How do I pass Torch Tensor data to C code?

[Marc]

I am binding C (and C++) code to Lua 5.2. I have been able to pass primitives back and forth no problem with the Lua 5.2 C API. However, I've struggled to figure out how to pass a Torch Tensor (like torch.DoubleTensor({1,2,3,4})) to my C program in order to operate on the data.

For example, my wrapped class now allows something like

mylib =require('mylib')
obj = mylib.new()

I would like to be able to have functions like setdata(data) and getdata() as follows.

data = torch.DoubleTensor({1,2,3,4})
obj:setdata(data)
--stores [1,2,3,4] in the back-end C++ object
obj:getdata()
--prints [1,2,3,4]

I know how to write the functions and make them work, but it's the passage of Tensors that I can't seem to figure out. I suspect luaT_checkudata(...) from Torch7's luaT API might be the right tool, but I can't figure it out. Can someone provide a simple example as to how to access the underlying array of a Torch Tensor in C?

Thanks!

[Jonghoon Jin]

luaT API does the job but perhaps using FFI is much simpler to use

https://github.com/Atcold/Torch-Developer-Guide/tree/master/FFI-so

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[Marc]

I'll keep that in mind, but I have already done a fair bit of work using the Lua C APIs and I'd like to finish in that realm. Do you have a solution to the OP?

On Sunday, August 21, 2016 at 4:09:37 PM UTC-4, Jonghoon Jin wrote:

luaT API does the job but perhaps using FFI is much simpler to use

https://github.com/Atcold/Torch-Developer-Guide/tree/master/FFI-so

On Sun, Aug 21, 2016 at 12:50 PM, Marc via torch7 <torch7+APn2wQdnymDJozVf8_V0EtywE2hwOHxh0LaHwrQ2ReSJTZnRirA-...@googlegroups.com> wrote:

I am binding C (and C++) code to Lua 5.2. I have been able to pass primitives back and forth no problem with the Lua 5.2 C API. However, I've struggled to figure out how to pass a Torch Tensor (like torch.DoubleTensor({1,2,3,4})) to my C program in order to operate on the data.

For example, my wrapped class now allows something like

mylib =require('mylib')
obj = mylib.new()

I would like to be able to have functions like setdata(data) and getdata() as follows.

data = torch.DoubleTensor({1,2,3,4})
obj:setdata(data)
--stores [1,2,3,4] in the back-end C++ object
obj:getdata()
--prints [1,2,3,4]

I know how to write the functions and make them work, but it's the passage of Tensors that I can't seem to figure out. I suspect luaT_checkudata(...) from Torch7's luaT API might be the right tool, but I can't figure it out. Can someone provide a simple example as to how to access the underlying array of a Torch Tensor in C?

Thanks!

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[alban desmaison]

Hi,

You can use luaT functions to get the elements in C with:

https://github.com/torch/torch7/tree/master/lib/luaT#void-luat_toudatalua_state-l-int-ud-const-char-tname

Here is a small example how to get the arguments from the stack to C objects https://github.com/albanD/c-in-lua/blob/refacto/cModule/src.c#L14-L26

You can then use https://github.com/torch/torch7/tree/master/lib/luaT#void-luat_pushudatalua_state-l-void-udata-const-char-tname to return a tensor.

[Marc]

This did the trick. Thanks for the pointers!

I am having a new, related issue however. Perhaps you have a suggestion for that as well? (link: https://groups.google.com/forum/#!topic/torch7/rmIcBYCiFG8)

On Monday, August 22, 2016 at 4:54:28 AM UTC-4, alban desmaison wrote:

Hi,

You can use luaT functions to get the elements in C with:

https://github.com/torch/torch7/tree/master/lib/luaT#void-luat_toudatalua_state-l-int-ud-const-char-tname

Here is a small example how to get the arguments from the stack to C objects https://github.com/albanD/c-in-lua/blob/refacto/cModule/src.c#L14-L26

You can then use https://github.com/torch/torch7/tree/master/lib/luaT#void-luat_pushudatalua_state-l-void-udata-const-char-tname to return a tensor.

On Sunday, August 21, 2016 at 11:30:08 PM UTC+1, Marc wrote:

I'll keep that in mind, but I have already done a fair bit of work using the Lua C APIs and I'd like to finish in that realm. Do you have a solution to the OP?

On Sunday, August 21, 2016 at 4:09:37 PM UTC-4, Jonghoon Jin wrote:

luaT API does the job but perhaps using FFI is much simpler to use

https://github.com/Atcold/Torch-Developer-Guide/tree/master/FFI-so