Lua

This page describes the detailed semantics underlying the FFI library and its interaction with both Lua and C code.

Given that the FFI library is designed to interface with C code and that declarations can be written in plain C syntax, it closely follows the C language semantics wherever possible. Some concessions are needed for smoother interoperation with Lua language semantics. But it should be straightforward to write applications using the LuaJIT FFI for developers with a C or C++ background.

C Language Support

The FFI library has a built-in C parser with a minimal memory footprint. It's used by the ffi.* library functions to declare C types or external symbols.

It's only purpose is to parse C declarations, as found e.g. in C header files. Although it does evaluate constant expressions, it's not a C compiler. The body of inline C function definitions is simply ignored.

Also, this is not a validating C parser. It expects and accepts correctly formed C declarations, but it may choose to ignore bad declarations or show rather generic error messages. If in doubt, please check the input against your favorite C compiler.

The C parser complies to the C99 language standard plus the following extensions:

The following C types are pre-defined by the C parser (like a typedef, except re-declarations will be ignored):

You're encouraged to use these types in preference to the compiler-specific extensions or the target-dependent standard types. E.g. char differs in signedness and long differs in size, depending on the target architecture and platform ABI.

The following C features are not supported:

C Type Conversion Rules

TODO

Conversions from C types to Lua objects

Conversions from Lua objects to C types

Conversions between C types

Initializers

Creating a cdata object with ffi.new() or the equivalent constructor syntax always initializes its contents, too. Different rules apply, depending on the number of optional initializers and the C types involved:

Operations on cdata Objects

TODO

Garbage Collection of cdata Objects

All explicitly (ffi.new(), ffi.cast() etc.) or implicitly (accessors) created cdata objects are garbage collected. You need to ensure to retain valid references to cdata objects somewhere on a Lua stack, an upvalue or in a Lua table while they are still in use. Once the last reference to a cdata object is gone, the garbage collector will automatically free the memory used by it (at the end of the next GC cycle).

Please note that pointers themselves are cdata objects, however they are not followed by the garbage collector. So e.g. if you assign a cdata array to a pointer, you must keep the cdata object holding the array alive as long as the pointer is still in use: 

ffi.cdef[[
typedef struct { int *a; } foo_t;
]]

local s = ffi.new("foo_t", ffi.new("int[10]")) -- WRONG!

local a = ffi.new("int[10]") -- OK
local s = ffi.new("foo_t", a)
-- Now do something with 's', but keep 'a' alive until you're done.

Similar rules apply for Lua strings which are implicitly converted to "const char *": the string object itself must be referenced somewhere or it'll be garbage collected eventually. The pointer will then point to stale data, which may have already beeen overwritten. Note that string literals are automatically kept alive as long as the function containing it (actually its prototype) is not garbage collected.

Objects which are passed as an argument to an external C function are kept alive until the call returns. So it's generally safe to create temporary cdata objects in argument lists. This is a common idiom for passing specific C types to vararg functions: 

ffi.cdef[[
int printf(const char *fmt, ...);
]]
ffi.C.printf("integer value: %d\n", ffi.new("int", x)) -- OK

Memory areas returned by C functions (e.g. from malloc()) must be manually managed, of course. Pointers to cdata objects are indistinguishable from pointers returned by C functions (which is one of the reasons why the GC cannot follow them).

C Library Namespaces

A C library namespace is a special kind of object which allows access to the symbols contained in shared libraries or the default symbol namespace. The default ffi.C namespace is automatically created when the FFI library is loaded. C library namespaces for specific shared libraries may be created with the ffi.load() API function.

Indexing a C library namespace object with a symbol name (a Lua string) automatically binds it to the library. First the symbol type is resolved — it must have been declared with ffi.cdef. Then the symbol address is resolved by searching for the symbol name in the associated shared libraries or the default symbol namespace. Finally, the resulting binding between the symbol name, the symbol type and its address is cached. Missing symbol declarations or nonexistent symbol names cause an error.

This is what happens on a read access for the different kinds of symbols:

This is what happens on a write access:

C library namespaces themselves are garbage collected objects. If the last reference to the namespace object is gone, the garbage collector will eventually release the shared library reference and remove all memory associated with the namespace. Since this may trigger the removal of the shared library from the memory of the running process, it's generally not safe to use function cdata objects obtained from a library if the namespace object may be unreferenced.

Performance notice: the JIT compiler specializes to the identity of namespace objects and to the strings used to index it. This effectively turns function cdata objects into constants. It's not useful and actually counter-productive to explicitly cache these function objects, e.g. local strlen = ffi.C.strlen. OTOH it is useful to cache the namespace itself, e.g. local C = ffi.C.

No Hand-holding!

The FFI library has been designed as a low-level library. The goal is to interface with C code and C data types with a minimum of overhead. This means you can do anything you can do from C: access all memory, overwrite anything in memory, call machine code at any memory address and so on.

The FFI library provides no memory safety, unlike regular Lua code. It will happily allow you to dereference a NULL pointer, to access arrays out of bounds or to misdeclare C functions. If you make a mistake, your application might crash, just like equivalent C code would.

This behavior is inevitable, since the goal is to provide full interoperability with C code. Adding extra safety measures, like bounds checks, would be futile. There's no way to detect misdeclarations of C functions, since shared libraries only provide symbol names, but no type information. Likewise there's no way to infer the valid range of indexes for a returned pointer.

Again: the FFI library is a low-level library. This implies it needs to be used with care, but it's flexibility and performance often outweigh this concern. If you're a C or C++ developer, it'll be easy to apply your existing knowledge. OTOH writing code for the FFI library is not for the faint of heart and probably shouldn't be the first exercise for someone with little experience in Lua, C or C++.

As a corollary of the above, the FFI library is not safe for use by untrusted Lua code. If you're sandboxing untrusted Lua code, you definitely don't want to give this code access to the FFI library or to any cdata object (except 64 bit integers or complex numbers). Any properly engineered Lua sandbox needs to provide safety wrappers for many of the standard Lua library functions — similar wrappers need to be written for high-level operations on FFI data types, too.

Current Status

The initial release of the FFI library has some limitations and is missing some features. Most of these will be fixed in future releases.

C language support is currently incomplete:

The JIT compiler already handles a large subset of all FFI operations. It automatically falls back to the interpreter for unimplemented operations (you can check for this with the -jv command line option). The following operations are currently not compiled and may exhibit suboptimal performance, especially when used in inner loops:

Other missing features: