system.c

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

int system (const char *cmdstring)  /* with appropriate signal handling */
{
    pid_t                  pid;
    int                    status;
    struct sigaction       ignore, saveintr, savequit;
    sigset_t               childmask, 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 (&childmask);        /* now block SIGCHLD */
    sigaddset (&childmask, SIGCHLD);

    if (sigprocmask (SIG_BLOCK, &childmask, &savemask) < 0)
        return (-1);
    
    if ( (pid = fork ()) < 0) {
        status = -1;    /* probably out of processes */
    
    }
    else if (pid == 0) {            /* child */
        /* restore previous signal actions & rest 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;   /* errno other than EINTR form 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);
}