Standard C program structure

PlatformIO IDE
2

This is the Arduino main function of the AVR core (ATmega…) (see GitHub):

int main(void)
{
	init();

	initVariant();

#if defined(USBCON)
	USBDevice.attach();
#endif
	
	setup();
    
	for (;;) {
		loop();
		if (serialEventRun) serialEventRun();
	}
        
	return 0;
}

If you replace it with your own, several thing will be missing, e.g. the calls to init() and initVariant().

Furthermore, your code is no longer portable to other platforms (e.g. ESP32, STM32) as the main function looks different on each platform.

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