libc: newlib: limits.h: missing several definitions for required constant minimum values

[cfriedt]

Describe the bug

Newlib does a good job of being mostly posix conformant. I noticed that there were some inconsistencies though when implementing dirent.h functionality.

Namely, Newlib (at least Zephyr's fork) does not define the mandatory constant minimum values _POSIX_NAME_MAX and _XOPEN_NAME_MAX among many others.

Initially, I assumed that the Newlib C library did include many of the mandatory POSIX limit definitions, but that seems to not be the case today.

target platform: all
workaround: linked pr

Regression

• This is a regression.

Steps to reproduce

Apply the following diff to samples/hello_world.

diff --git a/samples/hello_world/src/main.c b/samples/hello_world/src/main.c
index c550ab461cb..df844b87e29 100644
--- a/samples/hello_world/src/main.c
+++ b/samples/hello_world/src/main.c
@@ -4,8 +4,31 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#define _POSIX_C_SOURCE 200809L
+#define _XOPEN_SOURCE 700
+
+#include <limits.h>
 #include <stdio.h>
 
+#include <zephyr/toolchain.h>
+
+#if !(defined(_POSIX_RE_DUP_MAX) || defined(_POSIX2_RE_DUP_MAX))
+/* there are in fact a number of mandatory macros missing from Newlib, not just those below. This one just happens to exist */
+#error "_POSIX_RE_DUP_MAX not defined: this implies the expected behaviour of limits.h is incorrect"
+#endif
+
+#ifndef _POSIX_NAME_MAX
+#error "_POSIX_NAME_MAX not defined"
+#else
+BUILD_ASSERT(_POSIX_NAME_MAX == 14, "non-conformant value of _POSIX_NAME_MAX");
+#endif
+
+#ifndef _XOPEN_NAME_MAX
+#error "_XOPEN_NAME_MAX not defined"
+#else
+BUILD_ASSERT(_XOPEN_NAME_MAX == 255, "non-conformant value of _XOPEN_NAME_MAX");
+#endif
+
 int main(void)
 {
        printf("Hello World! %s\n", CONFIG_BOARD_TARGET);

Build against picolibc (successful)

west build -p -b qemu_riscv64 samples/hello_world

Build for newlib (fails)

west build -p -b qemu_riscv64 samples/hello_world -- -DCONFIG_NEWLIB_LIBC=y

Relevant log output

/Users/cfriedt/zephyrproject/zephyr/samples/hello_world/src/main.c:16:2: error: #error "_POSIX_NAME_MAX not defined"
   16 | #error "_POSIX_NAME_MAX not defined"
      |  ^~~~~
/Users/cfriedt/zephyrproject/zephyr/samples/hello_world/src/main.c:22:2: error: #error "_XOPEN_NAME_MAX not defined"
   22 | #error "_XOPEN_NAME_MAX not defined"
      |  ^~~~~
[13/79] Building C object zephyr/CMakeFiles/zephyr.dir/lib/os/cbprintf_complete.c.obj
ninja: build stopped: subcommand failed.

Impact

Major – Severely degrades functionality; workaround is difficult or unavailable.

Environment

OS: Ubuntu
SDK: 0.17.2
Commit: f5cc5d8

Additional Context

Mentioned here #93387 (comment) and here #94612 (comment)

https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html