Operating Systems 2019W Lecture 6

Video

Video from the lecture given on January 23, 2019 is now available.

Notes

Signals

• processes can register signal handler functions for specific signals
• When the kernel delivers a signal to a process, it runs the specified handler
• The C library defines default handlers for all signals (except STOP and KILL)
• When a process gets a signal, current execution is interrupted and the handler is invoked. When the handler terminates, the process continues where it was
• Interrupted system calls return with an error EINTR.
• If the process was blocked on a system call, the system call is interrupted and the handler is run.
• The standard library can do different things with interrupted system calls. The SA_RESTART flag says to run the system call again. It only applies to a few system calls, primarily read-like ones. (So, when a read is interrupted, it is tried again.)

I/O redirection

Command-line arguments and environment variables

• They are both stored in the memory of the process.
• They are put there by execve, as part of loading a new executable into a process (and erasing everything that was in the process's address space).
• execve doesn't necessarily change other aspects of process's state, thus they should be initialized by something else (normally, the C library)
• In particular, the data structures associated with file descriptors (i.e., open files) are preserved across execve.

Code

reactive.c

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

void signal_handler(int the_signal)
{
        if (the_signal == SIGUSR1) {
                fprintf(stderr, "Ouch!\n");
                return;
        }
}

int main(int argc, char *argv[], char *envp[])
{
        int i = 1;
        
        struct sigaction signal_handler_struct;

        memset(&signal_handler_struct, 0, sizeof(signal_handler_struct));
        signal_handler_struct.sa_handler = signal_handler;
        signal_handler_struct.sa_flags = SA_RESTART;

        if (sigaction(SIGUSR1, &signal_handler_struct, NULL)) {
                fprintf(stderr, "Couldn't register SIGUSR1 handler.\n");
        }

        printf("Hello!\n");
        printf("Environment variables at %lx\n", (unsigned long) envp);
        printf("Argument variables at %lx\n", (unsigned long) argv);
        printf("Sitting around doing nothing...\n");

        while (1) {
                sleep(i);
                i++;
        }
        
        return 0;
}