Metakit is an efficient embedded database library with a small footprint. It
fills the gap between flat-file, relational, object-oriented, and
tree-structured databases, supporting relational joins, serialization, nested
structures, and instant schema evolution. There is a C++ API, a Python binding
called Mk4py, and a Tcl binding called Mk4tcl. You can manipulate and exchange
data between any of these.

Data files are portable. The library has been used on Unix, Windows, Macintosh,
VMS, and others, spanning a range of 16- to 64-bit architectures, from PDA's to
S390's.

Metakit is in use in various commercial projects and products on millions of
desktops.

---AOS4 NOTE
I concrete on C/C++ support only (not bother with Python and TCL support)
because its used more offten. If someone will have interst in python/tcl
support, write me back.