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//
// fd_passing_stream_server.cpp
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2025 Christopher M. Kohlhoff (chris at kohlhoff dot com)
// Copyright (c) 2021 Heiko Hund (heiko at openvpn dot net)
//
// 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)
//
// Demonstrates how to pass file descriptors between processes with Asio.
// The client sends a file name to the server. The server opens the file and
// passes the associated file descriptor back to the client.
#include <array>
#include <cstdio>
#include <cassert>
#include <iostream>
#include <memory>
#include <boost/asio.hpp>
#if defined(BOOST_ASIO_HAS_LOCAL_SOCKETS)
using boost::asio::local::stream_protocol;
class session
: public std::enable_shared_from_this<session>
{
public:
session(stream_protocol::socket sock)
: socket_(std::move(sock))
{
}
void start()
{
do_read();
}
private:
void do_read()
{
auto self(shared_from_this());
socket_.async_read_some(boost::asio::buffer(data_),
[this, self](boost::system::error_code ec, std::size_t length)
{
if (ec)
return;
assert(length < data_.size());
data_[length] = 0;
do_write(data_.data());
});
}
void do_write(const char* filename)
{
auto self(shared_from_this());
socket_.async_wait(stream_protocol::socket::wait_write,
[this, self, filename](boost::system::error_code ec)
{
if (ec)
return;
FILE* f(::fopen(filename, "w+"));
if (!f)
return;
struct msghdr msg = {};
char buf[] = { 0 };
struct iovec iov = { buf, sizeof(buf) };
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
union
{
struct cmsghdr align;
char buf[CMSG_SPACE(sizeof(int))];
} cmsgu;
msg.msg_control = cmsgu.buf;
msg.msg_controllen = sizeof(cmsgu.buf);
struct cmsghdr* cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_len = CMSG_LEN(sizeof(int));
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
int fd = ::fileno(f);
std::memcpy(CMSG_DATA(cmsg), &fd, sizeof(int));
ssize_t s(::sendmsg(socket_.native_handle(), &msg, 0));
::fclose(f);
if (s != -1)
do_read();
});
}
// The socket used to communicate with the client.
stream_protocol::socket socket_;
// Buffer used to store data received from the client.
std::array<char, 1024> data_;
};
class server
{
public:
server(boost::asio::io_context& io_context, const std::string& file)
: acceptor_(io_context, stream_protocol::endpoint(file))
{
do_accept();
}
private:
void do_accept()
{
acceptor_.async_accept(
[this](boost::system::error_code ec, stream_protocol::socket socket)
{
if (!ec)
{
std::make_shared<session>(std::move(socket))->start();
}
do_accept();
});
}
stream_protocol::acceptor acceptor_;
};
int main(int argc, char* argv[])
{
try
{
if (argc != 2)
{
std::cerr << "Usage: fd_passing_stream_server <socketfile>\n";
std::cerr << "*** WARNING: existing file is removed ***\n";
return 1;
}
boost::asio::io_context io_context;
std::remove(argv[1]);
server s(io_context, argv[1]);
io_context.run();
}
catch (std::exception& e)
{
std::cerr << "Exception: " << e.what() << "\n";
}
return 0;
}
#else // defined(BOOST_ASIO_HAS_LOCAL_SOCKETS)
# error Local sockets not available on this platform.
#endif // defined(BOOST_ASIO_HAS_LOCAL_SOCKETS)
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