两个版本的system 第二个版本用sigaction设置了信号处理

书中实现了两个版本的system函数，第二个版本中队信号进行了处理。

忽略了SIGINT和SIGQUIT，阻塞了SIGCHLD。

 

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

#include    <sys/wait.h>
#include    <errno.h>
#include    <unistd.h>

int
system1(const char *cmdstring)    /* version without signal handling */
{
    pid_t   pid;
    int     status;

    if (cmdstring == NULL)
        return(1);      /* always a command processor with UNIX */

    if ((pid = fork()) < 0) {
        status = -1;    /* probably out of processes */
    } else if (pid == 0) {              /* child */
        execl("/bin/sh", "sh", "-c", cmdstring, (char *)0);
        _exit(127);     /* execl error */
    } else {                             /* parent */
        while (waitpid(pid, &status, 0) < 0) {
            if (errno != EINTR) {
                status = -1; /* error other than EINTR from waitpid() */
                break;
            }
        }
    }

    return(status);
}

int
system2(const char *cmdstring)   /* with appropriate signal handling */
{
    pid_t               pid;
    int                 status;
    struct sigaction    ignore, saveintr, savequit;
    sigset_t            chldmask, savemask;

    if (cmdstring == NULL)
        return(1);      /* always a command processor with UNIX */

    ignore.sa_handler = SIG_IGN;    /* ignore SIGINT and SIGQUIT */
    sigemptyset(&ignore.sa_mask);
    ignore.sa_flags = 0;
    if (sigaction(SIGINT, &ignore, &saveintr) < 0)
        return(-1);
    if (sigaction(SIGQUIT, &ignore, &savequit) < 0)
        return(-1);
    sigemptyset(&chldmask);         /* now block SIGCHLD */
    sigaddset(&chldmask, SIGCHLD);
    if (sigprocmask(SIG_BLOCK, &chldmask, &savemask) < 0)
        return(-1);

    if ((pid = fork()) < 0) {
        status = -1;    /* probably out of processes */
    } else if (pid == 0) {          /* child */
        /* restore previous signal actions & reset signal mask */
        sigaction(SIGINT, &saveintr, NULL);
        sigaction(SIGQUIT, &savequit, NULL);
        sigprocmask(SIG_SETMASK, &savemask, NULL);

        execl("/bin/sh", "sh", "-c", cmdstring, (char *)0);
        _exit(127);     /* exec error */
    } else {                        /* parent */
       while (waitpid(pid, &status, 0) < 0)
           if (errno != EINTR) {
               status = -1; /* error other than EINTR from waitpid() */
               break;
           }
    }

    /* restore previous signal actions & reset signal mask */
    if (sigaction(SIGINT, &saveintr, NULL) < 0)
        return(-1);
    if (sigaction(SIGQUIT, &savequit, NULL) < 0)
        return(-1);
    if (sigprocmask(SIG_SETMASK, &savemask, NULL) < 0)
        return(-1);

    return(status);
}

static void
sig_int(int signo)
{
    printf("caught SIGINT/n");
}

static void
sig_chld(int signo)
{
    printf("caught SIGCHLD/n");
}

int
main(void)
{
     if (signal(SIGINT, sig_int) == SIG_ERR)
         perror("signal(SIGINT) error");
     if (signal(SIGCHLD, sig_chld) == SIG_ERR)
         perror("signal(SIGCHLD) error");
     if (system2("/bin/ed") < 0)
         perror("system() error");
     exit(0);
}