Linux does not have getpeereid(2).

src/socket.c

@@ -193,6 +193,7 @@ socket_family(int s)
 static mrb_value
 mrb_basicsocket_getpeereid(mrb_state *mrb, mrb_value self)
 { 
+#ifdef HAVE_GETPEEREID
   mrb_value ary;
   gid_t egid;
   uid_t euid;
@@ -206,6 +207,10 @@ mrb_basicsocket_getpeereid(mrb_state *mrb, mrb_value self)
   mrb_ary_push(mrb, ary, mrb_fixnum_value((mrb_int)euid));
   mrb_ary_push(mrb, ary, mrb_fixnum_value((mrb_int)egid));
   return ary;
+#else
+  mrb_raise(mrb, E_RUNTIME_ERROR, "getpeereid is not avaialble on this system");
+  return mrb_nil_value();
+#endif
 }
 
 static mrb_value