Zephyr: C++ Support doesn't work

[zigarrre]

Problem

When trying to use Zephyr with C++ I noticed that a lot of the headers contain code that isn't valid in C++ and causes compile errors when included in C++ files. I noticed this mainly with the Bluetooth subsystem but I wouldn't be surprised if other parts of are affected too as I haven't tested them.

So even though my examples focus on Bluetooth this issue concerns Zephyr as a whole.

The documentation claims that Zephyr has C++ support, but at the moment not even the most basic things work.

After asking on IRC about this @SebastianBoe confirmed those problems and submitted PR #5642 with some small fixes. While that was a first step in the right direction a lot more has to be done still.

As far as I know there are absolutely no automated tests in Zephyr for C++ compatibility at the moment. I would suggest changing that to properly ensure C++ compatibility in the future. If possible maybe just reuse/adopt existing C tests and additionally compile them in C++.

Solution

1) Fix any headers that contain invalid C++ code to be valid in both C and C++.
2) Add automated tests for C++ compatibility

Minimal Example

#include <zephyr.h>

#include <bluetooth/bluetooth.h>
#include <bluetooth/hci.h>
#include <bluetooth/conn.h>
#include <bluetooth/uuid.h>
#include <bluetooth/gatt.h>

static const struct bt_data ad[] = {
    BT_DATA_BYTES(BT_DATA_FLAGS, (BT_LE_AD_GENERAL | BT_LE_AD_NO_BREDR))
};

void on_bt_ready(int err) {
    bl_le_adv_start(BT_LE_ADV_CONN, ad, ARRAY_SIZE(ad), NULL, NULL);
}

int main() {
    bt_enable(on_bt_ready);
}

The following configuration was used in prj.conf:

CONFIG_CPLUSPLUS=y
CONFIG_BT=y
CONFIG_BT_DEBUG_LOG=y
CONFIG_BT_DEVICE_NAME="Test beacon"

Built with the current master branch of zephyr and zephyr-sdk-0.9.2 for the nrf52 platform using the following commands:

mkdir build && cd build
cmake -DBOARD=nrf52_pca10040 ..
make

This minimal example will generate the following errors:

In file included from /home/zigarrre/coding/zephyr_test/src/test.cpp:3:0:
/home/zigarrre/coding/zephyr_test/src/test.cpp: In function ‘void on_bt_ready(int)’:
/home/zigarrre/coding/z/zephyr/include/bluetooth/bluetooth.h:149:4: error: taking address of temporary [-fpermissive]
    })
    ^
/home/zigarrre/coding/z/zephyr/include/bluetooth/bluetooth.h:151:24: note: in expansion of macro ‘BT_LE_ADV_PARAM’
 #define BT_LE_ADV_CONN BT_LE_ADV_PARAM(BT_LE_ADV_OPT_CONNECTABLE, \
                        ^~~~~~~~~~~~~~~
/home/zigarrre/coding/zephyr_test/src/test.cpp:14:21: note: in expansion of macro ‘BT_LE_ADV_CONN’
     bl_le_adv_start(BT_LE_ADV_CONN, ad, ARRAY_SIZE(ad), NULL, NULL);
                     ^~~~~~~~~~~~~~
In file included from /home/zigarrre/coding/z/zephyr/include/kernel.h:27:0,
                 from /home/zigarrre/coding/z/zephyr/include/zephyr.h:18,
                 from /home/zigarrre/coding/zephyr_test/src/test.cpp:1:
/home/zigarrre/coding/z/zephyr/include/misc/util.h:42:35: error: ‘__builtin_types_compatible_p’ was not declared in this scope
            __typeof__(&(array)[0])))
                                   ^
/home/zigarrre/coding/z/zephyr/include/misc/util.h:34:66: note: in definition of macro ‘ZERO_OR_COMPILE_ERROR’
 #define ZERO_OR_COMPILE_ERROR(cond) ((int) sizeof(char[1 - 2 * !(cond)]) - 1)
                                                                  ^~~~
/home/zigarrre/coding/z/zephyr/include/misc/util.h:48:20: note: in expansion of macro ‘IS_ARRAY’
  ((unsigned long) (IS_ARRAY(array) + \
                    ^~~~~~~~
/home/zigarrre/coding/zephyr_test/src/test.cpp:14:41: note: in expansion of macro ‘ARRAY_SIZE’
     bl_le_adv_start(BT_LE_ADV_CONN, ad, ARRAY_SIZE(ad), NULL, NULL);
                                         ^~~~~~~~~~
/home/zigarrre/coding/z/zephyr/include/misc/util.h:34:51: error: expected primary-expression before ‘char’
 #define ZERO_OR_COMPILE_ERROR(cond) ((int) sizeof(char[1 - 2 * !(cond)]) - 1)
                                                   ^
/home/zigarrre/coding/z/zephyr/include/misc/util.h:40:2: note: in expansion of macro ‘ZERO_OR_COMPILE_ERROR’
  ZERO_OR_COMPILE_ERROR( \
  ^~~~~~~~~~~~~~~~~~~~~
/home/zigarrre/coding/z/zephyr/include/misc/util.h:48:20: note: in expansion of macro ‘IS_ARRAY’
  ((unsigned long) (IS_ARRAY(array) + \
                    ^~~~~~~~
/home/zigarrre/coding/zephyr_test/src/test.cpp:14:41: note: in expansion of macro ‘ARRAY_SIZE’
     bl_le_adv_start(BT_LE_ADV_CONN, ad, ARRAY_SIZE(ad), NULL, NULL);
                                         ^~~~~~~~~~
/home/zigarrre/coding/z/zephyr/include/misc/util.h:34:51: error: expected ‘)’ before ‘char’
 #define ZERO_OR_COMPILE_ERROR(cond) ((int) sizeof(char[1 - 2 * !(cond)]) - 1)
                                                   ^
/home/zigarrre/coding/z/zephyr/include/misc/util.h:40:2: note: in expansion of macro ‘ZERO_OR_COMPILE_ERROR’
  ZERO_OR_COMPILE_ERROR( \
  ^~~~~~~~~~~~~~~~~~~~~
/home/zigarrre/coding/z/zephyr/include/misc/util.h:48:20: note: in expansion of macro ‘IS_ARRAY’
  ((unsigned long) (IS_ARRAY(array) + \
                    ^~~~~~~~
/home/zigarrre/coding/zephyr_test/src/test.cpp:14:41: note: in expansion of macro ‘ARRAY_SIZE’
     bl_le_adv_start(BT_LE_ADV_CONN, ad, ARRAY_SIZE(ad), NULL, NULL);
                                         ^~~~~~~~~~
/home/zigarrre/coding/zephyr_test/src/test.cpp:14:68: error: expected ‘)’ before ‘;’ token
     bl_le_adv_start(BT_LE_ADV_CONN, ad, ARRAY_SIZE(ad), NULL, NULL);
                                                                    ^
/home/zigarrre/coding/zephyr_test/src/test.cpp:14:68: error: expected ‘)’ before ‘;’ token
/home/zigarrre/coding/zephyr_test/src/test.cpp:14:68: error: expected ‘)’ before ‘;’ token
/home/zigarrre/coding/zephyr_test/src/test.cpp: In function ‘int main()’:
/home/zigarrre/coding/zephyr_test/src/test.cpp:19:1: warning: no return statement in function returning non-void [-Wreturn-type]
 }
 ^
make[2]: *** [CMakeFiles/app.dir/build.make:303: CMakeFiles/app.dir/src/test.cpp.obj] Error 1
make[1]: *** [CMakeFiles/Makefile2:228: CMakeFiles/app.dir/all] Error 2
make: *** [Makefile:84: all] Error 2

Full Examples

For additional, more complete code samples just use the various (Bluetooth) examples that Zephyr already provides and compile them as C++ by changing the extensions of the files as well as in the CMakeFiles.txt to .cpp.

I would suggest starting with bluetooth/beacon which is the most simple example and working our way up from there to more complex examples.

[pfalcon]

Based on the title, this is duplicate of #1205, #4028.

[SebastianBoe]

Kind of, I would suggest that when this is resolved one should look into if the fix has affected #1205 or #4028.

4028 looks like a feature-request for supporting more C++ features.

1205 looks like a report that C++ is broken when -fpermissive is not applied (I have already tested that neither -fpermissive or -fno-permissive resolves the issues reported here).

[jhedberg]

Note that you don't need to use the BT_MESH_ADV_CONN macro if you don't want to. Creating a "normal" struct bt_mesh_adv_param stack variable and passing a reference to that to bt_mesh_adv_start() should make a C++ compiler happy.

[DC11011100]

As I was trying to port my freeRTOS C++ applications, I'm sad this has stagnated and that I won't be able to use the full functionality/abstraction zephyr has to offer.

Given issues like this still exist, I think it's only fair to ATLEAST put a caveat in the documentation, which seems to sell zephyr as conveniently c++ compatible (it's not at the moment), or just remove the claim completely.

[pabigot]

Much of this has been addressed in the past 18 months. As soon as #18243, #18245, and #18242 are merged Zephyr should be pretty compatible with C++.

[pabigot]

Per above comment #18243, #18245, and #18242 are closed so we'll call this done.

[pabigot]

Nope, sorry. This references Bluetooth specifically so #18551 blocks it.