pico_cyw43_arch FreeRTOS+lwIP: route errno through newlib (LWIP_ERRNO_STDINCLUDE) #2974

2026-05-27T21:03:05-05:00

skycmoon commented

2026-05-27 21:03:05 -05:00

(Migrated from github.com)

What

Swap LWIP_PROVIDE_ERRNO=1 for LWIP_ERRNO_STDINCLUDE=1 on the pico_cyw43_arch_lwip_sys_freertos variant. Single hunk in src/rp2_common/pico_cyw43_arch/CMakeLists.txt.

Why

LWIP_PROVIDE_ERRNO=1 is correct for bare-metal builds (no libc owns errno), but the FreeRTOS+lwIP variant always ships with newlib, which already provides a thread-local errno via <errno.h>. The unconditional LWIP_PROVIDE_ERRNO=1 makes lwIP declare its own extern int errno global, diverging from newlib's storage. Any code reading errno via <errno.h> — including the standard mbedTLS library/net_sockets.c BIO — misses the EWOULDBLOCK lwIP wrote, treats every non-blocking recv() poll as fatal, and tears the TLS session down right after handshake.

LWIP_ERRNO_STDINCLUDE=1 keeps lwIP's E* constants available but routes them via <errno.h>, so lwIP and the rest of the program share newlib's thread-local storage. Issue #2973 has the full diagnosis and an observable symptom description (TLS handshake completes → application sends e.g. MQTT CONNECT → server queues CONNACK → client immediately FINs without reading the response → loop forever at the application's backoff cadence).

Scope

Only pico_cyw43_arch_lwip_sys_freertos_headers is touched.
pico_cyw43_arch_lwip_threadsafe_background (bare-metal) does not currently set LWIP_PROVIDE_ERRNO=1, so no change needed there.
Comment block added inline explaining the choice for future maintainers.

Compatibility

Behaviour-preserving for users who don't touch errno directly — lwIP's E* constants still resolve, just through <errno.h>. Correcting for users on the stock mbedTLS BIO pattern (or any equivalent POSIX-style errno shim).

Test

I verified the equivalent fix locally as a project-side override in lwipopts.h: cancelling the SDK's -D LWIP_PROVIDE_ERRNO=1 and setting LWIP_ERRNO_STDINCLUDE=1 resolved a persistent MQTT CONNECT/CONNACK/close storm on Pico 2W (FreeRTOS + lwIP + mbedTLS + coreMQTT). This PR moves that fix from a per-project workaround to the SDK default for the variant.

Fixes #2973. ## What Swap `LWIP_PROVIDE_ERRNO=1` for `LWIP_ERRNO_STDINCLUDE=1` on the `pico_cyw43_arch_lwip_sys_freertos` variant. Single hunk in `src/rp2_common/pico_cyw43_arch/CMakeLists.txt`. ## Why `LWIP_PROVIDE_ERRNO=1` is correct for bare-metal builds (no libc owns `errno`), but the FreeRTOS+lwIP variant always ships with newlib, which already provides a thread-local `errno` via `<errno.h>`. The unconditional `LWIP_PROVIDE_ERRNO=1` makes lwIP declare its own `extern int errno` global, diverging from newlib's storage. Any code reading `errno` via `<errno.h>` — including the standard mbedTLS `library/net_sockets.c` BIO — misses the `EWOULDBLOCK` lwIP wrote, treats every non-blocking `recv()` poll as fatal, and tears the TLS session down right after handshake. `LWIP_ERRNO_STDINCLUDE=1` keeps lwIP's `E*` constants available but routes them via `<errno.h>`, so lwIP and the rest of the program share newlib's thread-local storage. Issue #2973 has the full diagnosis and an observable symptom description (TLS handshake completes → application sends e.g. MQTT `CONNECT` → server queues `CONNACK` → client immediately FINs without reading the response → loop forever at the application's backoff cadence). ## Scope - Only `pico_cyw43_arch_lwip_sys_freertos_headers` is touched. - `pico_cyw43_arch_lwip_threadsafe_background` (bare-metal) does not currently set `LWIP_PROVIDE_ERRNO=1`, so no change needed there. - Comment block added inline explaining the choice for future maintainers. ## Compatibility Behaviour-preserving for users who don't touch `errno` directly — lwIP's `E*` constants still resolve, just through `<errno.h>`. Correcting for users on the stock mbedTLS BIO pattern (or any equivalent POSIX-style errno shim). ## Test I verified the equivalent fix locally as a project-side override in `lwipopts.h`: cancelling the SDK's `-D LWIP_PROVIDE_ERRNO=1` and setting `LWIP_ERRNO_STDINCLUDE=1` resolved a persistent MQTT `CONNECT/CONNACK/close` storm on Pico 2W (FreeRTOS + lwIP + mbedTLS + coreMQTT). This PR moves that fix from a per-project workaround to the SDK default for the variant.

peterharperuk (Migrated from github.com) approved these changes

2026-06-11 05:42:36 -05:00

kilograham (Migrated from github.com) reviewed

2026-06-15 19:41:53 -05:00

src/rp2_common/pico_cyw43_arch/CMakeLists.txt

					
				@ -69,2 +69,3 @@

				                    CYW43_LWIP=1

				                    LWIP_PROVIDE_ERRNO=1

				                    # FreeRTOS+lwIP uses newlib, which provides a thread-local errno

				                    # via <errno.h>. LWIP_PROVIDE_ERRNO=1 would declare lwIP's own

um, who says it uses newlib? surely it is dependent on the compiler/toolchain?

kilograham (Migrated from github.com) requested changes

2026-06-19 16:55:40 -05:00

kilograham (Migrated from github.com) left a comment

i'm not convinced by this yet - as i say, who says that newlib is used? and this is non-overridable now as implemented

peterharperuk commented

2026-06-20 04:16:21 -05:00

(Migrated from github.com)

Yes. I think it was a mistake to add this to the library. It should have been left to be defined in lwipopts.h

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