5 // Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot com)
7 // Distributed under the Boost Software License, Version 1.0. (See accompanying
8 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
11 #include <boost/asio/io_context.hpp>
12 #include <boost/asio/ip/udp.hpp>
13 #include <boost/asio/signal_set.hpp>
20 using boost::asio::ip::udp
;
22 class udp_daytime_server
25 udp_daytime_server(boost::asio::io_context
& io_context
)
26 : socket_(io_context
, {udp::v4(), 13})
34 socket_
.async_receive_from(
35 boost::asio::buffer(recv_buffer_
), remote_endpoint_
,
36 [this](boost::system::error_code ec
, std::size_t /*n*/)
40 using namespace std
; // For time_t, time and ctime;
42 std::string message
= ctime(&now
);
44 boost::system::error_code ignored_ec
;
45 socket_
.send_to(boost::asio::buffer(message
),
46 remote_endpoint_
, 0, ignored_ec
);
54 udp::endpoint remote_endpoint_
;
55 std::array
<char, 1> recv_buffer_
;
62 boost::asio::io_context io_context
;
64 // Initialise the server before becoming a daemon. If the process is
65 // started from a shell, this means any errors will be reported back to the
67 udp_daytime_server
server(io_context
);
69 // Register signal handlers so that the daemon may be shut down. You may
70 // also want to register for other signals, such as SIGHUP to trigger a
71 // re-read of a configuration file.
72 boost::asio::signal_set
signals(io_context
, SIGINT
, SIGTERM
);
74 [&](boost::system::error_code
/*ec*/, int /*signo*/)
79 // Inform the io_context that we are about to become a daemon. The
80 // io_context cleans up any internal resources, such as threads, that may
81 // interfere with forking.
82 io_context
.notify_fork(boost::asio::io_context::fork_prepare
);
84 // Fork the process and have the parent exit. If the process was started
85 // from a shell, this returns control to the user. Forking a new process is
86 // also a prerequisite for the subsequent call to setsid().
87 if (pid_t pid
= fork())
91 // We're in the parent process and need to exit.
93 // When the exit() function is used, the program terminates without
94 // invoking local variables' destructors. Only global variables are
95 // destroyed. As the io_context object is a local variable, this means
96 // we do not have to call:
98 // io_context.notify_fork(boost::asio::io_context::fork_parent);
100 // However, this line should be added before each call to exit() if
101 // using a global io_context object. An additional call:
103 // io_context.notify_fork(boost::asio::io_context::fork_prepare);
105 // should also precede the second fork().
110 syslog(LOG_ERR
| LOG_USER
, "First fork failed: %m");
115 // Make the process a new session leader. This detaches it from the
119 // A process inherits its working directory from its parent. This could be
120 // on a mounted filesystem, which means that the running daemon would
121 // prevent this filesystem from being unmounted. Changing to the root
122 // directory avoids this problem.
125 // The file mode creation mask is also inherited from the parent process.
126 // We don't want to restrict the permissions on files created by the
127 // daemon, so the mask is cleared.
130 // A second fork ensures the process cannot acquire a controlling terminal.
131 if (pid_t pid
= fork())
139 syslog(LOG_ERR
| LOG_USER
, "Second fork failed: %m");
144 // Close the standard streams. This decouples the daemon from the terminal
150 // We don't want the daemon to have any standard input.
151 if (open("/dev/null", O_RDONLY
) < 0)
153 syslog(LOG_ERR
| LOG_USER
, "Unable to open /dev/null: %m");
157 // Send standard output to a log file.
158 const char* output
= "/tmp/asio.daemon.out";
159 const int flags
= O_WRONLY
| O_CREAT
| O_APPEND
;
160 const mode_t mode
= S_IRUSR
| S_IWUSR
| S_IRGRP
| S_IROTH
;
161 if (open(output
, flags
, mode
) < 0)
163 syslog(LOG_ERR
| LOG_USER
, "Unable to open output file %s: %m", output
);
167 // Also send standard error to the same log file.
170 syslog(LOG_ERR
| LOG_USER
, "Unable to dup output descriptor: %m");
174 // Inform the io_context that we have finished becoming a daemon. The
175 // io_context uses this opportunity to create any internal file descriptors
176 // that need to be private to the new process.
177 io_context
.notify_fork(boost::asio::io_context::fork_child
);
179 // The io_context can now be used normally.
180 syslog(LOG_INFO
| LOG_USER
, "Daemon started");
182 syslog(LOG_INFO
| LOG_USER
, "Daemon stopped");
184 catch (std::exception
& e
)
186 syslog(LOG_ERR
| LOG_USER
, "Exception: %s", e
.what());
187 std::cerr
<< "Exception: " << e
.what() << std::endl
;