Apache NuttX is a mature, real-time embedded operating system (RTOS) https://nuttx.apache.org/
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raiden00pl a6393fa320 drivers/serial: sample xmit.head once per iteration in uart_xmitchars()
uart_putxmitchar() advances xmit.head from thread context without
holding the critical section, so on SMP the head index can move, and
wrap around, while uart_xmitchars() runs in the TX interrupt on another
CPU.  Since commit b319c27f03 ("serial: Added APIs for receiving and
sending multiple chars") the sendbuf path of uart_xmitchars() reads
xmit.head twice: once to decide whether the pending data is contiguous
and again to compute its length.  If the producer wraps the index in
between, the computed length goes negative, is passed to sendbuf() as a
huge size_t and the driver transmits memory far beyond the ring buffer.

The per-byte path reads the index only once and is not affected, which
is why this went unnoticed: the batch path is only used by drivers that
implement sendbuf, and the 16550 driver gained it in commit 45c38d8592
("drivers/serial/16550: add polling mode support for serial drivers").
qemu-intel64 with SMP is the first configuration combining a sendbuf
driver with a producer running on another CPU.

On qemu-intel64 SMP this shows up as an endless stream of NUL bytes on
the console (captured with gdb: head = 1, tail = 8, size = 16, and
u16550_sendbuf() called with size = (size_t)-7), which makes the ntfc
test harness fail every test that runs while the flood lasts.

Read the head index once per loop iteration and use that snapshot for
both the contiguity test and the length.  The producer only ever moves
the index forward, so a stale snapshot merely sends less now.

Assisted-by: Claude Code
Signed-off-by: raiden00pl <raiden00@railab.me>
2026-09-10 23:09:16 +08:00
.github tools/ci: remove obsolete testrun harness 2026-09-02 11:44:23 -03:00
arch arch/intel64: don't clear the oneshot handler from the HPET ISR 2026-09-10 10:23:46 +08:00
audio audio: limit the buffer count guard to shared ring requests 2026-08-05 07:58:53 +02:00
binfmt libs/libc/elf, binfmt: Describe the GOT by base and size, not by index. 2026-09-08 16:31:16 -03:00
boards boards/esp32s3-xiao: wire up Wi-Fi, guarding PM light sleep during radio init 2026-09-10 10:23:15 +08:00
cmake cmake: Build FDPIC modules the way the make build does. 2026-09-09 10:21:21 +08:00
crypto tools: fix stale archive members surviving a Kconfig-driven CSRCS change 2026-07-28 21:26:03 -03:00
Documentation tools/refresh.sh: Fix multi-argument parsing 2026-09-10 16:26:53 +02:00
drivers drivers/serial: sample xmit.head once per iteration in uart_xmitchars() 2026-09-10 23:09:16 +08:00
dummy build: add initial cmake build system 2023-07-08 13:50:48 +08:00
fs libs/libc/elf: Place an FDPIC object's segments independently. 2026-09-08 16:31:16 -03:00
graphics graphics/nxterm: consume SGR escape sequences 2026-08-23 10:46:02 +08:00
include libs/libc/elf, binfmt: Describe the GOT by base and size, not by index. 2026-09-08 16:31:16 -03:00
libs arch/arm, libs/libc/elf: Build FDPIC modules in the normal ELF build. 2026-09-09 10:21:21 +08:00
mm cmake: Use NUTTX(_DIR/_BIN_DIR) instead CMAKE(_SRC_DIR/_BIN_DIR) 2026-08-09 11:13:08 -03:00
net net/tcp: add configurable delayed ACK threshold 2026-09-08 09:00:19 +08:00
openamp cmake: Use NUTTX(_DIR/_BIN_DIR) instead CMAKE(_SRC_DIR/_BIN_DIR) 2026-08-09 11:13:08 -03:00
pass1 tools: fix stale archive members surviving a Kconfig-driven CSRCS change 2026-07-28 21:26:03 -03:00
sched sched/environ: Serialize clearenv updates. 2026-09-09 10:22:01 +08:00
syscall fs: rename PSEUDOFS_SOFTLINKS to FS_LINKS 2026-08-27 01:12:33 +08:00
tools tools/refresh.sh: Fix multi-argument parsing 2026-09-10 16:26:53 +02:00
video video/videomode: Fix EDID parsing and formatting of video mode dumps 2026-08-29 11:09:11 -03:00
wireless wireless/bluetooth: fix inverted MTU cap in bt_conn_send() 2026-08-30 10:48:18 -03:00
.asf.yaml github: master branch protection tune. 2025-05-07 18:37:13 -05:00
.codespell-ignore-lines arch/arm: Reserve r10 via ARCHCFLAGS and hoist the PIC module flags. 2026-07-24 23:09:08 +08:00
.codespellrc Revert "zbus: Add linker support and documentation for the zbus port" 2026-08-30 10:45:19 -03:00
.editorconfig .editorconfig: fix character encoding property specification 2025-11-28 19:12:13 +08:00
.gitignore boards/risc-v/eic7700x: Adopt the common board layout. 2026-08-19 01:40:57 +08:00
.gitmessage docs/contributing: Add a commit message template 2025-06-03 17:33:24 +08:00
.pre-commit-config.yaml pre-commit: enable codespell checks 2025-05-05 12:34:39 +08:00
.yamllint feat: add a GitHub action to lint the YAML files 2020-12-15 09:52:04 -06:00
AUTHORS AUTHORS: add Jorge Guzman 2026-08-25 08:43:13 -04:00
CMakeLists.txt arch, boards, cmake: Build C++ ELF modules without __cxa_atexit. 2026-09-04 15:44:24 -03:00
CONTRIBUTING.md contributing: Add requirement for 'Assisted-by' commit field 2026-07-12 09:42:28 +08:00
INVIOLABLES.md INVIOLABLES.md: Fix a simple alignment and change occurrences of Nuttx 2020-09-03 01:33:05 +08:00
Kconfig include/nuttx: Add link-time iterable sections infrastructure 2026-08-27 01:04:05 +08:00
LICENSE libs/libdsp: Add Matrix operations 2026-07-11 14:55:59 -03:00
Makefile !boards: enforce secure ROMFS passwd and TEA key setup 2026-07-09 22:41:11 +08:00
NOTICE Remove the double blank line from source files 2022-02-20 20:10:14 +01:00
README.md ci/testing: Add MemBrowse Integration 2026-06-18 12:07:41 -03:00
ReleaseNotes Documentation: move ReleaseNotes 2023-09-26 20:41:00 +08:00

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Apache NuttX is a real-time operating system (RTOS) with an emphasis on standards compliance and small footprint. Scalable from 8-bit to 64-bit microcontroller environments, the primary governing standards in NuttX are POSIX and ANSI standards. Additional standard APIs from Unix and other common RTOSs (such as VxWorks) are adopted for functionality not available under these standards, or for functionality that is not appropriate for deeply-embedded environments (such as fork()).

For brevity, many parts of the documentation will refer to Apache NuttX as simply NuttX.

Getting Started

First time on NuttX? Read the Getting Started guide! If you don't have a board available, NuttX has its own simulator that you can run on terminal.

Documentation

You can find the current NuttX documentation on the Documentation Page.

Alternatively, you can build the documentation yourself by following the Documentation Build Instructions.

The old NuttX documentation is still available in the Apache wiki.

Supported Boards

NuttX supports a wide variety of platforms. See the full list on the Supported Platforms page.

Contributing

If you wish to contribute to the NuttX project, read the Contributing guidelines for information on Git usage, coding standard, workflow and the NuttX principles.

License

The code in this repository is under either the Apache 2 license, or a license compatible with the Apache 2 license. See the License Page for more information.