7 指针

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李延 LV6 2022-06-07

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# 1 指针

指针使用的是leaq指令

```c
#include <stdio.h>

int print(int *q) {

printf("leaq %llu\n", q);
    printf("value %d\n", *q);
    return 0;
}

int main() {
    int q = 55;
    print(&q);
    return 0;
}
```

```asm
    .file  "leaq.c"
   .section   .rodata.str1.1,"aMS",@progbits,1
.LC0:
   .string    "leaq %llu\n"
.LC1:
   .string    "value %d\n"
   .text
   .globl print
   .type  print, @function
print:
.LFB11:
   .cfi_startproc
   pushq  %rbx
   .cfi_def_cfa_offset 16
   .cfi_offset 3, -16
   movq   %rdi, %rbx
   movq   %rdi, %rsi
   movl   $.LC0, %edi
   movl   $0, %eax
   call   printf
   movl   (%rbx), %esi
   movl   $.LC1, %edi
   movl   $0, %eax
   call   printf
   movl   $0, %eax
   popq   %rbx
   .cfi_def_cfa_offset 8
   ret
   .cfi_endproc
.LFE11:
   .size  print, .-print
   .globl main
   .type  main, @function
main:
.LFB12:
   .cfi_startproc
   subq   $24, %rsp
   .cfi_def_cfa_offset 32
   movl   $55, 12(%rsp)
   leaq   12(%rsp), %rdi
   call   print
   movl   $0, %eax
   addq   $24, %rsp
   .cfi_def_cfa_offset 8
   ret
   .cfi_endproc
.LFE12:
   .size  main, .-main
   .ident "GCC: (GNU) 4.8.5 20150623 (Red Hat 4.8.5-44)"
   .section   .note.GNU-stack,"",@progbits
```

# 2 解析

这里我们主要关注3点

1. &q 获取 变量q的地址，对应汇编

```asm
   leaq   12(%rsp), %rdi
```

通过leaq 将地址加载到rdi中

2.     printf("leaq %llu\n", q);

这里我们没有加指针前面的* ，是直接打印的地址，我们看到

```asm
   movq   %rdi, %rsi
```

3.     printf("value %d\n", *q);

这里打印的是 *q指向的值，

```asm
   movl   (%rbx), %esi
```

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