【转载】main函数之前究竟发生了什么？
转载自：http://blog.csdn.net/norains/article/details/6052029
//=====================================================================
//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=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：
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。
