catchsignal

Applications Programming
1

how does the catchsignal know what signal it needs to ignore? thanks

2

catchsignal isn't part of POSIX UNIX, it sounds like a function inside a program - one somebody wrote.

Can you tell us what you are doing, and what you are trying to answer.

3

hey jim. my prof gave me this program...

#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>

void catchsignal(int signo) {
   char handmsg[] = "You cannot \"Ctrl-C\" me!\n";
   int msglen = sizeof(handmsg);

   write(STDERR_FILENO, handmsg, msglen);
}


int main() {

   struct sigaction act;
   act.sa_handler = catchsignal;
   act.sa_flags = 0;
   if ((sigemptyset(&act.sa_mask) == -1) ||
      (sigaction(SIGINT, &act, NULL) == -1)) {
      printf("Failed to set SIGINT to handle Ctrl-C");
   }

   while(1);
}

that program catches the signal Ctrl C which if I'm not mistaken is one of the kill commands in linux. i can't remember what specific kill command is that. my question is how does the catchsignal function learn which signal to ignore? thanks.

4

Only signal(s) you 'intercept' will be handled by catchsignal.
So you cant kill that program with ctrl-c but try delivering it signal like TERM

kill -TERM pid

and you'll see ...

5

yup, i know it wouldn't be able to ignore the sigterm...i just want to know how it intercept the ctrl-c...?

6 
struct sigaction act;
   act.sa_handler = catchsignal;
   act.sa_flags = 0;
   if ((sigemptyset(&act.sa_mask) == -1) ||
      (sigaction(SIGINT, &act, NULL) == -1))

This bunch of statements is where the signal handler is being setup. The "act.sa_handler=catchsignal;" statement spells out the function that will be used (catchsignal). And the statement signaction(SIGINT,&act,NULL) specifies that values from the act structure are to be associated with the reception of SIGINT.

7

thank you very much.

8

one more question...how could i get out of the if loop in the statement above. say i want to have another catchsignal...?

9

Here's another way of writing the code:

#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>

void catchsignal(int signo) {
        fprintf(stdout,"Ha! Caught signal %d\n",signo);
}


int main() {

        struct sigaction act;
        act.sa_handler = catchsignal;
        act.sa_flags = 0;
        if(sigemptyset(&act.sa_mask)==-1) {
                perror("error in sigemptyset!!\n");
        }
        if(sigaction(SIGINT,&act,NULL)==-1) {
                perror("Failed to set SIGINT to handle Ctrl-C\n");
        }
        if(sigaction(SIGQUIT,&act,NULL)==-1) {
                perror("Failed to set SIGQUIT to handle Ctrl-\\\n");
        }

while(1);
}

You can just keep adding signals to that list.

10

As a side note - you might be better off with using write() in a signal handler.