Interfacing with External C Code

One of the main uses of Cython is wrapping existing libraries of C code. Thisis achieved by using external declarations to declare the C functions andvariables from the library that you want to use.

You can also use public declarations to make C functions and variables definedin a Cython module available to external C code. The need for this is expectedto be less frequent, but you might want to do it, for example, if you areembedding Python in another application as a scripting language. Just as aCython module can be used as a bridge to allow Python code to call C code, itcan also be used to allow C code to call Python code.

External declarations

By default, C functions and variables declared at the module level are localto the module (i.e. they have the C static storage class). They can also bedeclared extern to specify that they are defined elsewhere, for example,:

cdef extern int spam_counter
 
cdef extern void order_spam(int tons)

Referencing C header files

When you use an extern definition on its own as in the examples above, Cythonincludes a declaration for it in the generated C file. This can cause problemsif the declaration doesn’t exactly match the declaration that will be seen byother C code. If you’re wrapping an existing C library, for example, it’simportant that the generated C code is compiled with exactly the samedeclarations as the rest of the library.

To achieve this, you can tell Cython that the declarations are to be found in aC header file, like this:

cdef extern from "spam.h":
 
    int spam_counter
 
    void order_spam(int tons)

The cdef extern from clause does three things:

• It directs Cython to place a #include statement for the named header file inthe generated C code.
• It prevents Cython from generating any C codefor the declarations found in the associated block.

• It treats all declarations within the block as though they started withcdef extern.
  It’s important to understand that Cython does not itself read the C headerfile, so you still need to provide Cython versions of any declarations from itthat you use. However, the Cython declarations don’t always have to exactlymatch the C ones, and in some cases they shouldn’t or can’t. In particular:

• Leave out any platform-specific extensions to C declarations such as__declspec().

• If the header file declares a big struct and you only want to use a fewmembers, you only need to declare the members you’re interested in. Leavingthe rest out doesn’t do any harm, because the C compiler will use the fulldefinition from the header file.

In some cases, you might not need any of the struct’s members, in whichcase you can just put pass in the body of the struct declaration, e.g.:

cdef extern from "foo.h":
    struct spam:
        pass

Note

you can only do this inside a cdef extern from block; structdeclarations anywhere else must be non-empty.

• If the header file uses typedef names such as word to referto platform-dependent flavours of numeric types, you will need acorresponding ctypedef statement, but you don’t need to matchthe type exactly, just use something of the right general kind (int, float,etc). For example,:

ctypedef int word

will work okay whatever the actual size of a word is (provided the headerfile defines it correctly). Conversion to and from Python types, if any, will alsobe used for this new type.

• If the header file uses macros to define constants, translate them into anormal external variable declaration. You can also declare them as anenum if they contain normal int values. Note thatCython considers enum to be equivalent to int, so donot do this for non-int values.

• If the header file defines a function using a macro, declare it as thoughit were an ordinary function, with appropriate argument and result types.

• For archaic reasons C uses the keyword void to declare a functiontaking no parameters. In Cython as in Python, simply declare such functionsas foo().

A few more tricks and tips:

• If you want to include a C header because it’s needed by another header, butdon’t want to use any declarations from it, put pass in the extern-fromblock:

cdef extern from "spam.h":
    pass

• If you want to include a system header, put angle brackets inside the quotes:

cdef extern from "<sysheader.h>":
    ...

• If you want to include some external declarations, but don’t want to specifya header file (because it’s included by some other header that you’vealready included) you can put * in place of the header file name:

cdef extern from *:
    ...

• If a cdef extern from "inc.h" block is not empty and contains onlyfunction or variable declarations (and no type declarations of any kind),Cython will put the #include "inc.h" statement after alldeclarations generated by Cython. This means that the included filehas access to the variables, functions, structures, … which aredeclared by Cython.

Implementing functions in C

When you want to call C code from a Cython module, usually that codewill be in some external library that you link your extension against.However, you can also directly compile C (or C++) code as part of yourCython module. In the .pyx file, you can put something like:

cdef extern from "spam.c":
    void order_spam(int tons)

Cython will assume that the function order_spam() is defined in thefile spam.c. If you also want to cimport this function from anothermodule, it must be declared (not extern!) in the .pxd file:

cdef void order_spam(int tons)

For this to work, the signature of order_spam() in spam.c mustmatch the signature that Cython uses, in particular the function mustbe static:

static void order_spam(int tons)
{
    printf("Ordered %i tons of spam!\n", tons);
}

Styles of struct, union and enum declaration

There are two main ways that structs, unions and enums can be declared in Cheader files: using a tag name, or using a typedef. There are also somevariations based on various combinations of these.

It’s important to make the Cython declarations match the style used in theheader file, so that Cython can emit the right sort of references to the typein the code it generates. To make this possible, Cython provides two differentsyntaxes for declaring a struct, union or enum type. The style introducedabove corresponds to the use of a tag name. To get the other style, you prefixthe declaration with ctypedef, as illustrated below.

The following table shows the various possible styles that can be found in aheader file, and the corresponding Cython declaration that you should put inthe cdef extern from block. Struct declarations are used as an example; thesame applies equally to union and enum declarations.

struct Foo {
  …
};

cdef struct Foo:
  …

typedef struct {
  …
} Foo;

ctypedef struct Foo:
  …

typedef struct foo {
  …
} Foo;

cdef struct foo:
  …
ctypedef foo Foo #optional

ctypedef struct Foo:
  …

typedef struct Foo {
  …
} Foo;

cdef struct Foo:
  …

See also use of External extension types.Note that in all the cases below, you refer to the type in Cython code simplyas Foo, not struct Foo.

Accessing Python/C API routines

One particular use of the cdef extern from statement is for gaining access toroutines in the Python/C API. For example,:

cdef extern from "Python.h":
 
    object PyString_FromStringAndSize(char *s, Py_ssize_t len)

will allow you to create Python strings containing null bytes.

Special Types

Cython predefines the name Py_ssize_t for use with Python/C API routines. Tomake your extensions compatible with 64-bit systems, you should always usethis type where it is specified in the documentation of Python/C API routines.

Windows Calling Conventions

The stdcall and cdecl calling convention specifiers can be used inCython, with the same syntax as used by C compilers on Windows, for example,:

cdef extern int __stdcall FrobnicateWindow(long handle)
 
cdef void (__stdcall *callback)(void *)

If stdcall is used, the function is only considered compatible withother stdcall functions of the same signature.

Resolving naming conflicts - C name specifications

Each Cython module has a single module-level namespace for both Python and Cnames. This can be inconvenient if you want to wrap some external C functionsand provide the Python user with Python functions of the same names.

Cython provides a couple of different ways of solving this problem. The bestway, especially if you have many C functions to wrap, is to put the externC function declarations into a .pxd file and thus a different namespace,using the facilities described in sharing declarations between Cythonmodules. Writing them into a .pxd file allowstheir reuse across modules, avoids naming collisions in the normal Python wayand even makes it easy to rename them on cimport. For example, if yourdecl.pxd file declared a C function eject_tomato:

cdef extern from "myheader.h":
    void eject_tomato(float speed)

then you can cimport and wrap it in a .pyx file as follows:

from decl cimport eject_tomato as c_eject_tomato
 
def eject_tomato(speed):
    c_eject_tomato(speed)

or simply cimport the .pxd file and use it as prefix:

cimport decl
 
def eject_tomato(speed):
    decl.eject_tomato(speed)

Note that this has no runtime lookup overhead, as it would in Python.Cython resolves the names in the .pxd file at compile time.

For special cases where namespacing or renaming on import is not enough,e.g. when a name in C conflicts with a Python keyword, you can use a C namespecification to give different Cython and C names to the C function atdeclaration time. Suppose, for example, that you want to wrap an externalC function called yield(). If you declare it as:

cdef extern from "myheader.h":
    void c_yield "yield" (float speed)

then its Cython visible name will be c_yield, whereas its name in Cwill be yield. You can then wrap it with:

def call_yield(speed):
    c_yield(speed)

As for functions, C names can be specified for variables, structs, unions,enums, struct and union members, and enum values. For example:

cdef extern int one "eins", two "zwei"
cdef extern float three "drei"
 
cdef struct spam "SPAM":
    int i "eye"
 
cdef enum surprise "inquisition":
    first "alpha"
    second "beta" = 3

Note that Cython will not do any validation or name mangling on the stringyou provide. It will inject the bare text into the C code unmodified, so youare entirely on your own with this feature. If you want to declare a namexyz and have Cython inject the text “make the C compiler fail here” intothe C file for it, you can do this using a C name declaration. Consider thisan advanced feature, only for the rare cases where everything else fails.

Including verbatim C code

For advanced use cases, Cython allows you to directly write C codeas “docstring” of a cdef extern from block:

cdef extern from *:
    """
    /* This is C code which will be put
     * in the .c file output by Cython */
    static long square(long x) {return x * x;}
    #define assign(x, y) ((x) = (y))
    """
    long square(long x)
    void assign(long& x, long y)

The above is essentially equivalent to having the C code in a fileheader.h and writing

cdef extern from "header.h":
    long square(long x)
    void assign(long& x, long y)

It is also possible to combine a header file and verbatim C code:

cdef extern from "badheader.h":
    """
    /* This macro breaks stuff */
    #undef int
    """
    # Stuff from badheader.h

In this case, the C code #undef int is put right after#include "badheader.h" in the C code generated by Cython.

Note that the string is parsed like any other docstring in Python.If you require character escapes to be passed into the C code file,use a raw docstring, i.e. r""" … """.

Using Cython Declarations from C

Cython provides two methods for making C declarations from a Cython moduleavailable for use by external C code—public declarations and C APIdeclarations.

Note

You do not need to use either of these to make declarations from oneCython module available to another Cython module – you should use thecimport statement for that. Sharing Declarations Between Cython Modules.

Public Declarations

You can make C types, variables and functions defined in a Cython moduleaccessible to C code that is linked together with the Cython-generated C file,by declaring them with the public keyword:

cdef public struct Bunny: # public type declaration
    int vorpalness
 
cdef public int spam # public variable declaration
 
cdef public void grail(Bunny *) # public function declaration

If there are any public declarations in a Cython module, a header file calledmodulename.h file is generated containing equivalent C declarations forinclusion in other C code.

A typical use case for this is building an extension module from multipleC sources, one of them being Cython generated (i.e. with something likeExtension("grail", sources=["grail.pyx", "grail_helper.c"]) in setup.py.In this case, the file grail_helper.c just needs to add#include "grail.h" in order to access the public Cython variables.

A more advanced use case is embedding Python in C using Cython.In this case, make sure to call Py_Initialize() and Py_Finalize().For example, in the following snippet that includes grail.h:

#include <Python.h>
#include "grail.h"
 
int main() {
    Py_Initialize();
    initgrail();  /* Python 2.x only ! */
    Bunny b;
    grail(b);
    Py_Finalize();
}

This C code can then be built together with the Cython-generated C codein a single program (or library).

In Python 3.x, calling the module init function directly should be avoided. Instead,use the inittab mechanismto link Cython modules into a single shared library or program.

err = PyImport_AppendInittab("grail", PyInit_grail);
Py_Initialize();
grail_module = PyImport_ImportModule("grail");

If the Cython module resides within a package, then the name of the .hfile consists of the full dotted name of the module, e.g. a module calledfoo.spam would have a header file called foo.spam.h.

Note

On some operating systems like Linux, it is also possible to firstbuild the Cython extension in the usual way and then link againstthe resulting .so file like a dynamic library.Beware that this is not portable, so it should be avoided.

C API Declarations

The other way of making declarations available to C code is to declare themwith the api keyword. You can use this keyword with C functions andextension types. A header file called modulename_api.h is producedcontaining declarations of the functions and extension types, and a functioncalled import_modulename().

C code wanting to use these functions or extension types needs to include theheader and call the import_modulename() function. The other functionscan then be called and the extension types used as usual.

If the C code wanting to use these functions is part of more than one sharedlibrary or executable, then import_modulename() function needs to becalled in each of the shared libraries which use these functions. If youcrash with a segmentation fault (SIGSEGV on linux) when calling into one ofthese api calls, this is likely an indication that the shared library whichcontains the api call which is generating the segmentation fault does not callthe import_modulename() function before the api call which crashes.

Any public C type or extension type declarations in the Cython module are alsomade available when you include modulename_api.h.:

# delorean.pyx
 
cdef public struct Vehicle:
    int speed
    float power
 
cdef api void activate(Vehicle *v):
    if v.speed >= 88 and v.power >= 1.21:
        print("Time travel achieved")

# marty.c
#include "delorean_api.h"
 
Vehicle car;
 
int main(int argc, char *argv[]) {
    Py_Initialize();
    import_delorean();
    car.speed = atoi(argv[1]);
    car.power = atof(argv[2]);
    activate(&car);
    Py_Finalize();
}

Note

Any types defined in the Cython module that are used as argument orreturn types of the exported functions will need to be declared public,otherwise they won’t be included in the generated header file, and you willget errors when you try to compile a C file that uses the header.

Using the api method does not require the C code using thedeclarations to be linked with the extension module in any way, as the Pythonimport machinery is used to make the connection dynamically. However, onlyfunctions can be accessed this way, not variables. Note also that for themodule import mechanism to be set up correctly, the user must callPy_Initialize() and Py_Finalize(); if you experience a segmentation fault inthe call to import_modulename(), it is likely that this wasn’t done.

You can use both public and api on the same function tomake it available by both methods, e.g.:

cdef public api void belt_and_braces():
    ...

However, note that you should include either modulename.h ormodulename_api.h in a given C file, not both, otherwise you may getconflicting dual definitions.

If the Cython module resides within a package, then:

• The name of the header file contains of the full dotted name of the module.
• The name of the importing function contains the full name with dots replacedby double underscores.

E.g. a module called foo.spam would have an API header file calledfoo.spam_api.h and an importing function calledimport_foo__spam().

Multiple public and API declarations

You can declare a whole group of items as public and/orapi all at once by enclosing them in a cdef block, forexample,:

cdef public api:
    void order_spam(int tons)
    char *get_lunch(float tomato_size)

This can be a useful thing to do in a .pxd file (seeSharing Declarations Between Cython Modules) to make the module’s public interfaceavailable by all three methods.

Acquiring and Releasing the GIL

Cython provides facilities for acquiring and releasing theGlobal Interpreter Lock (GIL).This may be useful when calling from multi-threaded code into(external C) code that may block, or when wanting to use Pythonfrom a (native) C thread callback. Releasing the GIL shouldobviously only be done for thread-safe code or for code thatuses other means of protection against race conditions andconcurrency issues.

Note that acquiring the GIL is a blocking thread-synchronisingoperation, and therefore potentially costly. It might not beworth releasing the GIL for minor calculations. Usually, I/Ooperations and substantial computations in parallel code willbenefit from it.

Releasing the GIL

You can release the GIL around a section of code using thewith nogil statement:

with nogil:
    <code to be executed with the GIL released>

Code in the body of the with-statement must not raise exceptions ormanipulate Python objects in any way, and must not call anything thatmanipulates Python objects without first re-acquiring the GIL. Cythonvalidates these operations at compile time, but cannot look intoexternal C functions, for example. They must be correctly declaredas requiring or not requiring the GIL (see below) in order to makeCython’s checks effective.

Acquiring the GIL

A C function that is to be used as a callback from C code that is executedwithout the GIL needs to acquire the GIL before it can manipulate Pythonobjects. This can be done by specifying with gil in the functionheader:

cdef void my_callback(void *data) with gil:
    ...

If the callback may be called from another non-Python thread,care must be taken to initialize the GIL first, through a call toPyEval_InitThreads().If you’re already using cython.parallel in your module, this will already have been taken care of.

The GIL may also be acquired through the with gil statement:

with gil:
    <execute this block with the GIL acquired>

Conditional Acquiring / Releasing the GIL

Sometimes it is helpful to use a condition to decide whether to run acertain piece of code with or without the GIL. This code would run anyway,the difference is whether the GIL will be held or released.The condition must be constant (at compile time).

This could be useful for profiling, debugging, performance testing, andfor fused types (see Conditional GIL Acquiring / Releasing).:

DEF FREE_GIL = True
 
with nogil(FREE_GIL):
    <code to be executed with the GIL released>
 
    with gil(False):
       <GIL is still released>

Declaring a function as callable without the GIL

You can specify nogil in a C function header or function type todeclare that it is safe to call without the GIL.:

cdef void my_gil_free_func(int spam) nogil:
    ...

When you implement such a function in Cython, it cannot have any Pythonarguments or Python object return type. Furthermore, any operationthat involves Python objects (including calling Python functions) mustexplicitly acquire the GIL first, e.g. by using a with gil blockor by calling a function that has been defined with gil. Theserestrictions are checked by Cython and you will get a compile errorif it finds any Python interaction inside of a nogil code section.

Note

The nogil function annotation declares that it is safeto call the function without the GIL. It is perfectly allowedto execute it while holding the GIL. The function does not initself release the GIL if it is held by the caller.

Declaring a function with gil (i.e. as acquiring the GIL on entry) alsoimplicitly makes its signature nogil.