callonce is released under the MIT License — as are its sibling libraries cparse and fson.

Use the code freely — including in closed-source and commercial products. There is no copyleft, no attribution beyond keeping the copyright line, and no CLA for contributions (inbound = outbound).

MIT License

Copyright (c) Fedem (Fehmi Demiralp)

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

callonce is a single header, and it carries SPDX-License-Identifier: MIT; the repository also ships the full text as LICENSE.

What the licence does not cover

The MIT grant is for the code. It does not grant rights to:

• the names FEDEM, FFS, callonce, and any other project or product name used by Fehmi Demiralp (FEDEM);
• the FEDEM / FFS logos and other brand assets;
• the domain fedem.eu and its subdomains.

If you publish a modified version, don't use these names or logos in a way that implies it is the official project or endorsed by FEDEM — rename your fork. Nominative use ("a fork of callonce", "uses callonce") is fine. See TRADEMARKS.md in the repository for the details.

Third-party components

The test suite uses GoogleTest / GoogleMock (BSD-3-Clause), build-time only. No component imposes terms beyond MIT on a consumer of callonce.