关于C99定义的external linkage

password636

C99中有这么一段话

1. 
   
2. For an identifier declared with the storage-class specifier extern in a scope in which a
   
3. prior declaration of that identifier is visible, if the prior declaration specifies internal or
   
4. external linkage, the linkage of the identifier at the later declaration is the same as the
   
5. linkage specified at the prior declaration. If no prior declaration is visible, or [b]if the prior
   
6. declaration specifies no linkage, then the identifier has external linkage.[/b]
   

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那么，如果有如下代码

1. 
   
2. int main(void)
   
3. {
   
4.      int a;  //1st
   
5.      extern int a; //2nd
   
6.      printf("%d\n", a);
   
7. }
   
8. 
   

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1st后，a是no linkage, 根据标准，2nd之后a是不是就是external linkage? 那为什么我用nm在符号表里看不到a？

下面是我的运行结果

1. 
   
2. [57 alx@local ~]cat test4.c
   
3. int main(void)
   
4. {
   
5.      int a;  //1st
   
6.      extern int a; //2nd
   
7.      printf("%d\n", a);
   
8. }
   
9. 
   
10. [57 alx@local ~]gcc test4.c -c -Wall
    
11. test4.c: In function `main':
    
12. test4.c:5: warning: implicit declaration of function `printf'
    
13. test4.c:6: warning: control reaches end of non-void function
    
14. [57 alx@local ~]nm -f s test4.o
    
15. 
    
16. 
    
17. Symbols from test4.o:
    
18. 
    
19. Name                  Value           Class        Type         Size             Line  Section
    
20. 
    
21. main                |0000000000000000|   T  |              FUNC|000000000000001c|     |.text
    
22. printf              |                |   U  |            NOTYPE|                |     |*UND*
    
23. [57 alx@local ~]
    

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