[SOLVED] Remove unused functions from shared library

Remove unused functions from shared library

Consider the following files:

foo.c

void
bar(void);

void
foo(void)
{
    bar();
}

bar.c

#include <stdio.h>

void
bar(void)
{
    printf("bar\n");
}

void
baz(void)
{
    printf("baz\n");
}

vis.map

{
    global:
        foo;
    local:
        *;
};

I've compiled them into a library by

gcc -c -ffunction-sections foo.c
gcc -c -ffunction-sections bar.c
gcc -Wl,--version-script,vis.map -shared -o libfoo.so foo.o bar.o

As expected, nm -D libfoo.so shows only the foo symbol. However, objdump -d libfoo.so shows the code for baz. I would have thought, given that it's unreachable from the only visible function, that the linker would have removed baz from the final output.

Is there a linker flag or something to accomplish this?

For context, bar.c represents code from another project that I'm trying to use in my project. So, it's infeasible to alter it.

Solution

The --gc-sections linker flag does the trick. According to man ld, it

[enables] garbage collection of unused input sections.

Since you're compiling with -ffunction-sections, each function will be put into its own section. Adding -Wl,--gc-sections to your final gcc invocation will remove the unused baz section.