While most of my free software is copyrighted and released under the terms of the GNU GPL v2 (or later), for some other programs I have instead described them as "public domain" and deliberately not included a copyright notice.
I have always intended for such "public domain" programs to be free for all to use and modify in any way. Unfortunately, it seems that this approach is not universally recognised. So this is my attempt to provide a fallback option.
I (Russell Marks) intend that for any software I wrote in any context and of any age that I explicitly labelled as "public domain" or "PD", in any case where copyright is still deemed to apply for any reason, the following BSD Zero Clause licence should also apply:
Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted.
THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.