C Module and buffers

steve64

I have a DLL with a function defined as follows:

int my_func(const char* input, char* output, int outputSize);

Is it correct the following definition in LuaRT?

local mylib= c.Library('mylib.dll')
mylib.my_func= "c(ZZi)i"

How can I pass arguments, in particular the second one that is a char buffer to be filled by the DLL?

Thanks in advance

Samir

The function definition seems right (no need to prefix it with "c" in most cases, the calling convention is autodetected most of the time).

For the output buffer you can use a sys.Buffer() instance as parameter of the function when calling it

steve64

Passing sys.Buffer() as second argument does not seem to work...
It is accepted but after the DLL call is still filled with zeroes and the return status of the call is a random integer

Samir

It's been a long time since I used this module...
Try to give a c.Pointer(buff) when calling the function ?

steve64

already tried, no way...

Samir

See this example :

--
-- LuaRT C FFI example calling Windows API function 'GetComputerNameW()' to retrieve the computer name
--

local c = require "c"

-- Load the kernel32.dll library
local kernel32 = c.Library("kernel32.dll")

-- Define the GetComputerNameW function
-- Signature: BOOL GetComputerNameW(LPWSTR lpBuffer, LPDWORD nSize)
kernel32.GetComputerNameA = "(Zp)B"

-- Create a buffer for the computer name (MAX_COMPUTERNAME_LENGTH is typically 31)
local size = c.size_t(64) -- Size of buffer
local buffer = sys.Buffer(64) -- Buffer for C string

-- Call GetComputerNameW
local success = kernel32.GetComputerNameA(buffer, size)

-- Check if the call was successful
if success then
    print("Computer Name: " .. buffer)
else
    print("Error retrieving computer name")
end

steve64

thanks for your quick feedback

however, I have now solved the mistery;
the issue was a wrong documentation from the DLL developer;
looking at his source code I discovered that the function signature takes a single input string
and returns a string; now it works perfectly in LuaRT just using Lua strings...

Samir

Great !

Samir

If you use a Lua string as a buffer you will alter the Lua string and its hash value which is not recommaended as Lua strings are immutable (content cannot change)