Linker Version Scripts

Using linker version scripts to control symbol visibility

Linker version scripts can be used to provide fine-grained control of symbol visibility.

Suppose we have the following three source files in a shared library:

vis_comm.c: defines the function vis_comm().
vis_f1.c: defines the function vis_f1(), which calls vis_comm().
vis_f2.c: defines the function vis_f2(), which calls vis_comm().

vis_comm() is only intended for use within the library.

But building in the usual way:

bash$ gcc -g -c -fPIC -Wall vis_comm.c vis_f1.c vis_f2.c
bash$ gcc -g -shared -o vis.so vis_comm.o vis_f1.o vis_f2.o

produces a library that exports vis_comm().

We can see this using readelf(1), which lists information from ELF executables.

The following lists the dynamic symbols exported by the library:

bash$ readelf --syms --use-dynamic vis.so | grep vis_
   30  12: 00000790    59    FUNC GLOBAL DEFAULT  10 vis_f1
   25  13: 000007d0    73    FUNC GLOBAL DEFAULT  10 vis_f2
   27  16: 00000770    20    FUNC GLOBAL DEFAULT  10 vis_comm