Linking on different version of shared libraries - c++

Question

I have two versions of a a shared library:
library version 2:
simple.h
#pragma once
int first(int x);
simple.c
#include "simple.h"
#include <stdio.h>
__asm__(".symver first_1_0,first#LIBSIMPLE_1.0");
int first_1_0(int x)
{
printf("lib: %s\n", __FUNCTION__);
return x + 1;
}
__asm__(".symver first_2_0,first##LIBSIMPLE_2.0");
int first_2_0(int x)
{
int y;
printf("lib: %d\n", y);
printf("lib: %s\n", __FUNCTION__);
return (x + 1) * 1000;
}
linker version script file:
LIBSIMPLE_1.0{
global:
first;
local:
*;
};
LIBSIMPLE_2.0{
global:
first;
local:
*;
};
gcc -Wall -g -O0 -fPIC -c simple.c
gcc -shared simple.o -Wl,--version-script,script -o libsimple.so.2.0.0
And library version 3:
simple.h
#pragma once
#ifdef SIMPLELIB_VERSION_3_0
int first(int x, int normfactor);
#else
int first(int x);
#endif //SIMPLELIB_VERSION_3_0
simple.c
#include "simple.h"
#include <stdio.h>
__asm__(".symver first_1_0,first#LIBSIMPLE_1.0");
int first_1_0(int x)
{
printf("lib: %s\n", __FUNCTION__);
return x + 1;
}
__asm__(".symver first_2_0,first#LIBSIMPLE_2.0");
int first_2_0(int x)
{
printf("lib: %s\n", __FUNCTION__);
return (x + 1) * 1000;
}
__asm__(".symver first_3_0,first##LIBSIMPLE_3.0");
int first_3_0(int x, int normfactor)
{
printf("lib: %s\n", __FUNCTION__);
return (x + 1) * normfactor;
}
Linker version script file:
LIBSIMPLE_1.0{
global:
first; second;
local:
*;
};
LIBSIMPLE_2.0{
global:
first;
local:
*;
};
LIBSIMPLE_3.0{
global:
first;
local:
*;
};
gcc -Wall -g -O0 -fPIC -c simple.c
gcc -shared simple.o -Wl,--version-script,script -o libsimple.so.3.0.0
So i end up with having two different libraries. Next i create a simple application, that eventually i want to link to a library version 3, so in it i use function first() that takes two arguments:
main.c
#include <stdio.h>
#include "simple.h"
int main(int argc, char* argv[])
{
int nFirst = first(1, 10);
printf("First(1) = %d\n", nFirst);
}
I compile app with next commands:
gcc -g -Wall -DSIMPLELIB_VERSION_3_0 -c main.c
And then, by accident, instead of linking to a library version 3, i linked against library version 2. I expected linking to fail, but it went through, and application was working.
gcc main.o -Wl,-L. -lsimple.2.0.0 -Wl,-R. -o demo
So my questions are:
Is it because library exports symbol with name 'function', and application tries to link to the same symbol name, and that is why linker didn't complain, and just linked against library version 2 ?
I thought since c++ mangles symbol names, such thing wouldn't happen, and linker wouldn't link to a library version 2. So i tried all the same, but instead of a gcc compiler, i tried to use g++. Everything went well, until i tried to link application to a library, and i received unresolved links error. Can not figure out why.
p.s. Sorry for a big amount of code. I was trying to make it clear.
Thanks

Answer 1

Is it because library exports symbol with name 'function', and application tries to link to the same symbol name, and that is why linker didn't complain, and just linked against library version 2 ?
Yes, since plain C does not have function overload there is no need for mangling and as a consequence only function name will be used as a symbol for linking. In the end your application code wants to link with function and your library code exports function and this is enough to keep linker happy (even though it is not valid from binary interface perspective).
I thought since c++ mangles symbol names, such thing wouldn't happen, and linker wouldn't link to a library version 2. So i tried all the same, but instead of a gcc compiler, i tried to use g++. Everything went well, until i tried to link application to a library, and i received unresolved links error. Can not figure out why.
Yes, this problem should not occur in C++ because of name mangling. However, this is true only if you have both your application code and library code in C++ or if you bridge your C and C++ code the right way.
It is hard to say (without full listing) what happened in your case when you used g++ but from the looks of it you ended up having application code in C++ and library code still in C. If that is the case your application code will now want to link with mangled function while your library code still exports unmangled function.
To verify this you can inspect your object file with something like:
nm main.o
... and see exactly what kind of symbol does it want. If you will get something like this:
...
U _Z3functionii
...
... instead of:
...
U function
...
... then that is the case.
To "fix" this and make your C++ application code link with unmangled function from library code you'll need to declare your function prototype as extern "C".