This is a plain .c implementation of sha256, adapted from picoSHA2, which is a C++ header only version.
I wanted an MIT-licensed version to replace the version I created from LibreSSL. That came with 142 text lines of license boilerplate: 3 copies of the BSD 4-clause license, one for each copyright predecessor, lots of confusing uses of preprocessor macros, and support for SHA224 (which is unecessary for SHA256.)
In making this adaptation, I made only mechanical and trivial changes, which I decided are not copyrightable. I left the copyright notice from picoSHA2.