Linking with multiple versions of a library

Question

I have an application that statically links with version X of a library, libfoo, from thirdparty vendor, VENDOR1. It also links with a dynamic (shared) library, libbar, from

Answer 1

Thanks for all the responses. I have a solution that seem to be working. Here's the problem in detail with an example.

In main.c we have:

#include <stdio.h>

extern int foo();

int bar()
{
    printf("bar in main.c called\n");
    return 0;
}

int main()
{
    printf("result from foo is %d\n", foo());
    printf("result from bar is %d\n", bar());
}

In foo.c we have:

extern int bar();

int foo()
{
    int x = bar();
    return x;
}

In bar.c we have:

#include <stdio.h>

int bar()
{
    printf("bar in bar.c called\n");
    return 2;
}

Compile bar.c and foo.c:

$ gcc -fPIC -c bar.c
$ gcc -fPIC -c foo.c

Add bar.o to a static library:

$ ar r libbar.a bar.o

Now create a shared library using foo.o and link with static libbar.a

$ gcc -shared -o libfoo.so foo.o -L. -lbar

Compile main.c and link with shared library libfoo.so

$ gcc -o main main.c -L. -lfoo

Set LD_LIBRARY_PATH to find libfoo.so and run main:

$ setenv LD_LIBRARY_PATH `pwd`
$ ./main
bar in main.c called
result from foo is 0
bar in main.c called
result from bar is 0

Notice that the version of bar in main.c is called, not the version linked into the shared library.

In main2.c we have:

#include <stdio.h>
#include <dlfcn.h>


int bar()
{
    printf("bar in main2.c called\n");
    return 0;
}

int main()
{
    int x;
    int (*foo)();
    void *handle = dlopen("libfoo.so", RTLD_GLOBAL|RTLD_LAZY);
    foo = dlsym(handle, "foo");
    printf("result from foo is %d\n", foo());
    printf("result from bar is %d\n", bar());
}

Compile and run main2.c (notice we dont need to explicitly link with libfoo.so):

$ gcc -o main2 main2.c -ldl
$ ./main2
bar in bar.c called
result from foo is 2
bar in main2.c called
result from bar is 0

Now foo in the shared library calls bar in the shared library and main calls bar in main.c

I don't think this behaviour is intuitive and it is more work to use dlopen/dlsym, but it does resolve my problem.

Thanks again for the comments.

Answer 2

Try a partial link so that you have an object file "partial.o" with libbar and libfoo-Y. Use objcopy with "--localize-symbols " to make the symbols in partial.o from libfoo-Y local. You should be able to generate by running nm on libfoo-Y and massaging the output. Then take the modified partial.o and link it to your app.

I've done something similar with gcc toolchain on vxWorks where dynamic libs are not a complication but two versions of the same lib needed to link cleanly into a monolithic app.

Answer 3

Sorry no. My understanding of the way that Linux (and possibly most *nixes) is that that is not possible. The only 'solution' for your problem I can think of, is if you create a proxy app, which exposes what you need from libbar in the form of some IPC. You can then make that proxy load the correct version using LD_LIBRARY_PATH or something simmilar.