1 //
2 // process_per_connection.cpp
3 // ~~~~~~~~~~~~~~~~~~~~~~~~~~
4 //
5 // Copyright (c) 2003-2021 Christopher M. Kohlhoff (chris at kohlhoff dot com)
6 //
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)
9 //
10
11 #include <boost/asio/io_context.hpp>
12 #include <boost/asio/ip/tcp.hpp>
13 #include <boost/asio/signal_set.hpp>
14 #include <boost/asio/write.hpp>
15 #include <cstdlib>
16 #include <iostream>
17 #include <sys/types.h>
18 #include <sys/wait.h>
19 #include <unistd.h>
20
21 using boost::asio::ip::tcp;
22
23 class server
24 {
25 public:
server(boost::asio::io_context & io_context,unsigned short port)26 server(boost::asio::io_context& io_context, unsigned short port)
27 : io_context_(io_context),
28 signal_(io_context, SIGCHLD),
29 acceptor_(io_context, {tcp::v4(), port}),
30 socket_(io_context)
31 {
32 wait_for_signal();
33 accept();
34 }
35
36 private:
wait_for_signal()37 void wait_for_signal()
38 {
39 signal_.async_wait(
40 [this](boost::system::error_code /*ec*/, int /*signo*/)
41 {
42 // Only the parent process should check for this signal. We can
43 // determine whether we are in the parent by checking if the acceptor
44 // is still open.
45 if (acceptor_.is_open())
46 {
47 // Reap completed child processes so that we don't end up with
48 // zombies.
49 int status = 0;
50 while (waitpid(-1, &status, WNOHANG) > 0) {}
51
52 wait_for_signal();
53 }
54 });
55 }
56
accept()57 void accept()
58 {
59 acceptor_.async_accept(
60 [this](boost::system::error_code ec, tcp::socket new_socket)
61 {
62 if (!ec)
63 {
64 // Take ownership of the newly accepted socket.
65 socket_ = std::move(new_socket);
66
67 // Inform the io_context that we are about to fork. The io_context
68 // cleans up any internal resources, such as threads, that may
69 // interfere with forking.
70 io_context_.notify_fork(boost::asio::io_context::fork_prepare);
71
72 if (fork() == 0)
73 {
74 // Inform the io_context that the fork is finished and that this
75 // is the child process. The io_context uses this opportunity to
76 // create any internal file descriptors that must be private to
77 // the new process.
78 io_context_.notify_fork(boost::asio::io_context::fork_child);
79
80 // The child won't be accepting new connections, so we can close
81 // the acceptor. It remains open in the parent.
82 acceptor_.close();
83
84 // The child process is not interested in processing the SIGCHLD
85 // signal.
86 signal_.cancel();
87
88 read();
89 }
90 else
91 {
92
93 // Inform the io_context that the fork is finished (or failed)
94 // and that this is the parent process. The io_context uses this
95 // opportunity to recreate any internal resources that were
96 // cleaned up during preparation for the fork.
97 io_context_.notify_fork(boost::asio::io_context::fork_parent);
98
99 // The parent process can now close the newly accepted socket. It
100 // remains open in the child.
101 socket_.close();
102
103 accept();
104 }
105 }
106 else
107 {
108 std::cerr << "Accept error: " << ec.message() << std::endl;
109 accept();
110 }
111 });
112 }
113
read()114 void read()
115 {
116 socket_.async_read_some(boost::asio::buffer(data_),
117 [this](boost::system::error_code ec, std::size_t length)
118 {
119 if (!ec)
120 write(length);
121 });
122 }
123
write(std::size_t length)124 void write(std::size_t length)
125 {
126 boost::asio::async_write(socket_, boost::asio::buffer(data_, length),
127 [this](boost::system::error_code ec, std::size_t /*length*/)
128 {
129 if (!ec)
130 read();
131 });
132 }
133
134 boost::asio::io_context& io_context_;
135 boost::asio::signal_set signal_;
136 tcp::acceptor acceptor_;
137 tcp::socket socket_;
138 std::array<char, 1024> data_;
139 };
140
main(int argc,char * argv[])141 int main(int argc, char* argv[])
142 {
143 try
144 {
145 if (argc != 2)
146 {
147 std::cerr << "Usage: process_per_connection <port>\n";
148 return 1;
149 }
150
151 boost::asio::io_context io_context;
152
153 using namespace std; // For atoi.
154 server s(io_context, atoi(argv[1]));
155
156 io_context.run();
157 }
158 catch (std::exception& e)
159 {
160 std::cerr << "Exception: " << e.what() << std::endl;
161 }
162 }
163