pfalcon / c2ffi

Clang-based FFI wrapper generator

Version Notice

You need to use the correct branch of c2ffi for your version of LLVM/Clang:

• 3.3: branch llvm-3.3
• 3.4: branch llvm-3.4
• 3.5: branch master

Note that development will take place on the branch relating to the current stable branch of LLVM/Clang. As of writing, that is 3.4.

c2ffi

This is a tool for extracting definitions from C and Objective C headers for use with foreign function call interfaces. For instance:

#define FOO (1 << 2)

const int BAR = FOO + 10;

typedef struct my_point {
    int x;
    int y;
    int odd_value[BAR + 1];
} my_point_t;

enum some_values {
    a_value,
    another_value,
    yet_another_value
};

void do_something(my_point_t *p, int x, int y);

Running c2ffi on this, we can get the following:

(const BAR :int 14)

(struct my_point
    (x :int)
    (y :int)
    (odd_value (:array :int 15)))
(typedef my_point_t (:struct my_point))

(enum some_values
    (a_value 0)
    (another_value 1)
    (yet_another_value 2))

(function "do_something" ((p (:pointer my_point_t)) (x :int) (y :int)) :void)

(const FOO __int128_t 4)

Because this uses Clang as a parser, the C is fully and correctly parsed, including complex array initializers and similar. For output, JSON is the default, but this is a bit less readable:

[
{ "tag": "const", "name": "BAR", "location": "/home/rpav/test.h:3:11", "type": { "tag": ":int" }, "value": 14 },
{ "tag": "struct", "name": "my_point", "id": 0, "location": "/home/rpav/test.h:5:16", "bit-size": 544, "bit-alignment": 32, "fields": [{ "tag": "field", "name": "x", "bit-offset": 0, "bit-size": 32, "bit-alignment": 32, "type": { "tag": ":int" } }, { "tag": "field", "name": "y", "bit-offset": 32, "bit-size": 32, "bit-alignment": 32, "type": { "tag": ":int" } }, { "tag": "field", "name": "odd_value", "bit-offset": 64, "bit-size": 480, "bit-alignment": 32, "type": { "tag": ":array", "type": { "tag": ":int" }, "size": 15 } }] },
{ "tag": "typedef", "name": "my_point_t", "location": "/home/rpav/test.h:9:3", "type": { "tag": ":struct", "name": "my_point", "id": 0 } },
{ "tag": "enum", "name": "some_values", "id": 0, "location": "/home/rpav/test.h:11:6", "fields": [{ "tag": "field", "name": "a_value", "value": 0 }, { "tag": "field", "name": "another_value", "value": 1 }, { "tag": "field", "name": "yet_another_value", "value": 2 }] },
{ "tag": "function", "name": "do_something", "location": "/home/rpav/test.h:17:6", "variadic": false, "parameters": [{ "tag": "parameter", "name": "p", "type": { "tag": ":pointer", "type": { "tag": "my_point_t" } } }, { "tag": "parameter", "name": "x", "type": { "tag": ":int" } }, { "tag": "parameter", "name": "y", "type": { "tag": ":int" } }], "return-type": { "tag": ":void" } }
]

Building

This requires Clang 3.3, which you can obtain from the repository. Once that is built, you should be able to build c2ffi:

c2ffi/ $ ./autogen
c2ffi/ $ mkdir build/ && cd build
build/ $ ../configure
   : # lots of output
build/ $ make
build/ $ ./src/c2ffi -h
Usage: c2ffi [options ...] FILE

Options:
      -I, --include        Add a "LOCAL" include path
      -i, --sys-include    Add a <system> include path
      -D, --driver         Specify an output driver (default: json)

      -o, --output         Specify an output file (default: stdout)
      -M, --macro-file     Specify a file for macro definition output

      -x, --lang           Specify language (c, c++, objc, objc++)

Drivers: json, sexp

Now you have a working c2ffi.

Usage

There are generally two steps to using c2ffi:

• Generate output for a particular header or file, gathering macro definitions (with the -M <file>.c parameter)

• Generate output for macro definitions by running c2ffi again on the generated file (without -M)

This is due to the preprocessor being a huge hack (see below). However, once this is done, you should have two files with all the necessary data for your FFI bindings.

Currently JSON is the default output. This is in a rather wordy hierarchical format, with each object having a "tag" field which describes it. All objects are contained in an array. This should make it fairly easy (or at least far easier than parsing C yourself) to transform into language-specific bindings.

This format may be documented at some point, but for now, you'll have to look at the input and the output! I recommend a pretty-printing reformatter for the JSON. Patches to produce prettier output will be accepted. ;-)

Errors

You may encounter errors if the code in question is not correct. Presumably, most of the time, you will be running c2ffi on existing, known-working code.

In this case, the most likely "error" you will encounter will look like this:

Skipping invalid Decl:
FunctionDecl 0x21e05f0 </usr/include/glib-2.0/glib/gmacros.h:328:22, /usr/include/glib-2.0/glib/deprecated/gthread.h:282:65> g_cond_timed_wait 'int (GCond *, GMutex *, GTimeVal *)' extern
|-ParmVarDecl 0x21e0480 <line:280:42, col:58> cond 'GCond *'
|-ParmVarDecl 0x21e04f0 <line:281:42, col:58> mutex 'GMutex *'
`-ParmVarDecl 0x21e0560 <line:282:42, col:58> timeval 'GTimeVal *' invalid

This usually means that Clang didn't find a header, and it doesn't know about one of the types referenced. Look at the top of your error output. Missing header errors will often appear there.

You should specify any necessary additional include paths with -i(for system headers, i.e. those using <brackets>) or -I (for local headers, i.e. those using "quotes").

Generally, any issue relating to an error with C, includes, or the like is not a bug with c2ffi. However, c2ffi should not abort or crash; any such error is certainly a bug with c2ffi.

Language Support

C

C support should be fairly complete. Formerly variadic functions and bitfield support was incomplete. These should now be fully-supported.

Note however that bitfield support is platform- and sometimes compiler-specific; if your platform ABI does not provide a strict definition, expect the layout of structs which use bitfields to be undefined.

C++

Not at all. It will parse a C++ file, but understand none of the C++-specific things. This is an eventual todo, but the output would not be immediately useful, since to my knowledge nothing other than C++ talks to C++.

ObjC

Basic support at least exists. I am not an Objective C person and don't really have a great way to use or test the output, or verify that all the useful features are included.

If you send me example source along with some information about what would be useful, I can try to accommodate. If you write a translator for the JSON to an ObjC bridge, let me know and I will link it below.

ObjC++

Same as C++.

Importing

Processing the JSON into a usable format is fairly straightforward. Some care must be given to handle anonymous types (e.g., typedef struct { ... } type_t;), but writing these is fairly trivial overall.

The following language bindings exist for c2ffi:

• cl-autowrap: Create bindings in Commonn Lisp from a .h with c2ffi using a simple (c-include "file.h")

• c2ffi-ruby: Uses the JSON from c2ffi to produce a nicely-formatted Ruby file for ruby-ffi.

New Output Drivers

If you're feeling motivated, it should be fairly simple to produce a new output driver. Look in src/drivers/ and you can see the source for JSON, Sexp (lisp symbolic expressions), and possibly some others.

You will need to do the following:

• Create a new subclass of OutputDriver in src/drivers/; copying one of the existing ones is probably the easiest.

• Add this file to src/Makefile.am

• Add the factory function to src/OutputDriver.cpp.

• Write your code!

The Preprocessor

The preprocessor handling is, as was noted, a huge hack. This is due entirely to the fact that #define macros can contain just about anything, and thus it's not easy to tell if they are useful values or syntax hackery.

For this, c2ffi uses a simple heuristic:

• If there are arithmetic operators (+, -, *, <<, etc), parens, numbers, and identifiers, it's treated as "useful".

• If only ints are found, it's treated as an __int128_t; if floats are found, it's treated as a double; if a string is found, a char*

Why the odd __int128_t? Because without more parsing (and technically, without context), it can't be determined as signed or unsigned. So this is declared with very large capacity which will hold the entire range of signed and unsigned 64-bit ints.

If you're dealing with unsigned 128-bit int constants, you'll have to do it yourself. I personally haven't seen any.

License

This is currently GPL2, but it will almost certainly be moved to LGPL2, as I would like to make a shared library which you can load definitions at runtime. It may be moved to BSD or similar at some point in the future.