non-blocking popen for Lua(using ffi)

non_blocking_popen.lua

-- Implements a basic binding for popen that allows non-blocking reads
-- returned "file" table only supports :read(with an optional size argument, no mode etc.) and :close
local function non_blocking_popen(cmd, read_buffer_size)
	local ffi = require("ffi")

	-- C functions that we need
	ffi.cdef([[
		void* popen(const char* cmd, const char* mode);
		int pclose(void* stream);
		int fileno(void* stream);
		int fcntl(int fd, int cmd, int arg);
		int *__errno_location ();
		ssize_t read(int fd, void* buf, size_t count);
	]])
	
	-- you can compile a simple C programm to find these values(Or look in the headers)
	local F_SETFL = 4
	local O_NONBLOCK = 2048
	local EAGAIN = 11
	
	-- this "array" holds the errno variable
	local _errno = ffi.C.__errno_location()

	-- the buffer for reading from the process
	local read_buffer_size = tonumber(read_buffer_size) or 2048
	local read_buffer = ffi.new('uint8_t[?]',read_buffer_size)

	-- get a FILE* for our command
	local file = assert(ffi.C.popen(cmd, "r"))
	
	-- turn the FILE* to a fd(int) for fcntl
	local fd = ffi.C.fileno(file)
	
	-- set non-blocking mode for read
	assert(ffi.C.fcntl(fd, F_SETFL, O_NONBLOCK)==0, "fcntl failed")

	-- close the process, prevent reading, allow garbage colletion
	function file_close(self)
		ffi.C.pclose(file)
		self.read_buffer = nil
		read_buffer = nil
		self.read = function() return nil, "closed"end
	end

	-- read up to size bytes from the process. Returns data(string) and number of bytes read if successfull,
	-- nil, "EAGAIN" if there is no data aviable, and
	-- nil, "closed" if the process has ended
	local read = ffi.C.read
	function file_read(self, size)
		local _size = math.min(read_buffer_size, size)
		while true do
			local nbytes = read(fd,read_buffer,_size)
			if nbytes > 0 then
				local data = ffi.string(read_buffer, nbytes)
				return data, nbytes
			elseif (nbytes == -1) and (_errno[0] == EAGAIN) then
				return nil, "EAGAIN"
			else
				file_close(self)
				return nil, "closed"
			end
		end
	end

	return {
		_fd = fd,
		_file = file,
		_read_buffer = read_buffer,
		_read_buffer_size = read_buffer_size,
		read = file_read,
		close = file_close
	}
end