How to stop a program while it's executing and save progress

[goron]

Hello,

I am writing a computationally heavy program which I know will need to run for many hours. Therefore, I need to make it possible to stop the program at any time while it's running and save my progress in an external file so that later the program can be started again and continue from where it left off. What I don't know how to do is how to actually get the program to stop, but make sure it executes some functions to save its progress before quitting.
I'm writing in C and using DOS on Windows. The only way I know how to stop a program in DOS is ctrl-c, and I've done some research but have been unable to catch this and act upon it before exiting.
If this helps - my program is recursive, so to save the progress it will need to backstep from whatever recursive depth it's on and save some data from each one.

Thank you.

[RRick]

The concept you're talking about is called signals. Take a look at the signal.h header file. This will tell you what signals are supported and how to set up callbacks for them.

Typically, you register a callback subroutine for a specific signal. When the signal happens, your routine is called. As for traversing your recursive structure, you'll have to figure that out yourself. The callback routine knows nothing about what was running before it was called.

Some signals are "non-interruptable" and when they happen, you have no control over what happens. Usually, the program ends and that's that. The other interruptable types are the ones you want to deal with. I'm not sure which category Ctl-C falls into.

[cypherzero]

Stopping a program with Ctrl+C actually causes an exception in the program, so all you should need to do is surround your code with a try ... catch procedure.
If it's running in a DOS window, and the user clicks the close button I don't think the program is given any warning however (I'm not entirely sure).

[goron]

Stopping a program with Ctrl+C actually causes an exception in the program, so all you should need to do is surround your code with a try ... catch procedure.
If it's running in a DOS window, and the user clicks the close button I don't think the program is given any warning however (I'm not entirely sure).

I'm writing in plain C. Can C (not ++) catch exceptions? How is this done? If it can't be done, and what ctrl-C sends is an exception, not a signal, what is a better way to gracefully halt a program running in DOS?

[smalpani]

Signalling is not the solution.

Checkpointing is.

I can give you an example, you modify toi your problem.

Suppose you want to calculate fibonacci numbers from 1 to 10000 and you will keep appending these numbers to a file.

The file may look like

1 0
2 1
3 1
4 2

....
....

But it may happen that you can't run the program continuously, so what do you do.

Every time you run the program, you read this file, read the last record and figure out the next fibonacci number to be calculated. Run the program for 100 numbers then close the file.

And so on.

Its never a good idea to use ctrl + c. Although you can handle them with signals in a better OS. You should give user the liberty to kill your program and let it die an un-natural death.

[smalpani]

exit() in stdlib.h is much better than a ctrl + c

[smalpani]

Also , DOS is a UGLY OS

[goron]

Also , DOS is a UGLY OS

If I'm programming on a windows machine for a windows machine, do I have an option?

As for the program - I see what you're saying with constantly saving to a file, but the amount of data I would have to constantly save is pretty large. Also, it would not be relevant to what the program is trying to produce. If we want all the fibonacci numbers, it makes sense to save them, but my program is just looking for the final result.
It just seems like a big waste of time to constantly be reading and saving files.

[smalpani]

a lot of applications do provide checkpointing facility.

Check pointing is writing to disk in some way.

If you have to proceed with DOS for any inevitable reason, stick to it. If you are learning then come to Linux.

Coming to your task. What and how you will checkpoing to file depends on your application.

In general you will have to save the state of your program so that when it is restarted, it knows how much has been done and what it has to start doing.

Without knowing the details, I can just tell you to give more time to intelligently design what you will write to disk, how will you write it and how will on a restart reach a point where you will know how much was accomplished.


All the best.