Using a shared library to partly replace an archive library - ld changed behaviour

[Graham Hudspith]

H. J. Lu wrote:

> I am not sure if I understand what you are saying. It seems to work
>
>for me with the testcase enclosed here:
>
>[hjl@gnu shared]$ make
>gcc -B./ -O -g   -c -o main.o main.c
>gcc -B./ -O -g -fPIC -c foo.c -o shared.o
>gcc -B./ -shared -o libshared.so shared.o
>gcc -B./ -O -g   -c -o bar.o bar.c
>ar rv libfoo.a bar.o
>ar: creating libfoo.a
>a - bar.o
>gcc -B./ -O -g   -c -o foo.o foo.c
>ar rv libfoo.a foo.o
>a - foo.o
>gcc -B./ -o main1 main.o libshared.so libfoo.a -Wl,-rpath,.
>gcc -B./ -o main2 main.o libfoo.a
>./main1
>Shared library
>./main2
>Static library
>rm foo.o bar.o
>
>Does it work for you? 
>
Thanks for the swift reply (and source code too!).

Rather annoyingly, your examples worked as expected ;-)

So I decided to investigate the compilation of the archive library 
further. Using "gcc -H" and "gcc -E", I have reproduced and translated 
the code they use inside your example bar.c (see attached).

It seems they are doing something wacky with weak symbols (thanks for 
hinting about this, btw Dave Korn!).

They are also compiling their source code with -fPIC and then adding it 
to the archive library !?

I have modified your Makefile and bar.c to mimic the behaviour of this 
archive library. main1 and main2 are the same as before, except that 
main1 no longer finds the correct foo(). I've also added a main3 that 
DOES find foo(), but this is by linking in the archive to the shared 
library beforehand. We DON'T want to do this.

What's going on? Are we at fault? Or the maintainers of the package that 
produces the archive library? Or gcc? Or ld?

Is there some simple (or complicated) fix we can do to our shared 
library that will make it work, unlinked, with the archive library?

Now, remember, there is a difference in behaviour between RH9 (gcc 
v3.2.2, ld v2.13.90.0.18) and FC3 (gcc v3.4.3, ld v2.15.92.0.2).

*** RH9 version ***
[opt-testing@fender modified]$ make
gcc -B./ -O -g   -c -o main.o main.c
gcc -B./ -O -g -fPIC -c foo.c -o shared.o
gcc -B./ -shared -o libshared1.so shared.o
gcc -B./ -O -g -fPIC -c bar.c -o bar.o
ar rv libfoo.a bar.o
a - bar.o
gcc -B./ -o main1 main.o libshared1.so libfoo.a -Wl,-rpath,.
gcc -B./ -o main2 main.o libfoo.a
gcc -B./ -shared -o libshared3.so shared.o libfoo.a libfoo.a
gcc -B./ -o main3 main.o libshared3.so libfoo.a -Wl,-rpath,.
./main1
Shared library
Real Foo (or Bar)
./main2
Real Foo (or Bar)
./main3
Shared library
Real Foo (or Bar)

*** FC3 version ***
[gwh@snowball modified]$ make
gcc -B./ -O -g   -c -o main.o main.c
gcc -B./ -O -g -fPIC -c foo.c -o shared.o
gcc -B./ -shared -o libshared1.so shared.o
gcc -B./ -O -g -fPIC -c bar.c -o bar.o
ar rv libfoo.a bar.o
ar: creating libfoo.a
a - bar.o
gcc -B./ -o main1 main.o libshared1.so libfoo.a -Wl,-rpath,.
gcc -B./ -o main2 main.o libfoo.a
gcc -B./ -shared -o libshared3.so shared.o libfoo.a libfoo.a
gcc -B./ -o main3 main.o libshared3.so libfoo.a -Wl,-rpath,.
./main1
Real Foo (or Bar)
./main2
Real Foo (or Bar)
./main3
Shared library
Real Foo (or Bar)

The RH9 version behaves as we want, the FC3 (or Gentoo, or Ubuntu) does not.

Hope this helps! And is interesting enough that you have got this far!

Graham.

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