c - How can I remove a symbol from a shared object?

Question

Using GCC, how can I remove a symbol from a shared object after I've created the shared object? If I have three files in C manipulating symbol foo() like:

// a.c
int foo() { return 0xdead; }
int baz() { return 1; }

and

// b.c
int foo() { return 0xbeef; }
int bar() { return 0; }

and

// c.c
#include "stdio.h"
extern int foo();
extern int bar();
extern int baz();
int main() { printf("0x%x, 0x%x, 0x%x
",foo(),bar(),baz()); return 0; }

Then I compile and run like:

% gcc a.c --shared -fPIC -o a.so
% gcc b.c --shared -fPIC -o b.so
% setenv LD_LIBRARY_PATH . # export LD_LIBRARY_PATH=. for bash systems
% gcc c.c a.so b.so -o c
% ./c
0xdead, 0x0, 0x1

How can I make it so that a.so no longer has symbol foo() after I've created a.so? I want the foo() defined in b.so to be used instead of a.so by deleting the foo() symbol from a.so. After foo() is deleted from a.so, rerunning c should generate a printout of:

0xbeef, 0x0, 0x1

In this toy example, I know I can simply re-order the libary names when I compile c.c with a.so and b.so, but how can I actually delete the symbol from a.so? I imagine that after deleting foo() from a.so, this grep of the nm output would yield nothing:

nm -a a.so | grep foo

Whereas right now it returns:

000000000000063c T foo

Answer 1

You should be able to use the -N (--strip-symbol) option of objcopy to achieve what you want:

$ objcopy -N foo a.so