libc: common: declare strnlen() and strtok_r() in string.h

Rules A.4 and A.5 of the coding guidelines allow the POSIX
strnlen() and strtok_r() functions to be used anywhere within
Zephyr, including in the kernel.

These are not part of ISO C and typically require
_POSIX_C_SOURCE to be declared. However, in Zephyr, POSIX
and the interfaces it provides, are optional, and should not
be used to compile the kernel and core components of the
system, which is also specified by Rules A.4 and A.5 of the
coding guidelines.

Declare the POSIX strnlen() function with _POSIX_C_SOURCE or
with _ZEPHYR_SOURCE so that it can be used easily by any
C library (including those without POSIX support or those
without knowledge of _ZEPHYR_SOURCE).

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

lib/libc/common/include/string.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2024, Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_LIB_LIBC_COMMON_INCLUDE_STRING_H_
+#define ZEPHYR_LIB_LIBC_COMMON_INCLUDE_STRING_H_
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#if _POSIX_C_SOURCE >= 200809L || defined(_ZEPHYR_SOURCE)
+size_t strnlen(const char *s, size_t maxlen);
+#endif
+
+#if _POSIX_C_SOURCE >= 200112L || defined(_ZEPHYR_SOURCE)
+char *strtok_r(char *str, const char *sep, char **state);
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#include_next <string.h>
+
+#endif /* ZEPHYR_LIB_LIBC_COMMON_INCLUDE_STRING_H_ */

lib/libc/newlib/CMakeLists.txt

@@ -37,6 +37,11 @@ zephyr_compile_definitions(_ANSI_SOURCE)
 # used by the network stack
 zephyr_compile_definitions(__LINUX_ERRNO_EXTENSIONS__)
 
+# define _ZEPHYR_SOURCE so we can get POSIX extensions to ISO C declared in
+# lib/libc/common/include/string.h, to satisfy Rule A.4 of Zephyr's Coding
+# Guidelines.
+zephyr_compile_definitions(_ZEPHYR_SOURCE)
+
 if(CMAKE_C_COMPILER_ID STREQUAL "GNU")
   # We are using
   # - ${CMAKE_C_LINKER_WRAPPER_FLAG}${CMAKE_LINK_LIBRARY_FLAG}c

tests/lib/c_lib/common/src/main.c

@@ -15,10 +15,6 @@
  * it guarantee that ALL functionality provided is working correctly.
  */
 
-#ifdef CONFIG_NEWLIB_LIBC
-#define _POSIX_C_SOURCE 200809
-#endif
-
 #include <zephyr/kernel.h>
 #include <zephyr/sys/__assert.h>
 #include <zephyr/ztest.h>