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====== Wrap package ======
The **wrap** package helps you to automate the generation of Lua/C wrappers
around existing C functions, such that these functions would be callable
from Lua. This package is used by the **torch** package, but does not depend on
anything, and could be used by anyone using Lua.
**DISCLAIMER** Before going any further, we assume the reader has a good
knowledge of how to interface C functions with Lua. A good start would be
the [[http://www.lua.org/manual/5.1|Lua reference manual]], or the book
[[http://www.inf.puc-rio.br/~roberto/pil2|Programming in Lua]].
As an example is often better than lengthy explanations, let's consider the
case of a function
<file c>
int numel(THDoubleTensor *t);
</file>
which returns the number of elements of ''t''.
Writing a complete wrapper of this function would look like:
<file c>
int wrapper_numel(lua_State *L)
{
THDoubleTensor *t;
/* always good to check the number of arguments */
if(lua_gettop(L) != 1)
error("invalid number of arguments: <tensor> expected");
/* check if we have a tensor on the stack */
/* we use the luaT library, which deals with Torch objects */
/* we assume the torch_DoubleTensor_id has been already initialized */
t = luaT_checkudata(L, 1, torch_DoubleTensor_id);
/* push result on stack */
lua_pushnumber(L, numel(t));
/* the number of returned variables */
return 1;
}
</file>
For anybody familiar with the Lua C API, this should look very simple (and
//it is simple//, Lua has been designed for that!). Nevertheless, the
wrapper contains about 7 lines of C code, for a quite simple
function. Writing wrappers for C functions with multiple arguments, where
some of them might be optional, can become very quickly a tedious task. The
**wrap** package is here to help the process. Remember however that even
though you might be able to treat most complex cases with **wrap**,
sometimes it is also good to do everything by hand yourself!
===== High Level Interface =====
**wrap** provides only one class: ''CInterface''. Considering our easy example, a typical usage
would be:
'
<file lua>
require 'wrap'
interface = wrap.CInterface.new()
interface:wrap(
"numel", -- the Lua name
"numel", -- the C function name, here the same
-- now we describe the 'arguments' (or possibly returned values)
{
{name="DoubleTensor"},
{name="int", creturned=true} -- this one is returned by the C function
}
)
print(interface:tostring())
</file>
The wrapper generated by **wrap** is quite similar to what one would write by hand:
<file c>
static int wrapper_numel(lua_State *L)
{
int narg = lua_gettop(L);
THDoubleTensor *arg1 = NULL;
int arg2 = 0;
if(narg == 1
&& (arg1 = luaT_toudata(L, 1, torch_DoubleTensor_id))
)
{
}
else
luaL_error(L, "expected arguments: DoubleTensor");
arg2 = numel(arg1);
lua_pushnumber(L, (lua_Number)arg2);
return 1;
}
</file>
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