Signals

This is Software Interrupt to processor. For Example: Cntl^C, sends SIGINT to running process. Process should handle this. All signals start from SIG.

Signal Examples

Signal Signal Number Details
SIGHUP 1 Hang Up
When terminal controlling the process is closed
SIGINT 2 keyboard Interrupt(Cntrl C)
Can be caught?Yes If not caught, Terminate the process. Used for graceful termination
SIGQUIT 3 Keyboard Quit Cntl +\ OR Cntl Z
Can be caught? Yes Similar to SIGINT but Generates core dump on termination, cleans up resources
SIGBUS signal is sent to a process when it causes a bus error(access memory that he CPU cannot physically address: an invalid address for the address bus)
SIGTRAP 5 Breakpoint
Debuggers register and handle this signal.This is sent to debugger process when trap() occurs, for example, when a particular function is executed, or when a particular variable changes value
SIGABRT 6 Abort current process abort() system call

$ kill  -SIGABRT  pid
void main(){
    abort();    //Generates SIGABRT
}
SIGKILL 9 This CANNOT be caught. Will always terminate the process. Terminates immediately, does not allows process to do clean up

kill -9 pid     OR    
kill -SIGKILL pid
SIGSEGV(Segmentation fault) 11 Program accessed a memory location that was not assigned.

kill -9 pid     OR    
kill -SIGKILL pid
SIGALRM 14 Alarm clock

fun(){  printf("Hi");  }
void main(){
     signal(SIGALRM, fun); 
     alarm(1);    //alarm(t) generates SIGALRM after t sec.
}
SIGTERM 15 If not caught, Terminate the process. Used for graceful termination. 

fun(){  printf("Hi");  }
void main(){
     signal(SIGALRM, fun); 
     alarm(1);    //alarm(t) generates SIGALRM after t sec.
}
SIGCHLD 17 Child Process has stopped or Exited. It can be caught.
When child dies it send SIGCHLD to parent. 

void signalHandler(int signal){
    if(signal == SIGCHLD){
        printf("Child Dead");
    }
}
main() {
     signal(SIGCHLD,signalHandler);
     if(fork() == 0){    //Child Process
        sleep(5);
        printf("Child dying")
    }
}
SIGCONT 18 Continue the suspended process. It cannot be caught
Resume the stopped/suspended process. It always resumes the process.
SIGSTOP 19 Suspend the process donot Kill kill -STOP pid
It cannot be caught
SIGPIPE Scenario-1: When process attempts to write to a pipe whose Read-end is closed.

Parent                                    Child
            Write-end    Read-end
                |-parent pipe--|
                |-child pipe---|

Scenario-2: Attempt to write to socket what is no longer open for reading.
SIGUSR1, SIGUSR2 User Defined Signals. They donot have any special meaning as SIGTERM, SIGKILL etc.
Kernel will never send SIGUSR to process. Meaning of signal can be set up by user as per need of application.
SIGWINCH Signals Window Change sent automatically when a terminal detects a change in it's windows size to allow for a redraw.

Signal Mask

Collection of Signals blocked is called SM of process. sigprocmask() API is used to define Signal mask but in Single Threaded App. use pthread_sigmask() instead of sigprocmask()

Catching Signals


// This program will not terminate on Cntrl^C.

#include <signal.h>
#include <iostream>

void SIGINT_Handler(int signalNumber){
  std::cout << "Got Cntrl^C\n";
  return;
}

int main(){
  struct sigaction stAction;
  stAction.sa_flags = 0;
  sigemptyset (&stAction.sa_mask);
  
  //Adding Handler for SIGINT
  stAction.sa_handler = SIGINT_Handler;
  sigaction (SIGINT, &stAction, NULL);
  
  while(1);
}