trns1997 commented on PR #17968:
URL: https://github.com/apache/nuttx/pull/17968#issuecomment-3765121575
@simbit18 I dug into this further and I believe I’ve identified the
underlying reason for the failure.
In tools/Config.mk, when building with the C++ toolchain, we always add
NuttX’s C++ compatibility headers to the include path:
```
# The default C/C++ search path
ARCHINCLUDES += ${INCSYSDIR_PREFIX}$(TOPDIR)$(DELIM)include
ifeq ($(CONFIG_LIBCXX),y)
ARCHXXINCLUDES += ${INCSYSDIR_PREFIX}$(TOPDIR)$(DELIM)include$(DELIM)libcxx
else ifeq ($(CONFIG_UCLIBCXX),y)
ARCHXXINCLUDES +=
${INCSYSDIR_PREFIX}$(TOPDIR)$(DELIM)include$(DELIM)uClibc++
else
ARCHXXINCLUDES += ${INCSYSDIR_PREFIX}$(TOPDIR)$(DELIM)include$(DELIM)cxx
ifeq ($(CONFIG_ETL),y)
ARCHXXINCLUDES += ${INCSYSDIR_PREFIX}$(TOPDIR)$(DELIM)include$(DELIM)etl
endif
endif
```
So for the “plain” CXX case, `include/cxx` is implicitly part of the system
include path.
This directory contains the NuttX C/C++ compatibility headers that are meant
to sit in front of the toolchain’s libc headers.
In the CMake build, however, `include/cxx` is currently not added to the C++
include directories. As a result, the toolchain’s `<stdlib.h>` gets pulled in
first via `<cstdlib>`, which then conflicts with NuttX’s own `stdlib.h`
(`div_t`, `ldiv_t`, `lldiv_t`, etc.), exactly as seen in the error log.
To harmonize Makefile and CMake behavior, I think we need explicitly add
`include/cxx` when C++ is enabled in CMake, e.g. in
`arch/arm/src/cmake/gcc.cmake` :
```
if(CONFIG_HAVE_CXX)
list(APPEND CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES
${CMAKE_SOURCE_DIR}/include/cxx)
endif()
```
This makes the CMake build consistent with what the Makefile-based build
already does today and resolves the conflicting type definitions.
That said, I do have one open question for CXX users; When building with the
external C++ toolchain, is it expected that NuttX’s `include/cxx` headers are
always used?
My understanding is that these headers are required to maintain NuttX C++
compatibility (wrapping or redefining parts of the standard library to coexist
with NuttX’s libc), so including them even with a full C++ toolchain seems
intentional and correct.
But if the long-term expectation is that a “pure” toolchain C++ build should
not rely on `include/cxx`, then the Makefile behavior itself might need
revisiting.
What do you think? I would also like @leducp @xiaoxiang781216 input on this
:)
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