Note that a.out shared libraries are a very tricky business. Consider upgrading your libraries to ELF shared libraries. Programs Are Very Large. There should be a link like libc.so for every library like libc.so.5.2.18 . As well as the Unix multiprocessing model involving heavyweight processes, which is of course part of the standard Linux kernel, there are several implementations of lightweight processes or threads.
Recent kernels implement a thread model, kthreads.
In addition, there are the following packages available for Linux. GNU glibc2 for Linux has optional support for threads. Newer Linux libc's contain the pthreads source. This is useful mainly because it has a lot of Postscript papers that you'll find useful in learning more about threads. This is not directly usable under Linux.