1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
|
//
// daemon.cpp
// ~~~~~~~~~~
//
// Copyright (c) 2003-2023 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
#include <boost/asio/io_context.hpp>
#include <boost/asio/ip/udp.hpp>
#include <boost/asio/signal_set.hpp>
#include <boost/array.hpp>
#include <boost/bind/bind.hpp>
#include <ctime>
#include <iostream>
#include <syslog.h>
#include <unistd.h>
using boost::asio::ip::udp;
class udp_daytime_server
{
public:
udp_daytime_server(boost::asio::io_context& io_context)
: socket_(io_context, udp::endpoint(udp::v4(), 13))
{
start_receive();
}
private:
void start_receive()
{
socket_.async_receive_from(
boost::asio::buffer(recv_buffer_), remote_endpoint_,
boost::bind(&udp_daytime_server::handle_receive,
this, boost::placeholders::_1));
}
void handle_receive(const boost::system::error_code& ec)
{
if (!ec)
{
using namespace std; // For time_t, time and ctime;
time_t now = time(0);
std::string message = ctime(&now);
boost::system::error_code ignored_ec;
socket_.send_to(boost::asio::buffer(message),
remote_endpoint_, 0, ignored_ec);
}
start_receive();
}
udp::socket socket_;
udp::endpoint remote_endpoint_;
boost::array<char, 1> recv_buffer_;
};
int main()
{
try
{
boost::asio::io_context io_context;
// Initialise the server before becoming a daemon. If the process is
// started from a shell, this means any errors will be reported back to the
// user.
udp_daytime_server server(io_context);
// Register signal handlers so that the daemon may be shut down. You may
// also want to register for other signals, such as SIGHUP to trigger a
// re-read of a configuration file.
boost::asio::signal_set signals(io_context, SIGINT, SIGTERM);
signals.async_wait(
boost::bind(&boost::asio::io_context::stop, &io_context));
// Inform the io_context that we are about to become a daemon. The
// io_context cleans up any internal resources, such as threads, that may
// interfere with forking.
io_context.notify_fork(boost::asio::io_context::fork_prepare);
// Fork the process and have the parent exit. If the process was started
// from a shell, this returns control to the user. Forking a new process is
// also a prerequisite for the subsequent call to setsid().
if (pid_t pid = fork())
{
if (pid > 0)
{
// We're in the parent process and need to exit.
//
// When the exit() function is used, the program terminates without
// invoking local variables' destructors. Only global variables are
// destroyed. As the io_context object is a local variable, this means
// we do not have to call:
//
// io_context.notify_fork(boost::asio::io_context::fork_parent);
//
// However, this line should be added before each call to exit() if
// using a global io_context object. An additional call:
//
// io_context.notify_fork(boost::asio::io_context::fork_prepare);
//
// should also precede the second fork().
exit(0);
}
else
{
syslog(LOG_ERR | LOG_USER, "First fork failed: %m");
return 1;
}
}
// Make the process a new session leader. This detaches it from the
// terminal.
setsid();
// A process inherits its working directory from its parent. This could be
// on a mounted filesystem, which means that the running daemon would
// prevent this filesystem from being unmounted. Changing to the root
// directory avoids this problem.
chdir("/");
// The file mode creation mask is also inherited from the parent process.
// We don't want to restrict the permissions on files created by the
// daemon, so the mask is cleared.
umask(0);
// A second fork ensures the process cannot acquire a controlling terminal.
if (pid_t pid = fork())
{
if (pid > 0)
{
exit(0);
}
else
{
syslog(LOG_ERR | LOG_USER, "Second fork failed: %m");
return 1;
}
}
// Close the standard streams. This decouples the daemon from the terminal
// that started it.
close(0);
close(1);
close(2);
// We don't want the daemon to have any standard input.
if (open("/dev/null", O_RDONLY) < 0)
{
syslog(LOG_ERR | LOG_USER, "Unable to open /dev/null: %m");
return 1;
}
// Send standard output to a log file.
const char* output = "/tmp/asio.daemon.out";
const int flags = O_WRONLY | O_CREAT | O_APPEND;
const mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
if (open(output, flags, mode) < 0)
{
syslog(LOG_ERR | LOG_USER, "Unable to open output file %s: %m", output);
return 1;
}
// Also send standard error to the same log file.
if (dup(1) < 0)
{
syslog(LOG_ERR | LOG_USER, "Unable to dup output descriptor: %m");
return 1;
}
// Inform the io_context that we have finished becoming a daemon. The
// io_context uses this opportunity to create any internal file descriptors
// that need to be private to the new process.
io_context.notify_fork(boost::asio::io_context::fork_child);
// The io_context can now be used normally.
syslog(LOG_INFO | LOG_USER, "Daemon started");
io_context.run();
syslog(LOG_INFO | LOG_USER, "Daemon stopped");
}
catch (std::exception& e)
{
syslog(LOG_ERR | LOG_USER, "Exception: %s", e.what());
std::cerr << "Exception: " << e.what() << std::endl;
}
}
|
