#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
 
//
// gcc -g -O2 -Wall -Wextra -o demo demo.c
// 
int main(void) {
    // 创建守护进程
    if (daemon(0, 0)) {
        perror("daemon error");
        exit(EXIT_FAILURE);
    }
    // 守护进程 构建文本任务
    FILE * txt = fopen("demo.log", "w");
    if (txt) {
        fprintf(txt, "%ld, hello, 世界", time(NULL));
        fclose(txt);
    }
    exit(EXIT_SUCCESS);
}

DAEMON(3)                  Linux Programmer's Manual                 DAEMON(3)
NAME
       daemon - run in the background
SYNOPSIS
       #include <unistd.h>
       int daemon(int nochdir, int noclose);
   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
       daemon():
           Since glibc 2.21:
               _DEFAULT_SOURCE
           In glibc 2.19 and 2.20:
               _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
           Up to and including glibc 2.19:
               _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
DESCRIPTION
       The daemon() function is for programs wishing to detach themselves from
       the controlling terminal and run in the background as system daemons.
       If nochdir is zero, daemon()  changes  the  process's  current  working
       directory  to  the root directory ("/"); otherwise, the current working
       directory is left unchanged.
       If noclose is zero, daemon() redirects standard input, standard  output
       and  standard  error  to  /dev/null;  otherwise, no changes are made to
       these file descriptors.
RETURN VALUE
       (This function forks, and if the fork(2)  succeeds,  the  parent  calls
       _exit(2),  so that further errors are seen by the child only.)  On suc‐
       cess daemon() returns zero.  If an error occurs,  daemon()  returns  -1
       and  sets errno to any of the errors specified for the fork(2) and set‐
       sid(2).
ATTRIBUTES
       For  an  explanation  of  the  terms  used   in   this   section,   see
       attributes(7).
       ┌──────────┬───────────────┬─────────┐
       │Interface │ Attribute     │ Value   │
       ├──────────┼───────────────┼─────────┤
       │daemon()  │ Thread safety │ MT-Safe │
       └──────────┴───────────────┴─────────┘
CONFORMING TO
       Not  in POSIX.1.  A similar function appears on the BSDs.  The daemon()
       function first appeared in 4.4BSD.
NOTES
       The glibc implementation can also return -1 when /dev/null  exists  but
       is  not  a  character device with the expected major and minor numbers.
       In this case, errno need not be set.
BUGS
       The GNU C library implementation of this function was taken  from  BSD,
       and  does  not  employ  the  double-fork technique (i.e., fork(2), set‐
       sid(2), fork(2)) that is necessary to ensure that the resulting  daemon
       process  is  not  a session leader.  Instead, the resulting daemon is a
       session leader.  On systems  that  follow  System  V  semantics  (e.g.,
       Linux),  this  means  that  if  the daemon opens a terminal that is not
       already a controlling terminal for another session, then that  terminal
       will inadvertently become the controlling terminal for the daemon.
SEE ALSO
       fork(2), setsid(2), daemon(7), logrotate(8)
COLOPHON
       This  page  is  part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.
GNU                               2017-11-26                         DAEMON(3)

NAME
       daemon - 运行在后台
SYNOPSIS
       #include <unistd.h>
       int daemon(int nochdir, int noclose);
DESCRIPTION
        daemon() 函数希望运行程序脱离控制终端, 作为系统守护进程在后台运行.
        如果 nochdir 是 0, daemon() 将更改当前进程工作目录到 "/" 根目录. 否则保持
        不变.
        如果 noclose 是 0, deamon() 将重定向 STDIN_FILENO 标准输入, STDOUT_FILENO
        标准输出, STDERR_FILENO 标准错误 到 /dev/null, 否则保持不变.
RETURN VALUE
        函数内部会执行 fork, 如果 fork 成功, 父进程会调用 _exit 退出. 执行成功返回 0. 
        发生错误时候将返回 -1, errno 的设置依赖 fork(), setsid(), daemon() 源码.

 1 /*-
 2  * Copyright (c) 1990, 1993
 3  *    The Regents of the University of California.  All rights reserved.
 4  *
 5  * Redistribution and use in source and binary forms, with or without
 6  * modification, are permitted provided that the following conditions
 7  * are met:
 8  * 1. Redistributions of source code must retain the above copyright
 9  *    notice, this list of conditions and the following disclaimer.
10  * 2. Redistributions in binary form must reproduce the above copyright
11  *    notice, this list of conditions and the following disclaimer in the
12  *    documentation and/or other materials provided with the distribution.
13  * 4. Neither the name of the University nor the names of its contributors
14  *    may be used to endorse or promote products derived from this software
15  *    without specific prior written permission.
16  *
17  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
18  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
19  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
20  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
21  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
22  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
23  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
24  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
25  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
26  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
27  * SUCH DAMAGE.
28  */
29 
30 #if defined(LIBC_SCCS) && !defined(lint)
31 static char sccsid[] = "@(#)daemon.c    8.1 (Berkeley) 6/4/93";
32 #endif /* LIBC_SCCS and not lint */
33 
34 #include <errno.h>
35 #include <fcntl.h>
36 #include <paths.h>
37 #include <unistd.h>
38 #include <sys/stat.h>
39 
40 #include <device-nrs.h>
41 #include <not-cancel.h>
42 
43 int
44 daemon (int nochdir, int noclose)
45 {
46     int fd;
47 
48     switch (__fork()) {
49     case -1:
50         return (-1);
51     case 0:
52         break;
53     default:
54         _exit(0);
55     }
56 
57     if (__setsid() == -1)
58         return (-1);
59 
60     if (!nochdir)
61         (void)__chdir("/");
62 
63     if (!noclose) {
64         struct stat64 st;
65 
66         if ((fd = __open_nocancel(_PATH_DEVNULL, O_RDWR, 0)) != -1
67             && (__builtin_expect (__fstat64 (fd, &st), 0)
68             == 0)) {
69             if (__builtin_expect (S_ISCHR (st.st_mode), 1) != 0
70 #if defined DEV_NULL_MAJOR && defined DEV_NULL_MINOR
71                 && (st.st_rdev
72                 == makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR))
73 #endif
74                 ) {
75                 (void)__dup2(fd, STDIN_FILENO);
76                 (void)__dup2(fd, STDOUT_FILENO);
77                 (void)__dup2(fd, STDERR_FILENO);
78                 if (fd > 2)
79                     (void)__close (fd);
80             } else {
81                 /* We must set an errno value since no
82                    function call actually failed.  */
83                 __close_nocancel_nostatus (fd);
84                 __set_errno (ENODEV);
85                 return -1;
86             }
87         } else {
88             __close_nocancel_nostatus (fd);
89             return -1;
90         }
91     }
92     return (0);
93 }

// sysdeps/generic/not-cancel.h
 
/* By default we have none.  Map the name to the normal functions.  */
#define __open_nocancel(...) \
  __open (__VA_ARGS__)
#define __close_nocancel(fd) \
  __close (fd)

BUGS
       The GNU C library implementation of this function was taken  from  BSD,
       and  does  not  employ  the  double-fork technique (i.e., fork(2), set‐
       sid(2), fork(2)) that is necessary to ensure that the resulting  daemon
       process  is  not  a session leader.  Instead, the resulting daemon is a
       session leader.  On systems  that  follow  System  V  semantics  (e.g.,
       Linux),  this  means  that  if  the daemon opens a terminal that is not
       already a controlling terminal for another session, then that  terminal
       will inadvertently become the controlling terminal for the daemon.
BUGS
        GNUC 库 这个 daemon() 函数的实现取自 BSD 源码. 没有采用两次 double fork
        设置 sid 机制, 来确保生成的守护进程不是会话负责人. 相反, 这里生成的守护
        进程是会话负责人 (session leader). 在遵循 System V 语义系统上, 创建的守
        护进程在重新打开终端时候, 新开终端会自动成为守护进程的控制终端.

/*
 * 创建守护进程
 */
void daemon_service(void) {
    // fork 后父进程 exit 退出, 保证子进程可以成功 setsid() 拥有一个新会话
    switch (fork()) {
    case -1:
        exit(EXIT_FAILURE);
    case 0:
        break;    
    default:
        exit(EXIT_SUCCESS);
    }
    // 子进程创建新会话
    // 执行成功后 Process ID(PID) == Process Group ID(PGID) == Session ID(SID)
    if (setsid() == -1)
        exit(EXIT_FAILURE);
    // 二次 fork 后孙进程不再是会话组首进程, 因而孙进程无法重新打开一个新的控制终端
    // 执行成功后 Process ID(PID) != Process Group ID(PGID) == Session ID(SID)
    switch (fork()) {
    case -1:
        exit(EXIT_FAILURE);
    case 0:
        break;    
    default:
        exit(EXIT_SUCCESS);
    }
    // 子进程设置新的工作目录是 根目录 "/", 避免存在挂载磁盘一直被占用的情况
    if (chdir("/")) {}
    // 子进程重置 创建文件 权限
    umask(0);
    // 相关句柄善后, 节省资源
    fflush(stderr);
    fflush(stdout);
    for (int fd = sysconf(_SC_OPEN_MAX); fd >= 0; fd--)
        close(fd);
}