<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html><head><meta content="text/html; charset=ISO-8859-1" http-equiv="content-type"><title>Interfacing with External C Code</title></head> <body><h1><hr style="width: 100%; height: 2px;"><a name="InterfacingWithExternal"></a>Interfacing with External C Code <hr width="100%"></h1><ul><li> <a href="#ExternDecls">External declarations</a></li><ul><li> <a href="#ReferencingHeaders">Referencing C header files</a></li><li> <a href="#StructDeclStyles">Styles of struct/union/enum declaration</a></li><li> <a href="#AccessingAPI">Accessing Python/C API routines</a></li><li><a href="#SpecialTypes">Special Types</a><span style="color: rgb(255, 0, 0);"> <span style="color: rgb(255, 102, 0);">(New in 0.9.6)</span></span></li><li><a href="#CallingConventions">Windows Calling Conventions</a><span style="color: rgb(255, 0, 0);"> <span style="color: rgb(255, 102, 0);">(New in 0.9.6)</span></span></li><li> <a href="#CNameSpecs">Resolving naming conflicts - C name specifications</a></li></ul><li><a href="#Using_Pyrex_Declarations_from_C">Using Pyrex declarations from C</a></li><ul><li> <a href="#PublicDecls">Public declarations</a></li></ul><ul><li><a href="#C_API_Declarations">C API declarations</a><span style="color: rgb(255, 0, 0);"> <span style="color: rgb(255, 102, 0);">(New in 0.9.6)</span></span></li><li><a href="#Multiple_public_and_api_declarations">Multiple public and API declarations</a><span style="color: rgb(255, 0, 0);"> (New in 0.9.6.3)</span></li><li><a href="#Acquiring_and_Releasing_the_GIL">Acquiring and Releasing the GIL</a><span style="color: rgb(255, 0, 0);"> <span style="color: rgb(255, 102, 0);">(New in 0.9.6)</span></span></li></ul></ul> One of the main uses of Pyrex is wrapping existing libraries of C code. This is achieved by using <a href="#ExternDecls">external declarations</a> to declare the C functions and variables from the library that you want to use. <p>You can also use <a href="#PublicDecls">public declarations</a> to make C functions and variables defined in a Pyrex module available to external C code. The need for this is expected to be less frequent, but you might want to do it, for example, if you are embedding Python in another application as a scripting language. Just as a Pyrex module can be used as a bridge to allow Python code to call C code, it can also be used to allow C code to call Python code.</p><hr style="width: 100%; height: 2px;"> <h2> <a name="ExternDecls"></a>External declarations</h2> By default, C functions and variables declared at the module level are local to the module (i.e. they have the C <b>static</b> storage class). They can also be declared <b>extern</b> to specify that they are defined elsewhere, for example: <blockquote> <pre>cdef extern int spam_counter</pre> <pre>cdef extern void order_spam(int tons)</pre></blockquote> <h3> <a name="ReferencingHeaders"></a>Referencing C header files</h3> When you use an extern definition on its own as in the examples above, Pyrex includes a declaration for it in the generated C file. This can cause problems if the declaration doesn't exactly match the declaration that will be seen by other C code. If you're wrapping an existing C library, for example, it's important that the generated C code is compiled with exactly the same declarations as the rest of the library. <p>To achieve this, you can tell Pyrex that the declarations are to be found in a C header file, like this: </p> <blockquote> <pre>cdef extern from "spam.h":</pre> <pre> int spam_counter</pre><pre> void order_spam(int tons)</pre> </blockquote> The <b>cdef extern from</b> clause does three things: <ol><li> It directs Pyrex to place a <b>#include</b> statement for the named header file in the generated C code.<br> </li> <li> It prevents Pyrex from generating any C code for the declarations found in the associated block.<br> </li> <li> It treats all declarations within the block as though they started with <b>cdef extern</b>.</li></ol> It's important to understand that Pyrex does <i>not</i> itself read the C header file, so you still need to provide Pyrex versions of any declarations from it that you use. However, the Pyrex declarations don't always have to exactly match the C ones, and in some cases they shouldn't or can't. In particular: <ol><li> Don't use <b>const</b>. Pyrex doesn't know anything about const, so just leave it out. Most of the time this shouldn't cause any problem, although on rare occasions you might have to use a cast.<sup><a href="#Footnote1"> 1</a></sup><br> </li> <li> Leave out any platform-specific extensions to C declarations such as <b>__declspec()</b>.<br> </li> <li> If the header file declares a big struct and you only want to use a few members, you only need to declare the members you're interested in. Leaving the rest out doesn't do any harm, because the C compiler will use the full definition from the header file.<br> <br> In some cases, you might not need <i>any</i> of the struct's members, in which case you can just put <tt>pass</tt> in the body of the struct declaration, e.g.<br> <br> <tt> cdef extern from "foo.h":<br> struct spam:<br> pass</tt><br> <br> Note that you can only do this inside a <b>cdef extern from</b> block; struct declarations anywhere else must be non-empty.<br> <br> </li><li> If the header file uses typedef names such as <b>size_t </b>to refer to platform-dependent flavours of numeric types, you will need a corresponding <b>ctypedef</b> statement, but you don't need to match the type exactly, just use something of the right general kind (int, float, etc). For example,</li><ol><pre>ctypedef int size_t</pre></ol> will work okay whatever the actual size of a size_t is (provided the header file defines it correctly). <br> <li> If the header file uses macros to define constants, translate them into a dummy <b>enum</b> declaration.<br> </li> <li> If the header file defines a function using a macro, declare it as though it were an ordinary function, with appropriate argument and result types.</li></ol> A few more tricks and tips: <ul><li> If you want to include a C header because it's needed by another header, but don't want to use any declarations from it, put <tt><font size="+1">pass</font></tt> in the extern-from block:</li></ul> <ul><ul><tt>cdef extern from "spam.h":</tt><br><tt> pass</tt></ul></ul> <ul><li> If you want to include some external declarations, but don't want to specify a header file (because it's included by some other header that you've already included) you can put <tt>*</tt> in place of the header file name:</li></ul> <blockquote> <blockquote><tt>cdef extern from *:</tt> <br> <tt> ...</tt></blockquote> </blockquote> <h3> <a name="StructDeclStyles"></a>Styles of struct, union and enum declaration</h3> There are two main ways that structs, unions and enums can be declared in C header files: using a tag name, or using a typedef. There are also some variations based on various combinations of these. <p>It's important to make the Pyrex declarations match the style used in the header file, so that Pyrex can emit the right sort of references to the type in the code it generates. To make this possible, Pyrex provides two different syntaxes for declaring a struct, union or enum type. The style introduced above corresponds to the use of a tag name. To get the other style, you prefix the declaration with <b>ctypedef</b>, as illustrated below. </p> <p>The following table shows the various possible styles that can be found in a header file, and the corresponding Pyrex declaration that you should put in the <b>cdef exern from </b>block. Struct declarations are used as an example; the same applies equally to union and enum declarations. </p> <p>Note that in all the cases below, you refer to the type in Pyrex code simply as <tt><font size="+1">Foo</font></tt>, not <tt><font size="+1">struct Foo</font></tt>. </p><table cellpadding="5"> <tbody> <tr bgcolor="#8cbc1c" valign="top"> <td bgcolor="#8cbc1c"> </td> <td bgcolor="#ff9933" nowrap="nowrap"><b>C code</b></td> <td bgcolor="#66cccc" valign="top"><b>Possibilities for corresponding Pyrex code</b></td> <td bgcolor="#99cc33" valign="top"><b>Comments</b></td> </tr> <tr bgcolor="#8cbc1c" valign="top"> <td>1</td> <td bgcolor="#ff9900"><tt>struct Foo {</tt> <br> <tt> ...</tt> <br> <tt>};</tt></td> <td bgcolor="#66cccc"><tt>cdef struct Foo:</tt> <br> <tt> ...</tt></td> <td>Pyrex will refer to the type as <tt>struct Foo </tt>in the generated C code<tt>.</tt></td> </tr> <tr bgcolor="#8cbc1c" valign="top"> <td valign="top">2</td> <td bgcolor="#ff9900" nowrap="nowrap"><tt>typedef struct {</tt> <br> <tt> ...</tt> <br> <tt>} Foo;</tt></td> <td bgcolor="#66cccc" valign="top"><tt>ctypedef struct Foo:</tt> <br> <tt> ...</tt></td> <td valign="top">Pyrex will refer to the type simply as <tt>Foo</tt> in the generated C code.</td> </tr> <tr bgcolor="#8cbc1c" valign="top"> <td rowspan="2">3</td> <td rowspan="2" bgcolor="#ff9900" nowrap="nowrap"><tt>typedef struct foo {</tt> <br> <tt> ...</tt> <br> <tt>} Foo;</tt></td> <td bgcolor="#66cccc" nowrap="nowrap" valign="top"><tt>cdef struct foo:</tt> <br> <tt> ...</tt> <br> <tt>ctypedef foo Foo #optional</tt></td> <td rowspan="2" valign="top">If the C header uses both a tag and a typedef with <i>different</i> names, you can use either form of declaration in Pyrex (although if you need to forward reference the type, you'll have to use the first form).</td> </tr> <tr> <td bgcolor="#66cccc"><tt>ctypedef struct Foo:</tt> <br> <tt> ...</tt></td> </tr> <tr bgcolor="#8cbc1c" valign="top"> <td>4</td> <td bgcolor="#ff9900" nowrap="nowrap"><tt>typedef struct Foo {</tt> <br> <tt> ...</tt> <br> <tt>} Foo;</tt></td> <td bgcolor="#66cccc" valign="top"><tt>cdef struct Foo:</tt> <br> <tt> ...</tt></td> <td>If the header uses the <i>same</i> name for the tag and the typedef, you won't be able to include a <b>ctypedef</b> for it -- but then, it's not necessary.</td> </tr> </tbody> </table> <h3> <a name="AccessingAPI"></a>Accessing Python/C API routines</h3> One particular use of the <b>cdef extern from</b> statement is for gaining access to routines in the Python/C API. For example, <blockquote> <pre>cdef extern from "Python.h":</pre> <pre> object PyString_FromStringAndSize(char *s, Py_ssize_t len)</pre></blockquote> will allow you to create Python strings containing null bytes. <h3> <a name="SpecialTypes"></a>Special Types</h3><p>Pyrex predefines the name <span style="font-family: monospace;">Py_ssize_t</span> for use with Python/C API routines. To make your extensions compatible with 64-bit systems, you should always use this type where it is specified in the documentation of Python/C API routines.</p><h3><a name="CallingConventions"></a>Windows Calling Conventions</h3><p>The <span style="font-family: monospace;">__stdcall</span> and <span style="font-family: monospace;">__cdecl</span> calling convention specifiers can be used in Pyrex, with the same syntax as used by C compilers on Windows, for example,</p><pre style="margin-left: 40px;">cdef extern int __stdcall FrobnicateWindow(long handle)<br><br>cdef void (__stdcall *callback)(void *)<br></pre>If __stdcall is used, the function is only considered compatible with other __stdcall functions of the same signature.<br><br> <hr width="100%"> <h2> <a name="CNameSpecs"></a>Resolving naming conflicts - C name specifications</h2> Each Pyrex module has a single module-level namespace for both Python and C names. This can be inconvenient if you want to wrap some external C functions and provide the Python user with Python functions of the same names. <p>Pyrex 0.8 provides a couple of different ways of solving this problem. The best way, especially if you have many C functions to wrap, is probably to put the extern C function declarations into a different namespace using the facilities described in the section on <a href="sharing.html">sharing declarations between Pyrex modules</a>. </p> <p>The other way is to use a <b>c name specification</b> to give different Pyrex and C names to the C function. Suppose, for example, that you want to wrap an external function called <tt>eject_tomato</tt>. If you declare it as </p> <blockquote> <pre>cdef extern void c_eject_tomato "eject_tomato" (float speed)</pre> </blockquote> then its name inside the Pyrex module will be <tt>c_eject_tomato</tt>, whereas its name in C will be <tt>eject_tomato</tt>. You can then wrap it with <blockquote> <pre>def eject_tomato(speed):<br> c_eject_tomato(speed)</pre> </blockquote> so that users of your module can refer to it as <tt>eject_tomato</tt>. <p>Another use for this feature is referring to external names that happen to be Pyrex keywords. For example, if you want to call an external function called <tt>print</tt>, you can rename it to something else in your Pyrex module. </p> <p>As well as functions, C names can be specified for variables, structs, unions, enums, struct and union members, and enum values. For example, </p> <blockquote> <pre>cdef extern int one "ein", two "zwei"<br>cdef extern float three "drei"<br><br>cdef struct spam "SPAM":<br> int i "eye"</pre><tt>cdef enum surprise "inquisition":</tt> <br> <tt> first "alpha"</tt> <br> <tt> second "beta" = 3</tt></blockquote> <hr width="100%"> <h2><a name="Using_Pyrex_Declarations_from_C"></a>Using Pyrex Declarations from C</h2>Pyrex provides two methods for making C declarations from a Pyrex module available for use by external C code – public declarations and C API declarations.<br><br><div style="margin-left: 40px;"><span style="font-weight: bold;">NOTE:</span> You do <span style="font-style: italic;">not</span> need to use either of these to make declarations from one Pyrex module available to another Pyrex module – you should use the <span style="font-weight: bold;">cimport</span> statement for that. <a href="sharing.html">Sharing Declarations Between Pyrex Modules</a>.</div><h3><a name="PublicDecls"></a>Public Declarations</h3> You can make C types, variables and functions defined in a Pyrex module accessible to C code that is linked with the module, by declaring them with the <b><tt>public</tt></b> keyword: <blockquote><tt>cdef public struct Bunny: # public type declaration<br> int vorpalness<br><br>cdef public int spam # public variable declaration</tt> <p><tt>cdef public void grail(Bunny *): # public function declaration</tt> <br> <tt> ...</tt></p> </blockquote> If there are any <tt>public</tt> declarations in a Pyrex module, a header file called <b><span style="font-style: italic;">modulename</span>.h</b> file is generated containing equivalent C declarations for inclusion in other C code.<br><br>Any C code wanting to make use of these declarations will need to be linked, either statically or dynamically, with the extension module.<br><br>If the Pyrex module resides within a package, then the name of the .h file consists of the full dotted name of the module, e.g. a module called <span style="font-weight: bold;">foo.spam</span> would have a header file called <span style="font-weight: bold;">foo.spam.h</span>. <h3><a name="C_API_Declarations"></a>C API Declarations</h3><p>The other way of making declarations available to C code is to declare them with the <span style="font-family: monospace; font-weight: bold;">api</span> keyword. You can use this keyword with C functions and extension types. A header file called "<span style="font-weight: bold;"><span style="font-style: italic;">modulename</span>_api.h</span>" is produced containing declarations of the functions and extension types, and a function called <span style="font-weight: bold;">import_<span style="font-style: italic;">modulename</span>()</span>.</p><p>C code wanting to use these functions or extension types needs to include the header and call the import_<span style="font-style: italic;">modulename</span>() function. The other functions can then be called and the extension types used as usual.</p><p>Any <span style="font-family: monospace;">public</span> C type or extension type declarations in the Pyrex module are also made available when you include <span style="font-style: italic;">modulename</span>_api.h.</p><table style="text-align: left; width: 100%;" border="0" cellpadding="5" cellspacing="2"><tbody><tr><td style="background-color: rgb(102, 204, 204);"><pre>delorean.pyx</pre></td><td style="background-color: rgb(255, 153, 0);"><pre>marty.c</pre></td></tr><tr><td style="vertical-align: top; background-color: rgb(102, 204, 204);"><pre>cdef public struct Vehicle:<br> int speed<br> float power<br><br>cdef api void activate(Vehicle *v):<br> if v.speed >= 88 \<br> and v.power >= 1.21:<br> print "Time travel achieved"</pre></td><td style="background-color: rgb(255, 153, 0);"><pre>#include "delorean_api.h"<br><br>Vehicle car;<br><br>int main(int argc, char *argv[]) {<br> import_delorean();<br> car.speed = atoi(argv[1]);<br> car.power = atof(argv[2]); <br> activate(&car);<br>}</pre></td></tr></tbody></table><br>Note that any types defined in the Pyrex module that are used as argument or return types of the exported functions will need to be declared <span style="font-family: monospace;">public</span>, otherwise they won't be included in the generated header file, and you will get errors when you try to compile a C file that uses the header.<br><br>Using the <span style="font-family: monospace;">api</span> method does not require the C code using the declarations to be linked with the extension module in any way, as the Python import machinery is used to make the connection dynamically. However, only functions can be accessed this way, not variables.<br><br>You can use both <span style="font-family: monospace;">public</span> and <span style="font-family: monospace;">api</span> on the same function to make it available by both methods, e.g.<br><pre style="margin-left: 40px;">cdef public api void belt_and_braces():<br> ...<br></pre>However, note that you should include <span style="font-weight: bold;">either</span> <span style="font-style: italic;">modulename</span>.h <span style="font-weight: bold;">or</span> <span style="font-style: italic;">modulename</span>_api.h in a given C file, <span style="font-style: italic;">not</span> both, otherwise you may get conflicting dual definitions.<br><br>If the Pyrex module resides within a package, then:<br><ul><li>The name of the header file contains of the full dotted name of the module.</li><li>The name of the importing function contains the full name with dots replaced by double underscores.</li></ul>E.g. a module called <span style="font-weight: bold;">foo.spam</span> would have an API header file called <span style="font-weight: bold;">foo.spam_api.h</span> and an importing function called <span style="font-weight: bold;">import_foo__spam()</span>.<br><h3><a name="Multiple_public_and_api_declarations"></a>Multiple public and api declarations</h3>You can declare a whole group of items as <span style="font-style: italic;">public</span> and/or <span style="font-style: italic;">api</span> all at once by enclosing them in a cdef block, for example,<br><pre style="margin-left: 40px;">cdef public api:<br> void order_spam(int tons)<br> char *get_lunch(float tomato_size)<br></pre>This can be a useful thing to do in a <span style="font-family: monospace;">.pxd</span> file (see <a href="sharing.html">Sharing Declarations Between Pyrex Modules</a>) to make the module's public interface available by all three methods.<br><br><hr style="width: 100%; height: 2px;"><h2><a name="Acquiring_and_Releasing_the_GIL"></a>Acquiring and Releasing the GIL</h2>Pyrex provides facilities for releasing the Global Interpreter Lock (GIL) before calling C code, and for acquiring the GIL in functions that are to be called back from C code that is executed without the GIL.<br><h3>Releasing the GIL</h3>You can release the GIL around a section of code using the<span style="font-family: monospace; font-weight: bold;"> with nogil </span>statement:<br><pre style="margin-left: 40px;">with nogil:<br> <code to be executed with the GIL released><br></pre>Code in the body of the statement <span style="font-style: italic;">must not manipulate Python objects in any way</span>, and must not call anything that manipulates Python objects without first re-acquiring the GIL. <span style="font-weight: bold;">Pyrex currently does not check this.</span><br><h3>Acquiring the GIL</h3>A C function that is to be used as a callback from C code that is executed without the GIL needs to acquire the GIL before it can manipulate Python objects. This can be done by specifying<span style="font-family: monospace; font-weight: bold;"> with gil </span>in the function header:<br><pre style="margin-left: 40px;">cdef void my_callback(void *data) with gil:<br> ...<br></pre><h3>Declaring a function as callable without the GIL</h3>You can specify <span style="font-family: monospace; font-weight: bold;">nogil</span> in a C function header or function type to declare that it is safe to call without the GIL. <br><pre style="margin-left: 40px;">cdef void my_gil_free_func(int spam) nogil:<br> ...</pre>If you are implementing such a function in Pyrex, it cannot have any Python arguments, Python local variables, or Python return type, and cannot manipulate Python objects in any way or call any function that does so without acquiring the GIL first. Some of these restrictions are currently checked by Pyrex, but not all. It is possible that more stringent checking will be performed in the future.<br><br>Declaring a function<span style="font-family: monospace;"> with gil </span>also implicitly makes its signature<span style="font-family: monospace;"> nogil</span>.<br><br> <hr style="width: 100%; height: 2px;"><span style="font-weight: bold;">Footnotes</span> <hr width="100%"><a name="Footnote1"></a>1. A problem with const could arise if you have something like <blockquote> <pre>cdef extern from "grail.h":<br> char *nun</pre> </blockquote> where grail.h actually contains <blockquote> <pre>extern const char *nun;</pre> </blockquote> and you do <blockquote> <pre>cdef void languissement(char *s):<br> #something that doesn't change s</pre> <pre>...</pre> <pre>languissement(nun)</pre> </blockquote>which will cause the C compiler to complain. You can work around it by casting away the constness: <blockquote> <pre>languissement(<char *>nun) <br></pre> </blockquote>---</body></html>