Jsctypes/api
1. opening a library and declaring a function
Cu.import("ctypes"); // imports the global ctypes object
// searches the path and opens "libmylib.so" on linux,
// "libmylib.dylib" on mac, and "mylib.dll" on windows
let mylib = ctypes.open("mylib", ctypes.SEARCH);
// declares the C prototype int32_t myfunc(int32_t)
// Int32 implies ctypes.Int32, shortened for brevity
let myfunc = mylib.declare("myfunc", DEFAULT_ABI, Int32(), Int32());
let ret = myfunc(2); // calls myfunc
Note that for simple types (integers and strings), we will autoconvert the argument at call time - there's no need to pass in an Int32 object. The consumer should never need to instantiate such an object explicitly, unless they're using it to back a pointer - in which case we require explicit, strong typing. See later for examples.
2. declaring and passing a simple type (by object)
let myfunc = mylib.declare("myfunc", DEFAULT, Int32, Int32);
let i = new Int32(); // instantiates an Int32 object with default value 0
let ret = myfunc(i);
An Int32 object, like all other type objects in ctypes, can be used for type specification when passed as an object, as above. declare() can look at the prototype JSObject* of its argument, and use this as a canonical JSObject representing the type, a pointer to which can be used for simple type equality comparisons. (This will work for user-defined types such as structs also - see later - though for pointer types we need to dig down to the underlying type.)
Int32() can have two modes depending on whether JS_IsConstructing(cx) is JS_TRUE ("new Int32()") or JS_FALSE ("Int32()"). Used as a function, we could perform a type conversion with range checking, for instance:
let n = Int32(4); // JSVAL_IS_INT(n) == JS_TRUE n = Int32(4e16); // RangeError - out of bounds n = Int32.max; // 2^31 - 1 // etc
For the new constructor, the resulting object stores three pieces of information internally in reserved slots. |new Int32()| creates a JSObject which allocates sizeof(int32_t) and stores that pointer in a private slot. It also stores its type, as a JSObject* pointing to the canonical Int32 prototype, and can store a parent JSObject* in case it refers to an Int32 that happens to be part of another object. Thus the slot layout of i above would be
i object:
slot 1 (parent): JSObject* -> NULL (no parent object)
slot 2 (type) : JSObject* -> Int32 prototype
slot 3 (value) : void* -> binary blob from malloc(sizeof(int32_t))
Do we need to provide an explicit set() method, to allow for efficient modification? For instance,
i.set(5); // cheaper than i = new Int32(5);
3. declaring and passing a pointer
// C prototype: int32_t myfunc(int32_t* p)
let myfunc = mylib.declare("myfunc", DEFAULT_ABI, Int32, Pointer(Int32));
let p = new Pointer(new Int32()); // instantiates an int and a pointer
let ret = myfunc(p); // the int is an outparam
let i = p.contents(); // i = *p (by reference)
let a = p.address(); // 0x...
// same thing, but with a named integer
let i = new Int32();
let p = new Pointer(i);
let ret = myfunc(p); // modifies i
// same thing, but with a pointer temporary
let i = new Int32();
let ret = myfunc(new Pointer(i)); // modifies i
// other examples
let q = new Pointer(); // instantiate a null pointer to a void type
q = new Pointer(5); // TypeError - require a ctypes type
Internally, a pointer requires a backing object (unless it's a null pointer). In the examples, the Pointer JSObject holds a reference to the Int32 JSObject for rooting purposes, and is laid out similarly to an Int32 object:
p object:
slot 1 (parent): JSObject* -> Int32 backing object
slot 2 (type) : JSObject* -> Pointer prototype
slot 3 (value) : void* -> pointer to binary int32_t blob inside backing object
4. declaring a pointer to opaque struct
const FILE = ctypes.Struct(); // creates a Struct() type with no allocated binary storage, and no fields to access
let fopen = mylib.declare("fopen", DEFAULT_ABI, Pointer(FILE), String);
let file = fopen("foo"); // creates a new Pointer() object
file.contents(); // will throw - type is unknown
file.address(); // ok
5. declaring a struct
// C prototype: struct s_t { int32_t a; int64_t b };
const s_t = Struct([{ a: Int32 }, { b: Int64 }]);
let myfunc = mylib.declare("myfunc", DEFAULT_ABI, Int32, s_t);
let s = new s_t(10, 20);
This creates an s_t object which allocates binary space for both fields, creates getters and setters to access the binary fields via their offset, assigns the values 10 and 20 to the fields, and whose prototype is s_t:
s object:
slot 1 (parent): JSObject* -> NULL
slot 2 (type) : JSObject* -> s_t prototype
slot 3 (value) : void* -> pointer to binary blob from malloc()
slot 4 (fields): array of data for each field:
{ JSObject* parent; JSObject* type; ptrdiff_t offset; }
The array of field information allows each field to be dependent on another JSObject (only for the case where the field is a pointer), have an associated type, and have an offset into the binary blob for ease of access.
let c = s.b; // invokes the getter for |b| to create an Int64 object like so:
c object:
slot 1 (parent): JSObject* -> s backing object
slot 2 (type) : JSObject* -> Int64 prototype
slot 3 (value) : void* -> pointer to binary int64_t blob inside backing object
let i = myfunc(s); // checks the type of s by JSObject* prototype equality
6. pointers to struct fields
let p = new Pointer(s.b);
Once the Int64 representing s.b is constructed, the Pointer object references it directly:
p object:
slot 1 (parent): JSObject* -> Int64 backing object (which, in turn, is backed by s)
slot 2 (type) : JSObject* -> Pointer prototype
slot 3 (value) : void* -> pointer to binary int64_t blob inside backing object
7. nested structs
const u_t = Struct([{ x: Int64 }, { y: s_t }]);
let u = new u_t(5e4, s); // copies data from s into u.y - no references
let u_field = u.y; // creates an s_t object that points directly to the offset of u.y within u.
const v_t = Struct([{ x: Pointer(s_t) }, { y: Pointer(s_t) }]);
let v = new v_t(new Pointer(s), new Pointer(s));
In this case, the fields array will each have their respective Pointer as the parent object, and both will point to the s binary blob.
js-ctypes is a library for calling C/C++ functions from JavaScript without having to write or generate any C/C++ "glue code".
js-ctypes is already in mozilla-central, but the API is subject to change. This page contains design proposals for the eventual js-ctypes API.
Types
A type maps JS values to C/C++ values and vice versa. They're used when declaring functions. They can also be used to create and populate C/C++ data structures entirely from JS.
The types provided by ctypes
ctypes.int8_t, uint8_t, int16_t, uint16_t, int32_t, uint32_t, int64_t, uint64_t, float32_t, float64_t - Primitive types that behave the same way on all platforms (with the usual caveat that every platform has slightly different floating-point behavior, in corner cases, and there's nothing we can realistically do about it).
ctypes.bool, short, unsigned_short, int, unsigned, unsigned_int, long, unsigned_long, float, double - Types that behave like the corresponding C types. Some or all of these might be aliases for the primitive types listed above. As in C, unsigned is always an alias for unsigned_int.
(TBD: Do we want char types? how should they behave?)
ctypes.string, ustring - String types. The C/C++ type for ctypes.string is const char *. C/C++ values of this type must be either null or pointers to null-terminated strings. ctypes.ustring is the same, but for const jschar *; that is, the code units of the string are uint16_t.
ctypes.void_t - The special C type void. This can be used as a return value type. (void is a keyword in JavaScript.)
ctypes.PointerType - Constructor for pointer types. new ctypes.PointerType(T), where T is a type, creates the type "pointer to T". Also new ctypes.PointerType(name), where name is a string, creates an opaque pointer type.
ctypes.ArrayType - Constructor for array types. new ctypes.ArrayType(T, n), where T is a type and n is a nonnegative integer, creates the type T[n].
ctypes.StructType - Constructor for struct types. new ctypes.StructType(name, fields) creates a new struct named "struct name" with the given fields (fields is an array of field descriptors), details TBD.
Examples:
const DWORD = ctypes.uint32_t;
const HANDLE = new ctypes.PointerType("HANDLE");
const FILE = new ctypes.PointerType("FILE *");
const IOBuf = new ctypes.ArrayType(ctypes.uint8_t, 4096);
const struct_tm = new ctypes.Struct('tm', [[ctypes.int, 'tm_sec'], ...]);
Calling types
js-ctypes types are JavaScript constructors. That is, they are functions, and they can be called in various different ways. (ctypes.Buffer and ctypes.Reference are to be described below.)
new t or new t() - Without arguments, these allocate a new ctypes.Buffer of t.size bytes, populate it with zeroes, and return a new ctypes.Reference to the complete object in that Buffer.
t() - The same, but if the resulting value can be precisely represented as a JavaScript primitive value (boolean, number, string, null, undefined), return that instead. (This is the case for all number types, string types, pointer types, ctypes.bool, and ctypes.void_t.)
new t(ref) - With a single argument that is a reference to an object of size t.size, this creates a new Buffer and Reference as above, but populates the buffer with a copy of the object referred to by ref rather than zeroing it out.
new t(val) - With an argument that is any other JavaScript value, this converts the value to type t, throwing a TypeError if the conversion is impossible, then creates a new Buffer and Reference as above, populating the new buffer with the converted value. Details of conversion depend on the type.
t(val) - Convert val to type t as above. If the result can be precisely represented as a JavaScript primitive value (boolean, number, string, null, or undefined) or a ctypes pointer, return that. Otherwise return a new Reference, exactly as for new t(val).
The special type ctypes.void_t throws a TypeError if called with new, but ctypes.void_t() and ctypes.void_t(x) are allowed. Both return undefined.
Properties of types
All the fields described here are read-only.
All types have these properties:
t.size - The C/C++ sizeof the type, in bytes.
ctypes.void_t.size is 0.
t.name - A string, the type's name. It's intended that in ordinary use, this will be a C/C++ type expression, but it's not really meant to be machine-readable in all cases.
For primitive types this is just the name of the corresponding C/C++ type, e.g. ctypes.int32_t.name == "int32_t" and ctypes.void_t == "void". But some of the builtin types are aliases for other types, so it might be that ctypes.unsigned_long.name == "uint32_t" (or something else). (Is that too astonishing?)
For struct types and opaque pointer types, this is simply the string that was passed to the constructor; e.g. FILE.name == "FILE *" and struct_tm.name == "tm". For other pointer types and array types this should try to generate valid C/C++ type expressions, which isn't exactly trivial.
(Open issue: This conflicts with the usual meaning of .name for functions, and types are functions.)
(Should you be able to assign to type.name for types you create, the effect being kind of like a typedef?)
Pointer types also have:
t.targetType - The pointed-to type, or null if t is an opaque pointer type.
Struct types also have:
t.fields - A sealed array of field descriptors, details TBD.
Array types also have:
t.elementType - The type of the elements of an array of this type. E.g. IOBuf.elementType === ctypes.uint8_t.
t.length - The number of elements, a nonnegative integer.