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 <sys/uio.h>
29		#include <unistd.h>
30
31		/* select backend traits.
32
33		Captures the platform-specific behavior of the portable select() backend:
34		manual fcntl for O_NONBLOCK/FD_CLOEXEC, FD_SETSIZE validation,
35		mandatory SO_NOSIGPIPE where the platform defines it,
36		sendmsg(MSG_NOSIGNAL) where available, and accept()+fcntl for
37		accepted connections.
38		*/
39
40		namespace boost::corosio::detail {
41
42		class select_scheduler;
43
44		struct select_traits
45		{
46		using scheduler_type = select_scheduler;
47		using desc_state_type = reactor_descriptor_state;
48
49		static constexpr bool needs_write_notification = true;
50
51		// No extra per-socket state or lifecycle hooks needed for select.
52		struct stream_socket_hook
53		{
54	107x	std::error_code on_set_option(
55		int fd,
56		int level,
57		int optname,
58		void const* data,
59		std::size_t size) noexcept
60		{
61	107x	if (::setsockopt(
62	107x	fd, level, optname, data, static_cast<socklen_t>(size)) !=
63		0)
64	4x	return make_err(errno);
65	103x	return {};
66		}
67	19276x	static void pre_shutdown(int) noexcept {}
68	6260x	static void pre_destroy(int) noexcept {}
69		};
70
71		struct write_policy
72		{
73	72x	static ssize_t write(int fd, iovec* iovecs, int count) noexcept
74		{
75	72x	msghdr msg{};
76	72x	msg.msg_iov = iovecs;
77	72x	msg.msg_iovlen = static_cast<std::size_t>(count);
78
79		#ifdef MSG_NOSIGNAL
80	72x	constexpr int send_flags = MSG_NOSIGNAL;
81		#else
82		constexpr int send_flags = 0;
83		#endif
84
85		ssize_t n;
86		do
87		{
88	73x	n = ::sendmsg(fd, &msg, send_flags);
89		}
90	73x	while (n < 0 && errno == EINTR);
91	72x	return n;
92		}
93
94		// Single-buffer fast path. Where MSG_NOSIGNAL exists we use
95		// send() to suppress SIGPIPE inline; otherwise fall back to
96		// write() and rely on the SO_NOSIGPIPE set in accept_policy
97		// and set_fd_options.
98		static ssize_t
99	103468x	write_one(int fd, void const* data, std::size_t size) noexcept
100		{
101		ssize_t n;
102		do
103		{
104		#ifdef MSG_NOSIGNAL
105	103469x	n = ::send(fd, data, size, MSG_NOSIGNAL);
106		#else
107		n = ::write(fd, data, size);
108		#endif
109		}
110	103469x	while (n < 0 && errno == EINTR);
111	103468x	return n;
112		}
113		};
114
115		// Descriptors are not sockets: sendmsg() fails with ENOTSOCK on a
116		// pipe or character device, so the write path is writev()/write()
117		// and SIGPIPE suppression is structurally unavailable -- MSG_NOSIGNAL
118		// is a send() flag and SO_NOSIGPIPE a socket option. A write to a
119		// pipe whose read end has closed raises SIGPIPE, exactly as a plain
120		// write(2) would; callers install SIG_IGN.
121		struct descriptor_write_policy
122		{
123	2x	static ssize_t write(int fd, iovec* iovecs, int count) noexcept
124		{
125		ssize_t n;
126		do
127		{
128	2x	n = ::writev(fd, iovecs, count);
129		}
130	2x	while (n < 0 && errno == EINTR);
131	2x	return n;
132		}
133
134		// Single-buffer fast path: skips the kernel's iov_iter setup.
135		static ssize_t
136	2x	write_one(int fd, void const* data, std::size_t size) noexcept
137		{
138		ssize_t n;
139		do
140		{
141	2x	n = ::write(fd, data, size);
142		}
143	2x	while (n < 0 && errno == EINTR);
144	2x	return n;
145		}
146		};
147
148		struct accept_policy
149		{
150		static int
151	4112x	do_accept(int fd, sockaddr_storage& peer, socklen_t& addrlen) noexcept
152		{
153	4112x	addrlen = sizeof(peer);
154		int new_fd;
155		do
156		{
157		new_fd =
158	4113x	::accept(fd, reinterpret_cast<sockaddr*>(&peer), &addrlen);
159		}
160	4113x	while (new_fd < 0 && errno == EINTR);
161
162	4112x	if (new_fd < 0)
163	2061x	return new_fd;
164
165	2051x	if (new_fd >= FD_SETSIZE)
166		{
167	1x	::close(new_fd);
168	1x	errno = EMFILE;
169	1x	return -1;
170		}
171
172	2050x	int flags = ::fcntl(new_fd, F_GETFL, 0);
173	2050x	if (flags == -1)
174		{
175	1x	int err = errno;
176	1x	::close(new_fd);
177	1x	errno = err;
178	1x	return -1;
179		}
180
181	2049x	if (::fcntl(new_fd, F_SETFL, flags \| O_NONBLOCK) == -1)
182		{
183	1x	int err = errno;
184	1x	::close(new_fd);
185	1x	errno = err;
186	1x	return -1;
187		}
188
189	2048x	if (::fcntl(new_fd, F_SETFD, FD_CLOEXEC) == -1)
190		{
191	1x	int err = errno;
192	1x	::close(new_fd);
193	1x	errno = err;
194	1x	return -1;
195		}
196
197		#ifdef SO_NOSIGPIPE
198		// MSG_NOSIGNAL is not universal across the platforms this
199		// portable backend covers, and the write() the fast path
200		// falls back to there takes no flag at all; SO_NOSIGPIPE is
201		// the per-descriptor guard that covers both. Treat failure
202		// as fatal, matching the kqueue backend.
203		int one = 1;
204		if (::setsockopt(
205		new_fd, SOL_SOCKET, SO_NOSIGPIPE, &one, sizeof(one)) != 0)
206		{
207		int err = errno;
208		::close(new_fd);
209		errno = err;
210		return -1;
211		}
212		#endif
213
214	2047x	return new_fd;
215		}
216		};
217
218		// Create a plain socket (no atomic flags -- select is POSIX-portable).
219	2656x	static int create_socket(int family, int type, int protocol) noexcept
220		{
221	2656x	return ::socket(family, type, protocol);
222		}
223
224		// Set O_NONBLOCK, FD_CLOEXEC; check FD_SETSIZE; optionally SO_NOSIGPIPE.
225		// Caller is responsible for closing fd on error.
226	2650x	static std::error_code set_fd_options(int fd) noexcept
227		{
228	2650x	int flags = ::fcntl(fd, F_GETFL, 0);
229	2650x	if (flags == -1)
230	2x	return make_err(errno);
231	2648x	if (::fcntl(fd, F_SETFL, flags \| O_NONBLOCK) == -1)
232	2x	return make_err(errno);
233	2646x	if (::fcntl(fd, F_SETFD, FD_CLOEXEC) == -1)
234	2x	return make_err(errno);
235
236	2644x	if (fd >= FD_SETSIZE)
237	2x	return make_err(EMFILE);
238
239		#ifdef SO_NOSIGPIPE
240		// MSG_NOSIGNAL is not universal across the platforms this
241		// portable backend covers, and the write() the fast path falls
242		// back to there takes no flag at all; SO_NOSIGPIPE is the
243		// per-descriptor guard that covers both. Treat failure as fatal,
244		// matching the kqueue backend. Caller closes fd on error.
245		{
246		int one = 1;
247		if (::setsockopt(fd, SOL_SOCKET, SO_NOSIGPIPE, &one, sizeof(one)) !=
248		0)
249		return make_err(errno);
250		}
251		#endif
252
253	2642x	return {};
254		}
255
256		// Apply protocol-specific options after socket creation.
257		// For IP sockets, sets IPV6_V6ONLY on AF_INET6 (best-effort).
258	2265x	static std::error_code configure_ip_socket(int fd, int family) noexcept
259		{
260	2265x	if (family == AF_INET6)
261		{
262	23x	int one = 1;
263		std::ignore =
264	23x	::setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, &one, sizeof(one));
265		}
266
267	2265x	return set_fd_options(fd);
268		}
269
270		// Apply protocol-specific options for acceptor sockets.
271		// For IP acceptors, sets IPV6_V6ONLY=0 (dual-stack, best-effort).
272	276x	static std::error_code configure_ip_acceptor(int fd, int family) noexcept
273		{
274	276x	if (family == AF_INET6)
275		{
276	11x	int val = 0;
277		std::ignore =
278	11x	::setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, &val, sizeof(val));
279		}
280
281	276x	return set_fd_options(fd);
282		}
283
284		// Apply options for local (unix) sockets.
285	109x	static std::error_code configure_local_socket(int fd) noexcept
286		{
287	109x	return set_fd_options(fd);
288		}
289
290		// Non-mutating validation for fds adopted via assign(). Select's
291		// reactor cannot handle fds above FD_SETSIZE, so reject them up
292		// front instead of letting FD_SET clobber unrelated memory.
293	155x	static std::error_code validate_assigned_fd(int fd) noexcept
294		{
295	155x	if (fd >= FD_SETSIZE)
296	2x	return make_err(EMFILE);
297	153x	return {};
298		}
299		};
300
301		} // namespace boost::corosio::detail
302
303		#endif // BOOST_COROSIO_HAS_SELECT
304
305		#endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_TRAITS_HPP
306