1. Scott Tsai
  2. cffi


cffi / doc / source / index.rst

CFFI documentation

Foreign Function Interface for Python calling C code. The aim of this project is to provide a convenient and reliable way of calling C code from Python. The interface is based on LuaJIT's FFI and follows a few principles:

  • The goal is to call C code from Python. You should be able to do so without learning a 3rd language: every alternative requires you to learn their own language (Cython, SWIG) or API (ctypes). So we tried to assume that you know Python and C and minimize the extra bits of API that you need to learn.
  • Keep all the Python-related logic in Python so that you don't need to write much C code (unlike CPython native C extensions).
  • Work either at the level of the ABI (Application Binary Interface) or the API (Application Programming Interface). Usually, C libraries have a specified C API but often not an ABI (e.g. they may document a "struct" as having at least these fields, but maybe more). (ctypes works at the ABI level, whereas Cython and native C extensions work at the API level.)
  • We try to be complete. For now some C99 constructs are not supported, but all C89 should be, including macros (and including macro "abuses", which you can manually wrap in saner-looking C functions).
  • We attempt to support both PyPy and CPython (although PyPy support is not complete yet) with a reasonable path for other Python implementations like IronPython and Jython.
  • Note that this project is not about embedding executable C code in Python, unlike Weave. This is about calling existing C libraries from Python.

Installation and Status

This code has been developed on Linux but should work on any POSIX platform as well as on Win32. There are some Windows-specific issues left.

It currently supports CPython 2.x. Support for CPython 3.x should not be too hard. Support for PyPy is coming soon. (In fact, the authors of CFFI are also on the PyPy team; we plan to make it the first (and fastest) choice for PyPy.)


  • CPython 2.6 or 2.7 (you need python-dev)
  • pycparser 2.06: http://code.google.com/p/pycparser/
  • libffi (you need libffi-dev); or, on Windows, you need to copy the directory Modules\_ctypes\libffi_msvc from the CPython sources (2.6 or 2.7) into the top-level directory.

Download and Installation:

  • https://bitbucket.org/cffi/cffi/downloads
  • python setup.py install or python setup_base.py install
  • or you can directly import and use cffi, but if you don't compile the _ffi_backend extension module, it will fall back to using internally ctypes (slower and does not support verify()).



Simple example (ABI level)

>>> from cffi import FFI
>>> ffi = FFI()
>>> ffi.cdef("""
...     int printf(const char *format, ...);   // copy-pasted from the man page
... """)
>>> C = ffi.dlopen(None)                     # loads the entire C namespace
>>> arg = ffi.new("char[]", "world")         # equivalent to C code: char arg[] = "world";
>>> C.printf("hi there, %s!\n", arg);        # call printf
hi there, world!

Real example (API level)

from cffi import FFI
ffi = FFI()
ffi.cdef("""     // some declarations from the man page
    struct passwd {
        char *pw_name;
    struct passwd *getpwuid(int uid);
C = ffi.verify("""   // passed to the real C compiler
#include <sys/types.h>
#include <pwd.h>
p = C.getpwuid(0)
assert str(p.pw_name) == 'root'

Note that the above example works independently of the exact layout of struct passwd, but so far require a C compiler at runtime. (We plan to improve with caching and a way to distribute the compiled code.)

Struct/Array Example

from cffi import FFI
ffi = FFI()
    typedef struct {
        unsigned char r, g, b;
    } pixel_t;
image = ffi.new("pixel_t[]", 800*600)

f = open('data', 'rb')     # binary mode -- important

image[100].r = 255
image[100].g = 192
image[100].b = 128

f = open('data', 'wb')

This can be used as a more flexible replacement of the struct and array modules. You could also call ffi.new("pixel_t[600][800]") and get a two-dimensional array.

What actually happened?

The CFFI interface operates on the same level as C - you declare types and functions using the same syntax as you would define them in C. This means that most of the documentation or examples can be copied straight from the man pages.

The declarations can contain types, functions and global variables. The cdef in the above examples are just that - they declared "there is a function in the C level with this given signature", or "there is a struct type with this shape".

The dlopen() line loads libraries. C has multiple namespaces - a global one and local ones per library. In this example we load the global one (None as argument to dlopen()) which always contains the standard C library. You get as a result a <FFILibrary> object that has as attributes all symbols declared in the cdef() and coming from this library.

The verify() line in the second example is an alternative: instead of doing a dlopen, it generates and compiles a piece of C code. When using verify() you have the advantage that you can use ... at various places in the cdef(), and the missing information will be completed with the help of the C compiler. It also does checking, to verify that your declarations are correct. If the C compiler gives warnings or errors, they are reported here.

Finally, the ffi.new() lines allocate C objects. They are filled with zeroes initially, unless the optional second argument is used. If specified, this argument gives an "initializer", like you can use with C code to initialize global variables.

The actual function calls should be obvious. It's like C. Did you even notice that we accidentally left a trailing semicolon? That's fine because it is valid Python syntax as well :-)


As a guideline: you have already seen in the above examples all the major pieces except maybe ffi.cast(). The rest of this documentation gives a more complete reference.

Declaring types and functions

ffi.cdef(source) parses the given C source. This should be done first. It registers all the functions, types, and global variables in the C source. The types can be used immediately in ffi.new() and other functions. Before you can access the functions and global variables, you need to give ffi another piece of information: where they actually come from (which you do with either ffi.dlopen() or ffi.verify()).

The C source is parsed internally (using pycparser). This code cannot contain #include. It should typically be a self-contained piece of declarations extracted from a man page. The only things it can assume to exist are the standard types:

  • char, short, int, long, long long (both signed and unsigned)
  • float, double
  • intN_t, uintN_t (for N=8,16,32,64), intptr_t, uintptr_t, ptrdiff_t, size_t, ssize_t

As we will see on the verification step below, the declarations can also contain ... at various places; there are placeholders that will be completed by a call to verify().

Loading libraries

ffi.dlopen(libpath): this function opens a shared library and returns a module-like library object. You can use the library object to call the functions previously declared by ffi.cdef(), and to read or write global variables. Note that you can use a single cdef() to declare functions from multiple libraries, as long as you load each of them with dlopen() and access the functions from the correct one.

The libpath is the file name of the shared library, which can contain a full path or not (in which case it is searched in standard locations, as described in man dlopen). Alternatively, if libpath is None, it returns the standard C library (which can be used to access the functions of glibc, on Linux).

This gives ABI-level access to the library: you need to have all types declared manually exactly as they were while the library was made. No checking is done. For this reason, we recommend to use ffi.verify() instead when possible.

Note that only functions and global variables are in library objects; types exist in the ffi instance independently of library objects. This is due to the C model: the types you declare in C are not tied to a particular library, as long as you #include their headers; but you cannot call functions from a library without linking it in your program, as dlopen() does dynamically in C.

The verification step

ffi.verify(source, **kwargs): verifies that the current ffi signatures compile on this machine, and return a dynamic library object. The dynamic library can be used to call functions and access global variables declared by a previous ffi.cdef(). The library is compiled by the C compiler: it gives you C-level API compatibility (including calling macros, as long as you declared them as functions in ffi.cdef()). This differs from ffi.dlopen(), which requires ABI-level compatibility and must be called several times to open several shared libraries.

On top of CPython, the new library is actually a CPython C extension module. This solution constrains you to have a C compiler (future work will cache the compiled C code and let you distribute it to other systems which don't have a C compiler).

The arguments to ffi.verify() are:

  • source: C code that is pasted verbatim in the generated code (it is not parsed internally). It should contain at least the necessary #include. It can also contain the complete implementation of some functions declared in cdef(); this is useful if you really need to write a piece of C code, e.g. to access some advanced macros (see the example of getyx() in demo/_curses.py).
  • include_dirs, define_macros, undef_macros, libraries, library_dirs, extra_objects, extra_compile_args, extra_link_args (keyword arguments): these are used when compiling the C code, and are passed directly to distutils.

On the plus side, this solution gives more "C-like" flexibility:

  • functions taking or returning integer or float-point arguments can be misdeclared: if e.g. a function is declared by cdef() as taking a int, but actually takes a long, then the C compiler handles the difference.
  • other arguments are checked: you get a compilation warning or error if you pass a int * argument to a function expecting a long *.

Moreover, you can use ... in the following places in the cdef() for leaving details unspecified (filled in by the C compiler):

  • structure declarations: any struct that ends with "...;" is partial. It will be completed by the compiler. (But note that you can only access fields that you declared.) Any struct declaration without ...; is assumed to be exact, and this is checked: you get a VerificationError if it is not.
  • unknown types: the syntax "typedef ... foo_t;" declares the type foo_t as opaque.
  • array lengths: when used as structure fields, arrays can have an unspecified length, as in "int n[];". The length is completed by the C compiler.
  • enums: in "enum foo { A, B, C, ... };" (with a trailing ...), the enumerated values are not necessarily in order; the C compiler will reorder them as needed and skip any unmentioned value. Like with structs, an enum that does not end in ... is assumed to be exact, and this is checked.
  • integer macros: you can write in the cdef the line "#define FOO ...", with any macro name FOO. Provided the macro is defined to be an integer value, this value will be available via an attribute of the library object returned by verify(). The same effect can be achieved by writing a declaration static const int FOO;. The latter is more general because it supports other types than integer types (note: the syntax is then to write the const together with the variable name, as in static char *const FOO;).

Working with pointers, structures and arrays

The C code's integers and floating-point values are mapped to Python's regular int, long and float. Moreover, the C type char correspond to single-character strings in Python. (If you want it to map to small integers, use either signed char or unsigned char.)

Pointers, structures and arrays are more complex: they don't have an obvious Python equivalent. Thus, they correspond to objects of type cdata, which are printed for example as <cdata 'struct foo_s *'>.

ffi.new(ctype, [initializer]): this function builds a new cdata object of the given ctype. The ctype is usually some constant string describing the C type. This is similar to a malloc: it allocates the memory needed to store an object of the given C type, and returns a pointer to it. The memory is initially filled with zeros. An initializer can be given too, as described later.


>>> ffi.new("int")
<cdata 'int *' owning 4 bytes>
>>> ffi.new("int[10]")
<cdata 'int[10]' owning 40 bytes>

Unlike C, the returned pointer object has ownership on the allocated memory: when this exact object is garbage-collected, then the memory is freed. If, at the level of C, you store a pointer to the memory somewhere else, then make sure you also keep the object alive for as long as needed. (This also applies if you immediately cast the returned pointer to a pointer of a different type: only the original object has ownership, so you must keep it alive. As soon as you forget it, then the casted pointer will point to garbage.)

The cdata objects support mostly the same operations as in C: you can read or write from pointers, arrays and structures. Dereferencing a pointer is done usually in C with the syntax *p, which is not valid Python, so instead you have to use the alternative syntax p[0] (which is also valid C). Additionally, the p.x and p->x syntaxes in C both become p.x in Python. And instead of NULL you use None.

There is no equivalent to the & operator in C (because it would not fit nicely in the model, and it does not seem to be needed here).

Any operation that would in C return a pointer or array or struct type gives you a new cdata object. Unlike the "original" one, these new cdata objects don't have ownership: they are merely references to existing memory.


ffi.cdef("void somefunction(int *);")
lib = ffi.verify("#include <foo.h>")

x = ffi.new("int")        # allocate one int, and return a pointer to it
x[0] = 42                 # fill it
lib.somefunction(x)       # call the C function
print x[0]                # read the possibly-changed value

The equivalent of C casts are provided with ffi.cast("type", value). They should work in the same cases as they do in C. Additionally, this is the only way to get cdata objects of integer or floating-point type:

>>> x = ffi.cast("int", 42)
>>> x
<cdata 'int'>
>>> int(x)

Similarly, this is the only way to get cdata objects for NULL pointers, which are normally returned as None.

The initializer given as the optional second argument to ffi.new() can be mostly anything that you would use as an initializer for C code, with lists or tuples instead of using the C syntax { .., .., .. }. Example:

typedef struct { int x, y; } foo_t;

foo_t v = { 1, 2 };            // C syntax
v = ffi.new("foo_t", [1, 2])   # CFFI equivalent

foo_t v = { .y=1, .x=2 };               // C syntax
v = ffi.new("foo_t", {'y': 1, 'x': 2})  # CFFI equivalent

Like C, arrays of chars can also be initialized from a string, in which case a terminating null character is appended implicitly:

>>> x = ffi.new("char[]", "hello")
>>> x
<cdata 'char[]' owning 6 bytes>
>>> len(x)        # the actual size of the array
>>> x[5]          # the last item in the array
>>> x[0] = 'H'    # change the first item
>>> str(x)        # interpret 'x' as a regular null-terminated string

Note that unlike Python lists or tuples, but like C, you cannot index in a C array from the end using negative numbers.

More generally, the C array types can have their length unspecified in C types, as long as their length can be derived from the initializer, like in C:

int array[] = { 1, 2, 3, 4 };           // C syntax
array = ffi.new("int[]", [1, 2, 3, 4])  # CFFI equivalent

As an extension, the initializer can also be just a number, giving the length (in case you just want zero-initialization):

int array[1000];                  // C syntax
array = ffi.new("int[1000]")      # CFFI 1st equivalent
array = ffi.new("int[]", 1000)    # CFFI 2nd equivalent

This is useful if the length is not actually a constant, to avoid doing things like ffi.new("int[%d]"%x). Indeed, this is not recommended: ffi normally caches the string "int[]" to not need to re-parse it all the time.

Variadic function calls

Variadic functions in C (which end with "..." as their last argument) can be declared and called normally, with the exception that all the arguments passed in the variable part must be cdata objects, or possibly None for NULL. This is because it would not be possible to guess, if you wrote this:

C.printf("hello, %d\n", 42)

that you really meant the 42 to be passed as a C int, and not a long or long long. The same issue occurs with float versus double. So you have to force cdata objects of the C type you want, if necessary with ffi.cast():

C.printf("hello, %d\n", ffi.cast("int", 42))
C.printf("hello, %ld\n", ffi.cast("long", 42))
C.printf("hello, %f\n", ffi.cast("double", 42))


C functions can also be viewed as cdata objects, and so can be passed as callbacks. To make new C callback objects that will invoke a Python function, you need to use:

>>> def myfunc(x, y):
...    return x + y
>>> ffi.callback("int(*)(int, int)", myfunc)
<cdata 'int(*)(int, int)' calling <function myfunc at 0xf757bbc4>>

Warning: like ffi.new(), ffi.callback() returns a cdata that has ownership of its C data. (In this case, the necessary C data contains the libffi data structures to do a callback.) This means that the callback can only be invoked as long as this cdata object is alive. If you store the function pointer into C code, then make sure you also keep this object alive for as long as the callback may be invoked. (If you want the callback to remain valid forever, store the object in a fresh global variable somewhere.)

Note that callbacks of a variadic function type are not supported.

Windows: you can't yet specify the calling convention of callbacks. (For regular calls, the correct calling convention should be automatically inferred by the C backend.)

Be careful when writing the Python callback function: if it returns an object of the wrong type, or more generally raises an exception, then the exception cannot be propagated. Instead, it is printed to stderr and the C-level callback is made to return a null value (this is a random choice).


ffi.errno: the value of errno received from the most recent C call in this thread, and passed to the following C call, is available via reads and writes of the property ffi.errno.

ffi.buffer(pointer, [size]): return a read-write buffer object that references the raw C data pointed to by the given 'cdata', of 'size' bytes. The 'cdata' must be a pointer or an array. To get a copy of it in a regular string, call str() on the result. If unspecified, the default size of the buffer is sizeof(*pointer) or the whole size of the array. Getting a buffer is useful because you can read from it without an extra copy, or write into it to change the original value; you can use for example file.write() and file.readinto() with such a buffer (for files opened in binary mode).

ffi.typeof("C type" or cdata object): return an object of type <ctype> corresponding to the parsed string, or to the C type of the cdata instance. Usually you don't need to call this function or to explicitly manipulate <ctype> objects in your code: any place that accepts a C type can receive either a string or a pre-parsed ctype object (and because of caching of the string, there is no real performance difference). It can still be useful in writing typechecks, e.g.:

def myfunction(ptr):
    assert ffi.typeof(ptr) is ffi.typeof("foo_t*")

ffi.sizeof("C type" or cdata object): return the size of the argument in bytes. The argument can be either a C type, or a cdata object, like in the equivalent sizeof operator in C.

ffi.alignof("C type"): return the alignment of the C type. Corresponds to the __alignof__ operator in GCC.

ffi.offsetof("C struct type", "fieldname"): return the offset within the struct of the given field. Corresponds to offsetof() in C.

ffi.getcname("C type" or <ctype>, ["extra"]): return the string representation of the given C type. If non-empty, the "extra" string is appended (or inserted at the right place in more complicated cases); it can be the name of a variable to declare, or an extra part of the type like "*" or "[5]", so that for example ffi.getcname(ffi.typeof(x), "*") returns the string representation of the C type "pointer to the same type than x".

Comments and bugs

Please report to the issue tracker any bugs. Feel free to discuss matters in the mailing list. As a general rule, when there is a design issue to resolve, we pick the solution that is the "most C-like". We hope that this module has got everything you need to access C code and nothing more.

--- the authors, Armin Rigo and Maciej Fijalkowski

Indices and tables