Calling a custom C function from the startup file (STM32F479) CubeMX

[JGrab.2]

Hi, how can I call C function defined as uint32_t getValue(void) inside startup file (in Reset_Handler routine)? What is needed syntax? I tried importing it and setting it as extern (other standard keywords are not working), but nothing seems to work.

I generated the project using CubeMX and use gcc for compilation. The uC is STM32F479VIT (cortex m4).

[Tesla DeLorean]

bl getValue

[JGrab.2]

Thank you for your reply. When I call it I have linking error undefined reference to `getValue'. Do I need to import it somehow into startup file, or mark it in C source to get it to work?

[gbm]

Check how SystemInit() and main() are called from startup and do the same. However, someho I believe you are on a crash course. Do you realize that you have almost no chance to use the result of this function?

[JGrab.2]

Thanks, but as I understand, my function is defined exactly as SystemInit. That is one of the reasons for asking the question. Yes, maybe I omitted something in the process. I thought there is a list of requirements or something that I could use to double-check.

On the other hand, why is almost no chance to use the result? It wouldn't be stored in one of the Rx registers?

[gbm]

The result cannot be stored in memory unless you fiddle with linker script.

[JGrab.2]

I don't need it in RAM, but as I understand it will be written to the Rx register, so until something overrides the value, it can be used. Or am I mistaken?

[KnarfB · Accepted solution]

Correct. An int will be returned in r0, no issues.

Check the map file for getValue or similar entries.

If it is a static function that is never called from C, the compiler might optimize it away at higher opt levels?

hth

KnarfB

[JGrab.2]

Thank you for the map file hint. The name I found inside was _Z15getValuev and when I used it everything worked as intended.

[KnarfB]

Oh, this looks like C++ name mangling. You may prevent this by defining the function "extern C" in C++. You're not mistaken with the registers. It's also easy to define RAM space in assembler if needed.

hth

KnarfB