foo.c

#include <assert.h>
#include <stdlib.h>
#include <dlfcn.h>

int main() {
    void *lib = dlopen("./libfoo.so", RTLD_NOW | RTLD_DEEPBIND);
    assert(lib);

    void* (*foo_fn)(void) = dlsym(lib, "foo");
    assert(foo_fn);
    void *foo_ptr = foo_fn();
    assert(foo_ptr);
    free(foo_ptr);

    void* (*bar_fn)(void) = dlsym(lib, "bar");
    assert(bar_fn);
    void *bar_ptr = bar_fn();
    assert(bar_ptr);
    free(bar_ptr);
}

foo.rs

extern crate libc;

extern {
    fn strdup(a: *mut u8) -> *mut u8;
}

#[no_mangle]
pub unsafe extern fn foo() -> *mut u8 {
    strdup("foo\0".as_ptr() as *mut _)
}

#[no_mangle]
pub unsafe extern fn bar() -> *mut u8 {
    libc::malloc(8) as *mut _
}