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//
// process_per_connection.cpp
// ~~~~~~~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2025 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/tcp.hpp>
#include <boost/asio/signal_set.hpp>
#include <boost/asio/write.hpp>
#include <cstdlib>
#include <iostream>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
using boost::asio::ip::tcp;
class server
{
public:
server(boost::asio::io_context& io_context, unsigned short port)
: io_context_(io_context),
signal_(io_context, SIGCHLD),
acceptor_(io_context, {tcp::v4(), port}),
socket_(io_context)
{
wait_for_signal();
accept();
}
private:
void wait_for_signal()
{
signal_.async_wait(
[this](boost::system::error_code /*ec*/, int /*signo*/)
{
// Only the parent process should check for this signal. We can
// determine whether we are in the parent by checking if the acceptor
// is still open.
if (acceptor_.is_open())
{
// Reap completed child processes so that we don't end up with
// zombies.
int status = 0;
while (waitpid(-1, &status, WNOHANG) > 0) {}
wait_for_signal();
}
});
}
void accept()
{
acceptor_.async_accept(
[this](boost::system::error_code ec, tcp::socket new_socket)
{
if (!ec)
{
// Take ownership of the newly accepted socket.
socket_ = std::move(new_socket);
// Inform the io_context that we are about to fork. 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);
if (fork() == 0)
{
// Inform the io_context that the fork is finished and that this
// is the child process. The io_context uses this opportunity to
// create any internal file descriptors that must be private to
// the new process.
io_context_.notify_fork(boost::asio::io_context::fork_child);
// The child won't be accepting new connections, so we can close
// the acceptor. It remains open in the parent.
acceptor_.close();
// The child process is not interested in processing the SIGCHLD
// signal.
signal_.cancel();
read();
}
else
{
// Inform the io_context that the fork is finished (or failed)
// and that this is the parent process. The io_context uses this
// opportunity to recreate any internal resources that were
// cleaned up during preparation for the fork.
io_context_.notify_fork(boost::asio::io_context::fork_parent);
// The parent process can now close the newly accepted socket. It
// remains open in the child.
socket_.close();
accept();
}
}
else
{
std::cerr << "Accept error: " << ec.message() << std::endl;
accept();
}
});
}
void read()
{
socket_.async_read_some(boost::asio::buffer(data_),
[this](boost::system::error_code ec, std::size_t length)
{
if (!ec)
write(length);
});
}
void write(std::size_t length)
{
boost::asio::async_write(socket_, boost::asio::buffer(data_, length),
[this](boost::system::error_code ec, std::size_t /*length*/)
{
if (!ec)
read();
});
}
boost::asio::io_context& io_context_;
boost::asio::signal_set signal_;
tcp::acceptor acceptor_;
tcp::socket socket_;
std::array<char, 1024> data_;
};
int main(int argc, char* argv[])
{
try
{
if (argc != 2)
{
std::cerr << "Usage: process_per_connection <port>\n";
return 1;
}
boost::asio::io_context io_context;
using namespace std; // For atoi.
server s(io_context, atoi(argv[1]));
io_context.run();
}
catch (std::exception& e)
{
std::cerr << "Exception: " << e.what() << std::endl;
}
}
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