D-Bus  1.8.16
dbus-socket-set-epoll.c
1 /* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
2 /* dbus-socket-set-epoll.c - a socket set implemented via Linux epoll(4)
3  *
4  * Copyright © 2011 Nokia Corporation
5  *
6  * Licensed under the Academic Free License version 2.1
7  *
8  * This program is free software; you can redistribute it and/or modify
9  * it under the terms of the GNU General Public License as published by
10  * the Free Software Foundation; either version 2 of the License, or
11  * (at your option) any later version.
12  *
13  * This program is distributed in the hope that it will be useful,
14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16  * GNU General Public License for more details.
17  *
18  * You should have received a copy of the GNU General Public License
19  * along with this program; if not, write to the Free Software
20  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
21  * MA 02110-1301 USA
22  *
23  */
24 
25 #include <config.h>
26 #include "dbus-socket-set.h"
27 
28 #include <dbus/dbus-internals.h>
29 #include <dbus/dbus-sysdeps.h>
30 
31 #ifndef __linux__
32 # error This file is for Linux epoll(4)
33 #endif
34 
35 #include <errno.h>
36 #include <fcntl.h>
37 #include <sys/epoll.h>
38 #include <unistd.h>
39 
40 #ifndef DOXYGEN_SHOULD_SKIP_THIS
41 
42 typedef struct {
43  DBusSocketSet parent;
44  int epfd;
45 } DBusSocketSetEpoll;
46 
47 static inline DBusSocketSetEpoll *
48 socket_set_epoll_cast (DBusSocketSet *set)
49 {
50  _dbus_assert (set->cls == &_dbus_socket_set_epoll_class);
51  return (DBusSocketSetEpoll *) set;
52 }
53 
54 /* this is safe to call on a partially-allocated socket set */
55 static void
56 socket_set_epoll_free (DBusSocketSet *set)
57 {
58  DBusSocketSetEpoll *self = socket_set_epoll_cast (set);
59 
60  if (self == NULL)
61  return;
62 
63  if (self->epfd != -1)
64  close (self->epfd);
65 
66  dbus_free (self);
67 }
68 
69 DBusSocketSet *
70 _dbus_socket_set_epoll_new (void)
71 {
72  DBusSocketSetEpoll *self;
73 
74  self = dbus_new0 (DBusSocketSetEpoll, 1);
75 
76  if (self == NULL)
77  return NULL;
78 
79  self->parent.cls = &_dbus_socket_set_epoll_class;
80 
81  self->epfd = epoll_create1 (EPOLL_CLOEXEC);
82 
83  if (self->epfd == -1)
84  {
85  int flags;
86 
87  /* the size hint is ignored unless you have a rather old kernel,
88  * but must be positive on some versions, so just pick something
89  * arbitrary; it's a hint, not a limit */
90  self->epfd = epoll_create (42);
91 
92  flags = fcntl (self->epfd, F_GETFD, 0);
93 
94  if (flags != -1)
95  fcntl (self->epfd, F_SETFD, flags | FD_CLOEXEC);
96  }
97 
98  if (self->epfd == -1)
99  {
100  socket_set_epoll_free ((DBusSocketSet *) self);
101  return NULL;
102  }
103 
104  return (DBusSocketSet *) self;
105 }
106 
107 static uint32_t
108 watch_flags_to_epoll_events (unsigned int flags)
109 {
110  uint32_t events = 0;
111 
112  if (flags & DBUS_WATCH_READABLE)
113  events |= EPOLLIN;
114  if (flags & DBUS_WATCH_WRITABLE)
115  events |= EPOLLOUT;
116 
117  return events;
118 }
119 
120 static unsigned int
121 epoll_events_to_watch_flags (uint32_t events)
122 {
123  short flags = 0;
124 
125  if (events & EPOLLIN)
126  flags |= DBUS_WATCH_READABLE;
127  if (events & EPOLLOUT)
128  flags |= DBUS_WATCH_WRITABLE;
129  if (events & EPOLLHUP)
130  flags |= DBUS_WATCH_HANGUP;
131  if (events & EPOLLERR)
132  flags |= DBUS_WATCH_ERROR;
133 
134  return flags;
135 }
136 
137 static dbus_bool_t
138 socket_set_epoll_add (DBusSocketSet *set,
139  int fd,
140  unsigned int flags,
141  dbus_bool_t enabled)
142 {
143  DBusSocketSetEpoll *self = socket_set_epoll_cast (set);
144  struct epoll_event event;
145  int err;
146 
147  event.data.fd = fd;
148 
149  if (enabled)
150  {
151  event.events = watch_flags_to_epoll_events (flags);
152  }
153  else
154  {
155  /* We need to add *something* to reserve space in the kernel's data
156  * structures: see socket_set_epoll_disable for more details */
157  event.events = EPOLLET;
158  }
159 
160  if (epoll_ctl (self->epfd, EPOLL_CTL_ADD, fd, &event) == 0)
161  return TRUE;
162 
163  /* Anything except ENOMEM, ENOSPC means we have an internal error. */
164  err = errno;
165  switch (err)
166  {
167  case ENOMEM:
168  case ENOSPC:
169  /* be silent: this is basically OOM, which our callers are expected
170  * to cope with */
171  break;
172 
173  case EBADF:
174  _dbus_warn ("Bad fd %d\n", fd);
175  break;
176 
177  case EEXIST:
178  _dbus_warn ("fd %d added and then added again\n", fd);
179  break;
180 
181  default:
182  _dbus_warn ("Misc error when trying to watch fd %d: %s\n", fd,
183  strerror (err));
184  break;
185  }
186 
187  return FALSE;
188 }
189 
190 static void
191 socket_set_epoll_enable (DBusSocketSet *set,
192  int fd,
193  unsigned int flags)
194 {
195  DBusSocketSetEpoll *self = socket_set_epoll_cast (set);
196  struct epoll_event event;
197  int err;
198 
199  event.data.fd = fd;
200  event.events = watch_flags_to_epoll_events (flags);
201 
202  if (epoll_ctl (self->epfd, EPOLL_CTL_MOD, fd, &event) == 0)
203  return;
204 
205  err = errno;
206 
207  /* Enabling a file descriptor isn't allowed to fail, even for OOM, so we
208  * do our best to avoid all of these. */
209  switch (err)
210  {
211  case EBADF:
212  _dbus_warn ("Bad fd %d\n", fd);
213  break;
214 
215  case ENOENT:
216  _dbus_warn ("fd %d enabled before it was added\n", fd);
217  break;
218 
219  case ENOMEM:
220  _dbus_warn ("Insufficient memory to change watch for fd %d\n", fd);
221  break;
222 
223  default:
224  _dbus_warn ("Misc error when trying to watch fd %d: %s\n", fd,
225  strerror (err));
226  break;
227  }
228 }
229 
230 static void
231 socket_set_epoll_disable (DBusSocketSet *set,
232  int fd)
233 {
234  DBusSocketSetEpoll *self = socket_set_epoll_cast (set);
235  struct epoll_event event;
236  int err;
237 
238  /* The naive thing to do would be EPOLL_CTL_DEL, but that'll probably
239  * free resources in the kernel. When we come to do socket_set_epoll_enable,
240  * there might not be enough resources to bring it back!
241  *
242  * The next idea you might have is to set the flags to 0. However, events
243  * always trigger on EPOLLERR and EPOLLHUP, even if libdbus isn't actually
244  * delivering them to a DBusWatch. Because epoll is level-triggered by
245  * default, we'll busy-loop on an unhandled error or hangup; not good.
246  *
247  * So, let's set it to be edge-triggered: then the worst case is that
248  * we return from poll immediately on one iteration, ignore it because no
249  * watch is enabled, then go back to normal. When we re-enable a watch
250  * we'll switch back to level-triggered and be notified again (verified to
251  * work on 2.6.32). Compile this file with -DTEST_BEHAVIOUR_OF_EPOLLET for
252  * test code.
253  */
254  event.data.fd = fd;
255  event.events = EPOLLET;
256 
257  if (epoll_ctl (self->epfd, EPOLL_CTL_MOD, fd, &event) == 0)
258  return;
259 
260  err = errno;
261  _dbus_warn ("Error when trying to watch fd %d: %s\n", fd,
262  strerror (err));
263 }
264 
265 static void
266 socket_set_epoll_remove (DBusSocketSet *set,
267  int fd)
268 {
269  DBusSocketSetEpoll *self = socket_set_epoll_cast (set);
270  int err;
271  /* Kernels < 2.6.9 require a non-NULL struct pointer, even though its
272  * contents are ignored */
273  struct epoll_event dummy = { 0 };
274 
275  if (epoll_ctl (self->epfd, EPOLL_CTL_DEL, fd, &dummy) == 0)
276  return;
277 
278  err = errno;
279  _dbus_warn ("Error when trying to remove fd %d: %s\n", fd, strerror (err));
280 }
281 
282 /* Optimally, this should be the same as in DBusLoop: we use it to translate
283  * between struct epoll_event and DBusSocketEvent without allocating heap
284  * memory. */
285 #define N_STACK_DESCRIPTORS 64
286 
287 static int
288 socket_set_epoll_poll (DBusSocketSet *set,
289  DBusSocketEvent *revents,
290  int max_events,
291  int timeout_ms)
292 {
293  DBusSocketSetEpoll *self = socket_set_epoll_cast (set);
294  struct epoll_event events[N_STACK_DESCRIPTORS];
295  int n_ready;
296  int i;
297 
298  _dbus_assert (max_events > 0);
299 
300  n_ready = epoll_wait (self->epfd, events,
301  MIN (_DBUS_N_ELEMENTS (events), max_events),
302  timeout_ms);
303 
304  if (n_ready <= 0)
305  return n_ready;
306 
307  for (i = 0; i < n_ready; i++)
308  {
309  revents[i].fd = events[i].data.fd;
310  revents[i].flags = epoll_events_to_watch_flags (events[i].events);
311  }
312 
313  return n_ready;
314 }
315 
316 DBusSocketSetClass _dbus_socket_set_epoll_class = {
317  socket_set_epoll_free,
318  socket_set_epoll_add,
319  socket_set_epoll_remove,
320  socket_set_epoll_enable,
321  socket_set_epoll_disable,
322  socket_set_epoll_poll
323 };
324 
325 #ifdef TEST_BEHAVIOUR_OF_EPOLLET
326 /* usage: cat /dev/null | ./epoll
327  *
328  * desired output:
329  * ctl ADD: 0
330  * wait for HUP, edge-triggered: 1
331  * wait for HUP again: 0
332  * ctl MOD: 0
333  * wait for HUP: 1
334  */
335 
336 #include <sys/epoll.h>
337 
338 #include <stdio.h>
339 
340 int
341 main (void)
342 {
343  struct epoll_event input;
344  struct epoll_event output;
345  int epfd = epoll_create1 (EPOLL_CLOEXEC);
346  int fd = 0; /* stdin */
347  int ret;
348 
349  input.events = EPOLLHUP | EPOLLET;
350  ret = epoll_ctl (epfd, EPOLL_CTL_ADD, fd, &input);
351  printf ("ctl ADD: %d\n", ret);
352 
353  ret = epoll_wait (epfd, &output, 1, -1);
354  printf ("wait for HUP, edge-triggered: %d\n", ret);
355 
356  ret = epoll_wait (epfd, &output, 1, 1);
357  printf ("wait for HUP again: %d\n", ret);
358 
359  input.events = EPOLLHUP;
360  ret = epoll_ctl (epfd, EPOLL_CTL_MOD, fd, &input);
361  printf ("ctl MOD: %d\n", ret);
362 
363  ret = epoll_wait (epfd, &output, 1, -1);
364  printf ("wait for HUP: %d\n", ret);
365 
366  return 0;
367 }
368 
369 #endif /* TEST_BEHAVIOUR_OF_EPOLLET */
370 
371 #endif /* !DOXYGEN_SHOULD_SKIP_THIS */
#define NULL
A null pointer, defined appropriately for C or C++.
void dbus_free(void *memory)
Frees a block of memory previously allocated by dbus_malloc() or dbus_malloc0().
Definition: dbus-memory.c:701
#define _dbus_assert(condition)
Aborts with an error message if the condition is false.
#define dbus_new0(type, count)
Safe macro for using dbus_malloc0().
Definition: dbus-memory.h:59
As in POLLOUT.
dbus_uint32_t dbus_bool_t
A boolean, valid values are TRUE and FALSE.
Definition: dbus-types.h:35
void _dbus_warn(const char *format,...)
Prints a warning message to stderr.
#define _DBUS_N_ELEMENTS(array)
Computes the number of elements in a fixed-size array using sizeof().
As in POLLERR (can't watch for this, but can be present in current state passed to dbus_watch_handle(...
#define TRUE
Expands to "1".
As in POLLHUP (can't watch for it, but can be present in current state passed to dbus_watch_handle())...
#define FALSE
Expands to "0".
As in POLLIN.