ESP32 Forum

Hi all -

I was hoping not to have to drag this subject up again, but...I've reviewed my old posts on this topic and can't find what I'm doing wrong.

I began with a copy of a C project (https://github.com/OLIMEX/ESP32-POE/tre ... E_Ethernet) and renamed the two C files to .cpp. I added the extern "C" linkage as follows: and did the same for olimex_ethernet.cpp and olimex_ethernet.h. Ran a make using msys2, and got this error:

$ make
CXX build/main/main.o
AR build/main/libmain.a
LD build/ESP32-PoE.elf
C:/esp32_projects/esp32_poe_ethernet/build/main\libmain.a(main.o):(.literal.app_main+0x28): undefined reference to `testEthernet'
C:/esp32_projects/esp32_poe_ethernet/build/main\libmain.a(main.o): In function `app_main':
C:/esp32_projects/esp32_poe_ethernet/main/main.cpp:64: undefined reference to `testEthernet'

Can someone remind me what I'm forgetting? Thanks.

Do a make clean. I don't see where it's building the other dot cpp file.

Already tried a make clean...no change. You're right, though -- make doesn't seem to have any knowledge of olimex_ethernet.cpp. I touched it and did another make, and it didn't try to compile it. Isn't the make facility suppose to automatically pick up all source files? This one is in a subdirectory under ~project/main.

Not sure if this is the issue, but there is an option for c++ in menuconfig.

The only C++ options I found in menuconfig were:

CONFIG_CXX_EXCEPTIONS=y
CONFIG_CXX_EXCEPTIONS_EMG_POOL_SIZE=0

Was there another one you saw?

As an update, the problem doesn't appear to be C vs. C++ after all. The project uses the following source files:

main/main.cpp
main/ethernet/olimex_ethernet.cpp
main/ethernet/olimex_ethernet.h

And main doesn't seem to be looking in main/ethernet/.

Do I need to do something to get make to look in a subdirectory?

Does the ethernet .h file also have the extern c linkage things?

Yes it does.

When I have problems like this, I usually end up solving by adding something to component.mk telling it where to look for files.

Hey Fly - that was good advice. I'd been assuming all along that the build system knew to look in subdirectories, but that turned out not to be the case. in main/component.mk, I added this line:
And it builds now. The "." is needed if you have source files in your main/ directory.

It's good to have this working, even if Cmake is the wave of the (esp32's) future. Thanks for the suggestion.