lib/service.c

   1 /*
   2  * libwebsockets - small server side websockets and web server implementation
   3  *
   4  * Copyright (C) 2010-2015 Andy Green <andy@warmcat.com>
   5  *
   6  *  This library is free software; you can redistribute it and/or
   7  *  modify it under the terms of the GNU Lesser General Public
   8  *  License as published by the Free Software Foundation:
   9  *  version 2.1 of the License.
  10  *
  11  *  This library is distributed in the hope that it will be useful,
  12  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14  *  Lesser General Public License for more details.
  15  *
  16  *  You should have received a copy of the GNU Lesser General Public
  17  *  License along with this library; if not, write to the Free Software
  18  *  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
  19  *  MA  02110-1301  USA
  20  */
  21
  22 #include "private-libwebsockets.h"
  23
  24 static int
  25 lws_calllback_as_writeable(struct lws *wsi)
  26 {
  27         int n;
  28
  29         switch (wsi->mode) {
  30         case LWS_CONNMODE_WS_CLIENT:
  31                 n = LWS_CALLBACK_CLIENT_WRITEABLE;
  32                 break;
  33         case LWS_CONNMODE_WS_SERVING:
  34                 n = LWS_CALLBACK_SERVER_WRITEABLE;
  35                 break;
  36         default:
  37                 n = LWS_CALLBACK_HTTP_WRITEABLE;
  38                 break;
  39         }
  40         lwsl_info("%s: %p (user=%p)\n", __func__, wsi, wsi->user_space);
  41         return user_callback_handle_rxflow(wsi->protocol->callback,
  42                                            wsi, (enum lws_callback_reasons) n,
  43                                            wsi->user_space, NULL, 0);
  44 }
  45
  46 int
  47 lws_handle_POLLOUT_event(struct lws *wsi, struct lws_pollfd *pollfd)
  48 {
  49         int write_type = LWS_WRITE_PONG;
  50         struct lws_tokens eff_buf;
  51 #ifdef LWS_USE_HTTP2
  52         struct lws *wsi2;
  53 #endif
  54         int ret, m, n;
  55
  56         /* pending truncated sends have uber priority */
  57
  58         if (wsi->truncated_send_len) {
  59                 if (lws_issue_raw(wsi, wsi->truncated_send_malloc +
  60                                 wsi->truncated_send_offset,
  61                                                 wsi->truncated_send_len) < 0) {
  62                         lwsl_info("%s signalling to close\n", __func__);
  63                         return -1;
  64                 }
  65                 /* leave POLLOUT active either way */
  66                 return 0;
  67         } else
  68                 if (wsi->state == WSI_STATE_FLUSHING_STORED_SEND_BEFORE_CLOSE)
  69                         return -1; /* retry closing now */
  70
  71 #ifdef LWS_USE_HTTP2
  72         /* protocol packets are next */
  73         if (wsi->pps) {
  74                 lwsl_info("servicing pps %d\n", wsi->pps);
  75                 switch (wsi->pps) {
  76                 case LWS_PPS_HTTP2_MY_SETTINGS:
  77                 case LWS_PPS_HTTP2_ACK_SETTINGS:
  78                         lws_http2_do_pps_send(lws_get_ctx(wsi), wsi);
  79                         break;
  80                 default:
  81                         break;
  82                 }
  83                 wsi->pps = LWS_PPS_NONE;
  84                 lws_rx_flow_control(wsi, 1);
  85
  86                 return 0; /* leave POLLOUT active */
  87         }
  88 #endif
  89         /* pending control packets have next priority */
  90
  91         if ((wsi->state == WSI_STATE_ESTABLISHED &&
  92              wsi->u.ws.ping_pending_flag) ||
  93             (wsi->state == WSI_STATE_RETURNED_CLOSE_ALREADY &&
  94              wsi->u.ws.payload_is_close)) {
  95
  96                 if (wsi->u.ws.payload_is_close)
  97                         write_type = LWS_WRITE_CLOSE;
  98
  99                 n = lws_write(wsi, &wsi->u.ws.ping_payload_buf[
 100                                         LWS_SEND_BUFFER_PRE_PADDING],
 101                               wsi->u.ws.ping_payload_len, write_type);
 102                 if (n < 0)
 103                         return -1;
 104
 105                 /* well he is sent, mark him done */
 106                 wsi->u.ws.ping_pending_flag = 0;
 107                 if (wsi->u.ws.payload_is_close)
 108                         /* oh... a close frame was it... then we are done */
 109                         return -1;
 110
 111                 /* otherwise for PING, leave POLLOUT active either way */
 112                 return 0;
 113         }
 114
 115         /* if we are closing, don't confuse the user with writeable cb */
 116
 117         if (wsi->state == WSI_STATE_RETURNED_CLOSE_ALREADY)
 118                 goto user_service;
 119
 120         /* if nothing critical, user can get the callback */
 121
 122         m = lws_ext_callback_for_each_active(wsi, LWS_EXT_CALLBACK_IS_WRITEABLE,
 123                                                                        NULL, 0);
 124 #ifndef LWS_NO_EXTENSIONS
 125         if (!wsi->extension_data_pending)
 126                 goto user_service;
 127 #endif
 128         /*
 129          * check in on the active extensions, see if they
 130          * had pending stuff to spill... they need to get the
 131          * first look-in otherwise sequence will be disordered
 132          *
 133          * NULL, zero-length eff_buf means just spill pending
 134          */
 135
 136         ret = 1;
 137         while (ret == 1) {
 138
 139                 /* default to nobody has more to spill */
 140
 141                 ret = 0;
 142                 eff_buf.token = NULL;
 143                 eff_buf.token_len = 0;
 144
 145                 /* give every extension a chance to spill */
 146
 147                 m = lws_ext_callback_for_each_active(wsi,
 148                                         LWS_EXT_CALLBACK_PACKET_TX_PRESEND,
 149                                                                    &eff_buf, 0);
 150                 if (m < 0) {
 151                         lwsl_err("ext reports fatal error\n");
 152                         return -1;
 153                 }
 154                 if (m)
 155                         /*
 156                          * at least one extension told us he has more
 157                          * to spill, so we will go around again after
 158                          */
 159                         ret = 1;
 160
 161                 /* assuming they gave us something to send, send it */
 162
 163                 if (eff_buf.token_len) {
 164                         n = lws_issue_raw(wsi, (unsigned char *)eff_buf.token,
 165                                                              eff_buf.token_len);
 166                         if (n < 0) {
 167                                 lwsl_info("closing from POLLOUT spill\n");
 168                                 return -1;
 169                         }
 170                         /*
 171                          * Keep amount spilled small to minimize chance of this
 172                          */
 173                         if (n != eff_buf.token_len) {
 174                                 lwsl_err("Unable to spill ext %d vs %s\n",
 175                                                           eff_buf.token_len, n);
 176                                 return -1;
 177                         }
 178                 } else
 179                         continue;
 180
 181                 /* no extension has more to spill */
 182
 183                 if (!ret)
 184                         continue;
 185
 186                 /*
 187                  * There's more to spill from an extension, but we just sent
 188                  * something... did that leave the pipe choked?
 189                  */
 190
 191                 if (!lws_send_pipe_choked(wsi))
 192                         /* no we could add more */
 193                         continue;
 194
 195                 lwsl_info("choked in POLLOUT service\n");
 196
 197                 /*
 198                  * Yes, he's choked.  Leave the POLLOUT masked on so we will
 199                  * come back here when he is unchoked.  Don't call the user
 200                  * callback to enforce ordering of spilling, he'll get called
 201                  * when we come back here and there's nothing more to spill.
 202                  */
 203
 204                 return 0;
 205         }
 206 #ifndef LWS_NO_EXTENSIONS
 207         wsi->extension_data_pending = 0;
 208 #endif
 209 user_service:
 210         /* one shot */
 211
 212         if (pollfd) {
 213                 if (lws_change_pollfd(wsi, LWS_POLLOUT, 0)) {
 214                         lwsl_info("failed at set pollfd\n");
 215                         return 1;
 216                 }
 217
 218                 lws_libev_io(wsi, LWS_EV_STOP | LWS_EV_WRITE);
 219         }
 220
 221 #ifdef LWS_USE_HTTP2
 222         /*
 223          * we are the 'network wsi' for potentially many muxed child wsi with
 224          * no network connection of their own, who have to use us for all their
 225          * network actions.  So we use a round-robin scheme to share out the
 226          * POLLOUT notifications to our children.
 227          *
 228          * But because any child could exhaust the socket's ability to take
 229          * writes, we can only let one child get notified each time.
 230          *
 231          * In addition children may be closed / deleted / added between POLLOUT
 232          * notifications, so we can't hold pointers
 233          */
 234
 235         if (wsi->mode != LWS_CONNMODE_HTTP2_SERVING) {
 236                 lwsl_info("%s: non http2\n", __func__);
 237                 goto notify;
 238         }
 239
 240         wsi->u.http2.requested_POLLOUT = 0;
 241         if (!wsi->u.http2.initialized) {
 242                 lwsl_info("pollout on uninitialized http2 conn\n");
 243                 return 0;
 244         }
 245
 246         lwsl_info("%s: doing children\n", __func__);
 247
 248         wsi2 = wsi;
 249         do {
 250                 wsi2 = wsi2->u.http2.next_child_wsi;
 251                 lwsl_info("%s: child %p\n", __func__, wsi2);
 252                 if (!wsi2)
 253                         continue;
 254                 if (!wsi2->u.http2.requested_POLLOUT)
 255                         continue;
 256                 wsi2->u.http2.requested_POLLOUT = 0;
 257                 if (lws_calllback_as_writeable(wsi2)) {
 258                         lwsl_debug("Closing POLLOUT child\n");
 259                         lws_close_free_wsi(wsi2, LWS_CLOSE_STATUS_NOSTATUS);
 260                 }
 261                 wsi2 = wsi;
 262         } while (wsi2 != NULL && !lws_send_pipe_choked(wsi));
 263
 264         lwsl_info("%s: completed\n", __func__);
 265
 266         return 0;
 267 notify:
 268 #endif
 269         return lws_calllback_as_writeable(wsi);
 270 }
 271
 272 int
 273 lws_service_timeout_check(struct lws *wsi, unsigned int sec)
 274 {
 275         /*
 276          * if extensions want in on it (eg, we are a mux parent)
 277          * give them a chance to service child timeouts
 278          */
 279         if (lws_ext_callback_for_each_active(wsi, LWS_EXT_CALLBACK_1HZ,
 280                                              NULL, sec) < 0)
 281                 return 0;
 282
 283         if (!wsi->pending_timeout)
 284                 return 0;
 285
 286         /*
 287          * if we went beyond the allowed time, kill the
 288          * connection
 289          */
 290         if ((time_t)sec > wsi->pending_timeout_limit) {
 291                 lwsl_info("wsi %p: TIMEDOUT WAITING on %d\n",
 292                           (void *)wsi, wsi->pending_timeout);
 293                 /*
 294                  * Since he failed a timeout, he already had a chance to do
 295                  * something and was unable to... that includes situations like
 296                  * half closed connections.  So process this "failed timeout"
 297                  * close as a violent death and don't try to do protocol
 298                  * cleanup like flush partials.
 299                  */
 300                 wsi->socket_is_permanently_unusable = 1;
 301                 lws_close_free_wsi(wsi, LWS_CLOSE_STATUS_NOSTATUS);
 302
 303                 return 1;
 304         }
 305
 306         return 0;
 307 }
 308
 309 int lws_rxflow_cache(struct lws *wsi, unsigned char *buf, int n, int len)
 310 {
 311         /* his RX is flowcontrolled, don't send remaining now */
 312         if (wsi->rxflow_buffer) {
 313                 /* rxflow while we were spilling prev rxflow */
 314                 lwsl_info("stalling in existing rxflow buf\n");
 315                 return 1;
 316         }
 317
 318         /* a new rxflow, buffer it and warn caller */
 319         lwsl_info("new rxflow input buffer len %d\n", len - n);
 320         wsi->rxflow_buffer = lws_malloc(len - n);
 321         wsi->rxflow_len = len - n;
 322         wsi->rxflow_pos = 0;
 323         memcpy(wsi->rxflow_buffer, buf + n, len - n);
 324
 325         return 0;
 326 }
 327
 328 /**
 329  * lws_service_fd() - Service polled socket with something waiting
 330  * @context:    Websocket context
 331  * @pollfd:     The pollfd entry describing the socket fd and which events
 332  *              happened.
 333  *
 334  *      This function takes a pollfd that has POLLIN or POLLOUT activity and
 335  *      services it according to the state of the associated
 336  *      struct lws.
 337  *
 338  *      The one call deals with all "service" that might happen on a socket
 339  *      including listen accepts, http files as well as websocket protocol.
 340  *
 341  *      If a pollfd says it has something, you can just pass it to
 342  *      lws_service_fd() whether it is a socket handled by lws or not.
 343  *      If it sees it is a lws socket, the traffic will be handled and
 344  *      pollfd->revents will be zeroed now.
 345  *
 346  *      If the socket is foreign to lws, it leaves revents alone.  So you can
 347  *      see if you should service yourself by checking the pollfd revents
 348  *      after letting lws try to service it.
 349  */
 350
 351 LWS_VISIBLE int
 352 lws_service_fd(struct lws_context *context, struct lws_pollfd *pollfd)
 353 {
 354 #if LWS_POSIX
 355         int listen_socket_fds_index = 0;
 356 #endif
 357         lws_sockfd_type our_fd = 0;
 358         struct lws_tokens eff_buf;
 359         unsigned int pending = 0;
 360         char draining_flow = 0;
 361         lws_sockfd_type mfd;
 362         int timed_out = 0;
 363         struct lws *wsi;
 364         time_t now;
 365         int n, m;
 366         int more;
 367
 368 #if LWS_POSIX
 369         if (context->listen_service_fd)
 370                 listen_socket_fds_index = wsi_from_fd(context,
 371                         context->listen_service_fd)->position_in_fds_table;
 372 #endif
 373          /*
 374          * you can call us with pollfd = NULL to just allow the once-per-second
 375          * global timeout checks; if less than a second since the last check
 376          * it returns immediately then.
 377          */
 378
 379         time(&now);
 380
 381         /* TODO: if using libev, we should probably use timeout watchers... */
 382         if (context->last_timeout_check_s != now) {
 383                 context->last_timeout_check_s = now;
 384
 385                 lws_plat_service_periodic(context);
 386
 387                 /* global timeout check once per second */
 388
 389                 if (pollfd)
 390                         our_fd = pollfd->fd;
 391
 392                 for (n = 0; n < context->fds_count; n++) {
 393                         mfd = context->fds[n].fd;
 394                         wsi = wsi_from_fd(context, mfd);
 395                         if (!wsi)
 396                                 continue;
 397
 398                         if (lws_service_timeout_check(wsi, (unsigned int)now))
 399                                 /* he did time out... */
 400                                 if (mfd == our_fd)
 401                                         /* it was the guy we came to service! */
 402                                         timed_out = 1;
 403                                         /* he's gone, no need to mark as handled */
 404                 }
 405         }
 406
 407         /* the socket we came to service timed out, nothing to do */
 408         if (timed_out)
 409                 return 0;
 410
 411         /* just here for timeout management? */
 412         if (!pollfd)
 413                 return 0;
 414
 415         /* no, here to service a socket descriptor */
 416         wsi = wsi_from_fd(context, pollfd->fd);
 417         if (!wsi)
 418                 /* not lws connection ... leave revents alone and return */
 419                 return 0;
 420
 421         /*
 422          * so that caller can tell we handled, past here we need to
 423          * zero down pollfd->revents after handling
 424          */
 425
 426 #if LWS_POSIX
 427         /*
 428          * deal with listen service piggybacking
 429          * every listen_service_modulo services of other fds, we
 430          * sneak one in to service the listen socket if there's anything waiting
 431          *
 432          * To handle connection storms, as found in ab, if we previously saw a
 433          * pending connection here, it causes us to check again next time.
 434          */
 435
 436         if (context->listen_service_fd && pollfd !=
 437                                        &context->fds[listen_socket_fds_index]) {
 438                 context->listen_service_count++;
 439                 if (context->listen_service_extraseen ||
 440                                 context->listen_service_count ==
 441                                                context->listen_service_modulo) {
 442                         context->listen_service_count = 0;
 443                         m = 1;
 444                         if (context->listen_service_extraseen > 5)
 445                                 m = 2;
 446                         while (m--) {
 447                                 /*
 448                                  * even with extpoll, we prepared this
 449                                  * internal fds for listen
 450                                  */
 451                                 n = lws_poll_listen_fd(
 452                                         &context->fds[listen_socket_fds_index]);
 453                                 if (n > 0) { /* there's a conn waiting for us */
 454                                         lws_service_fd(context,
 455                                                 &context->
 456                                                   fds[listen_socket_fds_index]);
 457                                         context->listen_service_extraseen++;
 458                                 } else {
 459                                         if (context->listen_service_extraseen)
 460                                                 context->
 461                                                      listen_service_extraseen--;
 462                                         break;
 463                                 }
 464                         }
 465                 }
 466
 467         }
 468
 469         /* handle session socket closed */
 470
 471         if ((!(pollfd->revents & LWS_POLLIN)) &&
 472                         (pollfd->revents & LWS_POLLHUP)) {
 473
 474                 lwsl_debug("Session Socket %p (fd=%d) dead\n",
 475                                                        (void *)wsi, pollfd->fd);
 476
 477                 goto close_and_handled;
 478         }
 479 #endif
 480
 481         /* okay, what we came here to do... */
 482
 483         switch (wsi->mode) {
 484         case LWS_CONNMODE_HTTP_SERVING:
 485         case LWS_CONNMODE_HTTP_SERVING_ACCEPTED:
 486         case LWS_CONNMODE_SERVER_LISTENER:
 487         case LWS_CONNMODE_SSL_ACK_PENDING:
 488                 n = lws_server_socket_service(context, wsi, pollfd);
 489                 if (n) /* closed by above */
 490                         return 1;
 491                 pending = lws_ssl_pending(wsi);
 492                 if (pending)
 493                         goto handle_pending;
 494                 goto handled;
 495
 496         case LWS_CONNMODE_WS_SERVING:
 497         case LWS_CONNMODE_WS_CLIENT:
 498         case LWS_CONNMODE_HTTP2_SERVING:
 499
 500                 /* the guy requested a callback when it was OK to write */
 501
 502                 if ((pollfd->revents & LWS_POLLOUT) &&
 503                     (wsi->state == WSI_STATE_ESTABLISHED ||
 504                      wsi->state == WSI_STATE_HTTP2_ESTABLISHED ||
 505                      wsi->state == WSI_STATE_HTTP2_ESTABLISHED_PRE_SETTINGS ||
 506                      wsi->state == WSI_STATE_RETURNED_CLOSE_ALREADY ||
 507                      wsi->state == WSI_STATE_FLUSHING_STORED_SEND_BEFORE_CLOSE) &&
 508                            lws_handle_POLLOUT_event(wsi, pollfd)) {
 509                         if (wsi->state == WSI_STATE_RETURNED_CLOSE_ALREADY)
 510                                 wsi->state = WSI_STATE_FLUSHING_STORED_SEND_BEFORE_CLOSE;
 511                         lwsl_info("lws_service_fd: closing\n");
 512                         goto close_and_handled;
 513                 }
 514
 515                 if (wsi->rxflow_buffer &&
 516                               (wsi->rxflow_change_to & LWS_RXFLOW_ALLOW)) {
 517                         lwsl_info("draining rxflow\n");
 518                         /* well, drain it */
 519                         eff_buf.token = (char *)wsi->rxflow_buffer +
 520                                                 wsi->rxflow_pos;
 521                         eff_buf.token_len = wsi->rxflow_len - wsi->rxflow_pos;
 522                         draining_flow = 1;
 523                         goto drain;
 524                 }
 525
 526                 /* any incoming data ready? */
 527
 528                 if (!(pollfd->revents & LWS_POLLIN))
 529                         break;
 530 read:
 531
 532                 eff_buf.token_len = lws_ssl_capable_read(wsi,
 533                                         context->service_buffer,
 534                                         pending ? pending :
 535                                         sizeof(context->service_buffer));
 536                 switch (eff_buf.token_len) {
 537                 case 0:
 538                         lwsl_info("service_fd: closing due to 0 length read\n");
 539                         goto close_and_handled;
 540                 case LWS_SSL_CAPABLE_MORE_SERVICE:
 541                         lwsl_info("SSL Capable more service\n");
 542                         n = 0;
 543                         goto handled;
 544                 case LWS_SSL_CAPABLE_ERROR:
 545                         lwsl_info("Closing when error\n");
 546                         goto close_and_handled;
 547                 }
 548
 549                 /*
 550                  * give any active extensions a chance to munge the buffer
 551                  * before parse.  We pass in a pointer to an lws_tokens struct
 552                  * prepared with the default buffer and content length that's in
 553                  * there.  Rather than rewrite the default buffer, extensions
 554                  * that expect to grow the buffer can adapt .token to
 555                  * point to their own per-connection buffer in the extension
 556                  * user allocation.  By default with no extensions or no
 557                  * extension callback handling, just the normal input buffer is
 558                  * used then so it is efficient.
 559                  */
 560
 561                 eff_buf.token = (char *)context->service_buffer;
 562 drain:
 563
 564                 do {
 565                         more = 0;
 566
 567                         m = lws_ext_callback_for_each_active(wsi,
 568                                 LWS_EXT_CALLBACK_PACKET_RX_PREPARSE, &eff_buf, 0);
 569                         if (m < 0)
 570                                 goto close_and_handled;
 571                         if (m)
 572                                 more = 1;
 573
 574                         /* service incoming data */
 575
 576                         if (eff_buf.token_len) {
 577                                 n = lws_read(wsi, (unsigned char *)eff_buf.token,
 578                                              eff_buf.token_len);
 579                                 if (n < 0) {
 580                                         /* we closed wsi */
 581                                         n = 0;
 582                                         goto handled;
 583                                 }
 584                         }
 585
 586                         eff_buf.token = NULL;
 587                         eff_buf.token_len = 0;
 588                 } while (more);
 589
 590                 pending = lws_ssl_pending(wsi);
 591                 if (pending) {
 592 handle_pending:
 593                         pending = pending > sizeof(context->service_buffer) ?
 594                                 sizeof(context->service_buffer) : pending;
 595                         goto read;
 596                 }
 597
 598                 if (draining_flow && wsi->rxflow_buffer &&
 599                                  wsi->rxflow_pos == wsi->rxflow_len) {
 600                         lwsl_info("flow buffer: drained\n");
 601                         lws_free2(wsi->rxflow_buffer);
 602                         /* having drained the rxflow buffer, can rearm POLLIN */
 603 #ifdef LWS_NO_SERVER
 604                         n =
 605 #endif
 606                         _lws_rx_flow_control(wsi);
 607                         /* n ignored, needed for NO_SERVER case */
 608                 }
 609
 610                 break;
 611
 612         default:
 613 #ifdef LWS_NO_CLIENT
 614                 break;
 615 #else
 616                 n = lws_client_socket_service(context, wsi, pollfd);
 617                 goto handled;
 618 #endif
 619         }
 620
 621         n = 0;
 622         goto handled;
 623
 624 close_and_handled:
 625         lwsl_debug("Close and handled\n");
 626         lws_close_free_wsi(wsi, LWS_CLOSE_STATUS_NOSTATUS);
 627         /*
 628          * pollfd may point to something else after the close
 629          * due to pollfd swapping scheme on delete on some platforms
 630          * we can't clear revents now because it'd be the wrong guy's revents
 631          */
 632         return 1;
 633
 634 handled:
 635         pollfd->revents = 0;
 636         return n;
 637 }
 638
 639 /**
 640  * lws_service() - Service any pending websocket activity
 641  * @context:    Websocket context
 642  * @timeout_ms: Timeout for poll; 0 means return immediately if nothing needed
 643  *              service otherwise block and service immediately, returning
 644  *              after the timeout if nothing needed service.
 645  *
 646  *      This function deals with any pending websocket traffic, for three
 647  *      kinds of event.  It handles these events on both server and client
 648  *      types of connection the same.
 649  *
 650  *      1) Accept new connections to our context's server
 651  *
 652  *      2) Call the receive callback for incoming frame data received by
 653  *          server or client connections.
 654  *
 655  *      You need to call this service function periodically to all the above
 656  *      functions to happen; if your application is single-threaded you can
 657  *      just call it in your main event loop.
 658  *
 659  *      Alternatively you can fork a new process that asynchronously handles
 660  *      calling this service in a loop.  In that case you are happy if this
 661  *      call blocks your thread until it needs to take care of something and
 662  *      would call it with a large nonzero timeout.  Your loop then takes no
 663  *      CPU while there is nothing happening.
 664  *
 665  *      If you are calling it in a single-threaded app, you don't want it to
 666  *      wait around blocking other things in your loop from happening, so you
 667  *      would call it with a timeout_ms of 0, so it returns immediately if
 668  *      nothing is pending, or as soon as it services whatever was pending.
 669  */
 670
 671 LWS_VISIBLE int
 672 lws_service(struct lws_context *context, int timeout_ms)
 673 {
 674         return lws_plat_service(context, timeout_ms);
 675 }
 676