mirror of
https://github.com/python/cpython.git
synced 2024-12-01 11:15:56 +01:00
325 lines
16 KiB
HTML
325 lines
16 KiB
HTML
<HTML><HEAD><TITLE>Creating a C extension module on the Macintosh</TITLE></HEAD>
|
|
<BODY>
|
|
<H1>Creating a C extension module on the Macintosh</H1>
|
|
<HR>
|
|
|
|
This document gives a step-by-step example of how to create a new C
|
|
extension module on the mac. For this example, we will create a module
|
|
to interface to the programmers' API of InterSLIP, a package that
|
|
allows you to use MacTCP (and, hence, all internet services) over a
|
|
modem connection. The actual example does not work anymore, as both
|
|
MacTCP and Interslip died long ago, but the text is still mostly
|
|
correct.<p>
|
|
|
|
<H2>Prerequisites</H2>
|
|
|
|
There are a few things you need to pull this off. First and foremost,
|
|
you need a C development environment. Actually, you need a specific
|
|
development environment, CodeWarrior by <A
|
|
HREF="http://www.metrowerks.com/">MetroWerks</A>. You will
|
|
need Version 7 or later. You may be able to get by with an older
|
|
version of CodeWarrior or with another development environment (Up to
|
|
about 1994 python was developed with THINK C, and in the dim past it
|
|
was compiled with MPW C) assuming you have managed to get Python to
|
|
compile under your development environment, but the step-by-step
|
|
character of this document will be lost. <p>
|
|
|
|
Next, you need to install the Developer option in the MacPython installer.
|
|
You may also find that Guido's <A
|
|
HREF="http://www.python.org/doc/ext/ext.html">Extending and embedding
|
|
the Python interpreter</A> is a very handy piece of documentation. I
|
|
will skip lots of details that are handled there, like complete
|
|
descriptions of <CODE>Py_ParseTuple</CODE> and such utility routines, or
|
|
the general structure of extension modules. <p>
|
|
|
|
<H2>InterSLIP and the C API to it</H2>
|
|
|
|
InterSLIP, the utility to which we are going to create a python
|
|
interface, is a system extension that does all the work of connecting
|
|
to the internet over a modem connection. InterSLIP is provided
|
|
free-of-charge by <A
|
|
HREF="http://www.intercon.com/">InterCon</A>. First it connects to
|
|
your modem, then it goes through the whole process of dialling,
|
|
logging in and possibly starting the SLIP software on the remote
|
|
computer and finally it starts with the real work: packing up IP
|
|
packets handed to it by MacTCP and sending them to the remote side
|
|
(and, of course, the reverse action of receiving incoming packets,
|
|
unpacking them and handing them to MacTCP). InterSLIP is a device
|
|
driver, and you control it using a application supplied with it,
|
|
InterSLIP Setup. The API that InterSLIP Setup uses to talk to the
|
|
device driver is published in the documentation and, hence, also
|
|
useable by other applications. <p>
|
|
|
|
I happened to have a C interface to the API, which is all ugly
|
|
low-level device-driver calls by itself. The C interface is in <A
|
|
HREF="interslip/InterslipLib.c">InterslipLib.c</A> and <A
|
|
HREF="interslip/InterslipLib.h">InterslipLib.h</A>, we'll
|
|
concentrate here on how to build the Python wrapper module around
|
|
it. Note that this is the "normal" situation when you are writing a
|
|
Python extension module: you have some sort of functionality available
|
|
to C programmers and want to make a Python interface to it. <p>
|
|
|
|
<H2>Using Modulator</H2>
|
|
|
|
The method we describe in this document, using Modulator, is the best
|
|
method for small interfaces. For large interfaces there is another
|
|
tool, Bgen, which actually generates the complete module without you
|
|
lifting a single finger. Bgen, however, has the disadvantage of having
|
|
a very steep learning curve, so an example using it will have to wait
|
|
until another document, when I have more time. <p>
|
|
|
|
First, let us look at the <A
|
|
HREF="interslip/InterslipLib.h">InterslipLib.h</A> header file,
|
|
and see that the whole interface consists of six routines:
|
|
<CODE>is_open</CODE>, <CODE>is_connect</CODE>,
|
|
<CODE>is_disconnect</CODE>, <CODE>is_status</CODE>,
|
|
<CODE>is_getconfig</CODE> and <CODE>is_setconfig</CODE>. Our first
|
|
step will be to create a skeleton file <A
|
|
HREF="interslip/@interslipmodule.c">@interslipmodule.c</A>, a
|
|
dummy module that will contain all the glue code that python expects
|
|
of an extension module. Creating this glue code is a breeze with
|
|
modulator, a tool that we only have to tell that we want to create a
|
|
module with methods of the six names above and that will create the
|
|
complete skeleton C code for us. <p>
|
|
|
|
Why call this dummy module <CODE>@interslipmodule.c</CODE> and not
|
|
<CODE>interslipmodule.c</CODE>? Self-preservation: if ever you happen
|
|
to repeat the whole process after you have actually turned the
|
|
skeleton module into a real module you would overwrite your
|
|
hand-written code. By calling the dummy module a different name you
|
|
have to make <EM>two</EM> mistakes in a row before you do this. <p>
|
|
|
|
If you installed Tk support when you installed Python this is extremely
|
|
simple. You start modulator and are provided with a form in which you
|
|
fill out the details of the module you are creating. <p>
|
|
|
|
<IMG SRC="html.icons/modulator.gif" ALIGN=CENTER><p>
|
|
|
|
You'll need to supply a module name (<CODE>interslip</CODE>, in our
|
|
case), a module abbreviation (<CODE>pyis</CODE>, which is used as a
|
|
prefix to all the routines and data structures modulator will create
|
|
for you) and you enter the names of all the methods your module will
|
|
export (the list above, with <CODE>is_</CODE> stripped off). Note that
|
|
we use <CODE>pyis</CODE> as the prefix instead of the more logical
|
|
<CODE>is</CODE>, since the latter would cause our routine names to
|
|
collide with those in the API we are interfacing to! The method names
|
|
are the names as seen by the python program, and the C routine names
|
|
will have the prefix and an underscore prepended. Modulator can do
|
|
much more, like generating code for objects and such, but that is a
|
|
topic for a later example. <p>
|
|
|
|
Once you have told modulator all about the module you want to create
|
|
you press "check", which checks that you haven't omitted any
|
|
information and "Generate code". This will prompt you for a C output
|
|
file and generate your module for you. <p>
|
|
|
|
<H2>Using Modulator without Tk</H2>
|
|
|
|
|
|
Modulator actually uses a two-stage process to create your code: first
|
|
the information you provided is turned into a number of python
|
|
statements and then these statements are executed to generate your
|
|
code. This is done so that you can even use modulator if you don't
|
|
have Tk support in Python: you'll just have to write the modulator
|
|
python statements by hand (about 10 lines, in our example) and
|
|
modulator will generate the C code (about 150 lines, in our
|
|
example). Here is the Python code you'll want to execute to generate
|
|
our skeleton module: <p>
|
|
|
|
<CODE><PRE>
|
|
import addpack
|
|
addpack.addpack('Tools')
|
|
addpack.addpack('modulator')
|
|
import genmodule
|
|
|
|
m = genmodule.module()
|
|
m.name = 'interslip'
|
|
m.abbrev = 'pyis'
|
|
m.methodlist = ['open', 'connect', 'disconnect', 'status', \
|
|
'getconfig', 'setconfig']
|
|
m.objects = []
|
|
|
|
fp = open('@interslipmodule.c', 'w')
|
|
genmodule.write(fp, m)
|
|
</PRE></CODE>
|
|
|
|
Drop this program on the python interpreter and out will come your
|
|
skeleton module. <p>
|
|
|
|
Now, rename the file to interslipmodule.c and you're all set to start
|
|
developing. The module is complete in the sense that it should
|
|
compile, and that if you import it in a python program you will see
|
|
all the methods. It is, of course, not yet complete in a functional
|
|
way... <p>
|
|
|
|
<H2>Creating a plugin module</H2>
|
|
|
|
The easiest way to build a plugin module is to use the distutils package,
|
|
this works fine on MacOS with CodeWarrior. See the distutils documentation
|
|
for details. Keep in mind that even though you are on the Mac you specify
|
|
filenames with Unix syntax: they are actually URLs, not filenames.
|
|
<p>
|
|
|
|
Alternatively you can build the project file by hand.
|
|
Go to the ":Mac:Build" folder and copy the files xx.carbon.mcp,
|
|
and xx.carbon.mcp.exp to interslipmodule.carbon.mcp and
|
|
interslipmodule.carbon.mcp.exp, respectively. Edit
|
|
interslipmodule.carbon.mcp.exp and change the name of the exported routine
|
|
"initxx" to "initinterslip". Open interslipmodule.carbon.mcp with CodeWarrior,
|
|
remove the file xxmodule.c and add interslipmodule.c and make a number
|
|
of adjustments to the preferences:
|
|
<UL>
|
|
<LI> in PPC target, set the output file name to "interslipmodule.carbon.slb",
|
|
<LI> if you are working without a source distribution (i.e. with a normal
|
|
binary distribution plus a development distribution) you will not have
|
|
a file <code>PythonCoreCarbon</code>. The installation process has deposited this
|
|
file in the System <code>Extensions</code> folder under the name
|
|
<code>PythonCoreCarbon <i>version</i></code>. Add that file to the project, replacing
|
|
<code>PythonCoreCarbon</code>.
|
|
<LI> you must either download and build GUSI (if your extension module uses sockets
|
|
or other Unix I/O constructs) or remove GUSI references from the Access Paths
|
|
settings. See the <a href="building.html">Building</a> document for where to get GUSI
|
|
and how to build it.
|
|
</UL>
|
|
Next, compile and link your module, fire up python and test it. <p>
|
|
|
|
<H2>Getting the module to do real work</H2>
|
|
|
|
So far, so good. In half an hour or so we have created a complete new
|
|
extension module for Python. The downside, however, is that the module
|
|
does not do anything useful. So, in the next half hour we will turn
|
|
our beautiful skeleton module into something that is at least as
|
|
beautiful but also gets some serious work done. For this once,
|
|
<EM>I</EM> have spent that half hour for you, and you can see the
|
|
results in <A
|
|
HREF="interslip/interslipmodule.c">interslipmodule.c</A>. <p>
|
|
|
|
We add
|
|
<CODE><PRE>
|
|
#include "InterslipLib.h"
|
|
#include "macglue.h"
|
|
</PRE></CODE>
|
|
to the top of the file, and work our way through each of the methods
|
|
to add the functionality needed. Starting with open, we fill in the
|
|
template docstring, the value accessible from Python by looking at
|
|
<CODE>interslip.open.__doc__</CODE>. There are not many tools using
|
|
this information at the moment, but as soon as class browsers for
|
|
python become available having this minimal documentation available is
|
|
a good idea. We put "Load the interslip driver" as the comment
|
|
here. <p>
|
|
|
|
Next, we tackle the body of <CODE>pyis_open()</CODE>. Since it has no
|
|
arguments and no return value we don't need to mess with that, we just
|
|
have to add a call to <CODE>is_open()</CODE> and check the return for
|
|
an error code, in which case we raise an error:
|
|
<CODE><PRE>
|
|
err = is_open();
|
|
if ( err ) {
|
|
PyErr_Mac(ErrorObject, err);
|
|
return NULL;
|
|
}
|
|
</PRE></CODE>
|
|
The routine <CODE><A NAME="PyErr_Mac">PyErr_Mac()</A></CODE> is a
|
|
useful routine that raises the exception passed as its first
|
|
argument. The data passed with the exception is based on the standard
|
|
MacOS error code given, and PyErr_Mac() attempts to locate a textual
|
|
description of the error code (which sure beats the "error -14021"
|
|
messages that so many macintosh applications tell their poor
|
|
users). <p>
|
|
|
|
We will skip pyis_connect and pyis_disconnect here, which are pretty
|
|
much identical to pyis_open: no arguments, no return value, just a
|
|
call and an error check. With pyis_status() things get interesting
|
|
again: this call still takes 3 arguments, and all happen to be values
|
|
returned (a numeric connection status indicator, a message sequence
|
|
number and a pointer to the message itself, in MacOS pascal-style
|
|
string form). We declare variables to receive the returned values, do
|
|
the call, check the error and format the return value. <p>
|
|
|
|
Building the return value is done using <CODE><A
|
|
NAME="Py_BuildValue">Py_BuildValue</A></CODE>:
|
|
<CODE><PRE>
|
|
return Py_BuildValue("iiO&", (int)status, (int)seqnum, PyMac_BuildStr255, message);
|
|
</PRE></CODE>
|
|
Py_BuildValue() is a very handy routine that builds tuples according
|
|
to a format string, somewhat similar to the way <CODE>printf()</CODE>
|
|
works. The format string specifies the arguments expected after the
|
|
string, and turns them from C objects into python objects. The
|
|
resulting objects are put in a python tuple object and returned. The
|
|
"i" format specifier signifies an "int" (hence the cast: status and
|
|
seqnum are declared as "long", which is what the is_status() routine
|
|
wants, and even though we use a 4-byte project there is really no
|
|
reason not to put the cast here). Py_BuildValue and its counterpart
|
|
Py_ParseTuple have format codes for all the common C types like ints,
|
|
shorts, C-strings, floats, etc. Also, there is a nifty escape
|
|
mechanism to format values about which is does not know. This is
|
|
invoked by the "O&" format: it expects two arguments, a routine
|
|
pointer and an int-sized data object. The routine is called with the
|
|
object as a parameter and it should return a python objects
|
|
representing the data. <CODE>Macglue.h</CODE> declares a number of
|
|
such formatting routines for common MacOS objects like Str255, FSSpec,
|
|
OSType, Rect, etc. See the comments in the include file for
|
|
details. <p>
|
|
|
|
<CODE>Pyis_getconfig()</CODE> is again similar to pyis_getstatus, only
|
|
two minor points are worth noting here. First, the C API return the
|
|
input and output baudrate squashed together into a single 4-byte
|
|
long. We separate them out before returning the result to
|
|
python. Second, whereas the status call returned us a pointer to a
|
|
<CODE>Str255</CODE> it kept we are responsible for allocating the
|
|
<CODE>Str255</CODE> for getconfig. This is something that would have
|
|
been easy to get wrong had we not used prototypes everywhere. Morale:
|
|
always try to include the header files for interfaces to libraries and
|
|
other stuff, so that the compiler can catch any mistakes you make. <p>
|
|
|
|
<CODE>Pyis_setconfig()</CODE> finally shows off
|
|
<CODE>Py_ParseTuple</CODE>, the companion function to
|
|
<CODE>Py_BuildValue</CODE>. You pass it the argument tuple "args"
|
|
that your method gets as its second argument, a format string and
|
|
pointers to where you want the arguments stored. Again, standard C
|
|
types such as strings and integers Py_ParseTuple knows all about and
|
|
through the "O&" format you can extend the functionality. For each
|
|
"O&" you pass a function pointer and a pointer to a data area. The
|
|
function will be called with a PyObject pointer and your data pointer
|
|
and it should convert the python object to the correct C type. It
|
|
should return 1 on success and 0 on failure. Again, a number of
|
|
converters for standard MacOS types are provided, and declared in
|
|
<CODE>macglue.h</CODE>. <p>
|
|
|
|
Next in our source file comes the method table for our module, which
|
|
has been generated by modulator (and it did a good job too!), but
|
|
which is worth looking at for a moment. Entries are of the form
|
|
<CODE><PRE>
|
|
{"open", pyis_open, 1, pyis_open__doc__},
|
|
</PRE></CODE>
|
|
where the entries are python method name, C routine pointer, flags and
|
|
docstring pointer. The value to note is the 1 for the flags: this
|
|
signifies that you want to use "new-style" Py_ParseTuple behaviour. If
|
|
you are writing a new module always use this, but if you are modifying
|
|
old code which calls something like <CODE>getargs(args, "(ii)",
|
|
...)</CODE> you will have to put zero here. See "extending and
|
|
embedding" or possibly the getargs.c source file for details if you
|
|
need them. <p>
|
|
|
|
Finally, we add some code to the init module, to put some symbolic
|
|
constants (codes that can by returned by the status method) in the
|
|
module dictionary, so the python program can use "interslip.RUN"
|
|
instead of the cryptic "4" when it wants to check that the interslip
|
|
driver is in RUN state. Modulator has already generated code to get at
|
|
the module dictionary using PyModule_GetDict() to store the exception
|
|
object, so we simply call
|
|
<CODE><PRE>
|
|
PyDict_SetItemString(d, "IDLE", PyInt_FromLong(IS_IDLE));
|
|
</PRE></CODE>
|
|
for each of our items. Since the last bit of code in our init routine
|
|
checks for previous errors with <CODE>PyErr_Occurred()</CODE> and
|
|
since <CODE>PyDict_SetItemString()</CODE> gracefully handles the case
|
|
of <CODE>NULL</CODE> parameters (if <CODE>PyInt_FromLong()</CODE>
|
|
failed, for instance) we don't have to do error checking here. In some
|
|
other cases you may have to do error checking yourself. <p>
|
|
|
|
This concludes our crash-course on writing Python extensions in C on
|
|
the Macintosh. If you are not done reading yet I suggest you look
|
|
back at the <A HREF="index.html">MacPython Crashcourse index</A> to
|
|
find another topic to study. <p>
|