Re: Linker errors - calling ASM routines from C code

[Claudio Bley <bley at cs dot uni-magdeburg dot de>]

On Wed, Jul 30, 2003 at 11:19:31PM +0530, Nemo Physche wrote:
> Hello,
> 
> I'm having a problem compiling my C and ASM code using gcc on linux.
> I have a set of C files calling MMX/SSE/SSE2 optimized routines that
> are coded in ASM and compiled using GAS. The ASM function names are
> declared as global and have a leading underscore, which I believe is
> required when calling the function from my C code. It compiles and
> links as expected with DJGPP, gcc 3.2.3.
> 
> When some one else tried the compilation on a Linux machine with
> gcc 3.3 it belches at the linking stage, unable to find the ASM
> functions. Inspecting the C program's object files, I find that the
> function calls do not have a leading underscore. The problem is solved
> if I remove the leading underscore from my ASM routine name.
> 
> Why does the code not compile in one situation and work perfectly in the
> other?

The NASM Manual explains:

,----[ http://home.comcast.net/~fbkotler/nasmdoc8.html ]
| 8.1.1 External Symbol Names
| 
| Most 32-bit C compilers share the convention used by 16-bit compilers,
| that the names of all global symbols (functions or data) they define
| are formed by prefixing an underscore to the name as it appears in the
| C program. However, not all of them do: the ELF specification states
| that C symbols do not have a leading underscore on their
| assembly-language names.
| 
| The older Linux a.out C compiler, all Win32 compilers, DJGPP, and
| NetBSD and FreeBSD, all use the leading underscore; for these
| compilers, the macros cextern and cglobal, as given in section 7.4.1,
| will still work. For ELF, though, the leading underscore should not be
| used.
`----

,----[ man gcc ]
| 
| -fleading-underscore
|    This option and its counterpart, -fno-leading-underscore, forcibly
|    change the way C symbols are represented in the object file.  One
|    use is to help link with legacy assembly code.
| 
|    Warning: the -fleading-underscore switch causes GCC to generate
|             code that is not binary compatible with code generated
|             without that switch.  Use it to conform to a non-default
|             application binary interface.  Not all targets provide
|             complete support for this switch.
`----

-- 
Claudio Bley                                 ASCII ribbon campaign (")
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