Question

How to design a C/C++ program so that it can save some data after receiving interrupt signal.

I have a long running program that I might need to kill (say, by pressing Ctrl-C) before it finished running. When killed (as opposed to running to conclusion) the program should be able to save some variables to disk. I have several big Linux books, but not very sure where to start. A cookbook recipe would be very helpful.

Thank you.!

Answer 1

to do that, you need to make your program watch something, for example a global variable, that will tell him to stop what it is doing.

For example, supposing your long-running program execute a loop, you can do that :

g_shouldAbort = 0;
while(!finished)
{
   // (do some computing)

   if (g_shouldAbort)
   {
      // save variables and stuff
      break; // exit the loop
   }
}

with g_shouldAbort defined as a global volatile variable, like that :

static volatile int g_shouldAbort = 0;

(It is very important to declare it “volatile”, or else the compiler, seeing that no one write it in the loop, may consider that if (g_shouldAbort) will always be false and optimize it away.)

then, using for example the signal API that other users suggested, you can do that :

void signal_handler(int sig_code)
{
   if (sig_code == SIGUSR1) // user-defined signal 1
      g_shouldAbort = 1;
}

(you need to register this handler of course, cf. here.

signal(SIGUSR, signal_handler);

Then, when you “send” the SIGUSR1 signal to your program (with the kill command for example), g_shouldAbort will be set to 1 and your program will stop its computing.

Hope this help !

NOTE : this technique is easy but crude. Using signals and global variables makes it difficult to use multiple threads of course, as other users have outlined.