在Arduino IDE的安装目录下可以找到main.cpp这个代码模板文件，main函数就位于此。

文件位置：{Arduino安装目录}\hardware\arduino\avr\cores\arduino\main.cpp，内容如下：

/*

  main.cpp - Main loop for Arduino sketches

  Copyright (c) 2005-2013 Arduino Team.  All right reserved.

  This library is free software; you can redistribute it and/or

  modify it under the terms of the GNU Lesser General Public

  License as published by the Free Software Foundation; either

  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,

  but WITHOUT ANY WARRANTY; without even the implied warranty of

  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public

  License along with this library; if not, write to the Free Software

  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

*/

#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() { }

void setupUSB() __attribute__((weak));

void setupUSB() { }

int main(void)

{

    init();        //硬件初始化

    initVariant();  //特有硬件初始化。因为不同的开发板有自己独特的初始化逻辑。

#if defined(USBCON)

    USBDevice.attach();

#endif

    setup();

    for (;;) {

        loop();

        if (serialEventRun) serialEventRun();

    }

    return 0;

}