How to get around "multiple defined symbols" in linking with gcc.

Question

I am using an older system that has gcc 2.95.3, I have to link in two objects that although they have nothing to do with each other, they each have similarly named methods. I can't rename either of them, but I would hope there is a way to build them as to not have the linker complain. The methods it is complaining about are each internally called by classes within the object. What can I do?

Answer 1

If you have a complete GNU toolchain, you should be able to work around your problem using objcopy, like this (if I've understood your problem correctly):

Here are two very similar objects, "foo" and "bar", both of which export a symbol called clash - which is used internally, but doesn't really need to be exported at all:

$ cat foo.c
#include <stdio.h>
void clash(char *s) { printf("foo: %s\n", s); }
void foo(char *s) { clash(s); }
$ 

and

$ cat bar.c
#include <stdio.h>
void clash(char *s) { printf("bar: %s\n", s); }
void bar(char *s) { clash(s); }
$ 

And here's the main code, which wants to use both:

$ cat main.c
extern void foo(char *s);
extern void bar(char *s);

int main(void)
{
    foo("Hello");
    bar("world");
    return 0;
}
$ 

Linking them together doesn't work:

$ gcc -Wall -c foo.c  
$ gcc -Wall -c bar.c
$ gcc -Wall -c main.c
$ gcc -o test main.o foo.o bar.o
bar.o: In function `clash':
bar.c:(.text+0x0): multiple definition of `clash'
foo.o:foo.c:(.text+0x0): first defined here
collect2: ld returned 1 exit status
$ 

So, use objcopy to change the visibility of clash in one (or both, if you like!) of the objects:

$ objcopy --localize-symbol=clash bar.o bar2.o
$ 

Now you can link successfully with the modified object - and the program behaves as it should:

$ gcc -o test main.o foo.o bar2.o
$ ./test
foo: Hello
bar: world
$