Restarting threads in forked process
Threads execution state is not only the data in stack. It is also set of CPU registers, which is lost.
do_fork() system call just don't copy any thread other from thread, which executes a syscall do_fork -> copy_process and there is a single call to copy_thread at line 1181
retval = copy_thread(clone_flags, stack_start, stack_size, p, regs);
Guess What? Solaris already has such a function by the name of forkall! Would love to see a port of it in Linux!
I guess you are talking about continuations. I saw some hits on google (I asked for "continuation c"), but I can't recomend any link, since I don't know your specific needs.