diff --git a/README.md b/README.md
index 2053d52..14967a4 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,7 @@ Library for configuring and listing serial ports.
   - Handshake (none, hardware, software)
   - Byte Size (5, 6, 7, 8)
 - Flush serial port send/receive buffers
+- Query number of bytes in serial port send/receive buffers
 - List available serial ports
 - API: supports Windows, Linux and Mac
 
diff --git a/src/serial.zig b/src/serial.zig
index cdd0e89..25d0cd6 100644
--- a/src/serial.zig
+++ b/src/serial.zig
@@ -989,11 +989,11 @@ pub fn changeControlPins(port: std.Io.File, pins: ControlPins) !void {
             const SETRTS = 3;
 
             if (pins.dtr) |dtr| {
-                if (EscapeCommFunction(port.handle, if (dtr) SETDTR else CLRDTR) == 0)
+                if (EscapeCommFunction(port.handle, if (dtr) SETDTR else CLRDTR) == .FALSE)
                     return error.WindowsError;
             }
             if (pins.rts) |rts| {
-                if (EscapeCommFunction(port.handle, if (rts) SETRTS else CLRRTS) == 0)
+                if (EscapeCommFunction(port.handle, if (rts) SETRTS else CLRRTS) == .FALSE)
                     return error.WindowsError;
             }
         },
@@ -1030,12 +1030,72 @@ pub fn changeControlPins(port: std.Io.File, pins: ControlPins) !void {
                 return error.Unexpected;
         },
 
-        .macos => {},
-
         else => @compileError("changeControlPins not implemented for " ++ @tagName(builtin.os.tag)),
     }
 }
 
+/// TODO: This struct should probably be added to the Zig standard library at std.os.windows
+const COMSTAT = extern struct {
+    status: packed struct(std.os.windows.DWORD) {
+        fCtsHold: u1,
+        fDsrHold: u1,
+        fRlsdHold: u1,
+        fXoffHold: u1,
+        fXoffSent: u1,
+        fEof: u1,
+        fTxim: u1,
+        fReserved: u25,
+    },
+    cbInQue: std.os.windows.DWORD,
+    cbOutQue: std.os.windows.DWORD,
+};
+
+extern "kernel32" fn ClearCommError(hFile: std.os.windows.HANDLE, lpErrors: *std.os.windows.DWORD, lpStat: *COMSTAT) std.os.windows.BOOL;
+
+/// Returns the number of bytes waiting in the receive buffer
+pub fn receiveBufferCount(port: std.Io.File) !usize {
+    switch (builtin.os.tag) {
+        .windows => {
+            var ret_error: std.os.windows.DWORD = 0;
+            var ret_comstat: COMSTAT = std.mem.zeroes(COMSTAT);
+            if (ClearCommError(port.handle, &ret_error, &ret_comstat) == .FALSE)
+                return error.Unexpected;
+            return @intCast(ret_comstat.cbInQue);
+        },
+        .linux => {
+            // from /usr/include/asm-generic/ioctls.h
+            const FIONREAD = 0x541B;
+            var bytes_avail: usize = 0;
+            if (std.os.linux.ioctl(port.handle, FIONREAD, @intFromPtr(&bytes_avail)) != 0)
+                return error.Unexpected;
+            return bytes_avail;
+        },
+        else => @compileError("receiveBufferCount not implemented for " ++ @tagName(builtin.os.tag)),
+    }
+}
+
+/// Returns the number of bytes waiting in the transmit buffer
+pub fn transmitBufferCount(port: std.Io.File) !usize {
+    switch (builtin.os.tag) {
+        .windows => {
+            var ret_error: std.os.windows.DWORD = 0;
+            var ret_comstat: COMSTAT = std.mem.zeroes(COMSTAT);
+            if (ClearCommError(port.handle, &ret_error, &ret_comstat) == .FALSE)
+                return error.Unexpected;
+            return @intCast(ret_comstat.cbOutQue);
+        },
+        .linux => {
+            // from /usr/include/asm-generic/ioctls.h
+            const TIOCOUTQ = 0x5411;
+            var bytes_avail: usize = 0;
+            if (std.os.linux.ioctl(port.handle, TIOCOUTQ, @intFromPtr(&bytes_avail)) != 0)
+                return error.Unexpected;
+            return bytes_avail;
+        },
+        else => @compileError("transmitBufferCount not implemented for " ++ @tagName(builtin.os.tag)),
+    }
+}
+
 const PURGE_RXABORT = 0x0002;
 const PURGE_RXCLEAR = 0x0008;
 const PURGE_TXABORT = 0x0001;
@@ -1160,6 +1220,21 @@ test "iterate ports" {
     }
 }
 
+fn openTestSerialDeviceFile(io: std.Io) !std.Io.File {
+
+    // if any, these will likely exist on a machine
+    const tty = switch (builtin.os.tag) {
+        .windows => "\\\\.\\COM3",
+        .linux => "/dev/ttyUSB0",
+        .macos => "/dev/cu.usbmodem101",
+        else => unreachable,
+    };
+    return std.Io.Dir.openFileAbsolute(io, tty, .{ .mode = .read_write }) catch |err| switch (err) {
+        error.FileNotFound => return error.SkipZigTest,
+        else => |e| return e,
+    };
+}
+
 test "basic configuration test" {
     const cfg = SerialConfig{
         .handshake = .none,
@@ -1169,39 +1244,14 @@ test "basic configuration test" {
         .stop_bits = .one,
     };
 
-    var tty: []const u8 = undefined;
-
-    switch (builtin.os.tag) {
-        .windows => tty = "\\\\.\\COM3",
-        .linux => tty = "/dev/ttyUSB0",
-        .macos => tty = "/dev/cu.usbmodem101",
-        else => unreachable,
-    }
-
-    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,
-    }; 
+    const port = try openTestSerialDeviceFile(std.testing.io);
     defer port.close(std.testing.io);
-
     try configureSerialPort(port, cfg);
 }
 
 test "basic flush test" {
-    var tty: []const u8 = undefined;
-    // if any, these will likely exist on a machine
-    switch (builtin.os.tag) {
-        .windows => tty = "\\\\.\\COM3",
-        .linux => tty = "/dev/ttyUSB0",
-        .macos => tty = "/dev/cu.usbmodem101",
-        else => unreachable,
-    }
-    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,
-    };
+    const port = try openTestSerialDeviceFile(std.testing.io);
     defer port.close(std.testing.io);
-
     try flushSerialPort(port, .both);
     try flushSerialPort(port, .input);
     try flushSerialPort(port, .output);
@@ -1211,6 +1261,20 @@ test "change control pins" {
     _ = changeControlPins;
 }
 
+test "receive buffer counts" {
+    const port = try openTestSerialDeviceFile(std.testing.io);
+    defer port.close(std.testing.io);
+    try flushSerialPort(port, .input);
+    try std.testing.expectEqual(0, try receiveBufferCount(port));
+}
+
+test "transmit buffer counts" {
+    const port = try openTestSerialDeviceFile(std.testing.io);
+    defer port.close(std.testing.io);
+    try flushSerialPort(port, .output);
+    try std.testing.expectEqual(0, try transmitBufferCount(port));
+}
+
 test "bufPrint tests" {
     var buf: [32]u8 = undefined;