GDB报告静态C变量的意外内存

os-dev tutorial .
here

代码如下所示:

#define BUFSIZE 255
static char buf[BUFSIZE];

void foo() {
  // Making sure it's all zero.
  for (int i = 0; i < BUFSIZE; i++) buf[i] = 0;

  // Setting first char:
  buf[0] = 'a';

  // >> insert breakpoint right after setting the char <<

  // Prints 'a'.
  printf("%s", buf);
}

如果我在标记点处放置断点并用 p buf p &buf 我得到了一些看起来不正确的东西,有两个原因:

1. 如果我做一个 char* p_buf = buf 我和你核对了地址 p p_buf 它给了我一个完全不同的地址,在执行过程中是稳定的(另一个不是)。然后我用 x /255b 0x____ a

2. 下一个命令( printf("%s", buf); 一 .

我应该从哪里开始调试这个?

有关编译条件的详细信息:

• -g -Wall -Wextra -pedantic -nostdlib -nostdinc -fno-builtin -fno-stack-protector -nostartfiles -nodefaultlibs -m32
• qemu-系统-i386

GDB输出示例:

(gdb) p buf
$1 = "dfghjkl;'`\000\\zxcvbnm,./\000*\000 ", '\000' <repeats 198 times>...
(gdb) p p_buf
$2 = 0x40c0 <buf+224> "a"
(gdb) p &buf
$3 = (char (*)[255]) 0x3fe0 <buf>
(gdb) info address buf
Symbol "buf" is static storage at address 0x3fe0.

更新2:

反汇编显示差异的代码版本:

; void foo
0x19f1 <foo>            push   %ebp
0x19f2 <foo+1>          mov    %esp,%ebp
0x19f4 <foo+3>          sub    $0x10,%esp

; char* p_buf = char_buf; --> `p &char_buf` is 0x4040 (incorrect) but `p p_buf` is 0x4100
0x19f7 <foo+6>          movl   $0x4100,-0x4(%ebp)

; void* p_p_buf = (void*)p_buf; --> `p p_p_buf` gives 0x4100
0x19fe <foo+13>         mov    -0x4(%ebp),%eax
0x1a01 <foo+16>         mov    %eax,-0x8(%ebp)

; void* p_char_buf = (void*)&char_buf; --> `p p_char_buf` gives 0x4100
0x1a04 <foo+19>         movl   $0x4100,-0xc(%ebp)

; char_buf[0] = 'a'; --> correct address
0x1a0b <foo+26>         movb   $0x61,0x4100

; char_buf[1] = 'b'; --> correct address (asking `p &char_buf` here is still incorrectly 0x4040)
0x1a12 <foo+33>         movb   $0x62,0x4101

; void foo return
0x1a19 <foo+40>         nop
0x1a1a <foo+41>         leave
0x1a1b <foo+42>         ret

我的 Makefile 对于构建项目,如下所示:

C_SOURCES = $(wildcard kernel/*.c drivers/*.c)
C_HEADERS = $(wildcard kernel/*.h drivers/*.h)
OBJ = ${C_SOURCES:.c=.o kernel/interrupt_table.o}
CC = /home/itarato/code/os/i386elfgcc/bin/i386-elf-gcc
# GDB = /home/itarato/code/os/i386elfgcc/bin/i386-elf-gdb
GDB = /usr/bin/gdb
CFLAGS = -g -Wall -Wextra -ffreestanding -fno-exceptions -pedantic -fno-builtin -fno-stack-protector -nostartfiles -nodefaultlibs -m32
QEMU = qemu-system-i386

os-image.bin: boot/boot.bin kernel.bin
    cat $^ > $@

kernel.bin: boot/kernel_entry.o ${OBJ}
    i386-elf-ld -o $@ -Ttext 0x1000 $^ --oformat binary

kernel.elf: boot/kernel_entry.o ${OBJ}
    i386-elf-ld -o $@ -Ttext 0x1000 $^

kernel.dis: kernel.bin
    ndisasm -b 32 $< > $@

run: os-image.bin
    ${QEMU} -drive format=raw,media=disk,file=$<,index=0,if=floppy

debug: os-image.bin kernel.elf
    ${QEMU} -s -S -drive format=raw,media=disk,file=$<,index=0,if=floppy &
    ${GDB} -ex "target remote localhost:1234" -ex "symbol-file kernel.elf" -ex "tui enable" -ex "layout split" -ex "focus cmd"

%.o: %.c ${C_HEADERS}
    ${CC} ${CFLAGS} -c $< -o $@

%.o: %.asm
    nasm $< -f elf -o $@

%.bin: %.asm
    nasm $< -f bin -o $@

build: os-image.bin
    echo Pass

clean:
    rm -rf *.bin *.o *.dis *.elf
    rm -rf kernel/*.o boot/*.bin boot/*.o