Trying to keep N CPUs running threads - pthreads

Question

I want to run (say) 25 CPU intensive tasks in my 6 cores computer by using each time 5 cores (so 1 be left for other tasks). Each of the 25 CPU intensive tasks can finish at different times, for example one task can finish at 20 minutes while other task can take up to 4 hours.
Using pthreads I've managed to build a very simple algorithm which can launch the 5 threads, wait for its completion and launch another 5 threads again. The weak point of the algorithm is that it can not take advantage of the fact that different threads may finish at different times, instead it always waits for the job completion of all the 5 threads before launch another set.
Here is the code extract that launches the threads
#define MAX_REACTOR_THREADS 5
/* ....................... */
/* SOME LENGTHY STUFF HERE */
/* ....................... */
int cases = 25;
pthread_t *threads_REACTOR = calloc(cases, sizeof(pthread_t));
int index = 0;
/* CALL THREADS PROCEDURE FOR COMPUTING FUNCTION: REACTOR_THREAD */
int int_div = cases/MAX_REACTOR_THREADS;
int I, k;
for (I=0; I<int_div; I++){
/* LAUNCH THREADS */
for(k=0; k<MAX_REACTOR_THREADS; k++){
index = int_div*k+I;
pthread_create(&threads_REACTOR[index], NULL,
&REACTOR_THREAD, (void*) &inputs_array[index]);
}
/* JOIN THREADS */
for(k=0; k<MAX_REACTOR_THREADS; k++){
index = int_div*k+I;
pthread_join(threads_REACTOR[index], NULL);
}
/* HERE PRINT PROGRESS BAR */
Progress_Bar((I+1)*MAX_REACTOR_THREADS, cases, 40, COLOR_Y);
}
if((cases%MAX_REACTOR_THREADS) != 0){
/* LAUNCH REMAINING THREADS */
for(index=MAX_REACTOR_THREADS*int_div; index<cases; index++){
pthread_create(&threads_REACTOR[index], NULL,
&REACTOR_THREAD, (void*) &inputs_array[index]);
}
/* JOIN REMAINING THREADS */
for(index=MAX_REACTOR_THREADS*int_div; index<cases; index++){
pthread_join(threads_REACTOR[index], NULL);
/* HERE PRINT PROGRESS BAR */
Progress_Bar(index+1, cases, 40, COLOR_Y);
}
}
/* ....................... */
/* SOME LENGTHY STUFF HERE */
/* ....................... */
How can this be improved so it can launch a new task at the moment a thread has finished done its job?