I considered using leveldb initially, but that would require a C++ compiler and it also requires linking to libstdc++. So at that point, you might as well just write the project in C++, which defeats the point of this project.

Mako uses LMDB. Aside from Berkeley DB, it's pretty much the only key-value store in town if you want a pure C project.

In the end, it worked out well because I really like LMDB. It has a very intuitive API, and it's very small, which sort of matches the spirit of this project.

The zero-copy on reads feature is also amazing. When reading a UTXO from the database, we do no copying and no allocation whatsoever. The UTXO is simply parsed from the pointer LMDB returns. This is something LevelDB cannot do at all.