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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：
[c-sharp] view plain copy
- ;/*****************************************************************************/
- ;/* 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=3
- Stack_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_base
- Heap_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 Handler
- Reset_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 .
- ENDP
- HardFault_Handler/
- PROC
- EXPORT HardFault_Handler [WEAK]
- B .
- ENDP
- MemManage_Handler/
- PROC
- EXPORT MemManage_Handler [WEAK]
- B .
- ENDP
- BusFault_Handler/
- PROC
- EXPORT BusFault_Handler [WEAK]
- B .
- ENDP
- UsageFault_Handler/
- PROC
- EXPORT UsageFault_Handler [WEAK]
- B .
- ENDP
- SVC_Handler PROC
- EXPORT SVC_Handler [WEAK]
- B .
- ENDP
- DebugMon_Handler/
- PROC
- EXPORT DebugMon_Handler [WEAK]
- B .
- ENDP
- PendSV_Handler PROC
- EXPORT PendSV_Handler [WEAK]
- B .
- ENDP
- SysTick_Handler PROC
- EXPORT SysTick_Handler [WEAK]
- B .
- ENDP
- Default_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_IRQHandler
- PVD_IRQHandler
- TAMPER_IRQHandler
- RTC_IRQHandler
- FLASH_IRQHandler
- RCC_IRQHandler
- EXTI0_IRQHandler
- EXTI1_IRQHandler
- EXTI2_IRQHandler
- EXTI3_IRQHandler
- EXTI4_IRQHandler
- DMAChannel1_IRQHandler
- DMAChannel2_IRQHandler
- DMAChannel3_IRQHandler
- DMAChannel4_IRQHandler
- DMAChannel5_IRQHandler
- DMAChannel6_IRQHandler
- DMAChannel7_IRQHandler
- ADC_IRQHandler
- USB_HP_CAN_TX_IRQHandler
- USB_LP_CAN_RX0_IRQHandler
- CAN_RX1_IRQHandler
- CAN_SCE_IRQHandler
- EXTI9_5_IRQHandler
- TIM1_BRK_IRQHandler
- TIM1_UP_IRQHandler
- TIM1_TRG_COM_IRQHandler
- TIM1_CC_IRQHandler
- TIM2_IRQHandler
- TIM3_IRQHandler
- TIM4_IRQHandler
- I2C1_EV_IRQHandler
- I2C1_ER_IRQHandler
- I2C2_EV_IRQHandler
- I2C2_ER_IRQHandler
- SPI1_IRQHandler
- SPI2_IRQHandler
- USART1_IRQHandler
- USART2_IRQHandler
- USART3_IRQHandler
- EXTI15_10_IRQHandler
- RTCAlarm_IRQHandler
- USBWakeUp_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：
[c-sharp] view plain copy
- 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： [cpp] view plain copy
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。
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比8位单片机复杂多了