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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
parents a71d2fd90fa8
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1 <HTML><HEAD><TITLE>Creating a C extension module on the Macintosh</TITLE></HEAD>
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2 <BODY>
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3 <H1>Creating a C extension module on the Macintosh</H1>
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4 <HR>
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5
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6 This document gives a step-by-step example of how to create a new C
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7 extension module on the mac. For this example, we will create a module
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8 to interface to the programmers' API of InterSLIP, a package that
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9 allows you to use MacTCP (and, hence, all internet services) over a
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10 modem connection. <p>
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11
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12 <H2>Prerequisites</H2>
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13
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14 There are a few things you need to pull this off. First and foremost,
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15 you need a C development environment. Actually, you need a specific
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16 development environment, CodeWarrior by <A
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17 HREF="http://www.metrowerks.com/">MetroWerks</A>. You will probably
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18 need the latest version. You may be able to get by with an older
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19 version of CodeWarrior or with another development environment (Up to
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20 about 1994 python was developed with THINK C, and in the dim past it
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21 was compiled with MPW C) assuming you have managed to get Python to
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22 compile under your development environment, but the step-by-step
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23 character of this document will be lost. <p>
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24
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25 Next, you need a <A HREF="http://www.python.org/python/Sources.html">python
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26 source distribution</A>. There is a <A
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27 HREF="update-to-1.3/into-PlugIns.hqx"> fixed project template</A> that
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28 you also need if you are going to make a dynamically loaded
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29 module. For PowerPC development you can actually get by without a full
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30 source distribution, using the PPC Development distribution (if I have
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31 gotten around to putting it together by the time you read
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32 this). You'll also need a functional python interpreter, and the
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33 Modulator program (which lives in <CODE>Tools:Modulator</CODE> in the
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34 standard source distribution). You may also find that Guido's <A
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35 HREF="http://www.python.org/doc/ext/ext.html">Extending and embedding
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36 the Python interpreter</A> is a very handy piece of documentation. I
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37 will skip lots of details that are handled there, like complete
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38 descriptions of <CODE>Py_ParseTuple</CODE> and such utility routines,
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39 or the general structure of extension modules. <p>
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40
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41 <H2>InterSLIP and the C API to it</H2>
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42
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43 InterSLIP, the utility to which we are going to create a python
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44 interface, is a system extension that does all the work of connecting
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45 to the internet over a modem connection. InterSLIP is provided
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46 free-of-charge by <A
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47 HREF="http://www.intercon.com/">InterCon</A>. First it connects to
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48 your modem, then it goes through the whole process of dialling,
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49 logging in and possibly starting the SLIP software on the remote
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50 computer and finally it starts with the real work: packing up IP
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51 packets handed to it by MacTCP and sending them to the remote side
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52 (and, of course, the reverse action of receiving incoming packets,
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53 unpacking them and handing them to MacTCP). InterSLIP is a device
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54 driver, and you control it using a application supplied with it,
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55 InterSLIP Setup. The API that InterSLIP Setup uses to talk to the
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56 device driver is published in the documentation and, hence, also
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57 useable by other applications. <p>
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58
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59 I happened to have a C interface to the API, which is all ugly
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60 low-level device-driver calls by itself. The C interface is in <A
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61 HREF="interslip/InterslipLib.c">InterslipLib.c</A> and <A
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62 HREF="interslip/InterslipLib.h">InterslipLib.h</A>, we'll
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63 concentrate here on how to build the Python wrapper module around
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64 it. Note that this is the "normal" situation when you are writing a
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65 Python extension module: you have some sort of functionality available
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66 to C programmers and want to make a Python interface to it. <p>
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67
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68 <H2>Using Modulator</H2>
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69
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70 The method we describe in this document, using Modulator, is the best
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71 method for small interfaces. For large interfaces there is another
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72 tool, Bgen, which actually generates the complete module without you
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73 lifting a single finger. Bgen, however, has the disadvantage of having
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74 a very steep learning curve, so an example using it will have to wait
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75 until another document, when I have more time. <p>
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76
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77 First, let us look at the <A
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78 HREF="interslip/InterslipLib.h">InterslipLib.h</A> header file,
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79 and see that the whole interface consists of six routines:
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80 <CODE>is_open</CODE>, <CODE>is_connect</CODE>,
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81 <CODE>is_disconnect</CODE>, <CODE>is_status</CODE>,
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82 <CODE>is_getconfig</CODE> and <CODE>is_setconfig</CODE>. Our first
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83 step will be to create a skeleton file <A
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84 HREF="interslip/@interslipmodule.c">@interslipmodule.c</A>, a
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85 dummy module that will contain all the glue code that python expects
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86 of an extension module. Creating this glue code is a breeze with
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87 modulator, a tool that we only have to tell that we want to create a
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88 module with methods of the six names above and that will create the
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89 complete skeleton C code for us. <p>
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90
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91 Why call this dummy module <CODE>@interslipmodule.c</CODE> and not
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92 <CODE>interslipmodule.c</CODE>? Self-preservation: if ever you happen
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93 to repeat the whole process after you have actually turned the
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94 skeleton module into a real module you would overwrite your
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95 hand-written code. By calling the dummy module a different name you
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96 have to make <EM>two</EM> mistakes in a row before you do this. <p>
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97
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98 On systems with the Tk windowing API for Python (currently only
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99 unix/X11 systems, but mac support may be available when you read this)
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100 this is extremely simple. It is actually so simple that it pays to
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101 create the skeleton module under unix and ship the code to your
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102 mac. You start modulator and are provided with a form in which you
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103 fill out the details of the module you are creating. <p>
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104
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105 <IMG SRC="html.icons/modulator.gif" ALIGN=CENTER><p>
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106
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107 You'll need to supply a module name (<CODE>interslip</CODE>, in our
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108 case), a module abbreviation (<CODE>pyis</CODE>, which is used as a
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109 prefix to all the routines and data structures modulator will create
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110 for you) and you enter the names of all the methods your module will
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111 export (the list above, with <CODE>is_</CODE> stripped off). Note that
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112 we use <CODE>pyis</CODE> as the prefix instead of the more logical
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113 <CODE>is</CODE>, since the latter would cause our routine names to
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114 collide with those in the API we are interfacing to! The method names
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115 are the names as seen by the python program, and the C routine names
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116 will have the prefix and an underscore prepended. Modulator can do
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117 much more, like generating code for objects and such, but that is a
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118 topic for a later example. <p>
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119
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120 Once you have told modulator all about the module you want to create
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121 you press "check", which checks that you haven't omitted any
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122 information and "Generate code". This will prompt you for a C output
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123 file and generate your module for you. <p>
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124
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125 <H2>Using Modulator without Tk</H2>
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126
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127
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128 Modulator actually uses a two-stage process to create your code: first
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129 the information you provided is turned into a number of python
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130 statements and then these statements are executed to generate your
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131 code. This is done so that you can even use modulator if you don't
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132 have Tk support in Python: you'll just have to write the modulator
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133 python statements by hand (about 10 lines, in our example) and
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134 modulator will generate the C code (about 150 lines, in our
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135 example). Here is the Python code you'll want to execute to generate
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136 our skeleton module: <p>
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137
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138 <CODE><PRE>
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139 import addpack
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140 addpack.addpack('Tools')
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141 addpack.addpack('modulator')
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Jack Jansen <jack.jansen@cwi.nl>
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142 import genmodule
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143
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144 m = genmodule.module()
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145 m.name = 'interslip'
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146 m.abbrev = 'pyis'
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147 m.methodlist = ['open', 'connect', 'disconnect', 'status', \
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148 'getconfig', 'setconfig']
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149 m.objects = []
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150
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151 fp = open('@interslipmodule.c', 'w')
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152 genmodule.write(fp, m)
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153 </PRE></CODE>
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154
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Jack Jansen <jack.jansen@cwi.nl>
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155 Drop this program on the python interpreter and out will come your
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Jack Jansen <jack.jansen@cwi.nl>
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156 skeleton module. <p>
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157
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158 Now, rename the file to interslipmodule.c and you're all set to start
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159 developing. The module is complete in the sense that it should
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Jack Jansen <jack.jansen@cwi.nl>
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160 compile, and that if you import it in a python program you will see
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161 all the methods. It is, of course, not yet complete in a functional
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Jack Jansen <jack.jansen@cwi.nl>
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162 way... <p>
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163
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Jack Jansen <jack.jansen@cwi.nl>
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164 <H2>Adding a module to 68K Python</H2>
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165
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166 What you do now depends on whether you're developing for PowerPC (or
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167 for CFM68K) or for "traditional" mac. For a traditional 68K Python,
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168 you will have to add your new module to the project file of the Python
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169 interpreter, and you have to edit "config.c" to add the module to the
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170 set of builtin modules. In config.c you will add the module at two
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171 places: near the start of the file there is a list of external
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172 declarations for all init() routines. Add a line of the form
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173 <CODE><PRE>
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174 extern void initinterslip();
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175 </PRE></CODE>
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176 here. Further down the file there is an array that is initialized with
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177 modulename/initfunction pairs. Add a line of the form
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178 <CODE><PRE>
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179 {"interslip", initinterslip},
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180 </PRE></CODE>
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181 here. You may want to bracket these two lines with
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182 <CODE><PRE>
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183 #ifdef USE_INTERSLIP
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Jack Jansen <jack.jansen@cwi.nl>
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184 #endif
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185 </PRE></CODE>
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186 lines, that way you can easily control whether the module is
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187 incorporated into python at compile time. If you decide to do the
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188 latter edit your config file (you can find the name in the "C/C++
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189 language" section of the MW preferences dialog, it will probably be
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190 "mwerks_nonshared_config.h") and add a
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191 <CODE><PRE>
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192 #define USE_INTERSLIP
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193 </PRE></CODE>
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194
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195 Make the new interpreter and check that you can import the module, see
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196 the methods (with "dir(interslip)") and call them. <p>
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197
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198 <H2>Creating a PowerPC plugin module</H2>
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199
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200 For PowerPC development you could follow the same path, but it is
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201 actually a better idea to use a dynamically loadable module. The
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202 advantage of dynamically loadable modules is that they are not loaded
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203 until a python program actually uses them (resulting in less memory
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204 usage by the interpreter) and that development is a lot simpler (since
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205 your projects will all be smaller). Moreover, you can distribute a
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206 plugin module by itself without haveing to distribute a complete
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Jack Jansen <jack.jansen@cwi.nl>
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207 python interpreter. <p>
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208
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209 Go to the "PlugIns" folder and copy the files xxmodule.µ,
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210 xxmodule_config.h and xxmodule.µ.exp to interslipmodule.µ,
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211 interslipmodule_config.h and interslipmodule.µ.exp, respectively. Edit
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212 interslipmodule.µ.exp and change the name of the exported routine
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213 "initxx" to "initinterslip". Open interslipmodule.µ with CodeWarrior,
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Jack Jansen <jack.jansen@cwi.nl>
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214 remove the file xxmodule.c and add interslipmodule.c and make a number
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215 of adjustments to the preferences:
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216 <UL>
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217 <LI> in C/C++ language, set the header file to interslipmodule_config.h
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218 <LI> in PPC linker, set the entry point to "initinterslip"
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219 <LI> in PPC PEF, set the fragment name to "interslipmodule"
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220 <LI> in PPC Project, set the output file name to "interslipmodule.slb".
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221 </UL>
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Jack Jansen <jack.jansen@cwi.nl>
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222 Next, compile and link your module, fire up python and do the same
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Jack Jansen <jack.jansen@cwi.nl>
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223 tests as for 68K python. <p>
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224
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Jack Jansen <jack.jansen@cwi.nl>
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225 <H2>Getting the module to do real work</H2>
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226
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Jack Jansen <jack.jansen@cwi.nl>
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227 So far, so good. In half an hour or so we have created a complete new
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Jack Jansen <jack.jansen@cwi.nl>
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228 extension module for Python. The downside, however, is that the module
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229 does not do anything useful. So, in the next half hour we will turn
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Jack Jansen <jack.jansen@cwi.nl>
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230 our beautiful skeleton module into something that is at least as
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Jack Jansen <jack.jansen@cwi.nl>
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231 beautiful but also gets some serious work done. For this once,
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Jack Jansen <jack.jansen@cwi.nl>
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232 <EM>I</EM> have spent that half hour for you, and you can see the
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Jack Jansen <jack.jansen@cwi.nl>
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233 results in <A
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Jack Jansen <jack.jansen@cwi.nl>
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234 HREF="interslip/interslipmodule.c">interslipmodule.c</A>. <p>
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235
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236 We add
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Jack Jansen <jack.jansen@cwi.nl>
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237 <CODE><PRE>
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Jack Jansen <jack.jansen@cwi.nl>
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238 #include "InterslipLib.h"
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Jack Jansen <jack.jansen@cwi.nl>
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239 #include "macglue.h"
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Jack Jansen <jack.jansen@cwi.nl>
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240 </PRE></CODE>
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Jack Jansen <jack.jansen@cwi.nl>
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241 to the top of the file, and work our way through each of the methods
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Jack Jansen <jack.jansen@cwi.nl>
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242 to add the functionality needed. Starting with open, we fill in the
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Jack Jansen <jack.jansen@cwi.nl>
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243 template docstring, the value accessible from Python by looking at
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Jack Jansen <jack.jansen@cwi.nl>
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244 <CODE>interslip.open.__doc__</CODE>. There are not many tools using
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245 this information at the moment, but as soon as class browsers for
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Jack Jansen <jack.jansen@cwi.nl>
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246 python become available having this minimal documentation available is
a71d2fd90fa8 Explanations of the examples here
Jack Jansen <jack.jansen@cwi.nl>
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diff changeset
247 a good idea. We put "Load the interslip driver" as the comment
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248 here. <p>
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249
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250 Next, we tackle the body of <CODE>pyis_open()</CODE>. Since it has no
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251 arguments and no return value we don't need to mess with that, we just
a71d2fd90fa8 Explanations of the examples here
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252 have to add a call to <CODE>is_open()</CODE> and check the return for
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253 an error code, in which case we raise an error:
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Jack Jansen <jack.jansen@cwi.nl>
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254 <CODE><PRE>
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Jack Jansen <jack.jansen@cwi.nl>
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255 err = is_open();
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256 if ( err ) {
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257 PyErr_Mac(ErrorObject, err);
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Jack Jansen <jack.jansen@cwi.nl>
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258 return NULL;
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Jack Jansen <jack.jansen@cwi.nl>
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259 }
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260 </PRE></CODE>
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261 The routine <CODE><A NAME="PyErr_Mac">PyErr_Mac()</A></CODE> is a
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262 useful routine that raises the exception passed as its first
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263 argument. The data passed with the exception is based on the standard
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264 MacOS error code given, and PyErr_Mac() attempts to locate a textual
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265 description of the error code (which sure beats the "error -14021"
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266 messages that so many macintosh applications tell their poor
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Jack Jansen <jack.jansen@cwi.nl>
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267 users). <p>
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268
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269 We will skip pyis_connect and pyis_disconnect here, which are pretty
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270 much identical to pyis_open: no arguments, no return value, just a
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271 call and an error check. With pyis_status() things get interesting
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272 again: this call still takes 3 arguments, and all happen to be values
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273 returned (a numeric connection status indicator, a message sequence
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274 number and a pointer to the message itself, in MacOS pascal-style
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275 string form). We declare variables to receive the returned values, do
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276 the call, check the error and format the return value. <p>
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277
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278 Building the return value is done using <CODE><A
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279 NAME="Py_BuildValue">Py_BuildValue</A></CODE>:
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280 <CODE><PRE>
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281 return Py_BuildValue("iiO&", (int)status, (int)seqnum, PyMac_BuildStr255, message);
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282 </PRE></CODE>
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283 Py_BuildValue() is a very handy routine that builds tuples according
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284 to a format string, somewhat similar to the way <CODE>printf()</CODE>
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285 works. The format string specifies the arguments expected after the
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286 string, and turns them from C objects into python objects. The
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287 resulting objects are put in a python tuple object and returned. The
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288 "i" format specifier signifies an "int" (hence the cast: status and
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289 seqnum are declared as "long", which is what the is_status() routine
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290 wants, and even though we use a 4-byte project there is really no
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291 reason not to put the cast here). Py_BuildValue and its counterpart
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292 Py_ParseTuple have format codes for all the common C types like ints,
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293 shorts, C-strings, floats, etc. Also, there is a nifty escape
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294 mechanism to format values about which is does not know. This is
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295 invoked by the "O&" format: it expects two arguments, a routine
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296 pointer and an int-sized data object. The routine is called with the
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297 object as a parameter and it should return a python objects
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298 representing the data. <CODE>Macglue.h</CODE> declares a number of
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299 such formatting routines for common MacOS objects like Str255, FSSpec,
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300 OSType, Rect, etc. See the comments in the include file for
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301 details. <p>
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302
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303 <CODE>Pyis_getconfig()</CODE> is again similar to pyis_getstatus, only
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304 two minor points are worth noting here. First, the C API return the
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305 input and output baudrate squashed together into a single 4-byte
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306 long. We separate them out before returning the result to
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307 python. Second, whereas the status call returned us a pointer to a
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308 <CODE>Str255</CODE> it kept we are responsible for allocating the
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309 <CODE>Str255</CODE> for getconfig. This is something that would have
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310 been easy to get wrong had we not used prototypes everywhere. Morale:
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311 always try to include the header files for interfaces to libraries and
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312 other stuff, so that the compiler can catch any mistakes you make. <p>
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313
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314 <CODE>Pyis_setconfig()</CODE> finally shows off
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315 <CODE>Py_ParseTuple</CODE>, the companion function to
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316 <CODE>Py_BuildValue</CODE>. You pass it the argument tuple "args"
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317 that your method gets as its second argument, a format string and
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318 pointers to where you want the arguments stored. Again, standard C
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319 types such as strings and integers Py_ParseTuple knows all about and
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320 through the "O&" format you can extend the functionality. For each
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321 "O&" you pass a function pointer and a pointer to a data area. The
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322 function will be called with a PyObject pointer and your data pointer
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323 and it should convert the python object to the correct C type. It
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324 should return 1 on success and 0 on failure. Again, a number of
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325 converters for standard MacOS types are provided, and declared in
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326 <CODE>macglue.h</CODE>. <p>
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327
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328 Next in our source file comes the method table for our module, which
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329 has been generated by modulator (and it did a good job too!), but
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330 which is worth looking at for a moment. Entries are of the form
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331 <CODE><PRE>
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332 {"open", pyis_open, 1, pyis_open__doc__},
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333 </PRE></CODE>
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334 where the entries are python method name, C routine pointer, flags and
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335 docstring pointer. The value to note is the 1 for the flags: this
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336 signifies that you want to use "new-style" Py_ParseTuple behaviour. If
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337 you are writing a new module always use this, but if you are modifying
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338 old code which calls something like <CODE>getargs(args, "(ii)",
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339 ...)</CODE> you will have to put zero here. See "extending and
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340 embedding" or possibly the getargs.c source file for details if you
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341 need them. <p>
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342
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343 Finally, we add some code to the init module, to put some symbolic
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344 constants (codes that can by returned by the status method) in the
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345 module dictionary, so the python program can use "interslip.RUN"
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346 instead of the cryptic "4" when it wants to check that the interslip
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347 driver is in RUN state. Modulator has already generated code to get at
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348 the module dictionary using PyModule_GetDict() to store the exception
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349 object, so we simply call
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350 <CODE><PRE>
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351 PyDict_SetItemString(d, "IDLE", PyInt_FromLong(IS_IDLE));
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352 </PRE></CODE>
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353 for each of our items. Since the last bit of code in our init routine
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354 checks for previous errors with <CODE>PyErr_Occurred()</CODE> and
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355 since <CODE>PyDict_SetItemString()</CODE> gracefully handles the case
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356 of <CODE>NULL</CODE> parameters (if <CODE>PyInt_FromLong()</CODE>
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357 failed, for instance) we don't have to do error checking here. In some
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358 other cases you may have to do error checking yourself. <p>
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359
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360 This concludes our crash-course on writing Python extensions in C on
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361 the Macintosh. If you are not done reading yet I suggest you look
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362 back at the <A HREF="index.html">MacPython Crashcourse index</A> to
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363 find another topic to study. <p>