Using CFFI for embedding

You can use CFFI to generate a .so/.dll/.dylib which exports the API of your choice to any C application that wants to link with this .so/.dll/.dylib.

The general idea is as follows:

  • You write and execute a Python script, which produces a .so/.dll/.dylib file with the API of your choice. The script also gives some Python code to be “frozen” inside the .so.
  • At runtime, the C application loads this .so/.dll/.dylib without having to know that it was produced by Python and CFFI.
  • The first time a C function is called, Python is initialized and the frozen Python code is executed.
  • The frozen Python code attaches Python functions that implement the C functions of your API, which are then used for all subsequent C function calls.

One of the goals of this approach is to be entirely independent from the CPython C API: no Py_Initialize() nor PyRun_SimpleString() nor even PyObject. It works identically on CPython and PyPy.

Note

PyPy release 4.0.1 contains CFFI 1.4 only.

This is entirely new in version 1.5.

Usage

See the paragraph in the overview page for a quick introduction. In this section, we explain every step in more details. We will use here this slightly expanded example:

/* file plugin.h */
typedef struct { int x, y; } point_t;
extern int do_stuff(point_t *);

# file plugin_build.py
import cffi
ffi = cffi.FFI()

with open('plugin.h') as f:
    ffi.embedding_api(f.read())

ffi.set_source("my_plugin", '''
    #include "plugin.h"
''')

ffi.embedding_init_code("""
    from my_plugin import ffi

    @ffi.def_extern()
    def do_stuff(p):
        print("adding %d and %d" % (p.x, p.y))
        return p.x + p.y
""")

ffi.compile(target="plugin-1.5.*", verbose=True)

Running the code above produces a DLL, i,e, a dynamically-loadable library. It is a file with the extension .dll on Windows, .dylib on Mac OS/X, or .so on other platforms. As usual, it is produced by generating some intermediate .c code and then calling the regular platform-specific C compiler.

Here are some details about the methods used above:

  • ffi.embedding_api(source): parses the given C source, which declares functions that you want to be exported by the DLL. It can also declare types, constants and global variables that are part of the C-level API of your DLL.

    The functions that are found in source will be automatically defined in the .c file: they will contain code that initializes the Python interpreter the first time any of them is called, followed by code to call the attached Python function (with @ffi.def_extern(), see next point).

    The global variables, on the other hand, are not automatically produced. You have to write their definition explicitly in ffi.set_source(), as regular C code (see the point after next).

  • ffi.embedding_init_code(python_code): this gives initialization-time Python source code. This code is copied (“frozen”) inside the DLL. At runtime, the code is executed when the DLL is first initialized, just after Python itself is initialized. This newly initialized Python interpreter has got an extra “built-in” module that can be loaded magically without accessing any files, with a line like “from my_plugin import ffi, lib”. The name my_plugin comes from the first argument to ffi.set_source(). This module represents “the caller’s C world” from the point of view of Python.

    The initialization-time Python code can import other modules or packages as usual. You may have typical Python issues like needing to set up sys.path somehow manually first.

    For every function declared within ffi.embedding_api(), the initialization-time Python code or one of the modules it imports should use the decorator @ffi.def_extern() to attach a corresponding Python function to it.

    If the initialization-time Python code fails with an exception, then you get a traceback printed to stderr, along with more information to help you identify problems like wrong sys.path. If some function remains unattached at the time where the C code tries to call it, an error message is also printed to stderr and the function returns zero/null.

    Note that the CFFI module never calls exit(), but CPython itself contains code that calls exit(), for example if importing site fails. This may be worked around in the future.

  • ffi.set_source(c_module_name, c_code): set the name of the module from Python’s point of view. It also gives more C code which will be included in the generated C code. In trivial examples it can be an empty string. It is where you would #include some other files, define global variables, and so on. The macro CFFI_DLLEXPORT is available to this C code: it expands to the platform-specific way of saying “the following declaration should be exported from the DLL”. For example, you would put “extern int my_glob;” in ffi.embedding_api() and “CFFI_DLLEXPORT int my_glob = 42;” in ffi.set_source().

    Currently, any type declared in ffi.embedding_api() must also be present in the c_code. This is automatic if this code contains a line like #include "plugin.h" in the example above.

  • ffi.compile([target=...] [, verbose=True]): make the C code and compile it. By default, it produces a file called c_module_name.dll, c_module_name.dylib or c_module_name.so, but the default can be changed with the optional target keyword argument. You can use target="foo.*" with a literal * to ask for a file called foo.dll on Windows, foo.dylib on OS/X and foo.so elsewhere. One reason for specifying an alternate target is to include characters not usually allowed in Python module names, like “plugin-1.5.*”.

    For more complicated cases, you can call instead ffi.emit_c_code("foo.c") and compile the resulting foo.c file using other means. CFFI’s compilation logic is based on the standard library distutils package, which is really developed and tested for the purpose of making CPython extension modules, not other DLLs.

More reading

If you’re reading this page about embedding and you are not familiar with CFFI already, here are a few pointers to what you could read next:

  • For the @ffi.def_extern() functions, integer C types are passed simply as Python integers; and simple pointers-to-struct and basic arrays are all straightforward enough. However, sooner or later you will need to read about this topic in more details here.

  • @ffi.def_extern(): see documentation here, notably on what happens if the Python function raises an exception.

  • To create Python objects attached to C data, one common solution is to use ffi.new_handle(). See documentation here.

  • In embedding mode, the major direction is C code that calls Python functions. This is the opposite of the regular extending mode of CFFI, in which the major direction is Python code calling C. That’s why the page Using the ffi/lib objects talks first about the latter, and why the direction “C code that calls Python” is generally referred to as “callbacks” in that page. If you also need to have your Python code call C code, read more about Embedding and Extending below.

  • ffi.embedding_api(source): follows the same syntax as ffi.cdef(), documented here. You can use the “...” syntax as well, although in practice it may be less useful than it is for cdef(). On the other hand, it is expected that often the C sources that you need to give to ffi.embedding_api() would be exactly the same as the content of some .h file that you want to give to users of your DLL. That’s why the example above does this:

    with open('foo.h') as f:
    ffi.embedding_api(f.read())

    Note that a drawback of this approach is that ffi.embedding_api() doesn’t support #ifdef directives. You may have to use a more convoluted expression like:

    with open('foo.h') as f:
    lines = [line for line in f if not line.startswith('#')]
    ffi.embedding_api(''.join(lines))

    As in the example above, you can also use the same foo.h from ffi.set_source():

    ffi.set_source('module_name', '#include "foo.h"')

Troubleshooting

The error message

cffi extension module ‘c_module_name’ has unknown version 0x2701

means that the running Python interpreter located a CFFI version older than 1.5. CFFI 1.5 or newer must be installed in the running Python.

Using multiple CFFI-made DLLs

Multiple CFFI-made DLLs can be used by the same process.

Note that all CFFI-made DLLs in a process share a single Python interpreter. The effect is the same as the one you get by trying to build a large Python application by assembling a lot of unrelated packages. Some of these might be libraries that monkey-patch some functions from the standard library, for example, which might be unexpected from other parts.

Multithreading

Multithreading should work transparently, based on Python’s standard Global Interpreter Lock.

If two threads both try to call a C function when Python is not yet initialized, then locking occurs. One thread proceeds with initialization and blocks the other thread. The other thread will be allowed to continue only when the execution of the initialization-time Python code is done.

If the two threads call two different CFFI-made DLLs, the Python initialization itself will still be serialized, but the two pieces of initialization-time Python code will not. The idea is that there is a priori no reason for one DLL to wait for initialization of the other DLL to be complete.

After initialization, Python’s standard Global Interpreter Lock kicks in. The end result is that when one CPU progresses on executing Python code, no other CPU can progress on executing more Python code from another thread of the same process. At regular intervals, the lock switches to a different thread, so that no single thread should appear to block indefinitely.

Testing

For testing purposes, a CFFI-made DLL can be imported in a running Python interpreter instead of being loaded like a C shared library.

You might have some issues with the file name: for example, on Windows, Python expects the file to be called c_module_name.pyd, but the CFFI-made DLL is called target.dll instead. The base name target is the one specified in ffi.compile(), and on Windows the extension is .dll instead of .pyd. You have to rename or copy the file, or on POSIX use a symlink.

The module then works like a regular CFFI extension module. It is imported with “from c_module_name import ffi, lib” and exposes on the lib object all C functions. You can test it by calling these C functions. The initialization-time Python code frozen inside the DLL is executed the first time such a call is done.

Embedding and Extending

The embedding mode is not incompatible with the non-embedding mode of CFFI.

You can use both ffi.embedding_api() and ffi.cdef() in the same build script. You put in the former the declarations you want to be exported by the DLL; you put in the latter only the C functions and types that you want to share between C and Python, but not export from the DLL.

As an example of that, consider the case where you would like to have a DLL-exported C function written in C directly, maybe to handle some cases before calling Python functions. To do that, you must not put the function’s signature in ffi.embedding_api(). (Note that this requires more hacks if you use ffi.embedding_api(f.read()).) You must only write the custom function definition in ffi.set_source(), and prefix it with the macro CFFI_DLLEXPORT:

CFFI_DLLEXPORT int myfunc(int a, int b)
{
    /* implementation here */
}

This function can, if it wants, invoke Python functions using the general mechanism of “callbacks”—called this way because it is a call from C to Python, although in this case it is not calling anything back:

ffi.cdef("""
    extern "Python" int mycb(int);
""")

ffi.set_source("my_plugin", """

    static int mycb(int);   /* the callback: forward declaration, to make
                               it accessible from the C code that follows */

    CFFI_DLLEXPORT int myfunc(int a, int b)
    {
        int product = a * b;   /* some custom C code */
        return mycb(product);
    }
""")

and then the Python initialization code needs to contain the lines:

@ffi.def_extern()
def mycb(x):
    print "hi, I'm called with x =", x
    return x * 10

This @ffi.def_extern is attaching a Python function to the C callback mycb(), which in this case is not exported from the DLL. Nevertheless, the automatic initialization of Python occurs when mycb() is called, if it happens to be the first function called from C. More precisely, it does not happen when myfunc() is called: this is just a C function, with no extra code magically inserted around it. It only happens when myfunc() calls mycb().

As the above explanation hints, this is how ffi.embedding_api() actually implements function calls that directly invoke Python code; here, we have merely decomposed it explicitly, in order to add some custom C code in the middle.

In case you need to force, from C code, Python to be initialized before the first @ffi.def_extern() is called, you can do so by calling the C function cffi_start_python() with no argument. It returns an integer, 0 or -1, to tell if the initialization succeeded or not. Currently there is no way to prevent a failing initialization from also dumping a traceback and more information to stderr.