Fail pty opens that ask for parity or 7 bits

lib/circuits_uart.ex

@@ -192,6 +192,10 @@ defmodule Circuits.UART do
     of the Circuits.UART server pid as returned by `start_link/1`. The default
     value is `:name`.
 
+  A pseudo-terminal, such as the ones `socat` makes, can't do parity or any
+  other than 8 data bits. Opening one on Linux with other settings fails with
+  `{:error, :einval}`, so use `parity: :none` and `data_bits: 8` for those.
+
   The following options are supported on Linux only:
 
   * `:rs485_enabled` - (`true` or `false`) enable RS485 mode.

src/uart_comm_unix.c

@@ -230,6 +230,35 @@ static int clear_custom_speed(int fd)
 }
 
 
+/**
+ * @brief Check that the port can do the data bits and parity asked for
+ *
+ * A pseudo-terminal, such as the ones socat makes, can't do parity or any
+ * other than 8 data bits: the kernel drops the request. tcsetattr() still
+ * succeeds the first time, since it changes other settings, and then fails
+ * with EINVAL each time after, since it changes nothing. Fail them all, so
+ * that opening a pty that worked once doesn't stop working.
+ *
+ * @param fd
+ * @param config
+ * @return <0 on error, with errno set to EINVAL
+ */
+static int uart_check_pty(int fd, const struct uart_config *config)
+{
+    char name[64];
+
+    if (ttyname_r(fd, name, sizeof(name)) != 0 || strncmp(name, "/dev/pts/", 9) != 0)
+        return 0;
+
+    if (config->data_bits != 8 ||
+            (config->parity != UART_PARITY_NONE && config->parity != UART_PARITY_IGNORE)) {
+        errno = EINVAL;
+        return -1;
+    }
+
+    return 0;
+}
+
 /**
  * @brief Configure the speed, data bits, stop bits and parity for the port
  *
@@ -239,6 +268,9 @@ static int clear_custom_speed(int fd)
  */
 static int uart_config_line(int fd, const struct uart_config *config)
 {
+    if (uart_check_pty(fd, config) < 0)
+        return -1;
+
     struct termios options;
     tcgetattr(fd, &options);
 

test/pty_test.exs

@@ -0,0 +1,66 @@
+# SPDX-FileCopyrightText: 2026 Tallak Tveide
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+defmodule PtyTest do
+  use ExUnit.Case
+  alias Circuits.UART
+
+  @moduledoc """
+  A pseudo-terminal can't do parity or characters of other than 8 bits. These tests
+  make one with socat, so they are skipped where it isn't installed.
+  """
+
+  @moduletag skip: if(System.find_executable("socat") == nil, do: "socat is not installed")
+
+  setup do
+    socat = System.find_executable("socat")
+
+    port =
+      Port.open({:spawn_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)
+
+    {:ok, uart} = UART.start_link()
+    %{pty: pty_name(port, ""), uart: uart}
+  end
+
+  # socat prints "N PTY is /dev/pts/12" for each 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 "a pty opens again and again with 8 data bits and no parity", %{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.close(uart)
+    end
+  end
+
+  test "a pty refuses parity and 7 data bits every time, the first as well", %{
+    pty: pty,
+    uart: uart
+  } do
+    for options <- [[parity: :even], [parity: :odd], [parity: :mark], [data_bits: 7]],
+        _ <- 1..2 do
+      assert {:error, :einval} = UART.open(uart, pty, [speed: 19200] ++ options)
+    end
+
+    assert :ok = UART.open(uart, pty, speed: 19200, parity: :none)
+  end
+end