llvm dev - Jul 2015 - [LLVMdev] Request for help: why does the program link correctly?

Hi,
	I am a bit stumped on the following case, and hopefully missing 
something very basic. Can someone please help me understand why the 
following is allowed to link correctly?

$ cat -n main.c
      1  extern void foo();
      2
      3  int main() { foo(); }

$ cat -n def.c
      1  int foo = 99;

$ clang main.c def.c ; echo $?
0

I guess the answer depends on why is 'main.c::foo' not marked as func?

$ clang -c main.c def.c
$ readelf -s main.o def.o | grep foo
      9: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND foo
      7: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    2 foo

Thanks,
Soumitra

Linking typically doesn’t pay attention to types, just to symbol names. The
symbol is indeed defined, so it links OK. This is one of the problems name
mangling helps solve in C++.

-Jim
> On Jul 1, 2015, at 10:09 PM, Soumitra Chatterjee <soumitra at hp.com>
wrote:
> 
> Hi,
> 	I am a bit stumped on the following case, and hopefully missing something
very basic. Can someone please help me understand why the following is allowed
to link correctly?
> 
> $ cat -n main.c
>     1  extern void foo();
>     2
>     3  int main() { foo(); }
> 
> $ cat -n def.c
>     1  int foo = 99;
> 
> $ clang main.c def.c ; echo $?
> 0
> 
> I guess the answer depends on why is 'main.c::foo' not marked as
func?
> 
> $ clang -c main.c def.c
> $ readelf -s main.o def.o | grep foo
>     9: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND foo
>     7: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    2 foo
> 
> Thanks,
> Soumitra
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Jim, I didn’t understood why foo is not marked with type 'FUNC'.

At least as per elf standard symbol foo should be marked as 'FUNC'
STT_FUNC:    The symbol is associated with a function or other executable code.

Regards,
Ashutosh

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Jim Grosbach
Sent: Friday, July 03, 2015 3:31 AM
To: Soumitra Chatterjee
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Request for help: why does the program link correctly?

Linking typically doesn’t pay attention to types, just to symbol names. The
symbol is indeed defined, so it links OK. This is one of the problems name
mangling helps solve in C++.

-Jim
> On Jul 1, 2015, at 10:09 PM, Soumitra Chatterjee <soumitra at hp.com>
wrote:
> 
> Hi,
> 	I am a bit stumped on the following case, and hopefully missing something
very basic. Can someone please help me understand why the following is allowed
to link correctly?
> 
> $ cat -n main.c
>     1  extern void foo();
>     2
>     3  int main() { foo(); }
> 
> $ cat -n def.c
>     1  int foo = 99;
> 
> $ clang main.c def.c ; echo $?
> 0
> 
> I guess the answer depends on why is 'main.c::foo' not marked as
func?
> 
> $ clang -c main.c def.c
> $ readelf -s main.o def.o | grep foo
>     9: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND foo
>     7: 0000000000000000     4 OBJECT  GLOBAL DEFAULT    2 foo
> 
> Thanks,
> Soumitra
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

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