[LCP] How to do intremental linking with gcc and ld

[David Nugent]

On Sat, 20 Jun 2009 11:48:34 +1000, Bryan Christ <bryan.christ at gmail.com>  
wrote:

> I have a top-level directory from which I want to share as common code
> with two different applications (a client and a server).  I thought it
> would be quite easy to create a combined object file in the common
> directory and then link to it from each of the individual programs.
> While I don't have any problem compiling the larger common object
> file, I have having unresolved symbols when I try to link the two
> different programs to it.
>
> Here is how I am creating the common object file:
>
>  gcc -c $(CFLAGS) $(INCLUDES) $(DEFS) *.c
>  ld -r -o common_tools.o *.o
>
> And here is how I try to link against the common object file:
>
> gcc -c $(CFLAGS) $(INCLUDES) $(LIBS) $(NORMAL_DEFS) *.c
> ld -o myprogram *.o ../../common/common_tools.o
>
> I'm sure I'm missing something trivial but don't know what that is.


Don't use ld to link directly except in very special circumstances, but
use the gcc front-end instead. Just replace ld in the command line above
with gcc and it should work fine. In fact, you can just include the
"-o myprogram ../../../common/common_tools.o" to the gcc command above
it and it will do it all in one step.

Reason: the unix link editor usually needs a whole bunch more than you're
providing it to successfully link userland code, such as reference to the
standard C library, c startup object file and other platform specific
directives which the gcc front-end knows about but ld does not.  ld makes
no assumptions like that, it is used in a far wider variety of link-edit
scenarios than for userland applications.

I'm not sure what you're doing with the common files, although it looks
like you're attempting to build a shared library? The extension used
there is usually .so, not .o.  gcc can do this also with -shared in its
argument list. If you're just wanting to include the objects, then use ar
to build the static library instead as a .a file.

Regards,
David