The warning should not cause build failures (we are fixing the legacyaling warning, needs target update as well). If you use locally (mbed-cl) - what errors is shown?

Hi Martin, No can do. We've worked around the problem by backing off to a 3-week old rev of mbed-os. We found that the issue was caused by mismatched extern "C" declarations in our main.cpp conflicting with extern declarations for the same variables in functions.h and functions.c files. We regret that the problem files were not saved. The heterogeneous use of C++ and C in the same project has caused us enough grief to start converting everything to C++. The syntax for sharing of variables and procedures between the two is not apparent or straight forward in Mbed, and apparently the compiler gets confused with what should be mismatch errors and passes untenable executables off to the linker. I hope this helps. Steve