How to deal with symbol collisions between statically linked libraries?
One of the most important rules and best practices when writing a library, is putting all symbols of the
library into a library specific namespace. C++ makes this easy, due to the namespace keyword. In
C the usual approach is to prefix the identifiers with some library specific prefix.
Rules of the C standard put some constraints on those (for safe compilation): A C compiler may look at only the first
8 characters of an identifier, so foobar2k_eggs and foobar2k_spam may be interpreted as the same
identifiers validly – however every modern compiler allows for arbitrary long identifiers, so in our times
(the 21st century) we should not have to bother about this.
But what if you're facing some libraries of which you cannot change the symbol names / idenfiers? Maybe you got only a static binary and the headers or don't want to, or are not allowed to adjust and recompile yourself.
Answer
At least in the case of static libraries you can work around it quite conveniently.
Consider those headers of libraries foo and bar. For the sake of this tutorial I'll also give you the source files
examples/ex01/foo.h
int spam(void);
double eggs(void);
examples/ex01/foo.c (this may be opaque/not available)
int the_spams;
double the_eggs;
int spam()
{
return the_spams++;
}
double eggs()
{
return the_eggs--;
}
example/ex01/bar.h
int spam(int new_spams);
double eggs(double new_eggs);
examples/ex01/bar.c (this may be opaque/not available)
int the_spams;
double the_eggs;
int spam(int new_spams)
{
int old_spams = the_spams;
the_spams = new_spams;
return old_spams;
}
double eggs(double new_eggs)
{
double old_eggs = the_eggs;
the_eggs = new_eggs;
return old_eggs;
}
We want to use those in a program foobar
example/ex01/foobar.c
#include <stdio.h>
#include "foo.h"
#include "bar.h"
int main()
{
const int new_bar_spam = 3;
const double new_bar_eggs = 5.0f;
printf("foo: spam = %d, eggs = %f\n", spam(), eggs() );
printf("bar: old spam = %d, new spam = %d ; old eggs = %f, new eggs = %f\n",
spam(new_bar_spam), new_bar_spam,
eggs(new_bar_eggs), new_bar_eggs );
return 0;
}
One problem becomes apparent immediately: C doesn't know overloading. So we have two times two functions with identical name but of different signature. So we need some way to distinguish those. Anyway, lets see what a compiler has to say about this:
example/ex01/ $ make
cc -c -o foobar.o foobar.c
In file included from foobar.c:4:
bar.h:1: error: conflicting types for ‘spam’
foo.h:1: note: previous declaration of ‘spam’ was here
bar.h:2: error: conflicting types for ‘eggs’
foo.h:2: note: previous declaration of ‘eggs’ was here
foobar.c: In function ‘main’:
foobar.c:11: error: too few arguments to function ‘spam’
foobar.c:11: error: too few arguments to function ‘eggs’
make: *** [foobar.o] Error 1
Okay, this was no surprise, it just told us, what we already knew, or at least suspected.
So can we somehow resolve that identifer collision without modifying the original libraries' source code or headers? In fact we can.
First lets resolve the compile time issues. For this we surround the header includes with a
bunch of preprocessor #define directives that prefix all the symbols exported by the library.
Later we do this with some nice cozy wrapper-header, but just for the sake of demonstrating
what's going on were doing it verbatim in the foobar.c source file:
example/ex02/foobar.c
#include <stdio.h>
#define spam foo_spam
#define eggs foo_eggs
# include "foo.h"
#undef spam
#undef eggs
#define spam bar_spam
#define eggs bar_eggs
# include "bar.h"
#undef spam
#undef eggs
int main()
{
const int new_bar_spam = 3;
const double new_bar_eggs = 5.0f;
printf("foo: spam = %d, eggs = %f\n", foo_spam(), foo_eggs() );
printf("bar: old spam = %d, new spam = %d ; old eggs = %f, new eggs = %f\n",
bar_spam(new_bar_spam), new_bar_spam,
bar_eggs(new_bar_eggs), new_bar_eggs );
return 0;
}
Now if we compile this...
example/ex02/ $ make
cc -c -o foobar.o foobar.c
cc foobar.o foo.o bar.o -o foobar
bar.o: In function `spam':
bar.c:(.text+0x0): multiple definition of `spam'
foo.o:foo.c:(.text+0x0): first defined here
bar.o: In function `eggs':
bar.c:(.text+0x1e): multiple definition of `eggs'
foo.o:foo.c:(.text+0x19): first defined here
foobar.o: In function `main':
foobar.c:(.text+0x1e): undefined reference to `foo_eggs'
foobar.c:(.text+0x28): undefined reference to `foo_spam'
foobar.c:(.text+0x4d): undefined reference to `bar_eggs'
foobar.c:(.text+0x5c): undefined reference to `bar_spam'
collect2: ld returned 1 exit status
make: *** [foobar] Error 1
... it first looks like things got worse. But look closely: Actually the compilation stage went just fine. It's just the linker which is now complaining that there are symbols colliding and it tells us the location (source file and line) where this happens. And as we can see those symbols are unprefixed.
Let's take a look at the symbol tables with the nm utility:
example/ex02/ $ nm foo.o
0000000000000019 T eggs
0000000000000000 T spam
0000000000000008 C the_eggs
0000000000000004 C the_spams
example/ex02/ $ nm bar.o
0000000000000019 T eggs
0000000000000000 T spam
0000000000000008 C the_eggs
0000000000000004 C the_spams
So now we're challenged with the exercise to prefix those symbols in some opaque binary. Yes, I know in the course of this example we have the sources and could change this there. But for now, just assume you have only those .o files, or a .a (which actually is just a bunch of .o).
objcopy to the rescue
There is one tool particularily interesting for us: objcopy
objcopy works on temporary files, so we can use it as if it were operating in-place. There is one option/operation called --prefix-symbols and you have 3 guesses what it does.
So let's throw this fella onto our stubborn libraries:
example/ex03/ $ objcopy --prefix-symbols=foo_ foo.o
example/ex03/ $ objcopy --prefix-symbols=bar_ bar.o
nm shows us that this seemed to work:
example/ex03/ $ nm foo.o
0000000000000019 T foo_eggs
0000000000000000 T foo_spam
0000000000000008 C foo_the_eggs
0000000000000004 C foo_the_spams
example/ex03/ $ nm bar.o
000000000000001e T bar_eggs
0000000000000000 T bar_spam
0000000000000008 C bar_the_eggs
0000000000000004 C bar_the_spams
Lets try linking this whole thing:
example/ex03/ $ make
cc foobar.o foo.o bar.o -o foobar
And indeed, it worked:
example/ex03/ $ ./foobar
foo: spam = 0, eggs = 0.000000
bar: old spam = 0, new spam = 3 ; old eggs = 0.000000, new eggs = 5.000000
Now I leave it as an exercise to the reader to implement a tool/script that automatically extracts the symbols of a library using nm, writes a wrapper header file of the structure
/* wrapper header wrapper_foo.h for foo.h */
#define spam foo_spam
#define eggs foo_eggs
/* ... */
#include <foo.h>
#undef spam
#undef eggs
/* ... */
and applies the symbol prefix to the static library's object files using objcopy.
What about shared libraries?
In principle the same could be done with shared libraries. However shared libraries, the name tells it, are shared among multiple programs, so messing with a shared library in this way is not such a good idea.
You will not get around writing a trampoline wrapper. Even worse you cannot link against the shared library on the object file level, but are forced to do dynamic loading. But this deserves its very own article.
Stay tuned, and happy coding.