external linkage in a block?

This is a discussion on external linkage in a block? within the C Programming forums, part of the General Programming Boards category; Hi I was reading ISO C99 standard about linkage. Code: For an identifier declared with the storage-class specifier extern in ...

  1. #1
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    external linkage in a block?

    Hi

    I was reading ISO C99 standard about linkage.

    Code:
    For an identifier declared with the storage-class specifier extern in a scope in which a
    prior declaration of that identifier is visible, if the prior declaration specifies internal or
    external linkage, the linkage of the identifier at the later declaration is the same as the
    linkage specified at the prior declaration. If no prior declaration is visible, or if the prior
    declaration specifies no linkage, then the identifier has external linkage.
    So if I wrote

    Code:
    int main(void)
    {
         int a;  //1st
         extern int a; //2nd
         printf("%d\n", a);
    }
    The 1st a has no linkage. According to the standard, after 2nd declaration, 'a' has external linkage? Then why I can't find its information in the symbol table with the nm command?

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    Registered User slingerland3g's Avatar
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    What compiler errors are you getting? That should be your first clue.

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    Quote Originally Posted by slingerland3g View Post
    What compiler errors are you getting? That should be your first clue.
    There's no compiler error in my case. Also I don't think this should be an error. It is a syntactic problem. I want to understand what the standard really means. The standard states 'a' is external linkage so I think it should be seen using the nm command as an external symbol. But I can't see it.

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    Really? If I run gcc on your file, I get:
    Code:
    temp.c: In function ‘main’:
    temp.c:4: error: extern declaration of ‘a’ follows declaration with no linkage
    temp.c:3: error: previous declaration of ‘a’ was here
    temp.c:5: warning: implicit declaration of function ‘printf’
    temp.c:5: warning: incompatible implicit declaration of built-in function ‘printf’
    temp.c:3: warning: unused variable ‘a’
    temp.c:6: warning: control reaches end of non-void function

  5. #5
    DESTINY BEN10's Avatar
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    Quote Originally Posted by password636 View Post
    There's no compiler error in my case. Also I don't think this should be an error. It is a syntactic problem. I want to understand what the standard really means. The standard states 'a' is external linkage so I think it should be seen using the nm command as an external symbol. But I can't see it.
    In my compiler too I get the following error.
    Error 1 error C2086: 'int a' : redefinition
    And I guess it's because int a is a definition and when the compiler encounters extern declaration, it assumes that 'a' has been already defined somewhere. So it throws redefinition error.
    HOPE YOU UNDERSTAND.......

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    Quote Originally Posted by tabstop View Post
    Really? If I run gcc on your file, I get:
    Code:
    temp.c: In function ‘main’:
    ...
    Here is my output:
    Code:
    [tester@localhost apue2]$ cat test.c
    
    int main(void)
    {
         int a;  //1st
         extern int a; //2nd
         printf("%d\n", a);
    }
    [tester@localhost apue2]$ gcc test.c -Wall
    test.c: In function `main':
    test.c:6: warning: implicit declaration of function `printf'
    test.c:7: warning: control reaches end of non-void function
    [tester@localhost apue2]$ gcc --version
    gcc (GCC) 3.4.4 20050721 (Red Hat 3.4.4-2)
    Copyright (C) 2004 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
    
    [tester@localhost apue2]$

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