what is the #include <Arduino.h>

Projects Programming
1

some codes has #include <Arduino.h>
what the purpose of this library
is that part of Arduino built in or special library to download

Two problems come from LiquidCrystal_I2C library
2

It is part of the IDE. It is the magical include for all the arduino specific stuff and the AVR stuff, too.

3

For an application / program to be recognized by an operating system it HAS to have "main" function.
To get better answer to your question check the attached link.

Short answer - yes Arduino.h header file is ALWAYS there because it contains the supporting info the rest of the "main" needs to function.

https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/main.cpp

4

The gist of main.cpp

#include <Arduino.h>

//Declared weak in Arduino.h to allow user redefinitions.
int atexit(void (*func)()) { return 0; }

// Weak empty variant initialization function.
// May be redefined by variant files.
void initVariant() __attribute__((weak));
void initVariant() { }

int main(void)
{
	init();

	initVariant();

#if defined(USBCON)
	USBDevice.attach();
#endif
	
	setup();
    
	for (;;) {
		loop();
		if (serialEventRun) serialEventRun();
	}
        
	return 0;
}
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5

Another reason for including arduino.h is to get all the #DEFINES for the B000000 nonsense and stuff like INPUT_PULLUP

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