main函数之前究竟发生了什么？

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//=====================================================================
//TITLE:
//    main函数之前究竟发生了什么？
//AUTHOR:
//    norains
//DATE:
//    Friday 3-December-2010
//Environment:
//    MDK 4.1
//=====================================================================

当使用MFC时，我们会认为入口函数是:: InitInstance；当使用WIN32 API时，我们会认为入口函数是WinMain；当我们写个纯粹的C++程序时，入口函数又变成了main；可当我们进入到嵌入式领域，却发现main函数之前还有一段启动代码！

究竟在main函数之前，发生了什么？如果你觉得已经明白了这个过程，那么请试着回答这个问题：程序是存储到FLASH中的，运行时static变量地址是指向RAM，那么这些static变量的初始值是如何映射到RAM中的？

我们以STM32F10x的启动代码为例，先看看其完整的源码：

;/*****************************************************************************/;/* STM32F10x.s: Startup file for ST STM32F10x device series */;/*****************************************************************************/;/* <<< Use Configuration Wizard in Context Menu >>> */;/*****************************************************************************/;/* This file is part of the uVision/ARM development tools. */;/* Copyright (c) 2005-2007 Keil Software. All rights reserved. */;/* This software may only be used under the terms of a valid, current, */;/* end user licence from KEIL for a compatible version of KEIL software */;/* development tools. Nothing else gives you the right to use this software. */;/*****************************************************************************/;// <h> Stack Configuration;// <o> Stack Size (in Bytes) <0x0-0xFFFFFFFF:8>;// </h>Stack_Size EQU 0x00000200 AREA STACK, NOINIT, READWRITE, ALIGN=3Stack_Mem SPACE Stack_Size__initial_sp;// <h> Heap Configuration;// <o> Heap Size (in Bytes) <0x0-0xFFFFFFFF:8>;// </h>Heap_Size EQU 0x00000000 AREA HEAP, NOINIT, READWRITE, ALIGN=3__heap_baseHeap_Mem SPACE Heap_Size__heap_limit PRESERVE8 THUMB; Vector Table Mapped to Address 0 at Reset AREA RESET, DATA, READONLY EXPORT __Vectors__Vectors DCD __initial_sp ; Top of Stack DCD Reset_Handler ; Reset Handler DCD NMI_Handler ; NMI Handler DCD HardFault_Handler ; Hard Fault Handler DCD MemManage_Handler ; MPU Fault Handler DCD BusFault_Handler ; Bus Fault Handler DCD UsageFault_Handler ; Usage Fault Handler DCD 0 ; Reserved DCD 0 ; Reserved DCD 0 ; Reserved DCD 0 ; Reserved DCD SVC_Handler ; SVCall Handler DCD DebugMon_Handler ; Debug Monitor Handler DCD 0 ; Reserved DCD PendSV_Handler ; PendSV Handler DCD SysTick_Handler ; SysTick Handler ; External Interrupts DCD WWDG_IRQHandler ; Window Watchdog DCD PVD_IRQHandler ; PVD through EXTI Line detect DCD TAMPER_IRQHandler ; Tamper DCD RTC_IRQHandler ; RTC DCD FLASH_IRQHandler ; Flash DCD RCC_IRQHandler ; RCC DCD EXTI0_IRQHandler ; EXTI Line 0 DCD EXTI1_IRQHandler ; EXTI Line 1 DCD EXTI2_IRQHandler ; EXTI Line 2 DCD EXTI3_IRQHandler ; EXTI Line 3 DCD EXTI4_IRQHandler ; EXTI Line 4 DCD DMAChannel1_IRQHandler ; DMA Channel 1 DCD DMAChannel2_IRQHandler ; DMA Channel 2 DCD DMAChannel3_IRQHandler ; DMA Channel 3 DCD DMAChannel4_IRQHandler ; DMA Channel 4 DCD DMAChannel5_IRQHandler ; DMA Channel 5 DCD DMAChannel6_IRQHandler ; DMA Channel 6 DCD DMAChannel7_IRQHandler ; DMA Channel 7 DCD ADC_IRQHandler ; ADC DCD USB_HP_CAN_TX_IRQHandler ; USB High Priority or CAN TX DCD USB_LP_CAN_RX0_IRQHandler ; USB Low Priority or CAN RX0 DCD CAN_RX1_IRQHandler ; CAN RX1 DCD CAN_SCE_IRQHandler ; CAN SCE DCD EXTI9_5_IRQHandler ; EXTI Line 9..5 DCD TIM1_BRK_IRQHandler ; TIM1 Break DCD TIM1_UP_IRQHandler ; TIM1 Update DCD TIM1_TRG_COM_IRQHandler ; TIM1 Trigger and Commutation DCD TIM1_CC_IRQHandler ; TIM1 Capture Compare DCD TIM2_IRQHandler ; TIM2 DCD TIM3_IRQHandler ; TIM3 DCD TIM4_IRQHandler ; TIM4 DCD I2C1_EV_IRQHandler ; I2C1 Event DCD I2C1_ER_IRQHandler ; I2C1 Error DCD I2C2_EV_IRQHandler ; I2C2 Event DCD I2C2_ER_IRQHandler ; I2C2 Error DCD SPI1_IRQHandler ; SPI1 DCD SPI2_IRQHandler ; SPI2 DCD USART1_IRQHandler ; USART1 DCD USART2_IRQHandler ; USART2 DCD USART3_IRQHandler ; USART3 DCD EXTI15_10_IRQHandler ; EXTI Line 15..10 DCD RTCAlarm_IRQHandler ; RTC Alarm through EXTI Line DCD USBWakeUp_IRQHandler ; USB Wakeup from suspend AREA |.text|, CODE, READONLY; Reset HandlerReset_Handler PROC EXPORT Reset_Handler [WEAK] IMPORT __main LDR R0, =__main BX R0 ENDP; Dummy Exception Handlers (infinite loops which can be modified) NMI_Handler PROC EXPORT NMI_Handler [WEAK] B . ENDPHardFault_Handler/ PROC EXPORT HardFault_Handler [WEAK] B . ENDPMemManage_Handler/ PROC EXPORT MemManage_Handler [WEAK] B . ENDPBusFault_Handler/ PROC EXPORT BusFault_Handler [WEAK] B . ENDPUsageFault_Handler/ PROC EXPORT UsageFault_Handler [WEAK] B . ENDPSVC_Handler PROC EXPORT SVC_Handler [WEAK] B . ENDPDebugMon_Handler/ PROC EXPORT DebugMon_Handler [WEAK] B . ENDPPendSV_Handler PROC EXPORT PendSV_Handler [WEAK] B . ENDPSysTick_Handler PROC EXPORT SysTick_Handler [WEAK] B . ENDPDefault_Handler PROC EXPORT WWDG_IRQHandler [WEAK] EXPORT PVD_IRQHandler [WEAK] EXPORT TAMPER_IRQHandler [WEAK] EXPORT RTC_IRQHandler [WEAK] EXPORT FLASH_IRQHandler [WEAK] EXPORT RCC_IRQHandler [WEAK] EXPORT EXTI0_IRQHandler [WEAK] EXPORT EXTI1_IRQHandler [WEAK] EXPORT EXTI2_IRQHandler [WEAK] EXPORT EXTI3_IRQHandler [WEAK] EXPORT EXTI4_IRQHandler [WEAK] EXPORT DMAChannel1_IRQHandler [WEAK] EXPORT DMAChannel2_IRQHandler [WEAK] EXPORT DMAChannel3_IRQHandler [WEAK] EXPORT DMAChannel4_IRQHandler [WEAK] EXPORT DMAChannel5_IRQHandler [WEAK] EXPORT DMAChannel6_IRQHandler [WEAK] EXPORT DMAChannel7_IRQHandler [WEAK] EXPORT ADC_IRQHandler [WEAK] EXPORT USB_HP_CAN_TX_IRQHandler [WEAK] EXPORT USB_LP_CAN_RX0_IRQHandler [WEAK] EXPORT CAN_RX1_IRQHandler [WEAK] EXPORT CAN_SCE_IRQHandler [WEAK] EXPORT EXTI9_5_IRQHandler [WEAK] EXPORT TIM1_BRK_IRQHandler [WEAK] EXPORT TIM1_UP_IRQHandler [WEAK] EXPORT TIM1_TRG_COM_IRQHandler [WEAK] EXPORT TIM1_CC_IRQHandler [WEAK] EXPORT TIM2_IRQHandler [WEAK] EXPORT TIM3_IRQHandler [WEAK] EXPORT TIM4_IRQHandler [WEAK] EXPORT I2C1_EV_IRQHandler [WEAK] EXPORT I2C1_ER_IRQHandler [WEAK] EXPORT I2C2_EV_IRQHandler [WEAK] EXPORT I2C2_ER_IRQHandler [WEAK] EXPORT SPI1_IRQHandler [WEAK] EXPORT SPI2_IRQHandler [WEAK] EXPORT USART1_IRQHandler [WEAK] EXPORT USART2_IRQHandler [WEAK] EXPORT USART3_IRQHandler [WEAK] EXPORT EXTI15_10_IRQHandler [WEAK] EXPORT RTCAlarm_IRQHandler [WEAK] EXPORT USBWakeUp_IRQHandler [WEAK]WWDG_IRQHandlerPVD_IRQHandlerTAMPER_IRQHandlerRTC_IRQHandlerFLASH_IRQHandlerRCC_IRQHandlerEXTI0_IRQHandlerEXTI1_IRQHandlerEXTI2_IRQHandlerEXTI3_IRQHandlerEXTI4_IRQHandlerDMAChannel1_IRQHandlerDMAChannel2_IRQHandlerDMAChannel3_IRQHandlerDMAChannel4_IRQHandlerDMAChannel5_IRQHandlerDMAChannel6_IRQHandlerDMAChannel7_IRQHandlerADC_IRQHandlerUSB_HP_CAN_TX_IRQHandlerUSB_LP_CAN_RX0_IRQHandlerCAN_RX1_IRQHandlerCAN_SCE_IRQHandlerEXTI9_5_IRQHandlerTIM1_BRK_IRQHandlerTIM1_UP_IRQHandlerTIM1_TRG_COM_IRQHandlerTIM1_CC_IRQHandlerTIM2_IRQHandlerTIM3_IRQHandlerTIM4_IRQHandlerI2C1_EV_IRQHandlerI2C1_ER_IRQHandlerI2C2_EV_IRQHandlerI2C2_ER_IRQHandlerSPI1_IRQHandlerSPI2_IRQHandlerUSART1_IRQHandlerUSART2_IRQHandlerUSART3_IRQHandlerEXTI15_10_IRQHandlerRTCAlarm_IRQHandlerUSBWakeUp_IRQHandler B . ENDP ALIGN; User Initial Stack & Heap IF :DEF:__MICROLIB EXPORT __initial_sp EXPORT __heap_base EXPORT __heap_limit ELSE ; IMPORT __use_two_region_memory EXPORT __user_initial_stackheap__user_initial_stackheap LDR R0, = Heap_Mem LDR R1, =(Stack_Mem + Stack_Size) LDR R2, = (Heap_Mem + Heap_Size) LDR R3, = Stack_Mem BX LR ALIGN ENDIF END

一些旁枝末节和本文的主题无关，我们先不要去理会，只需要知道这个启动代码是设置向量表，然后跳转到__main函数。跳转具体到代码段部分如下：
Reset_Handler PROC EXPORT Reset_Handler [WEAK] IMPORT __main LDR R0, =__main BX R0 ENDP

当大家看到__main函数时，估计应该有不少人认为这个是main函数的别名或是编译之后的名字，否则在启动代码中再也无法找到和main相关的字眼了。可事实是，__main和main是完全两个不同的函数！如果这还不足以让你诧异，那么再告诉你另一个事实：你无法找到__main代码，因为这个是编译器自动创建的！

如果你对此还半信半疑，可以查看MDK的文档，会发现有这么一句说明：It is automatically created by the linker when it sees a definition of main()。简单点来说，当编译器发现定义了main函数，那么就会自动创建__main。

__main函数的出身我们基本搞清楚了，那么现在的问题是，它和main又有什么关系呢？其实__main主要做这么两件事：初始化C/C++所需的资源，调用main函数。初始化先暂时不说，但“调用main函数”这个功能能够让我们解决为什么之前的启动代码调用的是__main，最后却能转到main函数的疑惑。

初始化C/C++所需的资源，如果脱离了具体情况，实在很难解释清楚，还是先看看编译出来的汇编代码片段：

凡是以__rt开头的，都是用来初始化C/C++运行库的；而以__scatterload开头，则是根据离散文件的定义，将代码中的变量映射到相应的内存位置。而回答本文开头的问题，关键就在于__scatterload_copy函数！

我们在STM32F10x平台举个简单的例子，首先要明白一点是，该平台的的flash地址以0x08000000为起始，主要是存储代码；而SRAM是以0x20000000为起始，也就是内存。然后C/C++有这么一行代码：static int g_iVal = 12;

当我们程序开始跑起来的时候，通过IDE发现，g_iVal被映射到内存地址0x20000000，数值为一个随机数0xFFFFBE00，而不是代码中设置的12，如图：

我们让程序继续往下执行，当执行完毕__scatterload_copy之后，我们发现g_iVal这时候已经变成我们所需要的初始值了：

接下来就是C/C++库的初始化，最后就是进入到main函数，而此时已经是万事俱备。

如果大家只是局限于桌面应用的开发，因为编译出来的程序带有很多操作系统的特性，所以会给我们理解程序的运行带来很大的迷惑，也只有步入嵌入式领域，没有操作系统的支持下赤裸裸地奔跑在CPU之上，才能更好地理解软件是如何运行起来的，也只有这时候我们才能够更清楚知道，原来main函数并不是起点。