GCC 链接器，bss 部分符号等于零长度

Question

为了尝试理解基础知识，我编写了[或提取我猜想]以下 c 代码和链接器脚本。生成的二进制文件可以正常工作，并且 LED 会毫无问题地闪烁。但是，在调试时，我发现 bss_start 和 bss_end 符号的值都为 0x20000000。 bss归零功能基本被跳过。在做 objdump 时，我可以看到

20000000 g O .bss 00000004 timer_delayCount

所以该部分有 4 个字节长，位于 0x20000000 正确位置。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。

我现在想知道 bss_end 符号在我认为包含四个字节的 *(.bss) 被放置之后没有增加。

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
};

我的链接器文件：

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

这是因为一个单元化变量进入了COMMON instead of .bss。如果你用 0 初始化它，那么它会进入.bss。

查看链接器映射文件（如果没有，让链接器使用-Map 生成它），您应该会看到类似这样的内容

.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

当链接器脚本中没有指定COMMON 时，链接器会将它放在其他地方，可能会在最后转储它。

更完整的链接描述文件有以下.bss 部分

  .bss :
  {
    . = ALIGN(4);
    __bss_start__ = .;
    *(.bss)
    *(.bss*)
    *(COMMON)
    . = ALIGN(4);
    __bss_end__ = .;
  } >RAM

那么COMMON 部分将介于__bss_start__ 和__bss_end__ 之间。

仅供参考，*(.bss*) 用于覆盖-fdata-sections 选项，当每个变量都有自己的段时，以便链接器可以删除未引用的段。