Signals

1. Types of Signals

1.1 Special Keys in Terminal

• ctl + c generates SIGINT
• ctl + z generates SIGTSTP
• ctl + \ generates SIGQUIT
• kill -9 pid generates SIGKILL

1.2 Hardware Exceptions

• Illegal Instruction
• Privilege Instruction
• Division by Zero
• Segmentation Fault
• Bus Error

1.3 Software Exceptions

• Illegal Memory Access
• Out of Bounds Access
• Stack Overflow

1.4 Operating System Exceptions

• Illegal System Call
• Invalid System Command
• Insufficient System Resources

1.5 User-Defined Signals

• kill -sig pid
• kill -SIGINT pid
• kill -15 pid
• kill -SIGTERM pid
• kill -1 pid

Signal Functions

kill(pid_t pid,int sig)

raise(int sig) function

abort(void) function

Signal Generation

• Hardware Interrupt
• System Call
• Software Interrupt

Signal Capture and Handling

• signal function

#include <signal.h>
void (*signal(int signum, void (*func)(int)))(int);

• sigaction function

#include <signal.h>
int sigaction(int signum, const struct sigaction *act, struct sigaction *oact);
struct sigaction {
    void (*sa_handler)(int);
    sigset_t sa_mask;
    int sa_flags;
    void (*sa_sigaction)(int, siginfo_t *, void *);
};

Signal Set Handling Functions

sigset_t is a signal set, signal set operation functions

• sigemptyset initializes the signal set to empty
• sigfillset initializes the signal set to all signals
• sigaddset adds a signal to the signal set
• sigdelset removes a signal from the signal set
• sigismember checks if a signal is in the signal set
• sigprocmask blocks/unblocks the signal set
• sigpending checks the currently blocked signals
• sigsuspend blocks the signal set until a signal in the set is caught
• sigwait waits for a signal in the signal set
• sigwaitinfo waits for a signal in the signal set and retrieves detailed information about the signal
• sigtimedwait waits for a signal in the signal set and retrieves detailed information about the signal, while allowing a timeout
• sigaction captures signals
• siginterrupt sets whether the signal interrupts system calls

PCB saves: Pending Signal Set

Blocked Signal Set

Delivered State: Signal generated and responded to; Pending State: Signal generated but not responded to

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

void printsigset(const  __sigset_t *set)
{
    int i;
    // Iterate through signal set
    for (i = 1; i < NSIG; ++i) {
        // Check if signal set contains signal i
        if (sigismember(set, i) == 1)
            // If it contains, print signal number
            printf("%d ", i);
        else
            // If not, print 0
            printf("0  ");
    }
}

int main(int argc, char const *argv[])
{
    // Define signal sets s and p
    __sigset_t s,  p;

    // Clear signal set s
    __sigemptyset(&amp;s);

    // Add SIGINT signal to signal set s
    __sigaddset(&amp;s, SIGINT);

    // Block signals in signal set s
    sigprocmask(SIG_BLOCK, &amp;s, NULL);

    // Enter infinite loop
    while (1)
    {
        // Get currently blocked signals set p
        sigpending(&amp;p);

        // Print signal set p
        printsigset(&amp;p);

        // Sleep for 1 second
        sleep(1);
    }
    
    return 0;
}

Blocking and Unblocking

• sigprocmask
• sigpending
• sigsuspend

2. Process Behavior in Handling Signals

SIG_IGN ignores the signal

SIG_DFL default handling action

a signal handling function captures the signal

Default Signal Handling for 5 Cases

Term terminates the process

Ign ignores the signal

Core terminates the process and dumps memory

Stop pauses the process

Cont continues execution of the paused process