Is there a way to convert cdata to userdata? How can I use the C api to push/retrieve cdata to/from the stack?
Edit: The original title mentioned luajit but not Torch. In that context the other answers beside the one I finally had submitted myself (e.g wolfgang's) made perfect sense.
There is no extension to the Lua C API to access cdata objects as created by LuaJIT's FFI library.
So, the simple and recommended way is to do your marshalling from the Lua side if you're dealing with cdata. So, call a C function from Lua and pass that cdata as a parameter.
The cdata I was trying to access was the data of a tensor object from torch7, I finally found a way by using the torch7 C API, I will post it here just in case someone finds it useful:
#include <TH/TH.HW>
#include <TH/THStorage.h>
#include <TH/THTensor.h>
And then to get a tensor called "an_image" in the torch code:
lua_getglobal(L,"an_image");//assuming it goes on top of the stack
THDoubleTensor*data=(THDoubleTensor*)luaT_toudata(L,-1,"torch.DoubleTensor");
And finally given a buffer dest of doubles,
memcpy(dest,data->storage->data,n*sizeof(double));
Maybe you'll find useful information here: http://luajit.org/ext_ffi_semantics.html and more particularly here: http://luajit.org/ext_ffi_semantics.html#convert_fromlua
