[SysPro] SystemCall, Signal

// -------------------- System Call ------------

//mysys.c

#include <stdio.h>
int main(int argc, char *argv[]) {
    int status;
    if (argc < 2) {
        printf("\nCommand-line argument required!\n");
        exit(0);
    }
    if ((status = system(argv[1])) < 0)
        perror("system() error");
    return 0;
}

//showpid.c
#include <stdio.h>
int main(void) {
    printf("Real UID = %d,
        Eff UID = %d\n",
        getuid(), geteuid());
    return 0;
}

//-------------------- Signal --------------

//signal1.c

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

void my_alarm(int sig) {
    for (int i = 0; i < 5; i++)
    printf("Alarm!!! \n");
}
int main(void) {
    int pid;
    printf("Alarm clock is starting...\n");
    if ((pid = fork()) == 0) {
        sleep(3);
        kill(getppid(), SIGALRM);
        exit(0);
    }
    printf("Waiting for alarm...\n");
    (void) signal(SIGALRM, my_alarm);
    pause();
    printf("Done!\n");
    exit(0);
}

//signal2.c

#include <sys/types.h>
#include <signal.h>
#include <unistd.h>
#include <stdio.h>
/* one handler for both signals */
static void sig_usr(int);
int main(void) {
    if (signal(SIGUSR1, sig_usr) == SIG_ERR)
        perror("can't catch SIGUSR1");
    if (signal(SIGUSR2, sig_usr) == SIG_ERR)
        perror("can't catch SIGUSR2");
    for ( ; ; )
        pause();
}
/* argument is signal number */
static void sig_usr(int signo) {
    if (signo == SIGUSR1)
        printf("received SIGUSR1\n");
    else if (signo == SIGUSR2)
        printf("received SIGUSR2\n");
    else
        printf("received signal %d\n", signo);
}

//signal3.c

#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

static void sig_cld(int);
int main()
{
    pid_t pid;
    if (signal(SIGCLD, sig_cld) == SIG_ERR)
        perror("signal error");
    if ((pid = fork()) < 0)
        perror("fork error");
    else if (pid == 0)
    {
        /* child */ sleep(2);
        _exit(0);
    }
    pause(); /* parent */
    exit(0);
}
static void sig_cld(int signo)
/* interrupts pause() */
{
    pid_t pid;
    int status;
    printf("SIGCLD received\n");
    /* reestablish handler */
    if (signal(SIGCLD, sig_cld) == SIG_ERR)
        perror("signal error");
    /* fetch child status */
    if ((pid = wait(&status)) < 0)
        perror("wait error");
    printf("pid = %d\n", pid);
    return;
}

//sigaction.c

#include <signal.h>
#include <stdio.h>
#include <unistd.h>
void rx_int(int sig)
{
    printf("SIGINT - %d Received!!!\n", sig);
}
int main(void)
{
    struct sigaction act;
    act.sa_handler = rx_int;
    sigemptyset(&act.sa_mask);
    act.sa_flags = 0;
    sigaction(SIGINT, &act, 0);
    while(1)
    {
        printf("Running…\n");
        sleep(1);
    }
}

//killthatshit.c

#include <signal.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int total_length = 20;
    char line[total_length];
    FILE * command = popen("pidof sigaction","r");

    fgets(line,total_length,command);

    pid_t pid = strtoul(line,NULL,10);
    pclose(command);

    printf("PID IS : %d\n", pid);
    kill(pid, SIGTERM);
}