A non-relational embedded database with a permissive free software license?

many thanks in advance for taking the time to look at my question.

(I am aware of this question Nonrelational Databases for C++, but my needs are a bit different and it only has one answer.)

I am developing a commercial C++ library that must, among other things, persist messages. I would like to avoid reinventing the wheel by writing my own DBMS. Unfortunately, I have the following restricting criteria:

1. It must be usable from C++ - I'm writing a C++ library. Bindings are potentially acceptable, if the level of effort to make them work isn't too high.
2. I need an embedded database. Stand-alone will not work.
3. I want to avoid a relational database. In addition to concerns about performance overhead, there are technical politics beyond my control as a developer that discourage a relational database.
4. I need a permissive free software license. It'll be hard to buy licenses, but the client doesn't want to give his source away.
5. I'd like a solution that's established开发者_如何转开发 (been around for at least a little while, beyond the experimental stage, has been used by several projects).

Sadly, the two go-to choices don't work because of the above: -SQLite is relational -BerkeleyDB is GPL or commercial

Again, thanks for any help.

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Use SQLite in b-tree mode. Public domain. Avoids politics. Let's you work around the political issues by avoiding the SQL interface for performance-critical paths, and optionally using the SQL parser path for those queries that are not on the critical path.

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Both Tokyo Cabinet and QDBM are LGPL and have C APIs.