how to let a prognamm stop in random?

basically it is impossible to stop (suspend) a program.  (actually it is often possible to suspend a program on a multi-tasking operating system.  But how to do so is system-dependant and not what you want.)

Since it is impossible--fake it.

don't stop the program, instead make it appear stopped by having it execute a loop that does (almost) nothing.  Actually the loop can't do nothing or it will never end.  The loop should look for the key press that should allow the program to continue.

next problem, where do you put the loop?

That depends on you program.  If you program has a single central procedure for obtaining user input (like most windows programs, as well as others) then you can put the code to start the loop there.  Otherwise, you may need to alter the code at every location that does user input.

I can't provide more help without knowing more about your program its purpose and its design.  

If you need more, post some more information.

I see you accepted the answer, but I suspect that (as it is) it wasn't enough.  Do you need more help?

Thank you for helping me.
Can you help me?perhaps TSR can be solved this problem?
Do you think ?

A TSR could be used to suspend a DOS program.  But it seems like a very poor solution.  It is very system dependent.  What is it exactly that you are trying to do?  

Thank you for helping me.I plan to write a demo prognamm,I should let it stop when it is needed to stop.then press any key it should continue to run .

But why can't the program itself look for the key and enter a wait loop whent he key is pressed?  Doesn't that seem like a more natural design than envoking a TSR to suspend the program?  It is also more portable and as well as more extensible.

In BC++,how to realize stop?use getch()?I tried to do but failed.
Can you tell me?The computer can't correspond with this keyboard
interrupt.

You do not mean keyboard interrupt?  right.  I don't think you need to hook the keyboard interrupt and if you do you don't use getch().

you could do something like the following which holds up a loop whenever 'P' is typed.  The loop continues again when 'P' is typed again.

while (NotDoneDoingSomething())
{
   DoALittleSomething()
   if (_kbhit()) // If a key has been typed, then
       if (_getch() == 'P')
          while (_getch() != 'P');
}

Does this help?  I'm having a hard time understanding your situation.  If this doesn't help explain in more detail what it is your trying to do.

It IS GOOD!jUST What I need!Thank you!