include/boost/corosio/native/detail/select/select_traits.hpp

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select_traits.hpp

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1		//
2		// Copyright (c) 2026 Michael Vandeberg
3		//
4		// Distributed under the Boost Software License, Version 1.0. (See accompanying
5		// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6		//
7		// Official repository: https://github.com/cppalliance/corosio
8		//
9
10		#ifndef BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_TRAITS_HPP
11		#define BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_TRAITS_HPP
12
13		#include <boost/corosio/detail/platform.hpp>
14
15		#if BOOST_COROSIO_HAS_SELECT
16
17		#include <boost/corosio/native/detail/make_err.hpp>
18		#include <boost/corosio/native/detail/reactor/reactor_descriptor_state.hpp>
19
20		#include <system_error>
21		#include <tuple>
22
23		#include <errno.h>
24		#include <fcntl.h>
25		#include <netinet/in.h>
26		#include <sys/select.h>
27		#include <sys/socket.h>
28		#include <unistd.h>
29
30		/* select backend traits.
31
32		Captures the platform-specific behavior of the portable select() backend:
33		manual fcntl for O_NONBLOCK/FD_CLOEXEC, FD_SETSIZE validation,
34		mandatory SO_NOSIGPIPE where the platform defines it,
35		sendmsg(MSG_NOSIGNAL) where available, and accept()+fcntl for
36		accepted connections.
37		*/
38
39		namespace boost::corosio::detail {
40
41		class select_scheduler;
42
43		struct select_traits
44		{
45		using scheduler_type = select_scheduler;
46		using desc_state_type = reactor_descriptor_state;
47
48		static constexpr bool needs_write_notification = true;
49
50		// No extra per-socket state or lifecycle hooks needed for select.
51		struct stream_socket_hook
52		{
53	100x	std::error_code on_set_option(
54		int fd, int level, int optname,
55		void const* data, std::size_t size) noexcept
56		{
57	100x	if (::setsockopt(
58		fd, level, optname, data,
59	100x	static_cast<socklen_t>(size)) != 0)
60	4x	return make_err(errno);
61	96x	return {};
62		}
63	18529x	static void pre_shutdown(int) noexcept {}
64	6013x	static void pre_destroy(int) noexcept {}
65		};
66
67		struct write_policy
68		{
69	72x	static ssize_t write(int fd, iovec* iovecs, int count) noexcept
70		{
71	72x	msghdr msg{};
72	72x	msg.msg_iov = iovecs;
73	72x	msg.msg_iovlen = static_cast<std::size_t>(count);
74
75		#ifdef MSG_NOSIGNAL
76	72x	constexpr int send_flags = MSG_NOSIGNAL;
77		#else
78		constexpr int send_flags = 0;
79		#endif
80
81		ssize_t n;
82		do
83		{
84	73x	n = ::sendmsg(fd, &msg, send_flags);
85		}
86	73x	while (n < 0 && errno == EINTR);
87	72x	return n;
88		}
89
90		// Single-buffer fast path. Where MSG_NOSIGNAL exists we use
91		// send() to suppress SIGPIPE inline; otherwise fall back to
92		// write() and rely on the SO_NOSIGPIPE set in accept_policy
93		// and set_fd_options.
94	98344x	static ssize_t write_one(
95		int fd, void const* data, std::size_t size) noexcept
96		{
97		ssize_t n;
98		do
99		{
100		#ifdef MSG_NOSIGNAL
101	98345x	n = ::send(fd, data, size, MSG_NOSIGNAL);
102		#else
103		n = ::write(fd, data, size);
104		#endif
105		}
106	98345x	while (n < 0 && errno == EINTR);
107	98344x	return n;
108		}
109		};
110
111		struct accept_policy
112		{
113	3950x	static int do_accept(
114		int fd, sockaddr_storage& peer, socklen_t& addrlen) noexcept
115		{
116	3950x	addrlen = sizeof(peer);
117		int new_fd;
118		do
119		{
120	3951x	new_fd = ::accept(
121		fd, reinterpret_cast<sockaddr*>(&peer), &addrlen);
122		}
123	3951x	while (new_fd < 0 && errno == EINTR);
124
125	3950x	if (new_fd < 0)
126	1976x	return new_fd;
127
128	1974x	if (new_fd >= FD_SETSIZE)
129		{
130	1x	::close(new_fd);
131	1x	errno = EMFILE;
132	1x	return -1;
133		}
134
135	1973x	int flags = ::fcntl(new_fd, F_GETFL, 0);
136	1973x	if (flags == -1)
137		{
138	1x	int err = errno;
139	1x	::close(new_fd);
140	1x	errno = err;
141	1x	return -1;
142		}
143
144	1972x	if (::fcntl(new_fd, F_SETFL, flags \| O_NONBLOCK) == -1)
145		{
146	1x	int err = errno;
147	1x	::close(new_fd);
148	1x	errno = err;
149	1x	return -1;
150		}
151
152	1971x	if (::fcntl(new_fd, F_SETFD, FD_CLOEXEC) == -1)
153		{
154	1x	int err = errno;
155	1x	::close(new_fd);
156	1x	errno = err;
157	1x	return -1;
158		}
159
160		#ifdef SO_NOSIGPIPE
161		// MSG_NOSIGNAL is not universal across the platforms this
162		// portable backend covers, and the write() the fast path
163		// falls back to there takes no flag at all; SO_NOSIGPIPE is
164		// the per-descriptor guard that covers both. Treat failure
165		// as fatal, matching the kqueue backend.
166		int one = 1;
167		if (::setsockopt(
168		new_fd, SOL_SOCKET, SO_NOSIGPIPE,
169		&one, sizeof(one)) != 0)
170		{
171		int err = errno;
172		::close(new_fd);
173		errno = err;
174		return -1;
175		}
176		#endif
177
178	1970x	return new_fd;
179		}
180		};
181
182		// Create a plain socket (no atomic flags -- select is POSIX-portable).
183	2449x	static int create_socket(int family, int type, int protocol) noexcept
184		{
185	2449x	return ::socket(family, type, protocol);
186		}
187
188		// Set O_NONBLOCK, FD_CLOEXEC; check FD_SETSIZE; optionally SO_NOSIGPIPE.
189		// Caller is responsible for closing fd on error.
190	2443x	static std::error_code set_fd_options(int fd) noexcept
191		{
192	2443x	int flags = ::fcntl(fd, F_GETFL, 0);
193	2443x	if (flags == -1)
194	2x	return make_err(errno);
195	2441x	if (::fcntl(fd, F_SETFL, flags \| O_NONBLOCK) == -1)
196	2x	return make_err(errno);
197	2439x	if (::fcntl(fd, F_SETFD, FD_CLOEXEC) == -1)
198	2x	return make_err(errno);
199
200	2437x	if (fd >= FD_SETSIZE)
201	2x	return make_err(EMFILE);
202
203		#ifdef SO_NOSIGPIPE
204		// MSG_NOSIGNAL is not universal across the platforms this
205		// portable backend covers, and the write() the fast path falls
206		// back to there takes no flag at all; SO_NOSIGPIPE is the
207		// per-descriptor guard that covers both. Treat failure as fatal,
208		// matching the kqueue backend. Caller closes fd on error.
209		{
210		int one = 1;
211		if (::setsockopt(
212		fd, SOL_SOCKET, SO_NOSIGPIPE, &one, sizeof(one)) != 0)
213		return make_err(errno);
214		}
215		#endif
216
217	2435x	return {};
218		}
219
220		// Apply protocol-specific options after socket creation.
221		// For IP sockets, sets IPV6_V6ONLY on AF_INET6 (best-effort).
222		static std::error_code
223	2133x	configure_ip_socket(int fd, int family) noexcept
224		{
225	2133x	if (family == AF_INET6)
226		{
227	22x	int one = 1;
228	44x	std::ignore = ::setsockopt(
229	22x	fd, IPPROTO_IPV6, IPV6_V6ONLY, &one, sizeof(one));
230		}
231
232	2133x	return set_fd_options(fd);
233		}
234
235		// Apply protocol-specific options for acceptor sockets.
236		// For IP acceptors, sets IPV6_V6ONLY=0 (dual-stack, best-effort).
237		static std::error_code
238	212x	configure_ip_acceptor(int fd, int family) noexcept
239		{
240	212x	if (family == AF_INET6)
241		{
242	11x	int val = 0;
243	22x	std::ignore = ::setsockopt(
244	11x	fd, IPPROTO_IPV6, IPV6_V6ONLY, &val, sizeof(val));
245		}
246
247	212x	return set_fd_options(fd);
248		}
249
250		// Apply options for local (unix) sockets.
251		static std::error_code
252	98x	configure_local_socket(int fd) noexcept
253		{
254	98x	return set_fd_options(fd);
255		}
256
257		// Non-mutating validation for fds adopted via assign(). Select's
258		// reactor cannot handle fds above FD_SETSIZE, so reject them up
259		// front instead of letting FD_SET clobber unrelated memory.
260		static std::error_code
261	146x	validate_assigned_fd(int fd) noexcept
262		{
263	146x	if (fd >= FD_SETSIZE)
264	2x	return make_err(EMFILE);
265	144x	return {};
266		}
267		};
268
269		} // namespace boost::corosio::detail
270
271		#endif // BOOST_COROSIO_HAS_SELECT
272
273		#endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_TRAITS_HPP
274