#include <fstream.h>
#include <thread.h>
fstream* fp;

void *process_rtn(void*)
{
    // body of sub-threads which uses fp...
}

void multi_process(const char* filename, int numthreads)
{
    fp = new fstream(filename, ios::in); // create fstream object
                                         // before creating threads.
    // create threads
    for (int i=0; i<numthreads; i++)
            thr_create(0, STACKSIZE, process_rtn, 0, 0, 0);

        ...
    // wait for threads to finish
    for (int i=0; i<numthreads; i++)
            thr_join(0, 0, 0);

    delete fp;                          // delete fstream object after
    fp = NULL;                         // all threads have completed.
}