Is there a library for working with peripherals (COM-port) in LuaRT?

Alef

How can I organize communication with some peripheral device via a COM port?

Samir

Hi Alef,

There are no specific module for serial COM ports in LuaRT. You can use a Lua binary module like luaserial. I don't have tried to use sys.File objects but it may work.

Alef

Do you mean that I may to write a binary module (library) in C++ and register it in LuaRT?
OK, I will try...

Samir

Not necessary, Lua binary modules are compatible with LuaRT binary modules out of the box. Just be sure that the binary module is for Lua 5.4.

Alef

Thanks)

steve64

As an exercise in using the new FFI in LuART as well as a real use case, I'm trying to use a DLL for serial port handling.
However, I would need some help in finding proper signatures and using pointers for DLL functions like the following:

int sp_get_port_by_name(const char *portname, struct sp_port **port_ptr);
int sp_open(struct sp_port *port, enum sp_mode flags);

Consider that sp_port is an opaque struct type and I have also to manage a pointer to pointer.

local c = require 'c'
local sp_dll = c.Library('libserialport-0.dll')
...
sp_dll.sp_get_port_by_name = 'c(Zp)i'  --> ???
sp_dll.sp_open = 'c(pi)i'   --> ???

sp_dll.sp_get_port_by_name('COM4', p) --> ???
sp_dll.sp_open(p, 3) --> ???
...

Thanks in advance,

Samir

What is p ? A pointer to the opaque struct ?

steve64

The example is not perfect, there should be slightly different variables p1 and p2:
in the first call p1 should be a pointer to a pointer to the opaque struct.
in the second call p2 should be a pointer to the opaque struct as obtained by deferencing p1

Samir

steve64 ok so too get p2 from p1 :

p2 = p1:as("void*")

steve64

Ok, so to summarize:

local c = require 'c'
local sp_dll = c.Library('libserialport-0.dll')

sp_dll.sp_get_port_by_name = 'c(Zp)i'
sp_dll.sp_open = 'c(pi)i'

local p1, p2

-- do I need to create p1 here as some kind of object?

sp_dll.sp_get_port_by_name('COM4', p1)
p2 = p1:as('void*')
sp_dll.sp_open(p2, 3)

```.

Samir

Yes it should work that way:

sp_dll.sp_open(p2.content, 3)

steve64

There is an issue with this:

local c = require 'c'
local sp_dll = c.Library('libserialport-0.dll')
sp_dll.sp_get_port_by_name = 'c(pp)i' -- >function found in DLL
local port_ptr  -->NEED SOMETHING DIFFERENT ?
sp_ret = sp_dll.sp_get_port_by_name('COM3', port_ptr)
print(sp_ret) --> -1 (FAILURE)

The DLL is loaded, the sp_get_port_by_name function is found and the COM4 port does exist.
But the call fails.
Is the declaration sp_dll.sp_get_port_by_name = 'c(pp)i' correct if the second parameter is a pointer to pointer ?
Is there anything to be done to prepare/declare "port_ptr" before the call?

Samir

Port_ptr is nil so it's considered as a NULL pointer. Initialize it with a dummy value like c.Pointer(c.size_t(0))

steve64

It helped, but now I still I'm facing issues with the next steps:

local port_ptr = c.Pointer(c.size_t(0))
sp_ret = sp_dll.sp_get_port_by_name('COM3', port_ptr)
print('sp_get_port_by_name  ret=', sp_ret, 'port_ptr=', port_ptr) 
--> THIS IS OK: prints sp_get_port_by_name  ret=	0	port_ptr=	Pointer<size_t*>: 000001FA46B151A8

local port = c.Pointer(port_ptr.content.content)
sp_ret = sp_dll.sp_open(port, 3) --> call API function
print('sp_open:', sp_ret)
--> THIS IS NOT OK: prints sp_open:	-1

I also tried local port = c.Pointer(port_ptr.content) obtaining a failure with sp_open returning -2

Samir

Try c.Pointer(port_ptr:as("void*").content)

steve64

Just tried, I got this error message: "cannot get content of a void* Pointer"

Samir

Maybe just let the port_ptr value as it is and give it directly to the sp_open() function ?
If it does not work I have to update the C module to support this case of Pointer content

steve64

directly passing port_ptr or port_ptr.content to sp_open() do not work ...

Samir

Ok so I will need to update the Pointer object, sorry for that