Documented the MXChip Newlib runtime glue

Clarified the boundary between Newlib system calls, the BSP, ThreadX, and the ThreadX POSIX compatibility layer.

Assisted-by: Codex (GPT-5) <noreply@openai.com>

Author: fdesbiens

MXChip/AZ3166/deps/src/CMakeLists.txt

@@ -16,7 +16,7 @@ set(TARGET app_common)
 # Initialize sources
 set(SOURCES "")
 
-# Allow to disable the newlib stubbing
+# Add the sample's Newlib system-call glue unless the BSP provides its own.
 if(NOT DEFINED DISABLE_NEWLIB_STUB) 
     list(APPEND SOURCES
         newlib_nano.c
@@ -32,4 +32,4 @@ target_link_libraries(${TARGET}
     azrtos::netxduo
     jsmn
     nanoprintf
-)
+)

MXChip/AZ3166/deps/src/newlib_nano.c

@@ -1,6 +1,7 @@
 /* 
  * Copyright (c) Microsoft
  * Copyright (c) 2024 Eclipse Foundation
+ * Copyright (c) 2026 Eclipse ThreadX contributors
  * 
  *  This program and the accompanying materials are made available 
  *  under the terms of the MIT license which is available at
@@ -13,6 +14,29 @@
  *     Frédéric Desbiens - 2024 version.
  */
 
+// Some portions generated by Codex (GPT-5).
+
+/*
+ * Bare-metal C library integration for the MXChip sample.
+ *
+ * Newlib and newlib-nano implement the C library, but delegate services that
+ * normally come from an operating system to target-provided system calls.
+ * This file supplies the subset needed by this sample. The board console in
+ * app/common/console.c supplies _read() and _write() separately.
+ *
+ * These functions are not part of ThreadX or its POSIX compatibility layer.
+ * The exact system calls required by an application depend on the Newlib
+ * version and configuration, the linker specifications, and the C library
+ * routines that the application uses.
+ *
+ * Several functions below are minimal stubs because the sample has no file
+ * system or process model. The _sbrk() implementation is also sample-specific:
+ * it starts the C library heap at the linker-defined _end symbol, has no upper
+ * bound, and provides no synchronization. Applications that allocate from
+ * multiple threads, use a different memory layout, or require the unsupported
+ * services must provide suitable replacements in their BSP.
+ */
+
 #ifdef __GNUC__
 
 #include <stdio.h>