main函数之前究竟发生了什么？

发布时间：2020-12-15 07:02:07 所属栏目：百科来源：网络整理

导读：http://blog.csdn.net/norains/article/details/6052029 main函数之前究竟发生了什么？分类：?STM32F?开发工具 2010-12-03 10:12 ? 5878人阅读 ? 评论(11) ? 收藏 ? 举报 flash import wizard linker 编译器 exception //=================================

http://blog.csdn.net/norains/article/details/6052029

main函数之前究竟发生了什么？

分类：?STM32F?开发工具 2010-12-03 10:12? 5878人阅读? 评论(11)? 收藏? 举报

 
 flash 
 import 
 wizard 
 linker 
 编译器 
 exception 

//=====================================================================
//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,?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?????SVC_Handler???????????????;?SVCall?Handler??
 ????????????????DCD?????DebugMon_Handler??????????;?Debug?Monitor?Handler??
 ????????????????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???????.??
 HardFault_Handler/??
 ????????????????PROC??
 ????????????????EXPORT??HardFault_Handler?????????[WEAK]??
 ????????????????B???????.??
 ????????????????ENDP??
 MemManage_Handler/??
 ????????????????PROC??
 ????????????????EXPORT??MemManage_Handler?????????[WEAK]??
 BusFault_Handler/??
 ????????????????EXPORT??BusFault_Handler??????????[WEAK]??
 UsageFault_Handler/??
 ????????????????EXPORT??UsageFault_Handler????????[WEAK]??
 SVC_Handler?????PROC??
 ????????????????EXPORT??SVC_Handler???????????????[WEAK]??
 DebugMon_Handler/??
 ????????????????EXPORT??DebugMon_Handler??????????[WEAK]??
 PendSV_Handler??PROC??
 ????????????????EXPORT??PendSV_Handler????????????[WEAK]??
 SysTick_Handler?PROC??
 ????????????????EXPORT??SysTick_Handler???????????[WEAK]??
 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??
 ???
 ????????????????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?????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：

copy 
     
    

Reset_Handler???PROC??

????????????????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]? 
    copy

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。

（编辑：李大同）

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