main.cpp

#include <stdio.h>
#include <stdlib.h>
#include "gh.h"
int main()
{
    printf("%d \n",a);
    return 0;
}

fun.cpp

#include <stdio.h>
#include "./gh.h"
void fun2()
{
    printf("a2:%d \n",a);
}

gh.h

#ifndef GH_H_
#define GH_H_
int a=0;
int b=1;
#endif

(只編譯)沒錯誤，但這該死的水印．

連結階段出現錯誤，重複定義．

使用objdump -t 檢視　*.o檔案內的符號如下:

main.o：     檔案格式 elf64-x86-64

SYMBOL TABLE:
0000000000000000 l    df *ABS*	0000000000000000 main.cpp
0000000000000000 l    d  .text	0000000000000000 .text
0000000000000000 l    d  .data	0000000000000000 .data
0000000000000000 l    d  .bss	0000000000000000 .bss
0000000000000000 l    d  .rodata	0000000000000000 .rodata
0000000000000000 l    d  .note.GNU-stack	0000000000000000 .note.GNU-stack
0000000000000000 l    d  .eh_frame	0000000000000000 .eh_frame
0000000000000000 l    d  .comment	0000000000000000 .comment
0000000000000000 g     O .bss	0000000000000004 a
0000000000000000 g     O .data	0000000000000004 b
0000000000000000 g     F .text	0000000000000024 main
0000000000000000         *UND*	0000000000000000 _GLOBAL_OFFSET_TABLE_
0000000000000000         *UND*	0000000000000000 printf

;關鍵點注意a和b的第二項均為　g(global)


un.o：     檔案格式 elf64-x86-64

SYMBOL TABLE:
0000000000000000 l    df *ABS*	0000000000000000 fun.cpp
0000000000000000 l    d  .text	0000000000000000 .text
0000000000000000 l    d  .data	0000000000000000 .data
0000000000000000 l    d  .bss	0000000000000000 .bss
0000000000000000 l    d  .rodata	0000000000000000 .rodata
0000000000000000 l    d  .note.GNU-stack	0000000000000000 .note.GNU-stack
0000000000000000 l    d  .eh_frame	0000000000000000 .eh_frame
0000000000000000 l    d  .comment	0000000000000000 .comment
0000000000000000 g     O .bss	0000000000000004 a
0000000000000000 g     O .data	0000000000000004 b
0000000000000000 g     F .text	0000000000000020 _Z4fun2v
0000000000000000         *UND*	0000000000000000 _GLOBAL_OFFSET_TABLE_
0000000000000000         *UND*	0000000000000000 printf
 

l兩個*.o檔案連結的時候:同一作用域出現兩對屬性完全一致的符號自然會出錯，儘管你只定義了一個變數，但是在編譯器眼裡是２個同名變數．