Export -latomic even if BUILD_TESTING is disabled

[roehling]

On some architectures, libatomic is needed and should not be dependent on tests being enabled.

[fujitatomoya]

@roehling thanks for the PR, can you explain more details why we need this and what the actual problem you have? i think that would make it easier to understand for everyone.

[roehling]

The atomic operations are used by src/testing/fault_injection.c, which is compiled into the library unconditionally and may generate symbol references to libatomic on some architectures. However, as of now, the required -latomic linker flag is added only if BUILD_TESTING is enabled. This means that users may need to link against libatomic themselves, even if they do not use atomics at all, just because their rcutils library happens to be compiled with BUILD_TESTING disabled.

[fujitatomoya]

@roehling appreciate the explanation.

The atomic operations are used by src/testing/fault_injection.c, which is compiled into the library unconditionally and may generate symbol references to libatomic on some architectures.

is this because of the following? if that so, probably getting rid of unnecessary dependency would be solution?

rcutils/include/rcutils/error_handling.h

Line 39 in 9cf42fb

#include "rcutils/testing/fault_injection.h"

and, this should be no problem, since it includes only when it is test build.

rcutils/include/rcutils/macros.h

Lines 145 to 146 in 9cf42fb

	#if defined RCUTILS_ENABLE_FAULT_INJECTION
	#include "rcutils/testing/fault_injection.h"

thanks,

[roehling]

No, it is not.

It is because src/testing/fault_injection.c is compiled into the rcutils library here:

rcutils/CMakeLists.txt

Line 74 in 9cf42fb

src/testing/fault_injection.c

And said file then uses rcutils_atomic_ operations, which in turn generate symbol references to the atomic library.

I guess, if you wanted to avoid the unconditional dependency on the atomic library, you could also make fault_injection.c conditional, for example with the generator expression $<$<BOOL:BUILD_TESTING>:src/testing/fault_injection.c>, but that means you probably should not install the corresponding headers either.

I believe my PR is less intrusive, as GCC normally adds --as-needed to the linker call, so the atomic library will only be linked if it is actually needed.

[fujitatomoya]

but that means you probably should not install the corresponding headers either.

yeah right, currently it include this header, e.g) /opt/ros/humble/include/rcutils/rcutils/testing/fault_injection.h

I believe my PR is less intrusive, as GCC normally adds --as-needed to the linker call, so the atomic library will only be linked if it is actually needed.

agree.

[fujitatomoya]

@clalancette could you take a look?

[lopsided98]

I just ran into this problem while cross-compiling to armv6, and came up with pretty much the same solution. I did find that the ament_export_link_flags("-latomic") call doesn't appear to be necessary, at least for the packages that I've built (mavros and all its dependencies, which should include all the core ROS packages).

[fujitatomoya]

CI:

• Linux
• Linux-aarch64
• Windows