gistfile1.txt

$ cat so.cc
extern char** environ;
#include <iostream>

void f() {
        for (size_t i = 0; environ[i]; ++i) {
                std::cerr << environ[i] << std::endl;
        }
}
$ clang++ -shared -o so.so so.cc -fPIC
$ clang++ -shared -o so.so so.cc -fPIC -Wl,-z,defs
/tmp/so-693756.o: In function `f()':
so.cc:(.text+0x13): undefined reference to `environ'
so.cc:(.text+0x33): undefined reference to `environ'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
$