Break on user input, otherwise continue

Question

I have a program thats going to be processing large amounts of data. I already have things in place so that it can stop, and resume where it left off.

I want to be able to set it going for a while (e.g. process 1000 files), but be able to escape on user input, for example:

for(int i = 0; i < 1000; i++) { if( checkForUserInput() ) { break; } processFile(i); /* * ... * */ }

I know that you can prompt for user input, and if it matches some criteria then break... but I want it to default to keep running without user input.

I think I might have done this same thing a long time ago in java, and IIRC I ended up using threads --- would that be the only way to procede here as-well? Or is there a way to look for a keyboard escape sequence like Cntrl-C, and then behave a certain way?

Edit: If there is a reason why such a thing cant be done without threads, I'd be curious to know why, as-well.

Thanks!

oh whatever · Accepted Answer · 2012-12-22 01:05:35Z

3

User input is by definition blocking. So you need to set a time out at which you decide that there's no user input, break the block and continue. You can't do that without threading/interrupts.

Different platforms handle user interrupts differently. Read about signal handling.

answered Dec 22, 2012 at 1:05

oh whatever

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4 Comments

DilithiumMatrix Over a year ago

Okay. Presumably that was just a decision in c-history, right? I mean, couldn't you just check to see if the 'system' has any input for you, take action if it does, or continue if it doesn't?

oh whatever Over a year ago

@zhermes this is because C/C++ are OS agnostic. User input is handled and managed by the OS, as well as threads and interrupts (although threads are finally standardized in C++, its a recent addition).

Sebastian Hoffmann Over a year ago

One could write a wrapper stream performing asynchronus io but I dont know whether this is cleaner than using threads right away. The wrapper stream would have to work with threads nevertheless just abstracting their use.

oh whatever Over a year ago

@paranaix that's semantics of implementation. I think the OP was expecting built-in support in the language.

Mats Petersson · Accepted Answer · 2012-12-22 02:30:39Z

As mentioned, signal() can be used to catch CTRL-C in a console type application. But there is no standard way to read input without blocking.

There are some different solutions for different platforms, all of which are fairly cumbersome (of course, once you have the code, it's not that hard to copy it to another project).

As mentioned in the comments, C and C++ are designed to run on a large number of different machines, and the more requirements you put into the specification for "you must do this in this way", the more the language is restricted so that it can't work on a particular system - what if there is only a 110 bits per second, half duplex (data can only go one way at a time) line between your terminal and the computer? And you only have one processor, which runs at 1.2MHz...

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