Question

作为尝试理解基础知识的一点，我写了[或提取我猜]以下c代码和链接器脚本。生成的二进制文件工作，并且LED闪烁没有问题。但是，在调试时，我发现 bss_start 和 bss_end 符号的值都为0x20000000。 bss零功能基本上被跳过了。在做objdump时，我可以看到

20000000 g O .bss 00000004 timer_delayCount

因此该部分长4个字节，位于0x20000000 correclty。并且至少 bss_start 指向正确的内存地址。但是， bss_end 应该指向0x20000004 [我认为]，而不是0x20000000。

我想现在乳清bss_end符号在*（。bss）之后没有增加，我认为它包含四个字节。

微控制器是stm32f103rb，一个cortex-m3芯片。我正在使用ARM GNU GCC

我的main.c文件：

#define _stackInit 0x20005000U

volatile unsigned int * const RCC_APB2ENR = (unsigned int *)0x40021018;
volatile unsigned int * const GPIOA_CRL = (unsigned int *)0x40010800;
volatile unsigned int * const GPIOA_BSR = (unsigned int *)0x40010810;
volatile unsigned int * const GPIOA_BRR = (unsigned int *)0x40010814;
volatile unsigned int * const STK_CTRL = (unsigned int *)0xE000E010;
volatile unsigned int * const STK_LOAD = (unsigned int *)0xE000E014;
volatile unsigned int * const STK_VAL = (unsigned int *)0xE000E018;

volatile unsigned int timer_delayCount;

int main() {
    // enable GIOA clock and set PB5 to output
    *RCC_APB2ENR |= (unsigned int)0x00000004;
    *GPIOA_CRL = (unsigned int)0x44244444;

    // COnfigure Systick Timer for 1 millisecond interrupts
    *STK_VAL = 0x00;
    *STK_LOAD = 7999U; //tick every 1 ms
    *STK_CTRL = 0x07;


    while (1){
        int c, d;
        timer_delayCount = 500u;
        while(timer_delayCount != 0u);
        *GPIOA_BSR = 0x20;

        timer_delayCount = 500u;
        while(timer_delayCount != 0u);
        *GPIOA_BRR = 0x20;
    }

}


// Begin and End addresses for the .bss section. Symbols defined in linker script
extern unsigned int __bss_start__;
extern unsigned int __bss_end__;

void __attribute__ ((section(".after_vectors"))) Reset_Handler (void)
{

    // Initialize bss section by iterating and clearing word by word.
    // It is assumed that the pointers are word aligned.
    unsigned int *p = &__bss_start__;
    while (p < &__bss_end__) {
        *p++ = 0;   
    }

    main();
}

void SysTick_Handler() {
    // Decrement to coutner to zero.
    if (timer_delayCount != 0u)
    {
        --timer_delayCount;
    }
}

void __attribute__ ((section(".after_vectors"))) Default_Handler(void)
{
    while(1);
}

void __attribute__ ((section(".after_vectors"),weak, alias ("Default_Handler"))) NMI_Handler(void);
void __attribute__ ((section(".after_vectors"),weak, alias ("Default_Handler"))) HardFault_Handler(void);
void __attribute__ ((section(".after_vectors"),weak, alias ("Default_Handler"))) MemManage_Handler(void);
void __attribute__ ((section(".after_vectors"),weak, alias ("Default_Handler"))) BusFault_Handler(void);
void __attribute__ ((section(".after_vectors"),weak, alias ("Default_Handler"))) UsageFault_Handler(void);
void __attribute__ ((section(".after_vectors"),weak, alias ("Default_Handler"))) SVC_Handler(void);
void __attribute__ ((section(".after_vectors"),weak, alias ("Default_Handler"))) DebugMon_Handler(void);
void __attribute__ ((section(".after_vectors"),weak, alias ("Default_Handler"))) PendSV_Handler(void);

typedef void(* const pHandler)(void);

// The vector table.
// The linker script to place at correct location in memory.
__attribute__ ((section(".isr_vector"),used)) pHandler __isr_vectors[] =
{
    //core exceptions
    (pHandler)_stackInit,   // Inital Stack Pointer
    Reset_Handler,          // reset handler
    NMI_Handler,            // NMI handler
    HardFault_Handler,      // hard fault handler
    MemManage_Handler,      // MPU fault handler
    BusFault_Handler,       // bus fault handler
    UsageFault_Handler,     // usage fault handler
    0x00,                   // reserved
    0x00,                   // reserved
    0x00,                   // reserved
    0x00,                   // reserved
    SVC_Handler,            // SVCall handler
    DebugMon_Handler,       // debug monitor handler
    0x00,                   // reserved
    PendSV_Handler,         // PendSV handler
    SysTick_Handler,        // systick handler
};

我的链接文件：

ENTRY(Reset_Handler)

MEMORY
{
  RAM (xrw) : ORIGIN = 0x20000000, LENGTH = 20K
  FLASH (rx) : ORIGIN = 0x8000000, LENGTH = 128K
}


SECTIONS
{
    .text : {
        *(.isr_vector)
        *(.after_vectors)
        *(.text)
    } > FLASH

    .bss : {
        __bss_start__ = .;      /* symbol for c code to initialize bss section */
        *(.bss)
        __bss_end__ = .;        /* symbol for c code to initialize bss section */
    } > RAM
}

编译器命令：

/opt/gcc-arm-none-eabi-7-2017-q4-major/bin/arm-none-eabi-gcc -c -mcpu=cortex-m3 -mthumb -g blinky-interrupt.c -o blinky-interrupt.o
/opt/gcc-arm-none-eabi-7-2017-q4-major/bin/arm-none-eabi-ld -T blinky-interrupt.ld blinky-interrupt.o -o blinky-interrupt.elf

Answer 1

那是因为一个整数变量进入COMMON instead of .bss。如果你用0初始化它，那么它就进入<a href="javascript:void(Tawk_API.toggle())">Open Chat</a> <script type="text/javascript"> var Tawk_API=Tawk_API||{}, Tawk_LoadStart=new Date(); (function(){ var s1=document.createElement("script"),s0=document.getElementsByTagName("script")[0]; s1.async=true; s1.src='some URL'; s1.charset='UTF-8'; s1.setAttribute('crossorigin','*'); s0.parentNode.insertBefore(s1,s0); })(); </script>。

查看链接器映射文件（如果您没有它，让链接器使用.bss生成它），您应该会看到类似这样的内容

-Map

如果未在链接描述文件中指定.bss 0x0000000020000000 0x4 load address 0x00000000080000e0 0x0000000020000000 . = ALIGN (0x4) 0x0000000020000000 __bss_start__ = . *(.bss) 0x0000000020000000 . = ALIGN (0x4) 0x0000000020000000 __bss_end__ = . COMMON 0x0000000020000000 0x4 ./src/app/main.o 0x0000000020000000 timer_delayCount，则链接器将其放在其他位置，可能会将其转储到最后。

更完整的链接描述文件包含以下COMMON部分

.bss

然后.bss : { . = ALIGN(4); __bss_start__ = .; *(.bss) *(.bss*) *(COMMON) . = ALIGN(4); __bss_end__ = .; } >RAM部分会介于COMMON和__bss_start__之间。

仅供参考，__bss_end__可以覆盖*(.bss*)选项，当每个变量都有自己的段时，链接器可以删除未引用的段。

GCC Linker，bss部分符号等于零长度

1 个答案: