Fail on open if tcsetattr doesn't set bits/parity

lib/circuits_uart.ex

@@ -192,6 +192,11 @@ defmodule Circuits.UART do
     of the Circuits.UART server pid as returned by `start_link/1`. The default
     value is `:name`.
 
+  On Linux and macOS, opening or configuring a port returns `{:error, :einval}`
+  if the driver fails to set the requested data bits, stop bits, or parity.
+  Linux pseudo-terminals, such as those created by `socat`, require
+  `data_bits: 8` and `parity: :none`.
+
   The following options are supported on Linux only:
 
   * `:rs485_enabled` - (`true` or `false`) enable RS485 mode.

src/uart_comm_unix.c

@@ -240,7 +240,8 @@ static int clear_custom_speed(int fd)
 static int uart_config_line(int fd, const struct uart_config *config)
 {
     struct termios options;
-    tcgetattr(fd, &options);
+    if (tcgetattr(fd, &options) < 0)
+        return -1;
 
     // Set up the speed
     int baud_constant = to_baudrate_constant(config->speed);
@@ -317,7 +318,28 @@ static int uart_config_line(int fd, const struct uart_config *config)
     options.c_cc[VTIME] = 0;
 
     // Set everything
-    return tcsetattr(fd, TCSANOW, &options);
+    if (tcsetattr(fd, TCSANOW, &options) < 0)
+        return -1;
+
+    struct termios actual;
+    if (tcgetattr(fd, &actual) < 0)
+        return -1;
+
+    // tcsetattr() may succeed even when only some settings were applied.
+    tcflag_t mask = CSIZE | CSTOPB | PARENB;
+    if (options.c_cflag & PARENB) {
+        mask |= PARODD;
+#ifdef CMSPAR
+        mask |= CMSPAR;
+#endif
+    }
+
+    if ((actual.c_cflag & mask) != (options.c_cflag & mask)) {
+        errno = EINVAL;
+        return -1;
+    }
+
+    return 0;
 }
 
 /**

test/pty_test.exs

@@ -0,0 +1,81 @@
+# SPDX-FileCopyrightText: 2026 Tallak Tveide
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+defmodule PtyTest do
+  use ExUnit.Case
+  alias Circuits.UART
+
+  @skip_reason (cond do
+                  :os.type() != {:unix, :linux} -> "These tests require Linux pseudo-terminals"
+                  System.find_executable("socat") == nil -> "socat is not installed"
+                  true -> false
+                end)
+  @moduletag skip: @skip_reason
+
+  setup do
+    port =
+      Port.open({:spawn_executable, System.find_executable("socat")}, [
+        :binary,
+        :stderr_to_stdout,
+        args: ["-d", "-d", "pty,raw,echo=0", "pty,raw,echo=0"]
+      ])
+
+    {:os_pid, os_pid} = Port.info(port, :os_pid)
+    on_exit(fn -> System.cmd("kill", [to_string(os_pid)]) end)
+
+    uart = start_supervised!(UART)
+    %{pty: pty_name(port, ""), uart: uart}
+  end
+
+  defp pty_name(port, output) do
+    case Regex.run(~r/PTY is (\S+)/, output, capture: :all_but_first) do
+      [name] ->
+        name
+
+      nil ->
+        receive do
+          {^port, {:data, data}} -> pty_name(port, output <> data)
+        after
+          2000 -> flunk("socat did not open a pty: #{output}")
+        end
+    end
+  end
+
+  test "supported settings work on repeated opens and reconfiguration", %{pty: pty, uart: uart} do
+    for _ <- 1..3 do
+      assert :ok = UART.open(uart, pty, speed: 19200, data_bits: 8, parity: :none)
+      assert :ok = UART.configure(uart, stop_bits: 2)
+      assert :ok = UART.configure(uart, stop_bits: 1)
+      assert :ok = UART.close(uart)
+    end
+  end
+
+  for options <- [
+        [parity: :even],
+        [parity: :odd],
+        [parity: :mark],
+        [parity: :space],
+        [data_bits: 5],
+        [data_bits: 6],
+        [data_bits: 7]
+      ] do
+    test "unsupported settings #{inspect(options)} fail on every open", %{pty: pty, uart: uart} do
+      for _ <- 1..2 do
+        assert {:error, :einval} =
+                 UART.open(uart, pty, [speed: 19200] ++ unquote(options))
+      end
+
+      assert :ok = UART.open(uart, pty, data_bits: 8, parity: :none)
+    end
+
+    test "unsupported settings #{inspect(options)} fail on reconfiguration", %{
+      pty: pty,
+      uart: uart
+    } do
+      assert :ok = UART.open(uart, pty)
+      assert {:error, :einval} = UART.configure(uart, unquote(options))
+      assert :ok = UART.configure(uart, data_bits: 8, parity: :none)
+    end
+  end
+end