第1节 Gcc and Cgo
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[TOC]
我们默认为是C语言是占4个字节，这个和Go语言是不一样的。
在Go语言中，int默认是8个字节
- int32是四个字节
- int64是八个字节
- int是八个字节
/*************************************************************************
> File Name: a.go
> Author: smile
> Mail: 3293172751nss@gmail.com
> Created Time: Thu 30 Jun 2022 01:13:35 AM PDT
************************************************************************/
package main
import(
"fmt"
)
func main(){
var(
a = 40000
b = 50000
)
var c int
var d int32
c = 50000
d = 50000
fmt.Println(a*a)
fmt.Println(b*b)
fmt.Println(c*c)
fmt.Println(d*d)
}
我们对上面的代码进行编译：
root@ubuntu:/c# go run a.go
1600000000
2500000000
2500000000
-1794967296
对于C语言来说
编译为汇编文件
root@ubuntu:/c# gcc -S a.c
root@ubuntu:/c# cat a.s
.file "a.c"
.text
.section .rodata
.LC0:
.string "%d\n,%d\n"
.LC1:
.string "int c = %d\n,int64_t d = %lld\n"
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
endbr64
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $32, %rsp
movl $50000, -20(%rbp)
movl $40000, -16(%rbp)
movl -16(%rbp), %eax
imull %eax, %eax
movl %eax, %edx
movl -20(%rbp), %eax
imull %eax, %eax
movl %eax, %esi
leaq .LC0(%rip), %rdi
movl $0, %eax
call printf@PLT
movq -8(%rbp), %rax
imulq %rax, %rax
movq %rax, %rdx
movl -12(%rbp), %eax
imull %eax, %eax
movl %eax, %esi
leaq .LC1(%rip), %rdi
movl $0, %eax
call printf@PLT
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0"
.section .note.GNU-stack,"",@progbits
.section .note.gnu.property,"a"
.align 8
.long 1f - 0f
.long 4f - 1f
.long 5
0:
.string "GNU"
1:
.align 8
.long 0xc0000002
.long 3f - 2f
2:
.long 0x3
3:
.align 8
4:
编译为二进制代码
root@ubuntu:/c# gcc -Og -c a.c
root@ubuntu:/c# cat a.o E¹^_º5¿¸蹺H5¿¸.ed
,%d
int c = %d
,int64_t d = %lld
GCC: (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0GNUzRx
H R*a.c.LC0.LC1main_GLOBAL_OFFSET_TABLE___printf_chk ÿÿ$ÿÿ5
ÿÿDÿÿ .symtab.strtab.shstrtab.rela.text.data.bss.rodata.str1.1.comment.note.GNU-stack.note.gnu.property.rela.eh_frame @Rւ`
&12&@0¸,Il@8
8h
7P{root@ubuntu:/c# XshellXshellXshellXshellXshell
XshellXshellXshellXshellXshell: command not found
.o后缀的文件是二进制格式的，没有查看意义，我们可以得出：机器执行的程序只是一个字节序列， 它是对一系列指令的编码。机器对产生这些指令的源代码几乎一无所知。
如果需要显示出机器代码文件的内容，我们需要用到反汇编器（这个程序非常有用），在Linux中-d指令标志的objdump 可以充当这个角色
root@ubuntu:/c# objdump -d a.o
a.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <main>:
0: f3 0f 1e fa endbr64
4: 48 83 ec 08 sub $0x8,%rsp
8: b9 00 10 5e 5f mov $0x5f5e1000,%ecx
d: ba 00 f9 02 95 mov $0x9502f900,%edx
12: 48 8d 35 00 00 00 00 lea 0x0(%rip),%rsi # 19 <main+0x19>
19: bf 01 00 00 00 mov $0x1,%edi
1e: b8 00 00 00 00 mov $0x0,%eax
23: e8 00 00 00 00 callq 28 <main+0x28>
28: b9 00 00 00 00 mov $0x0,%ecx
2d: ba 00 00 00 00 mov $0x0,%edx
32: 48 8d 35 00 00 00 00 lea 0x0(%rip),%rsi # 39 <main+0x39>
39: bf 01 00 00 00 mov $0x1,%edi
3e: b8 00 00 00 00 mov $0x0,%eax
43: e8 00 00 00 00 callq 48 <main+0x48>
48: b8 00 00 00 00 mov $0x0,%eax
4d: 48 83 c4 08 add $0x8,%rsp
51: c3 retq
对于Go语言我们同样可以使用反汇编命令
root@ubuntu:/c# go build -o a a.go
root@ubuntu:/c# ./a
1600000000
2500000000
2500000000
-1794967296
root@ubuntu:/c# objdump -d a | tail -n 10
48d03c: 00 00
48d03e: 48 c7 44 24 20 01 00 movq $0x1,0x20(%rsp)
48d045: 00 00
48d047: e8 a4 96 ff ff callq 4866f0 <fmt.Fprintln>
48d04c: 48 8b ac 24 80 00 00 mov 0x80(%rsp),%rbp
48d053: 00
48d054: 48 81 c4 88 00 00 00 add $0x88,%rsp
48d05b: c3 retq
48d05c: e8 2f 47 fc ff callq 451790 <runtime.morestack_noctxt>
48d061: e9 2a fe ff ff jmpq 48ce90 <main.main>