Alien - Pure Lua extensions =========================== Status ------ This is Alien version 0.5.0. Changelog --------- * 0.5.0 * new functions alien.memcpy and alien.memset * new type "p" for alien.struct.pack and unpack, to pack pointers * new alien.struct.offset function to get the offset of a given record * buf:tostring now has optional "offset" argument * buf:topointer now has optional "offset" argument * added unsigned numbers: uint, ulong, ushort, and "ref uint" * basic support for declarative strucutures * unified treatment of funcitons and callbacks in the source * fixed segfault when collecting 0-arg functions * 0.4.1 * fixes bug where Alien was always using cdecl abi for Windows (except in callbacks) * fixes build on PPC OSX. * 0.4.0 * Windows support - stdcall ABI, including stdcall callbacks * alternative syntax for defining types * mutable buffers, wrapping lightuserdata in a buffer * alien.to*type* functions take optional length argument * callbacks are callable from Lua * alien.funcptr turns a function pointer into an alien function * improved library finding on Linux/FreeBSD, using ldconfig * alien.table utility function (wrapper for lua_createtable, useful for extensions) * alien.align utility function to get data structure alignment * arrays built on mutable buffers, with bounds checking * fixed a build bug on Linux ARM * 0.3.2 - fixes callback bug on NX-bit platforms * 0.3.1 - initial release with libffi * 0.3 - retracted due to license conflict What is Alien ------------- *Alien* is a Foreign Function Interface (FFI) for Lua. An FFI lets you call functions in dynamic libraries (.so, .dylib, .dll, etc.) from Lua code without having to write, compile and link a C binding from the library to Lua. In other words, it lets you write extensions that call out to native code using just Lua. Be careful when you use Alien, I tried to make it as safe as possible, but it is still very easy to crash Lua if you make a mistake. Alien itself is not as robust as a standard Lua extension, but you can use it to write extensions that won't crash if you code them well. Alien works on Unix-based systems and Windows. It has been tested on Linux x86, Linux x64, Linux ARM, FreeBSD x86, Windows x86, OSX x86, and OSX PPC. The Windows binary uses MSVCR80.DLL for compatibility with LuaBinaries. Installing Alien ---------------- The best way to install Alien is through [LuaRocks](http://luarocks.org). Just do `luarocks install alien`. You may need root permissions to do this, depending on your LuaRocks configuration. Go to the Alien rock directory to see local copies of this documentation, as well as the test suite. If you are in the path of the test suite (`tests`) you can run the suite with: lua -l luarocks.require test_alien.lua If everything is ok you should see no output. Alien installs to modules, `alien` and `alien.struct`. The latter is a slightly modified version of Roberto Ierusalimschy's [struct library](http://www.inf.puc-rio.br/~roberto/struct) that can unpack binary blobs (userdata) instead of just strings. Basic Usage ----------- Loading a dynamic library is very simple; Alien by default assumes a naming scheme of lib*name*.dylib for OSX and lib*name*.so for other Unix systems. If *name* is not one of the functions the `alien` module exports then you can get a reference to the library with `alien.`*`name`*. Otherwise (for example, to load a library called *libwrap.so*) you have to use `alien.load("wrap")`. You can also specify the full name of the library by calling `alien.load` with a path or with the appropriate extension, such as `alien.load("mylibs/libfoo.so")` or `alien.load("libfoo.so")`. Either way you get back a reference to the library that you will use to access its functions. You can also get a reference to the currently running module using `alien.default`, this lets you get references to any function exported by the module and its transitive dependencies on ELF and Mach-O systems. Once you have a reference to a library you can get a reference to an exported function with *libref.funcname*. For example: > def=alien.default > =def.puts alien function puts, library defaults > To use a function you first have to tell Alien the function prototype, using *func:types(ret_type, arg_types...)*, where the types are one of the following strings: "void", "int", "uint", "double", "char", "string", "pointer", "ref int", "ref uint", "ref double", "ref char", "callback", "short", "ushort", "byte", "long", "ulong", and "float". Most correspond directly to C types; *byte* is a signed char, *string* is *const char\**, *pointer* is *void\**, *callback* is a generic function pointer, and *ref char*, *ref int* and *ref double* are by reference versions of the C types. Continuing the previous example: > def.puts:types("int", "string") > def.puts("foo") foo > As you can see, after defining the prototype you can call the function just as a Lua function. Alien converts Lua numbers to the C numeric types, converts *nil* to *NULL* and Lua strings to *const char\** for *string*, and converts *nil* to *NULL* and userdata to *void\** for *pointer*. The conversions work in reverse for the return value (with the *pointer* type converted to a light userdata). By reference types are special; Alien allocates space on the stack for the argument, copies the Lua number you passed to it (converting appropriately), then calling the function with the address of this space. Then the value is converted back to a Lua number and returned after the function normal return value. An example, using *scanf*: > scanf = alien.default.scanf > scanf:types("int", "string", "ref int", "ref double") > _, x, y = scanf("%i %lf", 0, 0) 23 42.5 > =x 23 > =y 42.5 You have to pass a value even if the function does not use it, as you can see above. Another way to specify types is by passing a table to *func:types*. The array part of this table shoudl have one item for each parameter, and you can also pass two hash keys, *ret*, the function's return type (defaults to `int` as usual), and *abi*, the function's calling convention (useful for Windows, where you can specify "stdcall" as the ABI for `__stdcall` functions. The default ABI is always "default", and all systems also support "cdecl", the usual C calling convention. On systems that don't have the stdcall convention "stdcall" is the same as "default". This is the previous example using this alternate definition: > scanf = alien.default.scanf > scanf:types{ ret = "int", "string", "ref int", "ref double" } > _, x, y = scanf("%i %lf", 0, 0) 23 42.5 > =x 23 > =y 42.5 If you get raw function pointer (returned from a function, for example, or passed to a callback), you can turn it into an Alien function with `alien.funcptr(fptr)`. This returns an Alien function object that you can type and call function normally. Buffers ------- The basic usage is enough to do a lot of interfacing with C code, specially with well-behaved libraries that handle their own memory allocation (the Lua C API is a good example of such an API). But there are libraries that do not export such a well-behaved API, and require you to allocate memory that is mutated by the library. This prevents you from passing Lua strings to them, as Lua strings have to be immutable, so Alien provides a *buffer* abstraction. The function `alien.buffer` allocates a new buffer. If you call it with no arguments it will allocate a buffer with the standard buffer size for your platform. If call it with a number it will allocate a buffer with this number of bytes. If you pass it a string it will allocate a buffer that is a copy of the string. If you pass a light userdata it will use this userdata as the buffer (be careful with that). After making a buffer you can pass it in place of any argument of *string* or *pointer* type. To get back the contents of the buffer you use `buf:tostring`, and again you can either pass the number of characters to read from the buffer or don't pass anything, which treat the buffer as a C string (read until finding a *\0*). You can also call `buf:len`, which calls *strlen* on the buffer. Finally, `tostring(buf)` is the same as `buf:tostring()`. An example of how to use a buffer: > gets = alien.default.gets > gets:types("pointer", "string") > buf = alien.buffer() > gets(buf) Foo bar > =tostring(buf) Foo bar > You can access the i-th character of a buffer with `buf[i]`, and you can set its value with `buf[i] = v`. Notice that these are C characters (bytes), not Lua 1-character strings, so you need to use `string.char` and `string.byte` to convert between Lua characters and C characters. **Access to Alien buffers from Lua is 1-based instead of 0-based**. You can also get and set other values by using *buf:get(offset, type)*, and set it by *buf:set(offset, val, type)*. The offset is in bytes, *not* in elements, so if *buf* has three "int" values their offsets are 1, 5 and 9, respectively, assuming each "int" is four bytes long. All get and set operations do no bounds-checking, so be extra careful, or use the safer alien.array abstraction that is built on top of buffers. Arrays ------ Arrays are buffers with an extra layer of safety and sugar on top. You create an array with `alien.array(type, length)`, where *type* is the Alien type of the array's elements, and length is how many elements the array has. After creating an array *arr* you can get the type of its elements with *arr.type*, how many elements it has with *arr.length*, and the size (in bytes) of each element with *arr.size*. The underlying buffer is *arr.buffer*. You can access the i-th element with *arr[i]*, and set it with *arr[i] = val*. Type conversions are the same as with buffers, or function calls. Storing a string or userdata in an array pins it so it won't be collected while it is in the array. For convenience `alien.array` also accepts two other forms: `alien.array(type, tab)` creates an array with the same length as *tab* and initializes it with its values; `alien.array(type, length, buf)` creates an array with *buf* as the underlying buffer. You can also iterate over the array's contents with `arr:ipairs()`. The following example shows an use of arrays: local function sort(a, b) return a - b end local compare = alien.callback(sort, "int", "ref int", "ref int") local qsort = alien.default.qsort qsort:types("void", "pointer", "int", "int", "callback") local nums = alien.array(t, { 4, 5, 3, 2, 6, 1 }) qsort(nums.buffer, nums.length, nums.size, compare) for i, v in nums:ipairs() do print(v) end This prints numbers one to six on the console. Structs ------- Alien also has basic support for declarative structs that is also implemented as a layer of sugar on the basic buffers. The `alien.defstruct(description)` function creates a struct type with the given description, which is a list of pairs with the name and type of each field, where the type is any basic alien type (no structs inside structs yet). For example: rect = alien.defstruct{ { "left", "long" }, { "top", "long" }, { "right", "long" }, { "bottom", "long" } } This creates a new struct type with four fields of type "long", and stores it in `rect`. To create an instance of this structure (backed by a buffer) call `rect:new()`. You can then set the fields of the struct just like you do on a Lua table, like `r.left = 3`. To get the underlying buffer (to pass it to a C function, for example) you have to call the instance, `r()`. Continuing the example: r = rect:new() r.left = 2 doubleleft = alien.rectdll.double_left doubleleft:types("void", "pointer") doubleleft(r())) assert(r.left == 4) You can also pass a buffer or other userdata to the `new` method of your struct type, and in this case this will be the backing store of the struct instance you are creating. This is useful for unpacking a foreign struct that a C function returned. Pointer Unpacking ----------------- Alien also provides three convenience functions that let you dereference a pointer and convert the value to a Lua type: * `alien.tostring` takes a userdata (usually returned from a function that has a *pointer* return value), casts it to *char\**, and returns a Lua string. You can supply an optional size argument (if you don't Alien calls *strlen* on the buffer first). * `alien.toint` takes a userdata, casts it to *int\**, dereferences it and returns it as a number. If you pass it a number it assumes the userdata is an array with this number of elements. * `alien.toshort`, `alien.tolong`, `alien.tofloat`, and `alien.todouble` are like `alien.toint`, but works with with the respective typecasts. Unsigned versions are also available. The numeric alien.to*type* functions take an optional second argument that tells how many items to unpack from the userdata. For example, if ptr is a pointer to an array of four floats, the following code unpacks this array: > fs = alien.tofloat(ptr, 4) > =#fs 4 > Use these functions with extra care, they don't make any safety checks. For more advanced unmarshaling use the `alien.struct.unpack` function. Tags ---- A common pattern when wrapping objects from C libraries is to put a pointer to this object inside a full userdata, then associate this userdata with a metatable that is associated with a string tag. This tag is used to check if the userdata is a valid userdata in each function that uses it. As the userdata is a full userdata it also can have a `__gc` metamethod for resource reclamation. Alien has three functions that let you replicate this pattern on your extensions: * `alien.tag(*tagname*)` creates a new metatable with tag *tagname* if one does not exist, or returns the metatable with this tag. The namespace for tags is global, so a good pattern is to prefix the tag name with the name of your module (such as *mymod_mytag*). * `alien.wrap(*tagname*, ...)` creates a full userdata, tags it with the metatable associated with *tagname*, stores the values you passed, then returns the full userdata. Valid values are *nil*, integers and other userdata. * `alien.unwrap(*tagname*, obj)` tests if *obj* is tagged with *tagname*, throwing an error if it is not, then returns the values previously stored in it. * `alien.rewrap(*tagname*, obj, ...)` replaces the elements on *obj* with new values. If you pass more values than *obj* had previously the extra values are silently ignored. If you pass less tehn *obj* is filled with *nil*. For example, suppose *libfoo* has a `create_foo` function that returns a `Foo*` object. These objects have to be properly disposed by calling `destroy_foo` when they are not used anymore. This is easy to implement: local tag_foo = alien.tag("libfoo_foo") alien.foo.create_foo:types("pointer") alien.foo.destroy_foo_types("void", "pointer") function new_foo() local foo = alien.foo.create_foo() return alien.wrap("libfoo_foo", foo) end tag_foo = { __gc = function (obj) local foo = alien.unwrap("libfoo_foo", obj) alien.foo.destroy_foo(foo) end } Then on any function that operates on `Foo*` types you first unwrap to get the pointer, then pass it to the function in *libfoo*. Error Codes ----------- Several operating system functions return errors on a special variable called *errno*. To get the value of *errno* with Alien call `alien.errno()`. Callbacks --------- Some libraries have functions that take *callbacks*, functions that get called by the library. Most GUI libraries use callbacks, but even the C library has *qsort*. Alien lets you create a callback from a Lua function with `alien.callback`. You pass the function and the callback prototype that the library expects. Alien will return a callback object that you can pass in any argument of *callback* type. A simple example, using *qsort*: local function cmp(a, b) return a - b end local cmp_cb = alien.callback(sort, "int", "ref char", "ref char") local qsort = alien.default.qsort qsort:types("void", "pointer", "int", "int", "callback") local chars = alien.buffer("spam, spam, and spam") qsort(chars, chars:len(), alien.sizeof("char"), cmp_cb) assert(chars:tostring() == " ,,aaaadmmmnpppsss") The *qsort* function sorts an array in-place, so we have to use a buffer. Callbacks are callable from Lua just like any other Alien function, and you can freely change their types with their "types" method. Magic Numbers ------------- C libraries are full of symbolic constants that are in truth magic numbers, as they are replaced by the preprocessor before even the C compiler has a chance to see them. This means that all these constants are on header files. This also includes things such as the layout and size of strucutres the library depends on. All this information can change from version to version of the library, or from platform to platform. Alien provides a utility script called *constants* that makes it easier to work with these numbers. This utility takes three arguments on the command line: a *definitions file*, the name of the C file you want it to generate, and the name of a Lua file that the C file will generate when compiled and run. The definitions file can contain preprocessor directives, blank lines, and lines with definitions either of the form *identifier* or *lua_identifier* = *c_identifier*. The first form is equivalent to *identifier* = *identifier*. It is best to explain by example (from a libevent binding): #include #include EV_SIZE = sizeof(struct event) EV_READ EV_WRITE EV_TIMEOUT EVLOOP_NONBLOCK EVLOOP_ONCE Lines with preprocessor directives are copied verbatim to the C file *constants generates*. The above definitions file generates this C file: /* Generated by Alien constants */ #include #include #include #define LUA_FILE "event_constants.lua" int main() { FILE *f = fopen(LUA_FILE, "w+"); fprintf(f, "-- Generated by Alien constants\n\n"); fprintf(f, "%s = %i\n", "EV_SIZE ", sizeof(struct event)); fprintf(f, "%s = %i\n", "EV_READ", EV_READ); fprintf(f, "%s = %i\n", "EV_WRITE", EV_WRITE); fprintf(f, "%s = %i\n", "EV_TIMEOUT", EV_TIMEOUT); fprintf(f, "%s = %i\n", "EVLOOP_NONBLOCK", EVLOOP_NONBLOCK); fprintf(f, "%s = %i\n", "EVLOOP_ONCE", EVLOOP_ONCE); fclose(f); } Which, when compile and run, generates this file on a Linux/Intel system: -- Generated by Alien constants EV_SIZE = 84 EV_READ = 2 EV_WRITE = 4 EV_TIMEOUT = 1 EVLOOP_NONBLOCK = 2 EVLOOP_ONCE = 1 These steps (generating the C file, compiling, generating the Lua file) are best done in the build step of your extension. Miscellanea ----------- You can query what platform your extension is running on with `alien.platform`. Right now this can be either "unix" or "osx". Other platforms will be added as they are tested. You can use this information for conditional execution in your extensions. You can get the sizes of the types Alien supports using `alien.sizeof(*typename*)`, as the *qsort* example in the Callbacks section shows. You can also get strucutre aligment information with `alien.align(*typename*)`. Several extensions may need to create Lua tables with preallocated array and/or hash parts, for performance reasons (implementing a circular queue, for example). Alien exposes the `lua_createtable` function as `alien.table(narray, nhash)`. Credits ------- Alien is designed and implemented by Fabio Mascarenhas. It uses the great [libffi](http://sources.redhat.com/libffi) library by Anthony Green (and others) to do the heavy lifting of calling to and from C. The name is stolen from Common Lisp FFIs. License ------- Alien's uses the MIT license, reproduced below: Copyright (c) 2008-2009 Fabio Mascarenhas Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.