daemon_init函数：调用该函数把普通进程转变为守护进程

#include    <unistd.h>
#include    <syslog.h>
#include    <fcntl.h>
#include    <signal.h>#define    MAXFD    64extern int    daemon_proc;      /* defined in error.c */int daemon_init(const char *pname, int facility)
{
    int        i;
    pid_t    pid;    if ( (pid = fork()) < )
        return (-);
    else if (pid)
        _exit();               /* parent terminates */    /* child 1 continues... */    if (setsid() < )           /* become session leader */
        return (-);    signal(SIGHUP, SIG_IGN);
    if ( (pid = fork()) < )
        return (-);
    else if (pid)
        _exit();               /* child 1 terminates */    /* child 2 continues... */    daemon_proc = ;            /* for err_XXX() functions */    chdir("/");                 /* change working directory */    /* close off file descriptors */
    for (i = ; i < MAXFD; i++)
        close(i);    /* redirect stdin, stdout, and stderr to /dev/null */
    open("/dev/null", O_RDONLY);
    open("/dev/null", O_RDWR);
    open("/dev/null", O_RDWR);    openlog(pname, LOG_PID, facility);    return ();                 /* success */
}