From 3468da5cd72c54d5024b2c9c453b2f9589d28831 Mon Sep 17 00:00:00 2001
From: Chris Speciale <christopher.gaf@gmail.com>
Date: Thu, 20 Aug 2026 11:20:38 -0400
Subject: [PATCH] [windows] file_write: require a console, not just a character
 device

_isatty is true for any character device, so a write to NUL took the
WriteConsoleW path. WriteConsoleW fails on a non-console handle and the
file_error after it throws from inside a GC-free zone, so the process dies
instead of reporting anything:

    sys.io.File.write("NUL").writeString("x")
    prog.exe >NUL

GetConsoleMode only succeeds on a real console, and __hxcpp_print already uses
it. _isatty stays as the first check so files and pipes take the same fwrite
path they did before.

Came in with 1544ff5e (#1307), which fixed utf console output. That part is
unchanged here.
---
 Changes.md                                |  1 +
 src/hx/libs/std/File.cpp                  |  5 ++++-
 test/regression/NulDeviceWrite/Main.hx    | 13 +++++++++++++
 test/regression/NulDeviceWrite/build.hxml |  2 ++
 test/regression/NulDeviceWrite/stdout.txt |  1 +
 5 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 test/regression/NulDeviceWrite/Main.hx
 create mode 100644 test/regression/NulDeviceWrite/build.hxml
 create mode 100644 test/regression/NulDeviceWrite/stdout.txt

diff --git a/Changes.md b/Changes.md
index 816b4be7c..2e6d75d30 100644
--- a/Changes.md
+++ b/Changes.md
@@ -16,6 +16,7 @@
 * Updated sqlite to 3.40.1
 * Updated zlib to 1.2.13
 
+* Fixed crash writing to the null device on Windows
 * Fixed SSL socket non blocking handshake throwing an exception on 64bit Windows
 * Fixed Windows 64bit architecture detection
 * Fixed critial error handler returning the wrong callstack
diff --git a/src/hx/libs/std/File.cpp b/src/hx/libs/std/File.cpp
index a88318746..1a2435334 100644
--- a/src/hx/libs/std/File.cpp
+++ b/src/hx/libs/std/File.cpp
@@ -149,7 +149,10 @@ int _hx_std_file_write( Dynamic handle, Array<unsigned char> s, int p, int n )
 
    hx::AutoGCFreeZone zone;
 #ifdef HX_WINDOWS
-   if (_isatty(_fileno(f->io))) {
+   // _isatty is true for ANY character device - NUL, a serial port, a printer - and none of
+   // those accept WriteConsoleW. Only a real console has a console mode.
+   DWORD console_mode;
+   if (_isatty(_fileno(f->io)) && GetConsoleMode((HANDLE)_get_osfhandle(_fileno(f->io)), &console_mode)) {
       fflush(f->io);
       HANDLE win_handle = (HANDLE)_get_osfhandle(_fileno(f->io));
       static const int MAX_BUFFER_SIZE = 8192;
diff --git a/test/regression/NulDeviceWrite/Main.hx b/test/regression/NulDeviceWrite/Main.hx
new file mode 100644
index 000000000..5d3388d81
--- /dev/null
+++ b/test/regression/NulDeviceWrite/Main.hx
@@ -0,0 +1,13 @@
+// The null device is a character device, so _isatty reports it as a tty. file_write used that as
+// its test for a console and called WriteConsoleW, which only accepts a console handle; the failure
+// threw from inside a GC-free zone and took the process down instead of reporting an error.
+function main() {
+	final nul = Sys.systemName() == "Windows" ? "NUL" : "/dev/null";
+
+	final out = sys.io.File.write(nul);
+	out.writeString("written to the null device\n");
+	out.flush();
+	out.close();
+
+	Sys.println("ok");
+}
diff --git a/test/regression/NulDeviceWrite/build.hxml b/test/regression/NulDeviceWrite/build.hxml
new file mode 100644
index 000000000..f35632dbc
--- /dev/null
+++ b/test/regression/NulDeviceWrite/build.hxml
@@ -0,0 +1,2 @@
+-cpp bin
+-m Main
diff --git a/test/regression/NulDeviceWrite/stdout.txt b/test/regression/NulDeviceWrite/stdout.txt
new file mode 100644
index 000000000..9766475a4
--- /dev/null
+++ b/test/regression/NulDeviceWrite/stdout.txt
@@ -0,0 +1 @@
+ok