Apache portable runtime offers C developers a set of tools for resource allocation, threads, sockets, and abstract data types. Using APR you can quickly write C code which will compile cleanly on many platforms.
Get APR from Apache Portable Runtime Project
Here are some tutorials and resources on the web:
libapr(apache portable runtime) programming tutorial
Developer: Apache Tutor
Bucket Brigades: Data Management for Apache 2.0 [notes]
Writing Portable C Code with APR
Apache DBD API