第十三章：守护进程

#include "apue.h"
#include <syslog.h>
#icnldue <fcntl.h>
#include <sys/resource.h>

void daemonize(const char *cmd)
{
    int i, fd0, fd1, fd2;
    pid_t pid;
    struct rlimit rl;
    struct sigaction sa;
    
    // clear file creation mask
    umask(0);
    
    // get maximum number of file descriptors
    if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
    {
        err_quit("%s:can‘t get file limit", cmd);
    }
    
    // become a session leader to lose controlling tty
    if ((pid = fork()) < 0)
    {
        err_quit("%s: can‘t fork", cmd);
    }
    else if (pid != 0)
    {
        exit(0);
    }
    setsid();
    
    // ensure future opens won‘t allocate controlling ttya
    sa.sa_handle = SIG_IGN;
    sigemptyset(&sa.sa_mask);
    sa.sa_flag = 0;
    if (sigaction(SIGHUB, &sa, NULL) < 0)
    {
        err_quit("%s: can‘t ignore SIGHUB");
    }
    if ((pid = fork()) < 0)
    {
        err_quit("%s: can‘t fork", cmd);
    }
    else if (pid != 0)
    {
        exit(0);
    }
    
    // change the current working directory to the root so we won‘t prevent file systems from being unmounted
    if (chdir("/") < 0)
    {
        err_quit("%s: can‘t change directory to /");
    }
    
    // close all open file descriptors
    if (rl.rlim_max == RLIM_INFINITY)
    {
        rl.rlim_max = 1024;
    }
    for (i = 0; i < rl.rlim_max; i++)
    {
        close(i);
    }
    
    // attach file descriptors 0, 1, and 2 to /dev/null
    fd0 = open("/dev/null", O_RDWR);
    fd1 = dup(0);
    fd2 = dup(0);
    
    // initialize the log file
    openlog(cmd, LOG_CONS, LOG_DAEMON);
    if (fd0 != 0 || fd1 != 1 || fd2 != 2)
    {
        syslog(LOG_ERR, "unexpected file descriptors %d %d %d", fd0, fd1, fd2);
        exit(1);
    }
}

#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <syslog.h>
#include <string.h>
#include <errno.h>
#include <stdio.h>
#include <sys/stat.h>

#define LOCKFILE "/var/run/daemon.pid"
#define LOCKMODE (S_IRUSR | S_IWUSR | S_IRGRP | s_IPOTH)

extern int lockfile(int);

int already_running(void)
{
    int fd;
    char buf[16];
    
    fd = open(LOCKFILE, O_RDWR | O_CREAT, LOCKMODE);
    if (fd < 0)
    {
        syslog(LOG_ERR, "can‘t open %s: %s", LOCKFILE, strerror(errno));
        exit(1);
    }
    if (lockfile(fd) < 0)
    {
        if (errno == EACCES || errno == EAGAIN)
        {
            close(fd);
            return 1;
        }
        syslog(LOG_ERR, "can‘t lock %s: %s", LOCKFILE, strerror(errno));
        exit(1);
    }
    ftruncate(fd, 0);
    sprintf(buf, "%ld", (long)getpid());
    write(fd, buf, strlen(buf) + 1);
    return 0;
}