Incorrect newlib.h is used when nano.specs is used

[stephanosio]

newlib.h is responsible for defining the newlib build configurations so that the newlib headers can expose only the enabled features to the application code, and the nano.specs is responsible for configuring the include and library paths for the nano build of the newlib.

When --specs=nano.specs is specified to GCC, the newlib nano (libc_nano.a) is linked instead of the full newlib (libc.a) and the include/newlib-nano directory, which contains the newlib.h specific to the newlib nano build, is added as a system include directory, such that the nano newlib.h is included by application code instead of the full newlib.h, which resides in include.

The nano.specs file currently specifies the newlib.h include path as follows:

*cpp_unique_options:
-isystem =/include/newlib-nano %(nano_cpp_unique_options)

This is incorrect -- the = at the beginning of the include path is effectively no-op and GCC tries to literally resolve the path as =/include/newlib-nano, which obviously is not a valid path, and this leads to this include path being ignored and the newlib.h for the full newlib build from include/ being included by the application code:

$ echo | /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/arm-zephyr-eabi-gcc --specs=nano.specs -E -Wp,-v -
ignoring nonexistent directory "=/include/newlib-nano"
ignoring duplicate directory "/opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/../../lib/gcc/arm-zephyr-eabi/10.3.0/include"
ignoring duplicate directory "/opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/../../lib/gcc/arm-zephyr-eabi/10.3.0/include-fixed"
ignoring duplicate directory "/opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/../../lib/gcc/arm-zephyr-eabi/10.3.0/../../../../arm-zephyr-eabi/sys-include"
ignoring duplicate directory "/opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/../../lib/gcc/arm-zephyr-eabi/10.3.0/../../../../arm-zephyr-eabi/include"
#include "..." search starts here:
#include <...> search starts here:
 /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/10.3.0/include
 /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/10.3.0/include-fixed
 /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/10.3.0/../../../../arm-zephyr-eabi/sys-include
 /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/10.3.0/../../../../arm-zephyr-eabi/include
End of search list.
# 1 "<stdin>"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "<stdin>"

This can be corrected as follows so that the include/newlib-nano path is resolved based on the GCC_EXEC_PREFIX:

--- nano.specs
+++ nano.specs
@@ -3,7 +3,7 @@
 %rename cpp_unique_options  nano_cpp_unique_options

 *cpp_unique_options:
--isystem =/include/newlib-nano %(nano_cpp_unique_options)
+-isystem %:getenv(GCC_EXEC_PREFIX ../../arm-zephyr-eabi/include/newlib-nano) %(nano_cpp_unique_options)

 *nano_libc:
 -lc_nano

After applying the patch above, the include paths are as follows, which are correct:

$ echo | /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/arm-zephyr-eabi-gcc --specs=nano.specs -E -Wp,-v -
ignoring duplicate directory "/opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/../../lib/gcc/arm-zephyr-eabi/10.3.0/include"
ignoring duplicate directory "/opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/../../lib/gcc/arm-zephyr-eabi/10.3.0/include-fixed"
ignoring duplicate directory "/opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/../../lib/gcc/arm-zephyr-eabi/10.3.0/../../../../arm-zephyr-eabi/sys-include"
ignoring duplicate directory "/opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/../../lib/gcc/arm-zephyr-eabi/10.3.0/../../../../arm-zephyr-eabi/include"
#include "..." search starts here:
#include <...> search starts here:
 /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/../../arm-zephyr-eabi/include/newlib-nano
 /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/10.3.0/include
 /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/10.3.0/include-fixed
 /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/10.3.0/../../../../arm-zephyr-eabi/sys-include
 /opt/zephyr-sdk-0.14.1/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/10.3.0/../../../../arm-zephyr-eabi/include
End of search list.
# 1 "<stdin>"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "<stdin>"

[stephanosio]

Note that the nano.specs for the Zephyr SDK is generated by the crosstool-ng here:
https://github.com/crosstool-ng/crosstool-ng/blob/b6887d346fd5d54a9b778a5b27326c09ab774958/scripts/build/companion_libs/350-newlib_nano.sh#L205-L206

In the context of the above, the patch should be as follows:

--- nano.specs
+++ nano.specs
@@ -3,7 +3,7 @@
 %rename cpp_unique_options  nano_cpp_unique_options

 *cpp_unique_options:
--isystem =/include/newlib-nano %(nano_cpp_unique_options)
+-isystem %:getenv(GCC_EXEC_PREFIX ../../${CT_TARGET}/include/newlib-nano) %(nano_cpp_unique_options)

 *nano_libc:
 -lc_nano