Libmodbus currently provides modbus client support, plans are to provide RTU over TCPThe library is the work of many, started by Laurent LOPES, I have expanded it to meet some needs I had for a project. The library is quite terse and would be applicable to an embedded solution with the threading stripped out. The original format was threaded in a single library. The library has been split into a number of files one of these has the basic protocol handlers. This protocol file is suitable for inclusion into an embedded environment as it has no thread depenencies.
This is a heavily modified version of the source found on sourceforge.net