In this article, we present the AdOC (Adaptive Online Compression) library. It is a user-level set of functions that enables data transmission with compression. The compression is performed dynamically during the transmission and the compression level is constantly adapted according to the environment. In order to ease the integration of AdOC into existing software the API is very close to the read and write UNIX system calls and respects their semantic. Moreover this library is thread-safe and is ported to many UNIXlike systems. We have tested AdOC under various conditions and with various data types. Results show that the library outperforms the POSIX read/write system calls on a broad range of networks (up to 100 Mbit LAN), whereas on Gbit Ethernet, it provides similar performance.