Linux Signals

1. SIGNAL
UNIX SYSTEM
PROGRAMMING

2.  A signal is used to notify a process that a particular
event has occurred.
 Signals are software Interrupts
 A Signal may be received synchronously or
asynchronously.

3. Linux Signals
 In Linux, every signal has a name that begins with
characters SIG.
 A SIGINT signal that is generated when a user
presses ctrl+c. This is the way to terminate programs
from terminal.
 A SIGFPE is generated when the divide by zero
occurs.

4. Signal Handling
 Every signal may be handled by one of two possible
handlers.
 A default signal handler
 A user-defined signal handler

5. User-defined signal handler
 Every signal has a default signal handler that is run
by the kernel when handling that signal.
 The default action can be overridden by a user-
defined signal handler that is called to handle the
signal.
 Provide the handler function which has the syntax
 void handler_name(int signo);
 Register the signal handler with the signal
 Signal(Signal_Number , handler_name);

6. SIGSEGV

10. SIGINT

11. SIGALRM

12. SIGALRM
 #include <signal.h>
 void alarmhandler(int a)
 {
 printf("TIME OVER n");
 exit(0);
 }
 int main()
 {
 int i;
 signal(SIGALRM , alarmhandler);
 alarm(5);
 for(i = 0 ; i < 10 ; i++)
 {
 printf(" %dn" , i);
 sleep(1);
 }
 printf("END OF PROGRAM n");
 }

13. KILL - send signal to a process
 void handler(int a)
 {
 printf(" %d n" ,a);
 }
 int main()
 {
 signal(SIGALRM , handler);
 signal(SIGSEGV , handler);
 signal(SIGFPE , handler);
 #ifdef SEGV
 kill(getpid() , SIGSEGV);
 #endif
 #ifdef FPE
 kill(getpid() , SIGFPE);
 #endif
 }

15. SIGSEGV
 int main()
 {
 int x = 5 ;
 signal(SIGSEGV , myfunc);
 printf("Enter a number : n");
 scanf(" %d",x);
 printf("The number is %d",x );
 }
 void myfunc(int a)
 {
 printf("ENTERED MYHANDLER n");
 printf("signal recieved : %d n",a);
 exit(1);
 }

16. SIGCHLD – Indicates the death of a child
process.

17. Assignment
 Implement wait using SIGCHILD

18. Ignoring a Signal SIG_IGN
 void myhandler(int a)
 {
 printf("ENTERED MYHANDLER n");
 printf("signal recieved : %d n",a);
 exit(1);
 }
 int main()
 {
 int x = 0;
 printf("1. Default 2. Ignore 3. Handler n");
 scanf(" %d",&x);
 switch(x)
 {
 case 1 : signal(SIGINT , SIG_DFL); break;
 case 2 : signal(SIGINT , SIG_IGN); break;
 case 3 : signal(SIGINT , myhandler); break;
 }
 sleep(5);
 printf("END OF PROGRAM ");
 }