Thread: 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?

What compiler errors are you getting? That should be your first clue.

Originally Posted by slingerland3g

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.

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

Originally Posted by tabstop

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]$