I would like the option of changing the number of pthreads the program is allowed to run on without killing and rerunning the program.

Is there some mechanism for changing the number of pthreads while the program is mid run?

If so, could you explain how I would do so?
If not, is it at least possible?
General explanations are good enough, but specific would be nice if there isn't a possibility of doing so.

I have a feeling that if I could, it would require serious bookkeeping and a pausing mechanism. A partner program for changing the number of pthreads would also be better than a metafile.

Thanks in advance for any help

The best way to do it, I think, is to maintain two variables in a central, semaphore-protected table: the limit to the number of threads that you want, and the count of threads that you now have. If this number is increased, then the head-honcho thread simply adds more children. If the number is decreased, the head-honcho signals all the sleeping children to wake up.

Each child (entering the central table one-at-a-time due to the semaphore) notices if the limit is now exceeded. If so, the thread decrements the count and dies. Otherwise, it looks for work-to-do as usual.

In this way, the system will adjust itself in a matter of a few milliseconds to any reduction in the limit. But, no thread is killed: each thread is, as always, an obedient custodian of its own life-span. And, no thread fails to complete whatever unit of work it was in the process of doing at the time that the limit was lowered.