It's difficult to answer your query without looking at your code. Maybe you're making an invalid assumption and truncating the command line string without realizing it.
If you want to see how the process data are gathered look at the C source and headers in the /os directory of the source tree (Proc-ProcessTable-0.35/os/). On linux the process data are gathered from the /proc filesystem, which is the same method used by the ps util. On other OSs where the /proc filesystem isn't supported there are different methods for gathering process data, but all of the methods used should probably return the complete command line string.