What’s New


  • Fix Windows issue with managing the thread-state on CPython 3.0 to 3.5


  • Fix tests, remove deprecated C API usage
  • Fix (hack) for 3.6.0/3.6.1/3.6.2 giving incompatible binary extensions (cpython issue #29943)
  • Fix for 3.7.0a1+


  • Support the modern standard types char16_t and char32_t. These work like wchar_t: they represent one unicode character, or when used as charN_t * or charN_t[] they represent a unicode string. The difference with wchar_t is that they have a known, fixed size. They should work at all places that used to work with wchar_t (please report an issue if I missed something). Note that with set_source(), you need to make sure that these types are actually defined by the C source you provide (if used in cdef()).
  • Support the C99 types float _Complex and double _Complex. Note that libffi doesn’t support them, which means that in the ABI mode you still cannot call C functions that take complex numbers directly as arguments or return type.
  • Fixed a rare race condition when creating multiple FFI instances from multiple threads. (Note that you aren’t meant to create many FFI instances: in inline mode, you should write ffi = cffi.FFI() at module level just after import cffi; and in out-of-line mode you don’t instantiate FFI explicitly at all.)
  • Windows: using callbacks can be messy because the CFFI internal error messages show up to stderr—but stderr goes nowhere in many applications. This makes it particularly hard to get started with the embedding mode. (Once you get started, you can at least use @ffi.def_extern(onerror=...) and send the error logs where it makes sense for your application, or record them in log files, and so on.) So what is new in CFFI is that now, on Windows CFFI will try to open a non-modal MessageBox (in addition to sending raw messages to stderr). The MessageBox is only visible if the process stays alive: typically, console applications that crash close immediately, but that is also the situation where stderr should be visible anyway.
  • Progress on support for callbacks in NetBSD.
  • Functions returning booleans would in some case still return 0 or 1 instead of False or True. Fixed.
  • ffi.gc() now takes an optional third parameter, which gives an estimate of the size (in bytes) of the object. So far, this is only used by PyPy, to make the next GC occur more quickly (issue #320). In the future, this might have an effect on CPython too (provided the CPython issue 31105 is addressed).
  • Add a note to the documentation: the ABI mode gives function objects that are slower to call than the API mode does. For some reason it is often thought to be faster. It is not!


(only released inside PyPy 5.8.0)

  • Fixed the line numbers reported in case of cdef() errors. Also, I just noticed, but pycparser always supported the preprocessor directive # 42 "foo.h" to mean “from the next line, we’re in file foo.h starting from line 42”, which it puts in the error messages.


  • Issue #295: use calloc() directly instead of PyObject_Malloc()+memset() to handle ffi.new() with a default allocator. Speeds up ffi.new(large-array) where most of the time you never touch most of the array.
  • Some OS/X build fixes (“only with Xcode but without CLT”).
  • Improve a couple of error messages: when getting mismatched versions of cffi and its backend; and when calling functions which cannot be called with libffi because an argument is a struct that is “too complicated” (and not a struct pointer, which always works).
  • Add support for some unusual compilers (non-msvc, non-gcc, non-icc, non-clang)
  • Implemented the remaining cases for ffi.from_buffer. Now all buffer/memoryview objects can be passed. The one remaining check is against passing unicode strings in Python 2. (They support the buffer interface, but that gives the raw bytes behind the UTF16/UCS4 storage, which is most of the times not what you expect. In Python 3 this has been fixed and the unicode strings don’t support the memoryview interface any more.)
  • The C type _Bool or bool now converts to a Python boolean when reading, instead of the content of the byte as an integer. The potential incompatibility here is what occurs if the byte contains a value different from 0 and 1. Previously, it would just return it; with this change, CFFI raises an exception in this case. But this case means “undefined behavior” in C; if you really have to interface with a library relying on this, don’t use bool in the CFFI side. Also, it is still valid to use a byte string as initializer for a bool[], but now it must only contain \x00 or \x01. As an aside, ffi.string() no longer works on bool[] (but it never made much sense, as this function stops at the first zero).
  • ffi.buffer is now the name of cffi’s buffer type, and ffi.buffer() works like before but is the constructor of that type.
  • ffi.addressof(lib, "name") now works also in in-line mode, not only in out-of-line mode. This is useful for taking the address of global variables.
  • Issue #255: cdata objects of a primitive type (integers, floats, char) are now compared and ordered by value. For example, <cdata 'int' 42> compares equal to 42 and <cdata 'char' b'A'> compares equal to b'A'. Unlike C, <cdata 'int' -1> does not compare equal to ffi.cast("unsigned int", -1): it compares smaller, because -1 < 4294967295.
  • PyPy: ffi.new() and ffi.new_allocator()() did not record “memory pressure”, causing the GC to run too infrequently if you call ffi.new() very often and/or with large arrays. Fixed in PyPy 5.7.
  • Support in ffi.cdef() for numeric expressions with + or -. Assumes that there is no overflow; it should be fixed first before we add more general support for arbitrary arithmetic on constants.


  • Structs with variable-sized arrays as their last field: now we track the length of the array after ffi.new() is called, just like we always tracked the length of ffi.new("int[]", 42). This lets us detect out-of-range accesses to array items. This also lets us display a better repr(), and have the total size returned by ffi.sizeof() and ffi.buffer(). Previously both functions would return a result based on the size of the declared structure type, with an assumed empty array. (Thanks andrew for starting this refactoring.)
  • Add support in cdef()/set_source() for unspecified-length arrays in typedefs: typedef int foo_t[...];. It was already supported for global variables or structure fields.
  • I turned in v1.8 a warning from cffi/model.py into an error: 'enum xxx' has no values explicitly defined: refusing to guess which integer type it is meant to be (unsigned/signed, int/long). Now I’m turning it back to a warning again; it seems that guessing that the enum has size int is a 99%-safe bet. (But not 100%, so it stays as a warning.)
  • Fix leaks in the code handling FILE * arguments. In CPython 3 there is a remaining issue that is hard to fix: if you pass a Python file object to a FILE * argument, then os.dup() is used and the new file descriptor is only closed when the GC reclaims the Python file object—and not at the earlier time when you call close(), which only closes the original file descriptor. If this is an issue, you should avoid this automatic convertion of Python file objects: instead, explicitly manipulate file descriptors and call fdopen() from C (...via cffi).


  • When passing a void * argument to a function with a different pointer type, or vice-versa, the cast occurs automatically, like in C. The same occurs for initialization with ffi.new() and a few other places. However, I thought that char * had the same property—but I was mistaken. In C you get the usual warning if you try to give a char * to a char ** argument, for example. Sorry about the confusion. This has been fixed in CFFI by giving for now a warning, too. It will turn into an error in a future version.


  • Issue #283: fixed ffi.new() on structures/unions with nested anonymous structures/unions, when there is at least one union in the mix. When initialized with a list or a dict, it should now behave more closely like the { } syntax does in GCC.


  • CPython 3.x: experimental: the generated C extension modules now use the “limited API”, which means that, as a compiled .so/.dll, it should work directly on any version of CPython >= 3.2. The name produced by distutils is still version-specific. To get the version-independent name, you can rename it manually to NAME.abi3.so, or use the very recent setuptools 26.
  • Added ffi.compile(debug=...), similar to python setup.py build --debug but defaulting to True if we are running a debugging version of Python itself.


  • Removed the restriction that ffi.from_buffer() cannot be used on byte strings. Now you can get a char * out of a byte string, which is valid as long as the string object is kept alive. (But don’t use it to modify the string object! If you need this, use bytearray or other official techniques.)
  • PyPy 5.4 can now pass a byte string directly to a char * argument (in older versions, a copy would be made). This used to be a CPython-only optimization.


  • ffi.gc(p, None) removes the destructor on an object previously created by another call to ffi.gc()
  • bool(ffi.cast("primitive type", x)) now returns False if the value is zero (including -0.0), and True otherwise. Previously this would only return False for cdata objects of a pointer type when the pointer is NULL.
  • bytearrays: ffi.from_buffer(bytearray-object) is now supported. (The reason it was not supported was that it was hard to do in PyPy, but it works since PyPy 5.3.) To call a C function with a char * argument from a buffer object—now including bytearrays—you write lib.foo(ffi.from_buffer(x)). Additionally, this is now supported: p[0:length] = bytearray-object. The problem with this was that a iterating over bytearrays gives numbers instead of characters. (Now it is implemented with just a memcpy, of course, not actually iterating over the characters.)
  • C++: compiling the generated C code with C++ was supposed to work, but failed if you make use the bool type (because that is rendered as the C _Bool type, which doesn’t exist in C++).
  • help(lib) and help(lib.myfunc) now give useful information, as well as dir(p) where p is a struct or pointer-to-struct.


  • ffi.list_types()
  • ffi.unpack()
  • extern “Python+C”
  • in API mode, lib.foo.__doc__ contains the C signature now. On CPython you can say help(lib.foo), but for some reason help(lib) (or help(lib.foo) on PyPy) is still useless; I haven’t yet figured out the hacks needed to convince pydoc to show more. (You can use dir(lib) but it is not most helpful.)
  • Yet another attempt at robustness of ffi.def_extern() against CPython’s interpreter shutdown logic.


  • Fix 1.5.1 for Python 2.6.


  • A few installation-time tweaks (thanks Stefano!)
  • Issue #245: Win32: __stdcall was never generated for extern "Python" functions
  • Issue #246: trying to be more robust against CPython’s fragile interpreter shutdown logic



Nothing changed from v1.4.1.


  • Fix the compilation failure of cffi on CPython 3.5.0. (3.5.1 works; some detail changed that makes some underscore-starting macros disappear from view of extension modules, and I worked around it, thinking it changed in all 3.5 versions—but no: it was only in 3.5.1.)


  • A better way to do callbacks has been added (faster and more portable, and usually cleaner). It is a mechanism for the out-of-line API mode that replaces the dynamic creation of callback objects (i.e. C functions that invoke Python) with the static declaration in cdef() of which callbacks are needed. This is more C-like, in that you have to structure your code around the idea that you get a fixed number of function pointers, instead of creating them on-the-fly.
  • ffi.compile() now takes an optional verbose argument. When True, distutils prints the calls to the compiler.
  • ffi.compile() used to fail if given sources with a path that includes "..". Fixed.
  • ffi.init_once() added. See docs.
  • dir(lib) now works on libs returned by ffi.dlopen() too.
  • Cleaned up and modernized the content of the demo subdirectory in the sources (thanks matti!).
  • ffi.new_handle() is now guaranteed to return unique void * values, even if called twice on the same object. Previously, in that case, CPython would return two cdata objects with the same void * value. This change is useful to add and remove handles from a global dict (or set) without worrying about duplicates. It already used to work like that on PyPy. This change can break code that used to work on CPython by relying on the object to be kept alive by other means than keeping the result of ffi.new_handle() alive. (The corresponding warning in the docs of ffi.new_handle() has been here since v0.8!)


  • The optional typedefs (bool, FILE and all Windows types) were not always available from out-of-line FFI objects.
  • Opaque enums are phased out from the cdefs: they now give a warning, instead of (possibly wrongly) being assumed equal to unsigned int. Please report if you get a reasonable use case for them.
  • Some parsing details, notably volatile is passed along like const and restrict. Also, older versions of pycparser mis-parse some pointer-to-pointer types like char * const *: the “const” ends up at the wrong place. Added a workaround.


  • Added ffi.memmove().
  • Pull request #64: out-of-line API mode: we can now declare floating-point types with typedef float... foo_t;. This only works if foo_t is a float or a double, not long double.
  • Issue #217: fix possible unaligned pointer manipulation, which crashes on some architectures (64-bit, non-x86).
  • Issues #64 and #126: when using set_source() or verify(), the const and restrict keywords are copied from the cdef to the generated C code; this fixes warnings by the C compiler. It also fixes corner cases like typedef const int T; T a; which would previously not consider a as a constant. (The cdata objects themselves are never const.)
  • Win32: support for __stdcall. For callbacks and function pointers; regular C functions still don’t need to have their calling convention declared.
  • Windows: CPython 2.7 distutils doesn’t work with Microsoft’s official Visual Studio for Python, and I’m told this is not a bug. For ffi.compile(), we removed a workaround that was inside cffi but which had unwanted side-effects. Try saying import setuptools first, which patches distutils...


Nothing changed from v1.2.0.


  • Out-of-line mode: int a[][...]; can be used to declare a structure field or global variable which is, simultaneously, of total length unknown to the C compiler (the a[] part) and each element is itself an array of N integers, where the value of N is known to the C compiler (the int and [...] parts around it). Similarly, int a[5][...]; is supported (but probably less useful: remember that in C it means int (a[5])[...];).
  • PyPy: the lib.some_function objects were missing the attributes __name__, __module__ and __doc__ that are expected e.g. by some decorators-management functions from functools.
  • Out-of-line API mode: you can now do from _example.lib import x to import the name x from _example.lib, even though the lib object is not a standard module object. (Also works in from _example.lib import *, but this is even more of a hack and will fail if lib happens to declare a name called __all__. Note that * excludes the global variables; only the functions and constants make sense to import like this.)
  • lib.__dict__ works again and gives you a copy of the dict—assuming that lib has got no symbol called precisely __dict__. (In general, it is safer to use dir(lib).)
  • Out-of-line API mode: global variables are now fetched on demand at every access. It fixes issue #212 (Windows DLL variables), and also allows variables that are defined as dynamic macros (like errno) or __thread -local variables. (This change might also tighten the C compiler’s check on the variables’ type.)
  • Issue #209: dereferencing NULL pointers now raises RuntimeError instead of segfaulting. Meant as a debugging aid. The check is only for NULL: if you dereference random or dead pointers you might still get segfaults.
  • Issue #152: callbacks: added an argument ffi.callback(..., onerror=...). If the main callback function raises an exception and onerror is provided, then onerror(exception, exc_value, traceback) is called. This is similar to writing a try: except: in the main callback function, but in some cases (e.g. a signal) an exception can occur at the very start of the callback function—before it had time to enter the try: except: block.
  • Issue #115: added ffi.new_allocator(), which officializes support for alternative allocators.


  • ffi.gc(): fixed a race condition in multithreaded programs introduced in 1.1.1


  • Out-of-line mode: ffi.string(), ffi.buffer() and ffi.getwinerror() didn’t accept their arguments as keyword arguments, unlike their in-line mode equivalent. (It worked in PyPy.)
  • Out-of-line ABI mode: documented a restriction of ffi.dlopen() when compared to the in-line mode.
  • ffi.gc(): when called several times with equal pointers, it was accidentally registering only the last destructor, or even none at all depending on details. (It was correctly registering all of them only in PyPy, and only with the out-of-line FFIs.)


  • Out-of-line API mode: we can now declare integer types with typedef int... foo_t;. The exact size and signedness of foo_t is figured out by the compiler.
  • Out-of-line API mode: we can now declare multidimensional arrays (as fields or as globals) with int n[...][...]. Before, only the outermost dimension would support the ... syntax.
  • Out-of-line ABI mode: we now support any constant declaration, instead of only integers whose value is given in the cdef. Such “new” constants, i.e. either non-integers or without a value given in the cdef, must correspond to actual symbols in the lib. At runtime they are looked up the first time we access them. This is useful if the library defines extern const sometype somename;.
  • ffi.addressof(lib, "func_name") now returns a regular cdata object of type “pointer to function”. You can use it on any function from a library in API mode (in ABI mode, all functions are already regular cdata objects). To support this, you need to recompile your cffi modules.
  • Issue #198: in API mode, if you declare constants of a struct type, what you saw from lib.CONSTANT was corrupted.
  • Issue #196: ffi.set_source("package._ffi", None) would incorrectly generate the Python source to package._ffi.py instead of package/_ffi.py. Also fixed: in some cases, if the C file was in build/foo.c, the .o file would be put in build/build/foo.o.


  • Same as 1.0.2, apart from doc and test fixes on some platforms.


  • Variadic C functions (ending in a ”...” argument) were not supported in the out-of-line ABI mode. This was a bug—there was even a (non-working) example doing exactly that!


  • ffi.set_source() crashed if passed a sources=[..] argument. Fixed by chrippa on pull request #60.
  • Issue #193: if we use a struct between the first cdef() where it is declared and another cdef() where its fields are defined, then this definition was ignored.
  • Enums were buggy if you used too many ”...” in their definition.


  • The main news item is out-of-line module generation:
  • (this page will list what is new from all versions from 1.0.0 forward.)