GSK -- a library for writing servers.
It facilitates writing very efficient single-threaded servers in C. It has facilities which directly support some standard protocols, as well as some simple main-loop and parser abstractions.
It is licensed under the LGPL, to give you the maximum freedom in writing your application.
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In order to compile or use GSK, you must have Glib 2.0 installed. If you are using a standard linux distribution, a package is probably available. Otherwise, the source is available at ftp://ftp.gtk.org/.
If you have received a distributed .tar.gz file, please read INSTALL for generic installation instructions.