Detailed revision history can be found in the Git repository.

nvmutil 20221106

Very minor bugfix release:

Pledge now changed to stdio rpath (instead of stdio wpath), only when the dump command is used. (pledge is only used on OpenBSD systems; an ifdef rule prevents its use on other systems)
Documentation inaccuracies fixed (pertaining to nvmutil exit statuses)
Documentation generally tidied up a bit

nvmutil 20221103

Not much has changed, as this just fixes minor bugs and behavioural quirks seen in the previous release:

Prototypes now fully declared, with variable names
Fix implicit type conversion in readFromFile()
Always exit with zero status if the file was successfully modified. Previously, nvmutil would exit non-zero if one of the parts was invalid, even if the other was OK (and successfully modified)
Always exit with zero status when running dump, if at least one part contained a valid checksum and the file is of the correct size, fully readable. Previously, nvmutil would exit non-zero if one or both checksums was correct, but it now only does this if both are invalid

nvmutil 20220828

No new features have been added. This is a code cleanup and bugfix release.

General code cleanup (e.g. simpler argument handling)
Do not print errno 0 (fixes error when using the libc in OpenBSD)
Improved errno handling
Endianness portability re-implemented
The dump command no longer warns about multicast MAC addresses (such a warning is unnecessary, and up to the user to prevent)
The setmac command still prevents multicast MAC addresses being set, but no longer specifically warns about them (the documentation says not to use them already. No need to re-implement documentation in code)
Bug fix: errno now set, when an invalid file size is detected. The previous nvmutil version would exit (no operation) when the file size was wrong, but it would return with zero status. It now returns with non-zero status
The compiled binary size is still roughly the same as in the last release; the endianness checks increase the size, but other optimizations were made (e.g. cleaner argument handling). Tested with tcc on an x86_64 machine, where a 0.16% binary size increase was observed.

nvmutil 20220815

No new features have been added. This is a code cleanup and bugfix release.

Further 7% reduction in binary size, compared to the previous release. This is tested with tcc in an x86_64 machine. On my tests, tcc produces a 10540 byte binary in the previous release; in this release I got 9796 bytes.
It should be noted that nvmmac from the older nvmutils is 9892 bytes compiled with tcc on my test system; that's with the help/version output text removed from nvmmac. This nvmutil release contains so many more features and safety checks than just that lone nvmmac utility, yet the nvmutil binary is 1% smaller in size! That's how efficient the nvmutil code is, and there is probably room for further improvement of code efficiency.
Endianness portability code deleted; it was untested, and still is, so now nvmutil will detect the host endianness and exit, if the host is big endian (nearly all hosts these days are little endian, and it's very unlikely that you will use a big endian host). (the code will be properly tested and ported to big-endian hosts for the next release)
The status variable is no longer used; instead, errno is used exclusively and extensively. Error handling is much simpler, and much more unixy as a result.
Error messages are more terse
Fix build issues with clang, from the previous release
The dump command no longer states whether the MAC address is local or global; this can be easily done by yourself, and this change slightly reduces code bloat in nvmutil. The code still warns you if the MAC address is multicast

nvmutil 20220810

3.4% reduction in binary size (as tested with tcc on x86_64), due to code optimizations, while adding new checks and new features.
Random MAC address generation now takes multicast/unicast and local/global MAC addresses into account. The generator now forces local MAC addresses to be generated; the only way to set a global address is to set the corresponding nibble statically. Multicast and all-zero MAC addresses are no longer permitted in code.
The dump command now warns when the address is multicast, if set by another tool or older nvmutil that way (it will also return non-zero status upon exit). In addition, it will say whether the address is locally or globally assigned.
nvmutil now aborts when you try to open a directory
Even more terse error/feedback messages than in the last release, while still being friendly and informative
word/setWord now done in a more efficient, endian-specific way on x86_64 platforms; if non-x86_64, it falls back to the portable functions
The code has been simplified in general, and tidied up compared to the previous release
The setmac command can now be used without specifying a MAC address, which will cause the same behaviour as setmac ??:??:??:??:??:??

nvmutil 20220808

Released on 8 August 2022. Changes:

Vastly reduced binary size (21% reduction); the source line count has reduced by roughly the same amount (slightly higher than 21%, because of the extra stuff compilers add). This is with new features and behaviours added; the code is just that much more efficient, and I've thoroughly audited it.
OpenBSD pledge(2) now used, if available at build time (ifdef rule used, so it still compiles on Linux/FreeBSD)
New feature: setmac is now able to set random MAC addresses. This is done by reading from /dev/urandom. It is done conditionally, per-nibble, as specified by the user. For example, you can specify ??:??:??:??:??:?? and every nibble will be random, or you could set some of them statically. For example: 00:1f:16:??:4?:?2
The read() and pwrite() functions are now used for reading and writing files; older nvmutil versions used fopen/fseek/ftell and so on. The read/write functions are POSIX, and enable more robust file handling.
On showmac and dump, O_RDONLY is now set when executing read(). This means that these commands can now be used on read-only files.
Makefile: -Os flag used, instead of -O2
Error messages and feedback are much more user-friendly, and useful in general, while being much more terse.
The returned status on exit is much stricter. For example, the dump command will always cause an EXIT_FAILURE status when at least one part in the NVM image has a bad checksum. When writing a new MAC address, it will work only on valid parts like before, but now nvmutil will return with EXIT_FAILURE if one OR both parts are bad; older nvmutil versions always returned EXIT_SUCCESS after modifying the file.
The help and version commands have been removed; it is best to simply read the documentation. Programs should not include documentation inside themselves, but documentation should always be supplied separately, alongside that program. This change alone accounts for roughly 1/3 (33%) of the code size reduction; the other 2/3 of that reduction is due to increased code efficiency in general.

Regarding code size reduction

My test setup is an x86_64 machine with tcc used as the compiler; the libc doesn't really matter, if you use dynamic linking. Command:

make CC=tcc

Observations (dynamic linking with libc files):

20220808: 10.67KB
20220802 (unmodified): 13.51KB
20220802 (help/version command removed): 12.56KB

SLOC (source lines of code):

20220808: 321 lines
20220802 (unmodified): 421 lines
20220802 (help/version command removed): 373 lines

These numbers were obtained, using the sloccount program by David A. Wheeler, which you can find here:

https://dwheeler.com/sloccount/

This means that the actual reduction in compiled logic is about 1.89KB, or a 15% reduction, in nvmutil 20220808. By logic, I mean all code excluding the help function; this distinction is important, because the help/version commands are unavailable in nvmutil 20220808, but they were available in nvmutil 20220802.

Extra note: I also tested compressed sizes. With tar piped to xz -9e, I saw: about 3KB if compiled with tcc, and 5KB using gcc. Clang produces binaries of similar size, when compared with GCC.

Since the performance of nvmutil is largely disk-bound, I really recommend compiling it with tcc, not GCC or Clang because the binary sizes are much larger with those compilers, even with optimization flags; despite this, the Makefile in nvmutil assumes GCC/Clang and sets CFLAGS to -Os.

nvmutil 20220802

Released on 2 August 2022. Changes:

Another major code cleanup
More reliable argument handling
Files now loaded after verifying arguments
Files no longer written unless all checks pass (files were previously written unconditionally, even if no changes were made, wasting disk i/o)
Makefile now explicitly declares CFLAGS (strictest flags possible, warnings treated as errors)
More secure handling of strings; strnlen used instead of strlen, strncmp used instead of strcmp, number of characters limited
New command added: show mac (show what mac addresses are stored in both parts)
More human-friendly messages and help text
help/version commands actually listed in help output

nvmutil 20220731

Released on 31 July 2022. Changes:

Major code cleanup
Most operations now abort, if being performed on invalid files.
More robust error handling
More user-friendly error messages
The malloc function is no longer used

That's it. Bug fixes and safety features added. Enjoy!

nvmutil 20220728

Initial release. It is functionally equivalent to the older nvmutils, developed for the osboot project. This newer version is reduced to a single source file instead of many, and builds as a single binary instead of many.

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