and change the name of the exported routine
				208	"initxx" to "initinterslip". Open interslipmodule.prj with CodeWarrior,
Jack Jansen	a630813	1996-03-18 13:38:52 +0000	[diff] [blame]	209	remove the file xxmodule.c and add interslipmodule.c and make a number
				210	of adjustments to the preferences:
				211	<UL>
Jack Jansen	3412c5d	1997-08-27 14:08:22 +0000	[diff] [blame^]	212	<LI> in PPC target, set the output file name to "interslipmodule.pcc.slb",
				213	<LI> in cfm68k target set the output file name to "interslipmodule.cfm68k.slb".
Jack Jansen	a630813	1996-03-18 13:38:52 +0000	[diff] [blame]	214	</UL>
				215	Next, compile and link your module, fire up python and do the same
				216	tests as for 68K python. <p>
				217
				218	<H2>Getting the module to do real work</H2>
				219
				220	So far, so good. In half an hour or so we have created a complete new
				221	extension module for Python. The downside, however, is that the module
				222	does not do anything useful. So, in the next half hour we will turn
				223	our beautiful skeleton module into something that is at least as
				224	beautiful but also gets some serious work done. For this once,
				225	<EM>I</EM> have spent that half hour for you, and you can see the
				226	results in <A
				227	HREF="interslip/interslipmodule.c">interslipmodule.c</A>. <p>
				228
				229	We add
				230	<CODE><PRE>
				231	#include "InterslipLib.h"
				232	#include "macglue.h"
				233	</PRE></CODE>
				234	to the top of the file, and work our way through each of the methods
				235	to add the functionality needed. Starting with open, we fill in the
				236	template docstring, the value accessible from Python by looking at
				237	<CODE>interslip.open.__doc__</CODE>. There are not many tools using
				238	this information at the moment, but as soon as class browsers for
				239	python become available having this minimal documentation available is
				240	a good idea. We put "Load the interslip driver" as the comment
				241	here. <p>
				242
				243	Next, we tackle the body of <CODE>pyis_open()</CODE>. Since it has no
				244	arguments and no return value we don't need to mess with that, we just
				245	have to add a call to <CODE>is_open()</CODE> and check the return for
				246	an error code, in which case we raise an error:
				247	<CODE><PRE>
				248	err = is_open();
				249	if ( err ) {
				250	PyErr_Mac(ErrorObject, err);
				251	return NULL;
				252	}
				253	</PRE></CODE>
				254	The routine <CODE><A NAME="PyErr_Mac">PyErr_Mac()</A></CODE> is a
				255	useful routine that raises the exception passed as its first
				256	argument. The data passed with the exception is based on the standard
				257	MacOS error code given, and PyErr_Mac() attempts to locate a textual
				258	description of the error code (which sure beats the "error -14021"
				259	messages that so many macintosh applications tell their poor
				260	users). <p>
				261
				262	We will skip pyis_connect and pyis_disconnect here, which are pretty
				263	much identical to pyis_open: no arguments, no return value, just a
				264	call and an error check. With pyis_status() things get interesting
				265	again: this call still takes 3 arguments, and all happen to be values
				266	returned (a numeric connection status indicator, a message sequence
				267	number and a pointer to the message itself, in MacOS pascal-style
				268	string form). We declare variables to receive the returned values, do
				269	the call, check the error and format the return value. <p>
				270
				271	Building the return value is done using <CODE><A
				272	NAME="Py_BuildValue">Py_BuildValue</A></CODE>:
				273	<CODE><PRE>
				274	return Py_BuildValue("iiO&", (int)status, (int)seqnum, PyMac_BuildStr255, message);
				275	</PRE></CODE>
				276	Py_BuildValue() is a very handy routine that builds tuples according
				277	to a format string, somewhat similar to the way <CODE>printf()</CODE>
				278	works. The format string specifies the arguments expected after the
				279	string, and turns them from C objects into python objects. The
				280	resulting objects are put in a python tuple object and returned. The
				281	"i" format specifier signifies an "int" (hence the cast: status and
				282	seqnum are declared as "long", which is what the is_status() routine
				283	wants, and even though we use a 4-byte project there is really no
				284	reason not to put the cast here). Py_BuildValue and its counterpart
				285	Py_ParseTuple have format codes for all the common C types like ints,
				286	shorts, C-strings, floats, etc. Also, there is a nifty escape
				287	mechanism to format values about which is does not know. This is
				288	invoked by the "O&" format: it expects two arguments, a routine
				289	pointer and an int-sized data object. The routine is called with the
				290	object as a parameter and it should return a python objects
				291	representing the data. <CODE>Macglue.h</CODE> declares a number of
				292	such formatting routines for common MacOS objects like Str255, FSSpec,
				293	OSType, Rect, etc. See the comments in the include file for
				294	details. <p>
				295
				296	<CODE>Pyis_getconfig()</CODE> is again similar to pyis_getstatus, only
				297	two minor points are worth noting here. First, the C API return the
				298	input and output baudrate squashed together into a single 4-byte
				299	long. We separate them out before returning the result to
				300	python. Second, whereas the status call returned us a pointer to a
				301	<CODE>Str255</CODE> it kept we are responsible for allocating the
				302	<CODE>Str255</CODE> for getconfig. This is something that would have
				303	been easy to get wrong had we not used prototypes everywhere. Morale:
				304	always try to include the header files for interfaces to libraries and
				305	other stuff, so that the compiler can catch any mistakes you make. <p>
				306
				307	<CODE>Pyis_setconfig()</CODE> finally shows off
				308	<CODE>Py_ParseTuple</CODE>, the companion function to
				309	<CODE>Py_BuildValue</CODE>. You pass it the argument tuple "args"
				310	that your method gets as its second argument, a format string and
				311	pointers to where you want the arguments stored. Again, standard C
				312	types such as strings and integers Py_ParseTuple knows all about and
				313	through the "O&" format you can extend the functionality. For each
				314	"O&" you pass a function pointer and a pointer to a data area. The
				315	function will be called with a PyObject pointer and your data pointer
				316	and it should convert the python object to the correct C type. It
				317	should return 1 on success and 0 on failure. Again, a number of
				318	converters for standard MacOS types are provided, and declared in
				319	<CODE>macglue.h</CODE>. <p>
				320
				321	Next in our source file comes the method table for our module, which
				322	has been generated by modulator (and it did a good job too!), but
				323	which is worth looking at for a moment. Entries are of the form
				324	<CODE><PRE>
				325	{"open", pyis_open, 1, pyis_open__doc__},
				326	</PRE></CODE>
				327	where the entries are python method name, C routine pointer, flags and
				328	docstring pointer. The value to note is the 1 for the flags: this
				329	signifies that you want to use "new-style" Py_ParseTuple behaviour. If
				330	you are writing a new module always use this, but if you are modifying
				331	old code which calls something like <CODE>getargs(args, "(ii)",
				332	...)</CODE> you will have to put zero here. See "extending and
				333	embedding" or possibly the getargs.c source file for details if you
				334	need them. <p>
				335
				336	Finally, we add some code to the init module, to put some symbolic
				337	constants (codes that can by returned by the status method) in the
				338	module dictionary, so the python program can use "interslip.RUN"
				339	instead of the cryptic "4" when it wants to check that the interslip
				340	driver is in RUN state. Modulator has already generated code to get at
				341	the module dictionary using PyModule_GetDict() to store the exception
				342	object, so we simply call
				343	<CODE><PRE>
				344	PyDict_SetItemString(d, "IDLE", PyInt_FromLong(IS_IDLE));
				345	</PRE></CODE>
				346	for each of our items. Since the last bit of code in our init routine
				347	checks for previous errors with <CODE>PyErr_Occurred()</CODE> and
				348	since <CODE>PyDict_SetItemString()</CODE> gracefully handles the case
				349	of <CODE>NULL</CODE> parameters (if <CODE>PyInt_FromLong()</CODE>
				350	failed, for instance) we don't have to do error checking here. In some
				351	other cases you may have to do error checking yourself. <p>
				352
				353	This concludes our crash-course on writing Python extensions in C on
				354	the Macintosh. If you are not done reading yet I suggest you look
				355	back at the <A HREF="index.html">MacPython Crashcourse index</A> to
				356	find another topic to study. <p>