Signal Handling in C

Question

How can I implement signal Handling for Ctrl-C and Ctrl-D in C....So If Ctrl-C is pressed then the program will ignore and try to get the input from the user again...If Ctrl-D is pressed then the program will terminate...

My program follows:

int main(){
 char msg[400];

 while(1){
   printf("Enter: ");
   fgets(msg,400,stdin);
   printf("%s\n",msg); 
 }
}

Thanks,

Dave

Answer 1

In your example it seems you don't need CTRL-C handlind at all. A "signal(SIGINT,SIG_IGN)" seems enough for you, unless your application must handle a SIGINT coming from some other source. CTRL-D doesn't usually generates signals, it simply communicates the EOF condition. You can in general control the behavior of your terminal (we are talking about console input, it isn't?) by using the termios library (also here). You can enable, redefine or disable the "interrupt" character (CTRL-C), the EOF one and many other ones (XON, XOFF, modem control...)

Regards

Answer 2

When dealing with POSIX signals, you have two means at your disposal. First, the easy (but depreciated) way, signal(). Second, the more elegant, current but complex way, sigaction(). Please use sigaction() unless you find that it isn't available on some platform that you need to work on.

This chapter of the glibc manual explains differences between the two and gives good example code on how to use both. It also lists the signals that can be handled, recommends how they should be handled and goes more in depth on how to tell how any given signal is (or is not) currently being handled. That's way more code than I'd want to paste into an answer here, hence the links.

It really is worth the hour or two it would take you to read the links and work through the examples. Signal handling (especially in programs that daemonize) is extremely important. A good program should handle all fatal signals that can be handled (i.e. SIGHUP) and explicitly ignore signals that it might not be using (i.e. SIGUSR1 / SIGUSR2).

It also won't hurt to study the difference between normal and real time signals, at least up to the understanding of how the kernel merges the prior and not the latter.

Once you work through it, you'll probably feel inclined to write up an easy to modify set of functions to handle your signals and re-use that code over and over again.

Sorry for not giving a quick and dirty code snippet to show you how to solve your immediate need, but this isn't a quick and dirty topic :)