Hacking - The Art of Exploitation, 2nd Edition

Countermeasures 323

printf("Caught signal %d\t", signal);
if (signal == SIGTSTP)
printf("SIGTSTP (Ctrl-Z)");
else if (signal == SIGQUIT)
printf("SIGQUIT (Ctrl-\)");
else if (signal == SIGUSR1)
printf("SIGUSR1");
else if (signal == SIGUSR2)
printf("SIGUSR2");
printf("\n");
}

void sigint_handler(int x) {
printf("Caught a Ctrl-C (SIGINT) in a separate handler\nExiting.\n");
exit(0);
}

int main() {
/ Registering signal handlers /
signal(SIGQUIT, signal_handler); // Set signal_handler() as the
signal(SIGTSTP, signal_handler); // signal handler for these
signal(SIGUSR1, signal_handler); // signals.
signal(SIGUSR2, signal_handler);

signal(SIGINT, sigint_handler); // Set sigint_handler() for SIGINT.

while(1) {} // Loop forever.
}

When this program is compiled and executed, signal handlers are

registered, and the program enters an infinite loop. Even though the program

is stuck looping, incoming signals will interrupt execution and call the

registered signal handlers. In the output below, signals that can be triggered

from the controlling terminal are used. The signal_handler() function,

when finished, returns execution back into the interrupted loop, whereas

the sigint_handler() function exits the program.

reader@hacking:~/booksrc $ gcc -o signal_example signal_example.c
reader@hacking:~/booksrc $ ./signal_example
Caught signal 20 SIGTSTP (Ctrl-Z)
Caught signal 3 SIGQUIT (Ctrl-)
Caught a Ctrl-C (SIGINT) in a separate handler
Exiting.
reader@hacking:~/booksrc $

Specific signals can be sent to a process using the kill command. By

default, the kill command sends the terminate signal (SIGTERM) to a process.

With the -l command-line switch, kill lists all the possible signals. In the

output below, the SIGUSR1 and SIGUSR2 signals are sent to the signal_example

program being executed in another terminal.