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[urbanze]

See the docs for that.

Oh, I tried "target_link_libraries" but still failing... I can't understand why

Let me explain better my project.

My project is created using IDF 4.0 and use various custom modules created by me. Module for network, another module for NVS, RTC, webpage and etc.

[firmware_network] module have files for control WiFi, Ethernet, MQTT and so on.
[firmware_scheduler] module have files for control relay based on unix time.

However, to keep modules independents, in MQTT.cpp, when ESP receive [MQTT_EVENT_HANDLER -> data_event], this call 2 empty weak functions (weak functions inside mqtt.cpp contain a simple print inside).
- One weak function is called in main.cpp to process json to specific board. This work.
- Another weak function is called in [firmware_scheduler] to process specific scheduler jsons, but I can't do this work correctly.

I added "target_link_libraries(${COMPONENT_LIB} INTERFACE "-u mqtt_callback_scheduler")" in [firmware_network], but fail.
I tried to add this in command in [firmware_scheduler] but fail again.

In all tests of scheduler, weak function inside of mqtt.cpp keeps in use and function defined in scheduler.cpp is never called.


mqtt.h

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void mqtt_callback_scheduler(char *data) __attribute__((weak));

mqtt.cpp

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void mqtt_callback_scheduler(char *data)
{
    ESP_LOGW(__func__, "Replace me");
}

scheduler.cpp

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void mqtt_callback_scheduler(char *data)
{
    ESP_LOGI(__func__, "Scheduler test");
}

[ESP_Sprite]

No. You specifically need a *dummy function* (or symbol) to declare as undefined. It works like this.

- C compiler compiles all the objects
- Linker tries to recursively resolve all symbols (=functions and variable names that are used between functions). It starts with the assumption that there are no object files needed, but there is an unresolved symbol called 'app_main'. It searches through the objects it has until it finds it, in main.cpp. Cool, it'll include that in the binary. But now main.cpp has a bunch of unresolved symbols. It again searches throught he object files it has until it can resolve those symbols. But those files may have unresolved symbols, so it searches again, etc.
- When there are no more unresolved symbols, it will link the binary using *only the object files it needed to resolve all symbols*

The thing is, at this point the linker does not care about weak symbols that may or may not be also declared somewhere in a strong way. It takes your MQTT library thingy, sees the mqtt_callback_scheduler symbol is not undefined anymore (as that file has an internal weak symbol for it) and as such it never has an excuse to look at your scheduler file as it (presumably) only has functions in it that are otherwise declared as 'weak'. (Your main.cpp also has functions in it that actually are required to be resolved for the link to succeed, so the linker *does* take that into account.)

So how to solve it? You need to tell the linker to also take your scheduler file into account. Easiest way is to introduce an unlinked dummy symbol (which you do with the -u flag) which only can be resolved by taking the scheduler file into account. So you put '-u some_dummy_function_that_is_otherwise_entirely_unused' in the linker flags and put a corresponding 'void some_dummy_function_that_is_otherwise_entirely_unused() {}' in the C file.

[urbanze]

No. You specifically need a *dummy function* (or symbol) to declare as undefined. It works like this.

- C compiler compiles all the objects
- Linker tries to recursively resolve all symbols (=functions and variable names that are used between functions). It starts with the assumption that there are no object files needed, but there is an unresolved symbol called 'app_main'. It searches through the objects it has until it finds it, in main.cpp. Cool, it'll include that in the binary. But now main.cpp has a bunch of unresolved symbols. It again searches throught he object files it has until it can resolve those symbols. But those files may have unresolved symbols, so it searches again, etc.
- When there are no more unresolved symbols, it will link the binary using *only the object files it needed to resolve all symbols*

The thing is, at this point the linker does not care about weak symbols that may or may not be also declared somewhere in a strong way. It takes your MQTT library thingy, sees the mqtt_callback_scheduler symbol is not undefined anymore (as that file has an internal weak symbol for it) and as such it never has an excuse to look at your scheduler file as it (presumably) only has functions in it that are otherwise declared as 'weak'. (Your main.cpp also has functions in it that actually are required to be resolved for the link to succeed, so the linker *does* take that into account.)

So how to solve it? You need to tell the linker to also take your scheduler file into account. Easiest way is to introduce an unlinked dummy symbol (which you do with the -u flag) which only can be resolved by taking the scheduler file into account. So you put '-u some_dummy_function_that_is_otherwise_entirely_unused' in the linker flags and put a corresponding 'void some_dummy_function_that_is_otherwise_entirely_unused() {}' in the C file.

It's not possible, something strange is happening. It just doesn't work.

I added dummy empty function inside scheduler files and "target_link_libraries" on [firmware_network CMakeLists], but the weak function continues to be called!

Test 1:
Added "target_link_libraries(${COMPONENT_LIB} INTERFACE "-u mqtt_callback_scheduler_dummy")" to [Firmware_network CMakeLists].
Added "void mqtt_callback_scheduler_dummy();" in [scheduler.h]
Added "void mqtt_callback_scheduler_dummy(){}" in [scheduler.cpp]

Test 2:
Remove line from [scheduler.h].

Test 3:
Change "target_link..." to [firmware_scheduler CMakeLists] rather than [firmware_network CMakeLists], but not happened.

I can compile and run all tests but none worked. In all test, weak function is called.

Note1: Inside of main.cpp, I already call one "init_scheduler()" from scheduler file and this work normally. Only the weak function inside of [firmware_network mqtt.cpp] not working to call strong function in scheduler.cpp.

Note2: In all tests, I do "idf.py fullclean".

[ESP_Sprite]

That's odd... just to check if it's an issue with that file, can you put the strong function in main.cpp rather than an external file and see what happens? (Also, note that the dummy function doesn't need to be mentioned in the .h file as there's no other C files that need to use it.)

[boarchuz]

I suspect mangling is ruining your day, try:

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extern "C" void mqtt_callback_scheduler_dummy(){}

You will likely need to add

Code: Select all

extern "C"

to the default one as well, and you might need the weak attribute in its definition.

[urbanze]

That's odd... just to check if it's an issue with that file, can you put the strong function in main.cpp rather than an external file and see what happens? (Also, note that the dummy function doesn't need to be mentioned in the .h file as there's no other C files that need to use it.)

Putting [mqtt_callback_scheduler] in main.cpp work correctly. When drag this function to [scheduler.cpp] (with or without .h definition), never work. I really need this working

Firmware_network CMakeLists

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idf_component_register(SRCS "net_eth.c" "net_wifi.c" "net_mqtt.c" "net_ota.c" "net_common.c"
                    INCLUDE_DIRS "."
                    EMBED_FILES "certs/ca_certificate.pem" "certs/client_certificate.pem" "certs/client_private_key.pem" "certs/api-user-chain.pem" "certs/dev.cer" "certs/dev.key"
                    REQUIRES mqtt firmware_misc firmware_cjson esp_https_ota)

Firmware_scheduler CMakeLists

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idf_component_register(SRCS "scheduler.cpp" "relay.cpp"
                    INCLUDE_DIRS "."
                    REQUIRES firmware_misc firmware_nvs firmware_cjson firmware_network)

[urbanze]

I suspect mangling is ruining your day, try:

Code: Select all

extern "C" void mqtt_callback_scheduler_dummy(){}

You will likely need to add

Code: Select all

extern "C"

to the default one as well, and you might need the weak attribute in its definition.

I tried but doesnt work too.

[ESP_Sprite]

And what happens if you actually call the dummy function in your main.cpp?

[urbanze]

And what happens if you actually call the dummy function in your main.cpp?

Error. Dummy function exists only in scheduler.cpp.

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../main/main.cpp:20:5: error: 'mqtt_callback_scheduler_dummy' was not declared in this scope
     mqtt_callback_scheduler_dummy();
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../main/main.cpp:20:5: note: suggested alternative: 'mqtt_callback_scheduler'
     mqtt_callback_scheduler_dummy();
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     mqtt_callback_scheduler

[urbanze]

any idea to solve this? I need this feature working to keep modules more independents