[draft] Add InWaitingInput function

src/serial.zig

@@ -932,6 +932,23 @@ pub fn configureSerialPort(port: std.Io.File, config: SerialConfig) !void {
     }
 }
 
+/// Read the number input byte available from port `port`
+/// Only work on linux
+pub fn InWaintingSerialPort(port: std.Io.File) !usize {
+    switch (builtin.os.tag) {
+        .linux => {
+            var remaining: c_int = 0;
+            const FIONREAD: u32 = 0x541B; // same as TIOCINQ
+            if (std.os.linux.ioctl(port.handle, FIONREAD, @intFromPtr(&remaining)) == -1) {
+                // if (std.c.ioctl(port.handle, @bitCast(FIONREAD), &remaining) == -1) {
+                return error.InWaitingError;
+            }
+            return @intCast(remaining);
+        },
+        else => @compileError("unsupported OS, please implement!"),
+    }
+}
+
 const Flush = enum {
     input,
     output,
@@ -1178,10 +1195,10 @@ test "basic configuration test" {
         else => unreachable,
     }
 
-    var port = std.Io.Dir.openFileAbsolute(std.testing.io, tty, .{ .mode = .read_write }) catch |err| switch(err) {
+    var port = std.Io.Dir.openFileAbsolute(std.testing.io, tty, .{ .mode = .read_write }) catch |err| switch (err) {
         error.FileNotFound => return error.SkipZigTest,
         else => |e| return e,
-    }; 
+    };
     defer port.close(std.testing.io);
 
     try configureSerialPort(port, cfg);
@@ -1196,7 +1213,7 @@ test "basic flush test" {
         .macos => tty = "/dev/cu.usbmodem101",
         else => unreachable,
     }
-    var port = std.Io.Dir.openFileAbsolute(std.testing.io, tty, .{ .mode = .read_write }) catch |err| switch(err) {
+    var port = std.Io.Dir.openFileAbsolute(std.testing.io, tty, .{ .mode = .read_write }) catch |err| switch (err) {
         error.FileNotFound => return error.SkipZigTest,
         else => |e| return e,
     };