The libthread.so libraries are automatically linked into the respective libmpi.so libraries. This means that any thread-function calls in your program will be resolved by the libthread.so library. Simply omitting libthread.so from the link line will not cause thread calls to be stubbed out -- you must remove the thread calls yourself. For more information about the libthread.so library, see its man page. (For the location of Solaris man pages at your site, see your system administrator.)