main函数之前究竟发生了什么？

//=====================================================================
//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。