io.c 105 KB
Newer Older
Thiago C. Santini's avatar
Thiago C. Santini committed
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 1576 1577 1578 1579 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602 1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647 1648 1649 1650 1651 1652 1653 1654 1655 1656 1657 1658 1659 1660 1661 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678 1679 1680 1681 1682 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700 1701 1702 1703 1704 1705 1706 1707 1708 1709 1710 1711 1712 1713 1714 1715 1716 1717 1718 1719 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755 1756 1757 1758 1759 1760 1761 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776 1777 1778 1779 1780 1781 1782 1783 1784 1785 1786 1787 1788 1789 1790 1791 1792 1793 1794 1795 1796 1797 1798 1799 1800 1801 1802 1803 1804 1805 1806 1807 1808 1809 1810 1811 1812 1813 1814 1815 1816 1817 1818 1819 1820 1821 1822 1823 1824 1825 1826 1827 1828 1829 1830 1831 1832 1833 1834 1835 1836 1837 1838 1839 1840 1841 1842 1843 1844 1845 1846 1847 1848 1849 1850 1851 1852 1853 1854 1855 1856 1857 1858 1859 1860 1861 1862 1863 1864 1865 1866 1867 1868 1869 1870 1871 1872 1873 1874 1875 1876 1877 1878 1879 1880 1881 1882 1883 1884 1885 1886 1887 1888 1889 1890 1891 1892 1893 1894 1895 1896 1897 1898 1899 1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 2043 2044 2045 2046 2047 2048 2049 2050 2051 2052 2053 2054 2055 2056 2057 2058 2059 2060 2061 2062 2063 2064 2065 2066 2067 2068 2069 2070 2071 2072 2073 2074 2075 2076 2077 2078 2079 2080 2081 2082 2083 2084 2085 2086 2087 2088 2089 2090 2091 2092 2093 2094 2095 2096 2097 2098 2099 2100 2101 2102 2103 2104 2105 2106 2107 2108 2109 2110 2111 2112 2113 2114 2115 2116 2117 2118 2119 2120 2121 2122 2123 2124 2125 2126 2127 2128 2129 2130 2131 2132 2133 2134 2135 2136 2137 2138 2139 2140 2141 2142 2143 2144 2145 2146 2147 2148 2149 2150 2151 2152 2153 2154 2155 2156 2157 2158 2159 2160 2161 2162 2163 2164 2165 2166 2167 2168 2169 2170 2171 2172 2173 2174 2175 2176 2177 2178 2179 2180 2181 2182 2183 2184 2185 2186 2187 2188 2189 2190 2191 2192 2193 2194 2195 2196 2197 2198 2199 2200 2201 2202 2203 2204 2205 2206 2207 2208 2209 2210 2211 2212 2213 2214 2215 2216 2217 2218 2219 2220 2221 2222 2223 2224 2225 2226 2227 2228 2229 2230 2231 2232 2233 2234 2235 2236 2237 2238 2239 2240 2241 2242 2243 2244 2245 2246 2247 2248 2249 2250 2251 2252 2253 2254 2255 2256 2257 2258 2259 2260 2261 2262 2263 2264 2265 2266 2267 2268 2269 2270 2271 2272 2273 2274 2275 2276 2277 2278 2279 2280 2281 2282 2283 2284 2285 2286 2287 2288 2289 2290 2291 2292 2293 2294 2295 2296 2297 2298 2299 2300 2301 2302 2303 2304 2305 2306 2307 2308 2309 2310 2311 2312 2313 2314 2315 2316 2317 2318 2319 2320 2321 2322 2323 2324 2325 2326 2327 2328 2329 2330 2331 2332 2333 2334 2335 2336 2337 2338 2339 2340 2341 2342 2343 2344 2345 2346 2347 2348 2349 2350 2351 2352 2353 2354 2355 2356 2357 2358 2359 2360 2361 2362 2363 2364 2365 2366 2367 2368 2369 2370 2371 2372 2373 2374 2375 2376 2377 2378 2379 2380 2381 2382 2383 2384 2385 2386 2387 2388 2389 2390 2391 2392 2393 2394 2395 2396 2397 2398 2399 2400 2401 2402 2403 2404 2405 2406 2407 2408 2409 2410 2411 2412 2413 2414 2415 2416 2417 2418 2419 2420 2421 2422 2423 2424 2425 2426 2427 2428 2429 2430 2431 2432 2433 2434 2435 2436 2437 2438 2439 2440 2441 2442 2443 2444 2445 2446 2447 2448 2449 2450 2451 2452 2453 2454 2455 2456 2457 2458 2459 2460 2461 2462 2463 2464 2465 2466 2467 2468 2469 2470 2471 2472 2473 2474 2475 2476 2477 2478 2479 2480 2481 2482 2483 2484 2485 2486 2487 2488 2489 2490 2491 2492 2493 2494 2495 2496 2497 2498 2499 2500 2501 2502 2503 2504 2505 2506 2507 2508 2509 2510 2511 2512 2513 2514 2515 2516 2517 2518 2519 2520 2521 2522 2523 2524 2525 2526 2527 2528 2529 2530 2531 2532 2533 2534 2535 2536 2537 2538 2539 2540 2541 2542 2543 2544 2545 2546 2547 2548 2549 2550 2551 2552 2553 2554 2555 2556 2557 2558 2559 2560 2561 2562 2563 2564 2565 2566 2567 2568 2569 2570 2571 2572 2573 2574 2575 2576 2577 2578 2579 2580 2581 2582 2583 2584 2585 2586 2587 2588 2589 2590 2591 2592 2593 2594 2595 2596 2597 2598 2599 2600 2601 2602 2603 2604 2605 2606 2607 2608 2609 2610 2611 2612 2613 2614 2615 2616 2617 2618 2619 2620 2621 2622 2623 2624 2625 2626 2627 2628 2629 2630 2631 2632 2633 2634 2635 2636 2637 2638 2639 2640 2641 2642 2643 2644 2645 2646 2647 2648 2649 2650 2651 2652 2653 2654 2655 2656 2657 2658 2659 2660 2661 2662 2663 2664 2665 2666 2667 2668 2669 2670 2671 2672 2673 2674 2675 2676 2677 2678 2679 2680 2681 2682 2683 2684 2685 2686 2687 2688 2689 2690 2691 2692 2693 2694 2695 2696 2697 2698 2699 2700 2701 2702 2703 2704 2705 2706 2707 2708 2709 2710 2711 2712 2713 2714 2715 2716 2717 2718 2719 2720 2721 2722 2723 2724 2725 2726 2727 2728 2729 2730 2731 2732 2733 2734 2735 2736 2737 2738 2739 2740 2741 2742 2743 2744 2745 2746 2747 2748 2749 2750 2751 2752 2753 2754 2755 2756 2757 2758 2759 2760 2761 2762 2763 2764 2765 2766 2767 2768 2769 2770 2771 2772 2773 2774 2775 2776 2777 2778 2779 2780 2781 2782 2783 2784 2785 2786 2787 2788 2789 2790 2791 2792 2793 2794 2795 2796 2797 2798 2799 2800 2801 2802 2803 2804 2805 2806 2807 2808 2809 2810 2811 2812 2813 2814 2815 2816 2817 2818 2819
/* -*- Mode: C; indent-tabs-mode:t ; c-basic-offset:8 -*- */
/*
 * I/O functions for libusb
 * Copyright © 2007-2009 Daniel Drake <dsd@gentoo.org>
 * Copyright © 2001 Johannes Erdfelt <johannes@erdfelt.com>
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include <config.h>

#include <assert.h>
#include <errno.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#ifdef HAVE_SIGNAL_H
#include <signal.h>
#endif
#ifdef HAVE_SYS_TIME_H
#include <sys/time.h>
#endif
#ifdef USBI_TIMERFD_AVAILABLE
#include <sys/timerfd.h>
#endif

#include "libusbi.h"
#include "hotplug.h"

/**
 * \page libusb_io Synchronous and asynchronous device I/O
 *
 * \section io_intro Introduction
 *
 * If you're using libusb in your application, you're probably wanting to
 * perform I/O with devices - you want to perform USB data transfers.
 *
 * libusb offers two separate interfaces for device I/O. This page aims to
 * introduce the two in order to help you decide which one is more suitable
 * for your application. You can also choose to use both interfaces in your
 * application by considering each transfer on a case-by-case basis.
 *
 * Once you have read through the following discussion, you should consult the
 * detailed API documentation pages for the details:
 * - \ref libusb_syncio
 * - \ref libusb_asyncio
 *
 * \section theory Transfers at a logical level
 *
 * At a logical level, USB transfers typically happen in two parts. For
 * example, when reading data from a endpoint:
 * -# A request for data is sent to the device
 * -# Some time later, the incoming data is received by the host
 *
 * or when writing data to an endpoint:
 *
 * -# The data is sent to the device
 * -# Some time later, the host receives acknowledgement from the device that
 *    the data has been transferred.
 *
 * There may be an indefinite delay between the two steps. Consider a
 * fictional USB input device with a button that the user can press. In order
 * to determine when the button is pressed, you would likely submit a request
 * to read data on a bulk or interrupt endpoint and wait for data to arrive.
 * Data will arrive when the button is pressed by the user, which is
 * potentially hours later.
 *
 * libusb offers both a synchronous and an asynchronous interface to performing
 * USB transfers. The main difference is that the synchronous interface
 * combines both steps indicated above into a single function call, whereas
 * the asynchronous interface separates them.
 *
 * \section sync The synchronous interface
 *
 * The synchronous I/O interface allows you to perform a USB transfer with
 * a single function call. When the function call returns, the transfer has
 * completed and you can parse the results.
 *
 * If you have used the libusb-0.1 before, this I/O style will seem familar to
 * you. libusb-0.1 only offered a synchronous interface.
 *
 * In our input device example, to read button presses you might write code
 * in the following style:
\code
unsigned char data[4];
int actual_length;
int r = libusb_bulk_transfer(dev_handle, LIBUSB_ENDPOINT_IN, data, sizeof(data), &actual_length, 0);
if (r == 0 && actual_length == sizeof(data)) {
	// results of the transaction can now be found in the data buffer
	// parse them here and report button press
} else {
	error();
}
\endcode
 *
 * The main advantage of this model is simplicity: you did everything with
 * a single simple function call.
 *
 * However, this interface has its limitations. Your application will sleep
 * inside libusb_bulk_transfer() until the transaction has completed. If it
 * takes the user 3 hours to press the button, your application will be
 * sleeping for that long. Execution will be tied up inside the library -
 * the entire thread will be useless for that duration.
 *
 * Another issue is that by tieing up the thread with that single transaction
 * there is no possibility of performing I/O with multiple endpoints and/or
 * multiple devices simultaneously, unless you resort to creating one thread
 * per transaction.
 *
 * Additionally, there is no opportunity to cancel the transfer after the
 * request has been submitted.
 *
 * For details on how to use the synchronous API, see the
 * \ref libusb_syncio "synchronous I/O API documentation" pages.
 *
 * \section async The asynchronous interface
 *
 * Asynchronous I/O is the most significant new feature in libusb-1.0.
 * Although it is a more complex interface, it solves all the issues detailed
 * above.
 *
 * Instead of providing which functions that block until the I/O has complete,
 * libusb's asynchronous interface presents non-blocking functions which
 * begin a transfer and then return immediately. Your application passes a
 * callback function pointer to this non-blocking function, which libusb will
 * call with the results of the transaction when it has completed.
 *
 * Transfers which have been submitted through the non-blocking functions
 * can be cancelled with a separate function call.
 *
 * The non-blocking nature of this interface allows you to be simultaneously
 * performing I/O to multiple endpoints on multiple devices, without having
 * to use threads.
 *
 * This added flexibility does come with some complications though:
 * - In the interest of being a lightweight library, libusb does not create
 * threads and can only operate when your application is calling into it. Your
 * application must call into libusb from it's main loop when events are ready
 * to be handled, or you must use some other scheme to allow libusb to
 * undertake whatever work needs to be done.
 * - libusb also needs to be called into at certain fixed points in time in
 * order to accurately handle transfer timeouts.
 * - Memory handling becomes more complex. You cannot use stack memory unless
 * the function with that stack is guaranteed not to return until the transfer
 * callback has finished executing.
 * - You generally lose some linearity from your code flow because submitting
 * the transfer request is done in a separate function from where the transfer
 * results are handled. This becomes particularly obvious when you want to
 * submit a second transfer based on the results of an earlier transfer.
 *
 * Internally, libusb's synchronous interface is expressed in terms of function
 * calls to the asynchronous interface.
 *
 * For details on how to use the asynchronous API, see the
 * \ref libusb_asyncio "asynchronous I/O API" documentation pages.
 */


/**
 * \page libusb_packetoverflow Packets and overflows
 *
 * \section packets Packet abstraction
 *
 * The USB specifications describe how data is transmitted in packets, with
 * constraints on packet size defined by endpoint descriptors. The host must
 * not send data payloads larger than the endpoint's maximum packet size.
 *
 * libusb and the underlying OS abstract out the packet concept, allowing you
 * to request transfers of any size. Internally, the request will be divided
 * up into correctly-sized packets. You do not have to be concerned with
 * packet sizes, but there is one exception when considering overflows.
 *
 * \section overflow Bulk/interrupt transfer overflows
 *
 * When requesting data on a bulk endpoint, libusb requires you to supply a
 * buffer and the maximum number of bytes of data that libusb can put in that
 * buffer. However, the size of the buffer is not communicated to the device -
 * the device is just asked to send any amount of data.
 *
 * There is no problem if the device sends an amount of data that is less than
 * or equal to the buffer size. libusb reports this condition to you through
 * the \ref libusb_transfer::actual_length "libusb_transfer.actual_length"
 * field.
 *
 * Problems may occur if the device attempts to send more data than can fit in
 * the buffer. libusb reports LIBUSB_TRANSFER_OVERFLOW for this condition but
 * other behaviour is largely undefined: actual_length may or may not be
 * accurate, the chunk of data that can fit in the buffer (before overflow)
 * may or may not have been transferred.
 *
 * Overflows are nasty, but can be avoided. Even though you were told to
 * ignore packets above, think about the lower level details: each transfer is
 * split into packets (typically small, with a maximum size of 512 bytes).
 * Overflows can only happen if the final packet in an incoming data transfer
 * is smaller than the actual packet that the device wants to transfer.
 * Therefore, you will never see an overflow if your transfer buffer size is a
 * multiple of the endpoint's packet size: the final packet will either
 * fill up completely or will be only partially filled.
 */

/**
 * @defgroup libusb_asyncio Asynchronous device I/O
 *
 * This page details libusb's asynchronous (non-blocking) API for USB device
 * I/O. This interface is very powerful but is also quite complex - you will
 * need to read this page carefully to understand the necessary considerations
 * and issues surrounding use of this interface. Simplistic applications
 * may wish to consider the \ref libusb_syncio "synchronous I/O API" instead.
 *
 * The asynchronous interface is built around the idea of separating transfer
 * submission and handling of transfer completion (the synchronous model
 * combines both of these into one). There may be a long delay between
 * submission and completion, however the asynchronous submission function
 * is non-blocking so will return control to your application during that
 * potentially long delay.
 *
 * \section asyncabstraction Transfer abstraction
 *
 * For the asynchronous I/O, libusb implements the concept of a generic
 * transfer entity for all types of I/O (control, bulk, interrupt,
 * isochronous). The generic transfer object must be treated slightly
 * differently depending on which type of I/O you are performing with it.
 *
 * This is represented by the public libusb_transfer structure type.
 *
 * \section asynctrf Asynchronous transfers
 *
 * We can view asynchronous I/O as a 5 step process:
 * -# <b>Allocation</b>: allocate a libusb_transfer
 * -# <b>Filling</b>: populate the libusb_transfer instance with information
 *    about the transfer you wish to perform
 * -# <b>Submission</b>: ask libusb to submit the transfer
 * -# <b>Completion handling</b>: examine transfer results in the
 *    libusb_transfer structure
 * -# <b>Deallocation</b>: clean up resources
 *
 *
 * \subsection asyncalloc Allocation
 *
 * This step involves allocating memory for a USB transfer. This is the
 * generic transfer object mentioned above. At this stage, the transfer
 * is "blank" with no details about what type of I/O it will be used for.
 *
 * Allocation is done with the libusb_alloc_transfer() function. You must use
 * this function rather than allocating your own transfers.
 *
 * \subsection asyncfill Filling
 *
 * This step is where you take a previously allocated transfer and fill it
 * with information to determine the message type and direction, data buffer,
 * callback function, etc.
 *
 * You can either fill the required fields yourself or you can use the
 * helper functions: libusb_fill_control_transfer(), libusb_fill_bulk_transfer()
 * and libusb_fill_interrupt_transfer().
 *
 * \subsection asyncsubmit Submission
 *
 * When you have allocated a transfer and filled it, you can submit it using
 * libusb_submit_transfer(). This function returns immediately but can be
 * regarded as firing off the I/O request in the background.
 *
 * \subsection asynccomplete Completion handling
 *
 * After a transfer has been submitted, one of four things can happen to it:
 *
 * - The transfer completes (i.e. some data was transferred)
 * - The transfer has a timeout and the timeout expires before all data is
 * transferred
 * - The transfer fails due to an error
 * - The transfer is cancelled
 *
 * Each of these will cause the user-specified transfer callback function to
 * be invoked. It is up to the callback function to determine which of the
 * above actually happened and to act accordingly.
 *
 * The user-specified callback is passed a pointer to the libusb_transfer
 * structure which was used to setup and submit the transfer. At completion
 * time, libusb has populated this structure with results of the transfer:
 * success or failure reason, number of bytes of data transferred, etc. See
 * the libusb_transfer structure documentation for more information.
 *
 * <b>Important Note</b>: The user-specified callback is called from an event
 * handling context. It is therefore important that no calls are made into
 * libusb that will attempt to perform any event handling. Examples of such
 * functions are any listed in the \ref libusb_syncio "synchronous API" and any of
 * the blocking functions that retrieve \ref libusb_desc "USB descriptors".
 *
 * \subsection Deallocation
 *
 * When a transfer has completed (i.e. the callback function has been invoked),
 * you are advised to free the transfer (unless you wish to resubmit it, see
 * below). Transfers are deallocated with libusb_free_transfer().
 *
 * It is undefined behaviour to free a transfer which has not completed.
 *
 * \section asyncresubmit Resubmission
 *
 * You may be wondering why allocation, filling, and submission are all
 * separated above where they could reasonably be combined into a single
 * operation.
 *
 * The reason for separation is to allow you to resubmit transfers without
 * having to allocate new ones every time. This is especially useful for
 * common situations dealing with interrupt endpoints - you allocate one
 * transfer, fill and submit it, and when it returns with results you just
 * resubmit it for the next interrupt.
 *
 * \section asynccancel Cancellation
 *
 * Another advantage of using the asynchronous interface is that you have
 * the ability to cancel transfers which have not yet completed. This is
 * done by calling the libusb_cancel_transfer() function.
 *
 * libusb_cancel_transfer() is asynchronous/non-blocking in itself. When the
 * cancellation actually completes, the transfer's callback function will
 * be invoked, and the callback function should check the transfer status to
 * determine that it was cancelled.
 *
 * Freeing the transfer after it has been cancelled but before cancellation
 * has completed will result in undefined behaviour.
 *
 * When a transfer is cancelled, some of the data may have been transferred.
 * libusb will communicate this to you in the transfer callback. Do not assume
 * that no data was transferred.
 *
 * \section bulk_overflows Overflows on device-to-host bulk/interrupt endpoints
 *
 * If your device does not have predictable transfer sizes (or it misbehaves),
 * your application may submit a request for data on an IN endpoint which is
 * smaller than the data that the device wishes to send. In some circumstances
 * this will cause an overflow, which is a nasty condition to deal with. See
 * the \ref libusb_packetoverflow page for discussion.
 *
 * \section asyncctrl Considerations for control transfers
 *
 * The <tt>libusb_transfer</tt> structure is generic and hence does not
 * include specific fields for the control-specific setup packet structure.
 *
 * In order to perform a control transfer, you must place the 8-byte setup
 * packet at the start of the data buffer. To simplify this, you could
 * cast the buffer pointer to type struct libusb_control_setup, or you can
 * use the helper function libusb_fill_control_setup().
 *
 * The wLength field placed in the setup packet must be the length you would
 * expect to be sent in the setup packet: the length of the payload that
 * follows (or the expected maximum number of bytes to receive). However,
 * the length field of the libusb_transfer object must be the length of
 * the data buffer - i.e. it should be wLength <em>plus</em> the size of
 * the setup packet (LIBUSB_CONTROL_SETUP_SIZE).
 *
 * If you use the helper functions, this is simplified for you:
 * -# Allocate a buffer of size LIBUSB_CONTROL_SETUP_SIZE plus the size of the
 * data you are sending/requesting.
 * -# Call libusb_fill_control_setup() on the data buffer, using the transfer
 * request size as the wLength value (i.e. do not include the extra space you
 * allocated for the control setup).
 * -# If this is a host-to-device transfer, place the data to be transferred
 * in the data buffer, starting at offset LIBUSB_CONTROL_SETUP_SIZE.
 * -# Call libusb_fill_control_transfer() to associate the data buffer with
 * the transfer (and to set the remaining details such as callback and timeout).
 *   - Note that there is no parameter to set the length field of the transfer.
 *     The length is automatically inferred from the wLength field of the setup
 *     packet.
 * -# Submit the transfer.
 *
 * The multi-byte control setup fields (wValue, wIndex and wLength) must
 * be given in little-endian byte order (the endianness of the USB bus).
 * Endianness conversion is transparently handled by
 * libusb_fill_control_setup() which is documented to accept host-endian
 * values.
 *
 * Further considerations are needed when handling transfer completion in
 * your callback function:
 * - As you might expect, the setup packet will still be sitting at the start
 * of the data buffer.
 * - If this was a device-to-host transfer, the received data will be sitting
 * at offset LIBUSB_CONTROL_SETUP_SIZE into the buffer.
 * - The actual_length field of the transfer structure is relative to the
 * wLength of the setup packet, rather than the size of the data buffer. So,
 * if your wLength was 4, your transfer's <tt>length</tt> was 12, then you
 * should expect an <tt>actual_length</tt> of 4 to indicate that the data was
 * transferred in entirity.
 *
 * To simplify parsing of setup packets and obtaining the data from the
 * correct offset, you may wish to use the libusb_control_transfer_get_data()
 * and libusb_control_transfer_get_setup() functions within your transfer
 * callback.
 *
 * Even though control endpoints do not halt, a completed control transfer
 * may have a LIBUSB_TRANSFER_STALL status code. This indicates the control
 * request was not supported.
 *
 * \section asyncintr Considerations for interrupt transfers
 *
 * All interrupt transfers are performed using the polling interval presented
 * by the bInterval value of the endpoint descriptor.
 *
 * \section asynciso Considerations for isochronous transfers
 *
 * Isochronous transfers are more complicated than transfers to
 * non-isochronous endpoints.
 *
 * To perform I/O to an isochronous endpoint, allocate the transfer by calling
 * libusb_alloc_transfer() with an appropriate number of isochronous packets.
 *
 * During filling, set \ref libusb_transfer::type "type" to
 * \ref libusb_transfer_type::LIBUSB_TRANSFER_TYPE_ISOCHRONOUS
 * "LIBUSB_TRANSFER_TYPE_ISOCHRONOUS", and set
 * \ref libusb_transfer::num_iso_packets "num_iso_packets" to a value less than
 * or equal to the number of packets you requested during allocation.
 * libusb_alloc_transfer() does not set either of these fields for you, given
 * that you might not even use the transfer on an isochronous endpoint.
 *
 * Next, populate the length field for the first num_iso_packets entries in
 * the \ref libusb_transfer::iso_packet_desc "iso_packet_desc" array. Section
 * 5.6.3 of the USB2 specifications describe how the maximum isochronous
 * packet length is determined by the wMaxPacketSize field in the endpoint
 * descriptor.
 * Two functions can help you here:
 *
 * - libusb_get_max_iso_packet_size() is an easy way to determine the max
 *   packet size for an isochronous endpoint. Note that the maximum packet
 *   size is actually the maximum number of bytes that can be transmitted in
 *   a single microframe, therefore this function multiplies the maximum number
 *   of bytes per transaction by the number of transaction opportunities per
 *   microframe.
 * - libusb_set_iso_packet_lengths() assigns the same length to all packets
 *   within a transfer, which is usually what you want.
 *
 * For outgoing transfers, you'll obviously fill the buffer and populate the
 * packet descriptors in hope that all the data gets transferred. For incoming
 * transfers, you must ensure the buffer has sufficient capacity for
 * the situation where all packets transfer the full amount of requested data.
 *
 * Completion handling requires some extra consideration. The
 * \ref libusb_transfer::actual_length "actual_length" field of the transfer
 * is meaningless and should not be examined; instead you must refer to the
 * \ref libusb_iso_packet_descriptor::actual_length "actual_length" field of
 * each individual packet.
 *
 * The \ref libusb_transfer::status "status" field of the transfer is also a
 * little misleading:
 *  - If the packets were submitted and the isochronous data microframes
 *    completed normally, status will have value
 *    \ref libusb_transfer_status::LIBUSB_TRANSFER_COMPLETED
 *    "LIBUSB_TRANSFER_COMPLETED". Note that bus errors and software-incurred
 *    delays are not counted as transfer errors; the transfer.status field may
 *    indicate COMPLETED even if some or all of the packets failed. Refer to
 *    the \ref libusb_iso_packet_descriptor::status "status" field of each
 *    individual packet to determine packet failures.
 *  - The status field will have value
 *    \ref libusb_transfer_status::LIBUSB_TRANSFER_ERROR
 *    "LIBUSB_TRANSFER_ERROR" only when serious errors were encountered.
 *  - Other transfer status codes occur with normal behaviour.
 *
 * The data for each packet will be found at an offset into the buffer that
 * can be calculated as if each prior packet completed in full. The
 * libusb_get_iso_packet_buffer() and libusb_get_iso_packet_buffer_simple()
 * functions may help you here.
 *
 * <b>Note</b>: Some operating systems (e.g. Linux) may impose limits on the
 * length of individual isochronous packets and/or the total length of the
 * isochronous transfer. Such limits can be difficult for libusb to detect,
 * so the library will simply try and submit the transfer as set up by you.
 * If the transfer fails to submit because it is too large,
 * libusb_submit_transfer() will return
 * \ref libusb_error::LIBUSB_ERROR_INVALID_PARAM "LIBUSB_ERROR_INVALID_PARAM".
 *
 * \section asyncmem Memory caveats
 *
 * In most circumstances, it is not safe to use stack memory for transfer
 * buffers. This is because the function that fired off the asynchronous
 * transfer may return before libusb has finished using the buffer, and when
 * the function returns it's stack gets destroyed. This is true for both
 * host-to-device and device-to-host transfers.
 *
 * The only case in which it is safe to use stack memory is where you can
 * guarantee that the function owning the stack space for the buffer does not
 * return until after the transfer's callback function has completed. In every
 * other case, you need to use heap memory instead.
 *
 * \section asyncflags Fine control
 *
 * Through using this asynchronous interface, you may find yourself repeating
 * a few simple operations many times. You can apply a bitwise OR of certain
 * flags to a transfer to simplify certain things:
 * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_SHORT_NOT_OK
 *   "LIBUSB_TRANSFER_SHORT_NOT_OK" results in transfers which transferred
 *   less than the requested amount of data being marked with status
 *   \ref libusb_transfer_status::LIBUSB_TRANSFER_ERROR "LIBUSB_TRANSFER_ERROR"
 *   (they would normally be regarded as COMPLETED)
 * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_BUFFER
 *   "LIBUSB_TRANSFER_FREE_BUFFER" allows you to ask libusb to free the transfer
 *   buffer when freeing the transfer.
 * - \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_TRANSFER
 *   "LIBUSB_TRANSFER_FREE_TRANSFER" causes libusb to automatically free the
 *   transfer after the transfer callback returns.
 *
 * \section asyncevent Event handling
 *
 * An asynchronous model requires that libusb perform work at various
 * points in time - namely processing the results of previously-submitted
 * transfers and invoking the user-supplied callback function.
 *
 * This gives rise to the libusb_handle_events() function which your
 * application must call into when libusb has work do to. This gives libusb
 * the opportunity to reap pending transfers, invoke callbacks, etc.
 *
 * There are 2 different approaches to dealing with libusb_handle_events:
 *
 * -# Repeatedly call libusb_handle_events() in blocking mode from a dedicated
 *    thread.
 * -# Integrate libusb with your application's main event loop. libusb
 *    exposes a set of file descriptors which allow you to do this.
 *
 * The first approach has the big advantage that it will also work on Windows
 * were libusb' poll API for select / poll integration is not available. So
 * if you want to support Windows and use the async API, you must use this
 * approach, see the \ref eventthread "Using an event handling thread" section
 * below for details.
 *
 * If you prefer a single threaded approach with a single central event loop,
 * see the \ref libusb_poll "polling and timing" section for how to integrate libusb
 * into your application's main event loop.
 *
 * \section eventthread Using an event handling thread
 *
 * Lets begin with stating the obvious: If you're going to use a separate
 * thread for libusb event handling, your callback functions MUST be
 * threadsafe.
 *
 * Other then that doing event handling from a separate thread, is mostly
 * simple. You can use an event thread function as follows:
\code
void *event_thread_func(void *ctx)
{
    while (event_thread_run)
        libusb_handle_events(ctx);

    return NULL;
}
\endcode
 *
 * There is one caveat though, stopping this thread requires setting the
 * event_thread_run variable to 0, and after that libusb_handle_events() needs
 * to return control to event_thread_func. But unless some event happens,
 * libusb_handle_events() will not return.
 *
 * There are 2 different ways of dealing with this, depending on if your
 * application uses libusb' \ref libusb_hotplug "hotplug" support or not.
 *
 * Applications which do not use hotplug support, should not start the event
 * thread until after their first call to libusb_open(), and should stop the
 * thread when closing the last open device as follows:
\code
void my_close_handle(libusb_device_handle *dev_handle)
{
    if (open_devs == 1)
        event_thread_run = 0;

    libusb_close(dev_handle); // This wakes up libusb_handle_events()

    if (open_devs == 1)
        pthread_join(event_thread);

    open_devs--;
}
\endcode
 *
 * Applications using hotplug support should start the thread at program init,
 * after having successfully called libusb_hotplug_register_callback(), and
 * should stop the thread at program exit as follows:
\code
void my_libusb_exit(void)
{
    event_thread_run = 0;
    libusb_hotplug_deregister_callback(ctx, hotplug_cb_handle); // This wakes up libusb_handle_events()
    pthread_join(event_thread);
    libusb_exit(ctx);
}
\endcode
 */

/**
 * @defgroup libusb_poll Polling and timing
 *
 * This page documents libusb's functions for polling events and timing.
 * These functions are only necessary for users of the
 * \ref libusb_asyncio "asynchronous API". If you are only using the simpler
 * \ref libusb_syncio "synchronous API" then you do not need to ever call these
 * functions.
 *
 * The justification for the functionality described here has already been
 * discussed in the \ref asyncevent "event handling" section of the
 * asynchronous API documentation. In summary, libusb does not create internal
 * threads for event processing and hence relies on your application calling
 * into libusb at certain points in time so that pending events can be handled.
 *
 * Your main loop is probably already calling poll() or select() or a
 * variant on a set of file descriptors for other event sources (e.g. keyboard
 * button presses, mouse movements, network sockets, etc). You then add
 * libusb's file descriptors to your poll()/select() calls, and when activity
 * is detected on such descriptors you know it is time to call
 * libusb_handle_events().
 *
 * There is one final event handling complication. libusb supports
 * asynchronous transfers which time out after a specified time period.
 *
 * On some platforms a timerfd is used, so the timeout handling is just another
 * fd, on other platforms this requires that libusb is called into at or after
 * the timeout to handle it. So, in addition to considering libusb's file
 * descriptors in your main event loop, you must also consider that libusb
 * sometimes needs to be called into at fixed points in time even when there
 * is no file descriptor activity, see \ref polltime details.
 *
 * In order to know precisely when libusb needs to be called into, libusb
 * offers you a set of pollable file descriptors and information about when
 * the next timeout expires.
 *
 * If you are using the asynchronous I/O API, you must take one of the two
 * following options, otherwise your I/O will not complete.
 *
 * \section pollsimple The simple option
 *
 * If your application revolves solely around libusb and does not need to
 * handle other event sources, you can have a program structure as follows:
\code
// initialize libusb
// find and open device
// maybe fire off some initial async I/O

while (user_has_not_requested_exit)
	libusb_handle_events(ctx);

// clean up and exit
\endcode
 *
 * With such a simple main loop, you do not have to worry about managing
 * sets of file descriptors or handling timeouts. libusb_handle_events() will
 * handle those details internally.
 *
 * \section libusb_pollmain The more advanced option
 *
 * \note This functionality is currently only available on Unix-like platforms.
 * On Windows, libusb_get_pollfds() simply returns NULL. Applications which
 * want to support Windows are advised to use an \ref eventthread
 * "event handling thread" instead.
 *
 * In more advanced applications, you will already have a main loop which
 * is monitoring other event sources: network sockets, X11 events, mouse
 * movements, etc. Through exposing a set of file descriptors, libusb is
 * designed to cleanly integrate into such main loops.
 *
 * In addition to polling file descriptors for the other event sources, you
 * take a set of file descriptors from libusb and monitor those too. When you
 * detect activity on libusb's file descriptors, you call
 * libusb_handle_events_timeout() in non-blocking mode.
 *
 * What's more, libusb may also need to handle events at specific moments in
 * time. No file descriptor activity is generated at these times, so your
 * own application needs to be continually aware of when the next one of these
 * moments occurs (through calling libusb_get_next_timeout()), and then it
 * needs to call libusb_handle_events_timeout() in non-blocking mode when
 * these moments occur. This means that you need to adjust your
 * poll()/select() timeout accordingly.
 *
 * libusb provides you with a set of file descriptors to poll and expects you
 * to poll all of them, treating them as a single entity. The meaning of each
 * file descriptor in the set is an internal implementation detail,
 * platform-dependent and may vary from release to release. Don't try and
 * interpret the meaning of the file descriptors, just do as libusb indicates,
 * polling all of them at once.
 *
 * In pseudo-code, you want something that looks like:
\code
// initialise libusb

libusb_get_pollfds(ctx)
while (user has not requested application exit) {
	libusb_get_next_timeout(ctx);
	poll(on libusb file descriptors plus any other event sources of interest,
		using a timeout no larger than the value libusb just suggested)
	if (poll() indicated activity on libusb file descriptors)
		libusb_handle_events_timeout(ctx, &zero_tv);
	if (time has elapsed to or beyond the libusb timeout)
		libusb_handle_events_timeout(ctx, &zero_tv);
	// handle events from other sources here
}

// clean up and exit
\endcode
 *
 * \subsection polltime Notes on time-based events
 *
 * The above complication with having to track time and call into libusb at
 * specific moments is a bit of a headache. For maximum compatibility, you do
 * need to write your main loop as above, but you may decide that you can
 * restrict the supported platforms of your application and get away with
 * a more simplistic scheme.
 *
 * These time-based event complications are \b not required on the following
 * platforms:
 *  - Darwin
 *  - Linux, provided that the following version requirements are satisfied:
 *   - Linux v2.6.27 or newer, compiled with timerfd support
 *   - glibc v2.9 or newer
 *   - libusb v1.0.5 or newer
 *
 * Under these configurations, libusb_get_next_timeout() will \em always return
 * 0, so your main loop can be simplified to:
\code
// initialise libusb

libusb_get_pollfds(ctx)
while (user has not requested application exit) {
	poll(on libusb file descriptors plus any other event sources of interest,
		using any timeout that you like)
	if (poll() indicated activity on libusb file descriptors)
		libusb_handle_events_timeout(ctx, &zero_tv);
	// handle events from other sources here
}

// clean up and exit
\endcode
 *
 * Do remember that if you simplify your main loop to the above, you will
 * lose compatibility with some platforms (including legacy Linux platforms,
 * and <em>any future platforms supported by libusb which may have time-based
 * event requirements</em>). The resultant problems will likely appear as
 * strange bugs in your application.
 *
 * You can use the libusb_pollfds_handle_timeouts() function to do a runtime
 * check to see if it is safe to ignore the time-based event complications.
 * If your application has taken the shortcut of ignoring libusb's next timeout
 * in your main loop, then you are advised to check the return value of
 * libusb_pollfds_handle_timeouts() during application startup, and to abort
 * if the platform does suffer from these timing complications.
 *
 * \subsection fdsetchange Changes in the file descriptor set
 *
 * The set of file descriptors that libusb uses as event sources may change
 * during the life of your application. Rather than having to repeatedly
 * call libusb_get_pollfds(), you can set up notification functions for when
 * the file descriptor set changes using libusb_set_pollfd_notifiers().
 *
 * \subsection mtissues Multi-threaded considerations
 *
 * Unfortunately, the situation is complicated further when multiple threads
 * come into play. If two threads are monitoring the same file descriptors,
 * the fact that only one thread will be woken up when an event occurs causes
 * some headaches.
 *
 * The events lock, event waiters lock, and libusb_handle_events_locked()
 * entities are added to solve these problems. You do not need to be concerned
 * with these entities otherwise.
 *
 * See the extra documentation: \ref libusb_mtasync
 */

/** \page libusb_mtasync Multi-threaded applications and asynchronous I/O
 *
 * libusb is a thread-safe library, but extra considerations must be applied
 * to applications which interact with libusb from multiple threads.
 *
 * The underlying issue that must be addressed is that all libusb I/O
 * revolves around monitoring file descriptors through the poll()/select()
 * system calls. This is directly exposed at the
 * \ref libusb_asyncio "asynchronous interface" but it is important to note that the
 * \ref libusb_syncio "synchronous interface" is implemented on top of the
 * asynchonrous interface, therefore the same considerations apply.
 *
 * The issue is that if two or more threads are concurrently calling poll()
 * or select() on libusb's file descriptors then only one of those threads
 * will be woken up when an event arrives. The others will be completely
 * oblivious that anything has happened.
 *
 * Consider the following pseudo-code, which submits an asynchronous transfer
 * then waits for its completion. This style is one way you could implement a
 * synchronous interface on top of the asynchronous interface (and libusb
 * does something similar, albeit more advanced due to the complications
 * explained on this page).
 *
\code
void cb(struct libusb_transfer *transfer)
{
	int *completed = transfer->user_data;
	*completed = 1;
}

void myfunc() {
	struct libusb_transfer *transfer;
	unsigned char buffer[LIBUSB_CONTROL_SETUP_SIZE] __attribute__ ((aligned (2)));
	int completed = 0;

	transfer = libusb_alloc_transfer(0);
	libusb_fill_control_setup(buffer,
		LIBUSB_REQUEST_TYPE_VENDOR | LIBUSB_ENDPOINT_OUT, 0x04, 0x01, 0, 0);
	libusb_fill_control_transfer(transfer, dev, buffer, cb, &completed, 1000);
	libusb_submit_transfer(transfer);

	while (!completed) {
		poll(libusb file descriptors, 120*1000);
		if (poll indicates activity)
			libusb_handle_events_timeout(ctx, &zero_tv);
	}
	printf("completed!");
	// other code here
}
\endcode
 *
 * Here we are <em>serializing</em> completion of an asynchronous event
 * against a condition - the condition being completion of a specific transfer.
 * The poll() loop has a long timeout to minimize CPU usage during situations
 * when nothing is happening (it could reasonably be unlimited).
 *
 * If this is the only thread that is polling libusb's file descriptors, there
 * is no problem: there is no danger that another thread will swallow up the
 * event that we are interested in. On the other hand, if there is another
 * thread polling the same descriptors, there is a chance that it will receive
 * the event that we were interested in. In this situation, <tt>myfunc()</tt>
 * will only realise that the transfer has completed on the next iteration of
 * the loop, <em>up to 120 seconds later.</em> Clearly a two-minute delay is
 * undesirable, and don't even think about using short timeouts to circumvent
 * this issue!
 *
 * The solution here is to ensure that no two threads are ever polling the
 * file descriptors at the same time. A naive implementation of this would
 * impact the capabilities of the library, so libusb offers the scheme
 * documented below to ensure no loss of functionality.
 *
 * Before we go any further, it is worth mentioning that all libusb-wrapped
 * event handling procedures fully adhere to the scheme documented below.
 * This includes libusb_handle_events() and its variants, and all the
 * synchronous I/O functions - libusb hides this headache from you.
 *
 * \section Using libusb_handle_events() from multiple threads
 *
 * Even when only using libusb_handle_events() and synchronous I/O functions,
 * you can still have a race condition. You might be tempted to solve the
 * above with libusb_handle_events() like so:
 *
\code
	libusb_submit_transfer(transfer);

	while (!completed) {
		libusb_handle_events(ctx);
	}
	printf("completed!");
\endcode
 *
 * This however has a race between the checking of completed and
 * libusb_handle_events() acquiring the events lock, so another thread
 * could have completed the transfer, resulting in this thread hanging
 * until either a timeout or another event occurs. See also commit
 * 6696512aade99bb15d6792af90ae329af270eba6 which fixes this in the
 * synchronous API implementation of libusb.
 *
 * Fixing this race requires checking the variable completed only after
 * taking the event lock, which defeats the concept of just calling
 * libusb_handle_events() without worrying about locking. This is why
 * libusb-1.0.9 introduces the new libusb_handle_events_timeout_completed()
 * and libusb_handle_events_completed() functions, which handles doing the
 * completion check for you after they have acquired the lock:
 *
\code
	libusb_submit_transfer(transfer);

	while (!completed) {
		libusb_handle_events_completed(ctx, &completed);
	}
	printf("completed!");
\endcode
 *
 * This nicely fixes the race in our example. Note that if all you want to
 * do is submit a single transfer and wait for its completion, then using
 * one of the synchronous I/O functions is much easier.
 *
 * \section eventlock The events lock
 *
 * The problem is when we consider the fact that libusb exposes file
 * descriptors to allow for you to integrate asynchronous USB I/O into
 * existing main loops, effectively allowing you to do some work behind
 * libusb's back. If you do take libusb's file descriptors and pass them to
 * poll()/select() yourself, you need to be aware of the associated issues.
 *
 * The first concept to be introduced is the events lock. The events lock
 * is used to serialize threads that want to handle events, such that only
 * one thread is handling events at any one time.
 *
 * You must take the events lock before polling libusb file descriptors,
 * using libusb_lock_events(). You must release the lock as soon as you have
 * aborted your poll()/select() loop, using libusb_unlock_events().
 *
 * \section threadwait Letting other threads do the work for you
 *
 * Although the events lock is a critical part of the solution, it is not
 * enough on it's own. You might wonder if the following is sufficient...
\code
	libusb_lock_events(ctx);
	while (!completed) {
		poll(libusb file descriptors, 120*1000);
		if (poll indicates activity)
			libusb_handle_events_timeout(ctx, &zero_tv);
	}
	libusb_unlock_events(ctx);
\endcode
 * ...and the answer is that it is not. This is because the transfer in the
 * code shown above may take a long time (say 30 seconds) to complete, and
 * the lock is not released until the transfer is completed.
 *
 * Another thread with similar code that wants to do event handling may be
 * working with a transfer that completes after a few milliseconds. Despite
 * having such a quick completion time, the other thread cannot check that
 * status of its transfer until the code above has finished (30 seconds later)
 * due to contention on the lock.
 *
 * To solve this, libusb offers you a mechanism to determine when another
 * thread is handling events. It also offers a mechanism to block your thread
 * until the event handling thread has completed an event (and this mechanism
 * does not involve polling of file descriptors).
 *
 * After determining that another thread is currently handling events, you
 * obtain the <em>event waiters</em> lock using libusb_lock_event_waiters().
 * You then re-check that some other thread is still handling events, and if
 * so, you call libusb_wait_for_event().
 *
 * libusb_wait_for_event() puts your application to sleep until an event
 * occurs, or until a thread releases the events lock. When either of these
 * things happen, your thread is woken up, and should re-check the condition
 * it was waiting on. It should also re-check that another thread is handling
 * events, and if not, it should start handling events itself.
 *
 * This looks like the following, as pseudo-code:
\code
retry:
if (libusb_try_lock_events(ctx) == 0) {
	// we obtained the event lock: do our own event handling
	while (!completed) {
		if (!libusb_event_handling_ok(ctx)) {
			libusb_unlock_events(ctx);
			goto retry;
		}
		poll(libusb file descriptors, 120*1000);
		if (poll indicates activity)
			libusb_handle_events_locked(ctx, 0);
	}
	libusb_unlock_events(ctx);
} else {
	// another thread is doing event handling. wait for it to signal us that
	// an event has completed
	libusb_lock_event_waiters(ctx);

	while (!completed) {
		// now that we have the event waiters lock, double check that another
		// thread is still handling events for us. (it may have ceased handling
		// events in the time it took us to reach this point)
		if (!libusb_event_handler_active(ctx)) {
			// whoever was handling events is no longer doing so, try again
			libusb_unlock_event_waiters(ctx);
			goto retry;
		}

		libusb_wait_for_event(ctx, NULL);
	}
	libusb_unlock_event_waiters(ctx);
}
printf("completed!\n");
\endcode
 *
 * A naive look at the above code may suggest that this can only support
 * one event waiter (hence a total of 2 competing threads, the other doing
 * event handling), because the event waiter seems to have taken the event
 * waiters lock while waiting for an event. However, the system does support
 * multiple event waiters, because libusb_wait_for_event() actually drops
 * the lock while waiting, and reaquires it before continuing.
 *
 * We have now implemented code which can dynamically handle situations where
 * nobody is handling events (so we should do it ourselves), and it can also
 * handle situations where another thread is doing event handling (so we can
 * piggyback onto them). It is also equipped to handle a combination of
 * the two, for example, another thread is doing event handling, but for
 * whatever reason it stops doing so before our condition is met, so we take
 * over the event handling.
 *
 * Four functions were introduced in the above pseudo-code. Their importance
 * should be apparent from the code shown above.
 * -# libusb_try_lock_events() is a non-blocking function which attempts
 *    to acquire the events lock but returns a failure code if it is contended.
 * -# libusb_event_handling_ok() checks that libusb is still happy for your
 *    thread to be performing event handling. Sometimes, libusb needs to
 *    interrupt the event handler, and this is how you can check if you have
 *    been interrupted. If this function returns 0, the correct behaviour is
 *    for you to give up the event handling lock, and then to repeat the cycle.
 *    The following libusb_try_lock_events() will fail, so you will become an
 *    events waiter. For more information on this, read \ref fullstory below.
 * -# libusb_handle_events_locked() is a variant of
 *    libusb_handle_events_timeout() that you can call while holding the
 *    events lock. libusb_handle_events_timeout() itself implements similar
 *    logic to the above, so be sure not to call it when you are
 *    "working behind libusb's back", as is the case here.
 * -# libusb_event_handler_active() determines if someone is currently
 *    holding the events lock
 *
 * You might be wondering why there is no function to wake up all threads
 * blocked on libusb_wait_for_event(). This is because libusb can do this
 * internally: it will wake up all such threads when someone calls
 * libusb_unlock_events() or when a transfer completes (at the point after its
 * callback has returned).
 *
 * \subsection fullstory The full story
 *
 * The above explanation should be enough to get you going, but if you're
 * really thinking through the issues then you may be left with some more
 * questions regarding libusb's internals. If you're curious, read on, and if
 * not, skip to the next section to avoid confusing yourself!
 *
 * The immediate question that may spring to mind is: what if one thread
 * modifies the set of file descriptors that need to be polled while another
 * thread is doing event handling?
 *
 * There are 2 situations in which this may happen.
 * -# libusb_open() will add another file descriptor to the poll set,
 *    therefore it is desirable to interrupt the event handler so that it
 *    restarts, picking up the new descriptor.
 * -# libusb_close() will remove a file descriptor from the poll set. There
 *    are all kinds of race conditions that could arise here, so it is
 *    important that nobody is doing event handling at this time.
 *
 * libusb handles these issues internally, so application developers do not
 * have to stop their event handlers while opening/closing devices. Here's how
 * it works, focusing on the libusb_close() situation first:
 *
 * -# During initialization, libusb opens an internal pipe, and it adds the read
 *    end of this pipe to the set of file descriptors to be polled.
 * -# During libusb_close(), libusb writes some dummy data on this event pipe.
 *    This immediately interrupts the event handler. libusb also records
 *    internally that it is trying to interrupt event handlers for this
 *    high-priority event.
 * -# At this point, some of the functions described above start behaving
 *    differently:
 *   - libusb_event_handling_ok() starts returning 1, indicating that it is NOT
 *     OK for event handling to continue.
 *   - libusb_try_lock_events() starts returning 1, indicating that another
 *     thread holds the event handling lock, even if the lock is uncontended.
 *   - libusb_event_handler_active() starts returning 1, indicating that
 *     another thread is doing event handling, even if that is not true.
 * -# The above changes in behaviour result in the event handler stopping and
 *    giving up the events lock very quickly, giving the high-priority
 *    libusb_close() operation a "free ride" to acquire the events lock. All
 *    threads that are competing to do event handling become event waiters.
 * -# With the events lock held inside libusb_close(), libusb can safely remove
 *    a file descriptor from the poll set, in the safety of knowledge that
 *    nobody is polling those descriptors or trying to access the poll set.
 * -# After obtaining the events lock, the close operation completes very
 *    quickly (usually a matter of milliseconds) and then immediately releases
 *    the events lock.
 * -# At the same time, the behaviour of libusb_event_handling_ok() and friends
 *    reverts to the original, documented behaviour.
 * -# The release of the events lock causes the threads that are waiting for
 *    events to be woken up and to start competing to become event handlers
 *    again. One of them will succeed; it will then re-obtain the list of poll
 *    descriptors, and USB I/O will then continue as normal.
 *
 * libusb_open() is similar, and is actually a more simplistic case. Upon a
 * call to libusb_open():
 *
 * -# The device is opened and a file descriptor is added to the poll set.
 * -# libusb sends some dummy data on the event pipe, and records that it
 *    is trying to modify the poll descriptor set.
 * -# The event handler is interrupted, and the same behaviour change as for
 *    libusb_close() takes effect, causing all event handling threads to become
 *    event waiters.
 * -# The libusb_open() implementation takes its free ride to the events lock.
 * -# Happy that it has successfully paused the events handler, libusb_open()
 *    releases the events lock.
 * -# The event waiter threads are all woken up and compete to become event
 *    handlers again. The one that succeeds will obtain the list of poll
 *    descriptors again, which will include the addition of the new device.
 *
 * \subsection concl Closing remarks
 *
 * The above may seem a little complicated, but hopefully I have made it clear
 * why such complications are necessary. Also, do not forget that this only
 * applies to applications that take libusb's file descriptors and integrate
 * them into their own polling loops.
 *
 * You may decide that it is OK for your multi-threaded application to ignore
 * some of the rules and locks detailed above, because you don't think that
 * two threads can ever be polling the descriptors at the same time. If that
 * is the case, then that's good news for you because you don't have to worry.
 * But be careful here; remember that the synchronous I/O functions do event
 * handling internally. If you have one thread doing event handling in a loop
 * (without implementing the rules and locking semantics documented above)
 * and another trying to send a synchronous USB transfer, you will end up with
 * two threads monitoring the same descriptors, and the above-described
 * undesirable behaviour occurring. The solution is for your polling thread to
 * play by the rules; the synchronous I/O functions do so, and this will result
 * in them getting along in perfect harmony.
 *
 * If you do have a dedicated thread doing event handling, it is perfectly
 * legal for it to take the event handling lock for long periods of time. Any
 * synchronous I/O functions you call from other threads will transparently
 * fall back to the "event waiters" mechanism detailed above. The only
 * consideration that your event handling thread must apply is the one related
 * to libusb_event_handling_ok(): you must call this before every poll(), and
 * give up the events lock if instructed.
 */

int usbi_io_init(struct libusb_context *ctx)
{
	int r;

	usbi_mutex_init(&ctx->flying_transfers_lock);
	usbi_mutex_init(&ctx->events_lock);
	usbi_mutex_init(&ctx->event_waiters_lock);
	usbi_cond_init(&ctx->event_waiters_cond);
	usbi_mutex_init(&ctx->event_data_lock);
	usbi_tls_key_create(&ctx->event_handling_key);
	list_init(&ctx->flying_transfers);
	list_init(&ctx->ipollfds);
	list_init(&ctx->hotplug_msgs);
	list_init(&ctx->completed_transfers);

	/* FIXME should use an eventfd on kernels that support it */
	r = usbi_pipe(ctx->event_pipe);
	if (r < 0) {
		r = LIBUSB_ERROR_OTHER;
		goto err;
	}

	r = usbi_add_pollfd(ctx, ctx->event_pipe[0], POLLIN);
	if (r < 0)
		goto err_close_pipe;

#ifdef USBI_TIMERFD_AVAILABLE
	ctx->timerfd = timerfd_create(usbi_backend->get_timerfd_clockid(),
		TFD_NONBLOCK);
	if (ctx->timerfd >= 0) {
		usbi_dbg("using timerfd for timeouts");
		r = usbi_add_pollfd(ctx, ctx->timerfd, POLLIN);
		if (r < 0)
			goto err_close_timerfd;
	} else {
		usbi_dbg("timerfd not available (code %d error %d)", ctx->timerfd, errno);
		ctx->timerfd = -1;
	}
#endif

	return 0;

#ifdef USBI_TIMERFD_AVAILABLE
err_close_timerfd:
	close(ctx->timerfd);
	usbi_remove_pollfd(ctx, ctx->event_pipe[0]);
#endif
err_close_pipe:
	usbi_close(ctx->event_pipe[0]);
	usbi_close(ctx->event_pipe[1]);
err:
	usbi_mutex_destroy(&ctx->flying_transfers_lock);
	usbi_mutex_destroy(&ctx->events_lock);
	usbi_mutex_destroy(&ctx->event_waiters_lock);
	usbi_cond_destroy(&ctx->event_waiters_cond);
	usbi_mutex_destroy(&ctx->event_data_lock);
	usbi_tls_key_delete(ctx->event_handling_key);
	return r;
}

void usbi_io_exit(struct libusb_context *ctx)
{
	usbi_remove_pollfd(ctx, ctx->event_pipe[0]);
	usbi_close(ctx->event_pipe[0]);
	usbi_close(ctx->event_pipe[1]);
#ifdef USBI_TIMERFD_AVAILABLE
	if (usbi_using_timerfd(ctx)) {
		usbi_remove_pollfd(ctx, ctx->timerfd);
		close(ctx->timerfd);
	}
#endif
	usbi_mutex_destroy(&ctx->flying_transfers_lock);
	usbi_mutex_destroy(&ctx->events_lock);
	usbi_mutex_destroy(&ctx->event_waiters_lock);
	usbi_cond_destroy(&ctx->event_waiters_cond);
	usbi_mutex_destroy(&ctx->event_data_lock);
	usbi_tls_key_delete(ctx->event_handling_key);
	if (ctx->pollfds)
		free(ctx->pollfds);
}

static int calculate_timeout(struct usbi_transfer *transfer)
{
	int r;
	struct timespec current_time;
	unsigned int timeout =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer)->timeout;

	if (!timeout)
		return 0;

	r = usbi_backend->clock_gettime(USBI_CLOCK_MONOTONIC, &current_time);
	if (r < 0) {
		usbi_err(ITRANSFER_CTX(transfer),
			"failed to read monotonic clock, errno=%d", errno);
		return r;
	}

	current_time.tv_sec += timeout / 1000;
	current_time.tv_nsec += (timeout % 1000) * 1000000;

	while (current_time.tv_nsec >= 1000000000) {
		current_time.tv_nsec -= 1000000000;
		current_time.tv_sec++;
	}

	TIMESPEC_TO_TIMEVAL(&transfer->timeout, &current_time);
	return 0;
}

/** \ingroup libusb_asyncio
 * Allocate a libusb transfer with a specified number of isochronous packet
 * descriptors. The returned transfer is pre-initialized for you. When the new
 * transfer is no longer needed, it should be freed with
 * libusb_free_transfer().
 *
 * Transfers intended for non-isochronous endpoints (e.g. control, bulk,
 * interrupt) should specify an iso_packets count of zero.
 *
 * For transfers intended for isochronous endpoints, specify an appropriate
 * number of packet descriptors to be allocated as part of the transfer.
 * The returned transfer is not specially initialized for isochronous I/O;
 * you are still required to set the
 * \ref libusb_transfer::num_iso_packets "num_iso_packets" and
 * \ref libusb_transfer::type "type" fields accordingly.
 *
 * It is safe to allocate a transfer with some isochronous packets and then
 * use it on a non-isochronous endpoint. If you do this, ensure that at time
 * of submission, num_iso_packets is 0 and that type is set appropriately.
 *
 * \param iso_packets number of isochronous packet descriptors to allocate
 * \returns a newly allocated transfer, or NULL on error
 */
DEFAULT_VISIBILITY
struct libusb_transfer * LIBUSB_CALL libusb_alloc_transfer(
	int iso_packets)
{
	struct libusb_transfer *transfer;
	size_t os_alloc_size = usbi_backend->transfer_priv_size;
	size_t alloc_size = sizeof(struct usbi_transfer)
		+ sizeof(struct libusb_transfer)
		+ (sizeof(struct libusb_iso_packet_descriptor) * iso_packets)
		+ os_alloc_size;
	struct usbi_transfer *itransfer = calloc(1, alloc_size);
	if (!itransfer)
		return NULL;

	itransfer->num_iso_packets = iso_packets;
	usbi_mutex_init(&itransfer->lock);
	transfer = USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	usbi_dbg("transfer %p", transfer);
	return transfer;
}

/** \ingroup libusb_asyncio
 * Free a transfer structure. This should be called for all transfers
 * allocated with libusb_alloc_transfer().
 *
 * If the \ref libusb_transfer_flags::LIBUSB_TRANSFER_FREE_BUFFER
 * "LIBUSB_TRANSFER_FREE_BUFFER" flag is set and the transfer buffer is
 * non-NULL, this function will also free the transfer buffer using the
 * standard system memory allocator (e.g. free()).
 *
 * It is legal to call this function with a NULL transfer. In this case,
 * the function will simply return safely.
 *
 * It is not legal to free an active transfer (one which has been submitted
 * and has not yet completed).
 *
 * \param transfer the transfer to free
 */
void API_EXPORTED libusb_free_transfer(struct libusb_transfer *transfer)
{
	struct usbi_transfer *itransfer;
	if (!transfer)
		return;

	usbi_dbg("transfer %p", transfer);
	if (transfer->flags & LIBUSB_TRANSFER_FREE_BUFFER && transfer->buffer)
		free(transfer->buffer);

	itransfer = LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
	usbi_mutex_destroy(&itransfer->lock);
	free(itransfer);
}

#ifdef USBI_TIMERFD_AVAILABLE
static int disarm_timerfd(struct libusb_context *ctx)
{
	const struct itimerspec disarm_timer = { { 0, 0 }, { 0, 0 } };
	int r;

	usbi_dbg("");
	r = timerfd_settime(ctx->timerfd, 0, &disarm_timer, NULL);
	if (r < 0)
		return LIBUSB_ERROR_OTHER;
	else
		return 0;
}

/* iterates through the flying transfers, and rearms the timerfd based on the
 * next upcoming timeout.
 * must be called with flying_list locked.
 * returns 0 on success or a LIBUSB_ERROR code on failure.
 */
static int arm_timerfd_for_next_timeout(struct libusb_context *ctx)
{
	struct usbi_transfer *transfer;

	list_for_each_entry(transfer, &ctx->flying_transfers, list, struct usbi_transfer) {
		struct timeval *cur_tv = &transfer->timeout;

		/* if we've reached transfers of infinite timeout, then we have no
		 * arming to do */
		if (!timerisset(cur_tv))
			goto disarm;

		/* act on first transfer that has not already been handled */
		if (!(transfer->timeout_flags & (USBI_TRANSFER_TIMEOUT_HANDLED | USBI_TRANSFER_OS_HANDLES_TIMEOUT))) {
			int r;
			const struct itimerspec it = { {0, 0},
				{ cur_tv->tv_sec, cur_tv->tv_usec * 1000 } };
			usbi_dbg("next timeout originally %dms", USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer)->timeout);
			r = timerfd_settime(ctx->timerfd, TFD_TIMER_ABSTIME, &it, NULL);
			if (r < 0)
				return LIBUSB_ERROR_OTHER;
			return 0;
		}
	}

disarm:
	return disarm_timerfd(ctx);
}
#else
static int arm_timerfd_for_next_timeout(struct libusb_context *ctx)
{
	UNUSED(ctx);
	return 0;
}
#endif

/* add a transfer to the (timeout-sorted) active transfers list.
 * This function will return non 0 if fails to update the timer,
 * in which case the transfer is *not* on the flying_transfers list. */
static int add_to_flying_list(struct usbi_transfer *transfer)
{
	struct usbi_transfer *cur;
	struct timeval *timeout = &transfer->timeout;
	struct libusb_context *ctx = ITRANSFER_CTX(transfer);
	int r;
	int first = 1;

	r = calculate_timeout(transfer);
	if (r)
		return r;

	/* if we have no other flying transfers, start the list with this one */
	if (list_empty(&ctx->flying_transfers)) {
		list_add(&transfer->list, &ctx->flying_transfers);
		goto out;
	}

	/* if we have infinite timeout, append to end of list */
	if (!timerisset(timeout)) {
		list_add_tail(&transfer->list, &ctx->flying_transfers);
		/* first is irrelevant in this case */
		goto out;
	}

	/* otherwise, find appropriate place in list */
	list_for_each_entry(cur, &ctx->flying_transfers, list, struct usbi_transfer) {
		/* find first timeout that occurs after the transfer in question */
		struct timeval *cur_tv = &cur->timeout;

		if (!timerisset(cur_tv) || (cur_tv->tv_sec > timeout->tv_sec) ||
				(cur_tv->tv_sec == timeout->tv_sec &&
					cur_tv->tv_usec > timeout->tv_usec)) {
			list_add_tail(&transfer->list, &cur->list);
			goto out;
		}
		first = 0;
	}
	/* first is 0 at this stage (list not empty) */

	/* otherwise we need to be inserted at the end */
	list_add_tail(&transfer->list, &ctx->flying_transfers);
out:
#ifdef USBI_TIMERFD_AVAILABLE
	if (first && usbi_using_timerfd(ctx) && timerisset(timeout)) {
		/* if this transfer has the lowest timeout of all active transfers,
		 * rearm the timerfd with this transfer's timeout */
		const struct itimerspec it = { {0, 0},
			{ timeout->tv_sec, timeout->tv_usec * 1000 } };
		usbi_dbg("arm timerfd for timeout in %dms (first in line)",
			USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer)->timeout);
		r = timerfd_settime(ctx->timerfd, TFD_TIMER_ABSTIME, &it, NULL);
		if (r < 0) {
			usbi_warn(ctx, "failed to arm first timerfd (errno %d)", errno);
			r = LIBUSB_ERROR_OTHER;
		}
	}
#else
	UNUSED(first);
#endif

	if (r)
		list_del(&transfer->list);

	return r;
}

/* remove a transfer from the active transfers list.
 * This function will *always* remove the transfer from the
 * flying_transfers list. It will return a LIBUSB_ERROR code
 * if it fails to update the timer for the next timeout. */
static int remove_from_flying_list(struct usbi_transfer *transfer)
{
	struct libusb_context *ctx = ITRANSFER_CTX(transfer);
	int rearm_timerfd;
	int r = 0;

	usbi_mutex_lock(&ctx->flying_transfers_lock);
	rearm_timerfd = (timerisset(&transfer->timeout) &&
		list_first_entry(&ctx->flying_transfers, struct usbi_transfer, list) == transfer);
	list_del(&transfer->list);
	if (usbi_using_timerfd(ctx) && rearm_timerfd)
		r = arm_timerfd_for_next_timeout(ctx);
	usbi_mutex_unlock(&ctx->flying_transfers_lock);

	return r;
}

/** \ingroup libusb_asyncio
 * Submit a transfer. This function will fire off the USB transfer and then
 * return immediately.
 *
 * \param transfer the transfer to submit
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NO_DEVICE if the device has been disconnected
 * \returns LIBUSB_ERROR_BUSY if the transfer has already been submitted.
 * \returns LIBUSB_ERROR_NOT_SUPPORTED if the transfer flags are not supported
 * by the operating system.
 * \returns LIBUSB_ERROR_INVALID_PARAM if the transfer size is larger than
 * the operating system and/or hardware can support
 * \returns another LIBUSB_ERROR code on other failure
 */
int API_EXPORTED libusb_submit_transfer(struct libusb_transfer *transfer)
{
	struct usbi_transfer *itransfer =
		LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
	struct libusb_context *ctx = TRANSFER_CTX(transfer);
	int r;

	usbi_dbg("transfer %p", transfer);

	/*
	 * Important note on locking, this function takes / releases locks
	 * in the following order:
	 *  take flying_transfers_lock
	 *  take itransfer->lock
	 *  clear transfer
	 *  add to flying_transfers list
	 *  release flying_transfers_lock
	 *  submit transfer
	 *  release itransfer->lock
	 *  if submit failed:
	 *   take flying_transfers_lock
	 *   remove from flying_transfers list
	 *   release flying_transfers_lock
	 *
	 * Note that it takes locks in the order a-b and then releases them
	 * in the same order a-b. This is somewhat unusual but not wrong,
	 * release order is not important as long as *all* locks are released
	 * before re-acquiring any locks.
	 *
	 * This means that the ordering of first releasing itransfer->lock
	 * and then re-acquiring the flying_transfers_list on error is
	 * important and must not be changed!
	 *
	 * This is done this way because when we take both locks we must always
	 * take flying_transfers_lock first to avoid ab-ba style deadlocks with
	 * the timeout handling and usbi_handle_disconnect paths.
	 *
	 * And we cannot release itransfer->lock before the submission is
	 * complete otherwise timeout handling for transfers with short
	 * timeouts may run before submission.
	 */
	usbi_mutex_lock(&ctx->flying_transfers_lock);
	usbi_mutex_lock(&itransfer->lock);
	if (itransfer->state_flags & USBI_TRANSFER_IN_FLIGHT) {
		usbi_mutex_unlock(&ctx->flying_transfers_lock);
		usbi_mutex_unlock(&itransfer->lock);
		return LIBUSB_ERROR_BUSY;
	}
	itransfer->transferred = 0;
	itransfer->state_flags = 0;
	itransfer->timeout_flags = 0;
	r = add_to_flying_list(itransfer);
	if (r) {
		usbi_mutex_unlock(&ctx->flying_transfers_lock);
		usbi_mutex_unlock(&itransfer->lock);
		return r;
	}
	/*
	 * We must release the flying transfers lock here, because with
	 * some backends the submit_transfer method is synchroneous.
	 */
	usbi_mutex_unlock(&ctx->flying_transfers_lock);

	r = usbi_backend->submit_transfer(itransfer);
	if (r == LIBUSB_SUCCESS) {
		itransfer->state_flags |= USBI_TRANSFER_IN_FLIGHT;
		/* keep a reference to this device */
		libusb_ref_device(transfer->dev_handle->dev);
	}
	usbi_mutex_unlock(&itransfer->lock);

	if (r != LIBUSB_SUCCESS)
		remove_from_flying_list(itransfer);

	return r;
}

/** \ingroup libusb_asyncio
 * Asynchronously cancel a previously submitted transfer.
 * This function returns immediately, but this does not indicate cancellation
 * is complete. Your callback function will be invoked at some later time
 * with a transfer status of
 * \ref libusb_transfer_status::LIBUSB_TRANSFER_CANCELLED
 * "LIBUSB_TRANSFER_CANCELLED."
 *
 * \param transfer the transfer to cancel
 * \returns 0 on success
 * \returns LIBUSB_ERROR_NOT_FOUND if the transfer is not in progress,
 * already complete, or already cancelled.
 * \returns a LIBUSB_ERROR code on failure
 */
int API_EXPORTED libusb_cancel_transfer(struct libusb_transfer *transfer)
{
	struct usbi_transfer *itransfer =
		LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
	int r;

	usbi_dbg("transfer %p", transfer );
	usbi_mutex_lock(&itransfer->lock);
	if (!(itransfer->state_flags & USBI_TRANSFER_IN_FLIGHT)
			|| (itransfer->state_flags & USBI_TRANSFER_CANCELLING)) {
		r = LIBUSB_ERROR_NOT_FOUND;
		goto out;
	}
	r = usbi_backend->cancel_transfer(itransfer);
	if (r < 0) {
		if (r != LIBUSB_ERROR_NOT_FOUND &&
		    r != LIBUSB_ERROR_NO_DEVICE)
			usbi_err(TRANSFER_CTX(transfer),
				"cancel transfer failed error %d", r);
		else
			usbi_dbg("cancel transfer failed error %d", r);

		if (r == LIBUSB_ERROR_NO_DEVICE)
			itransfer->state_flags |= USBI_TRANSFER_DEVICE_DISAPPEARED;
	}

	itransfer->state_flags |= USBI_TRANSFER_CANCELLING;

out:
	usbi_mutex_unlock(&itransfer->lock);
	return r;
}

/** \ingroup libusb_asyncio
 * Set a transfers bulk stream id. Note users are advised to use
 * libusb_fill_bulk_stream_transfer() instead of calling this function
 * directly.
 *
 * Since version 1.0.19, \ref LIBUSB_API_VERSION >= 0x01000103
 *
 * \param transfer the transfer to set the stream id for
 * \param stream_id the stream id to set
 * \see libusb_alloc_streams()
 */
void API_EXPORTED libusb_transfer_set_stream_id(
	struct libusb_transfer *transfer, uint32_t stream_id)
{
	struct usbi_transfer *itransfer =
		LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);

	itransfer->stream_id = stream_id;
}

/** \ingroup libusb_asyncio
 * Get a transfers bulk stream id.
 *
 * Since version 1.0.19, \ref LIBUSB_API_VERSION >= 0x01000103
 *
 * \param transfer the transfer to get the stream id for
 * \returns the stream id for the transfer
 */
uint32_t API_EXPORTED libusb_transfer_get_stream_id(
	struct libusb_transfer *transfer)
{
	struct usbi_transfer *itransfer =
		LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);

	return itransfer->stream_id;
}

/* Handle completion of a transfer (completion might be an error condition).
 * This will invoke the user-supplied callback function, which may end up
 * freeing the transfer. Therefore you cannot use the transfer structure
 * after calling this function, and you should free all backend-specific
 * data before calling it.
 * Do not call this function with the usbi_transfer lock held. User-specified
 * callback functions may attempt to directly resubmit the transfer, which
 * will attempt to take the lock. */
int usbi_handle_transfer_completion(struct usbi_transfer *itransfer,
	enum libusb_transfer_status status)
{
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	struct libusb_device_handle *dev_handle = transfer->dev_handle;
	uint8_t flags;
	int r;

	r = remove_from_flying_list(itransfer);
	if (r < 0)
		usbi_err(ITRANSFER_CTX(itransfer), "failed to set timer for next timeout, errno=%d", errno);

	usbi_mutex_lock(&itransfer->lock);
	itransfer->state_flags &= ~USBI_TRANSFER_IN_FLIGHT;
	usbi_mutex_unlock(&itransfer->lock);

	if (status == LIBUSB_TRANSFER_COMPLETED
			&& transfer->flags & LIBUSB_TRANSFER_SHORT_NOT_OK) {
		int rqlen = transfer->length;
		if (transfer->type == LIBUSB_TRANSFER_TYPE_CONTROL)
			rqlen -= LIBUSB_CONTROL_SETUP_SIZE;
		if (rqlen != itransfer->transferred) {
			usbi_dbg("interpreting short transfer as error");
			status = LIBUSB_TRANSFER_ERROR;
		}
	}

	flags = transfer->flags;
	transfer->status = status;
	transfer->actual_length = itransfer->transferred;
	usbi_dbg("transfer %p has callback %p", transfer, transfer->callback);
	if (transfer->callback)
		transfer->callback(transfer);
	/* transfer might have been freed by the above call, do not use from
	 * this point. */
	if (flags & LIBUSB_TRANSFER_FREE_TRANSFER)
		libusb_free_transfer(transfer);
	libusb_unref_device(dev_handle->dev);
	return r;
}

/* Similar to usbi_handle_transfer_completion() but exclusively for transfers
 * that were asynchronously cancelled. The same concerns w.r.t. freeing of
 * transfers exist here.
 * Do not call this function with the usbi_transfer lock held. User-specified
 * callback functions may attempt to directly resubmit the transfer, which
 * will attempt to take the lock. */
int usbi_handle_transfer_cancellation(struct usbi_transfer *transfer)
{
	struct libusb_context *ctx = ITRANSFER_CTX(transfer);
	uint8_t timed_out;

	usbi_mutex_lock(&ctx->flying_transfers_lock);
	timed_out = transfer->timeout_flags & USBI_TRANSFER_TIMED_OUT;
	usbi_mutex_unlock(&ctx->flying_transfers_lock);

	/* if the URB was cancelled due to timeout, report timeout to the user */
	if (timed_out) {
		usbi_dbg("detected timeout cancellation");
		return usbi_handle_transfer_completion(transfer, LIBUSB_TRANSFER_TIMED_OUT);
	}

	/* otherwise its a normal async cancel */
	return usbi_handle_transfer_completion(transfer, LIBUSB_TRANSFER_CANCELLED);
}

/* Add a completed transfer to the completed_transfers list of the
 * context and signal the event. The backend's handle_transfer_completion()
 * function will be called the next time an event handler runs. */
void usbi_signal_transfer_completion(struct usbi_transfer *transfer)
{
	struct libusb_context *ctx = ITRANSFER_CTX(transfer);
	int pending_events;

	usbi_mutex_lock(&ctx->event_data_lock);
	pending_events = usbi_pending_events(ctx);
	list_add_tail(&transfer->completed_list, &ctx->completed_transfers);
	if (!pending_events)
		usbi_signal_event(ctx);
	usbi_mutex_unlock(&ctx->event_data_lock);
}

/** \ingroup libusb_poll
 * Attempt to acquire the event handling lock. This lock is used to ensure that
 * only one thread is monitoring libusb event sources at any one time.
 *
 * You only need to use this lock if you are developing an application
 * which calls poll() or select() on libusb's file descriptors directly.
 * If you stick to libusb's event handling loop functions (e.g.
 * libusb_handle_events()) then you do not need to be concerned with this
 * locking.
 *
 * While holding this lock, you are trusted to actually be handling events.
 * If you are no longer handling events, you must call libusb_unlock_events()
 * as soon as possible.
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \returns 0 if the lock was obtained successfully
 * \returns 1 if the lock was not obtained (i.e. another thread holds the lock)
 * \ref libusb_mtasync
 */
int API_EXPORTED libusb_try_lock_events(libusb_context *ctx)
{
	int r;
	unsigned int ru;
	USBI_GET_CONTEXT(ctx);

	/* is someone else waiting to close a device? if so, don't let this thread
	 * start event handling */
	usbi_mutex_lock(&ctx->event_data_lock);
	ru = ctx->device_close;
	usbi_mutex_unlock(&ctx->event_data_lock);
	if (ru) {
		usbi_dbg("someone else is closing a device");
		return 1;
	}

	r = usbi_mutex_trylock(&ctx->events_lock);
	if (r)
		return 1;

	ctx->event_handler_active = 1;
	return 0;
}

/** \ingroup libusb_poll
 * Acquire the event handling lock, blocking until successful acquisition if
 * it is contended. This lock is used to ensure that only one thread is
 * monitoring libusb event sources at any one time.
 *
 * You only need to use this lock if you are developing an application
 * which calls poll() or select() on libusb's file descriptors directly.
 * If you stick to libusb's event handling loop functions (e.g.
 * libusb_handle_events()) then you do not need to be concerned with this
 * locking.
 *
 * While holding this lock, you are trusted to actually be handling events.
 * If you are no longer handling events, you must call libusb_unlock_events()
 * as soon as possible.
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \ref libusb_mtasync
 */
void API_EXPORTED libusb_lock_events(libusb_context *ctx)
{
	USBI_GET_CONTEXT(ctx);
	usbi_mutex_lock(&ctx->events_lock);
	ctx->event_handler_active = 1;
}

/** \ingroup libusb_poll
 * Release the lock previously acquired with libusb_try_lock_events() or
 * libusb_lock_events(). Releasing this lock will wake up any threads blocked
 * on libusb_wait_for_event().
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \ref libusb_mtasync
 */
void API_EXPORTED libusb_unlock_events(libusb_context *ctx)
{
	USBI_GET_CONTEXT(ctx);
	ctx->event_handler_active = 0;
	usbi_mutex_unlock(&ctx->events_lock);

	/* FIXME: perhaps we should be a bit more efficient by not broadcasting
	 * the availability of the events lock when we are modifying pollfds
	 * (check ctx->device_close)? */
	usbi_mutex_lock(&ctx->event_waiters_lock);
	usbi_cond_broadcast(&ctx->event_waiters_cond);
	usbi_mutex_unlock(&ctx->event_waiters_lock);
}

/** \ingroup libusb_poll
 * Determine if it is still OK for this thread to be doing event handling.
 *
 * Sometimes, libusb needs to temporarily pause all event handlers, and this
 * is the function you should use before polling file descriptors to see if
 * this is the case.
 *
 * If this function instructs your thread to give up the events lock, you
 * should just continue the usual logic that is documented in \ref libusb_mtasync.
 * On the next iteration, your thread will fail to obtain the events lock,
 * and will hence become an event waiter.
 *
 * This function should be called while the events lock is held: you don't
 * need to worry about the results of this function if your thread is not
 * the current event handler.
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \returns 1 if event handling can start or continue
 * \returns 0 if this thread must give up the events lock
 * \ref fullstory "Multi-threaded I/O: the full story"
 */
int API_EXPORTED libusb_event_handling_ok(libusb_context *ctx)
{
	unsigned int r;
	USBI_GET_CONTEXT(ctx);

	/* is someone else waiting to close a device? if so, don't let this thread
	 * continue event handling */
	usbi_mutex_lock(&ctx->event_data_lock);
	r = ctx->device_close;
	usbi_mutex_unlock(&ctx->event_data_lock);
	if (r) {
		usbi_dbg("someone else is closing a device");
		return 0;
	}

	return 1;
}


/** \ingroup libusb_poll
 * Determine if an active thread is handling events (i.e. if anyone is holding
 * the event handling lock).
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \returns 1 if a thread is handling events
 * \returns 0 if there are no threads currently handling events
 * \ref libusb_mtasync
 */
int API_EXPORTED libusb_event_handler_active(libusb_context *ctx)
{
	unsigned int r;
	USBI_GET_CONTEXT(ctx);

	/* is someone else waiting to close a device? if so, don't let this thread
	 * start event handling -- indicate that event handling is happening */
	usbi_mutex_lock(&ctx->event_data_lock);
	r = ctx->device_close;
	usbi_mutex_unlock(&ctx->event_data_lock);
	if (r) {
		usbi_dbg("someone else is closing a device");
		return 1;
	}

	return ctx->event_handler_active;
}

/** \ingroup libusb_poll
 * Interrupt any active thread that is handling events. This is mainly useful
 * for interrupting a dedicated event handling thread when an application
 * wishes to call libusb_exit().
 *
 * Since version 1.0.21, \ref LIBUSB_API_VERSION >= 0x01000105
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \ref libusb_mtasync
 */
void API_EXPORTED libusb_interrupt_event_handler(libusb_context *ctx)
{
	USBI_GET_CONTEXT(ctx);

	usbi_dbg("");
	usbi_mutex_lock(&ctx->event_data_lock);
	if (!usbi_pending_events(ctx)) {
		ctx->event_flags |= USBI_EVENT_USER_INTERRUPT;
		usbi_signal_event(ctx);
	}
	usbi_mutex_unlock(&ctx->event_data_lock);
}

/** \ingroup libusb_poll
 * Acquire the event waiters lock. This lock is designed to be obtained under
 * the situation where you want to be aware when events are completed, but
 * some other thread is event handling so calling libusb_handle_events() is not
 * allowed.
 *
 * You then obtain this lock, re-check that another thread is still handling
 * events, then call libusb_wait_for_event().
 *
 * You only need to use this lock if you are developing an application
 * which calls poll() or select() on libusb's file descriptors directly,
 * <b>and</b> may potentially be handling events from 2 threads simultaenously.
 * If you stick to libusb's event handling loop functions (e.g.
 * libusb_handle_events()) then you do not need to be concerned with this
 * locking.
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \ref libusb_mtasync
 */
void API_EXPORTED libusb_lock_event_waiters(libusb_context *ctx)
{
	USBI_GET_CONTEXT(ctx);
	usbi_mutex_lock(&ctx->event_waiters_lock);
}

/** \ingroup libusb_poll
 * Release the event waiters lock.
 * \param ctx the context to operate on, or NULL for the default context
 * \ref libusb_mtasync
 */
void API_EXPORTED libusb_unlock_event_waiters(libusb_context *ctx)
{
	USBI_GET_CONTEXT(ctx);
	usbi_mutex_unlock(&ctx->event_waiters_lock);
}

/** \ingroup libusb_poll
 * Wait for another thread to signal completion of an event. Must be called
 * with the event waiters lock held, see libusb_lock_event_waiters().
 *
 * This function will block until any of the following conditions are met:
 * -# The timeout expires
 * -# A transfer completes
 * -# A thread releases the event handling lock through libusb_unlock_events()
 *
 * Condition 1 is obvious. Condition 2 unblocks your thread <em>after</em>
 * the callback for the transfer has completed. Condition 3 is important
 * because it means that the thread that was previously handling events is no
 * longer doing so, so if any events are to complete, another thread needs to
 * step up and start event handling.
 *
 * This function releases the event waiters lock before putting your thread
 * to sleep, and reacquires the lock as it is being woken up.
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \param tv maximum timeout for this blocking function. A NULL value
 * indicates unlimited timeout.
 * \returns 0 after a transfer completes or another thread stops event handling
 * \returns 1 if the timeout expired
 * \ref libusb_mtasync
 */
int API_EXPORTED libusb_wait_for_event(libusb_context *ctx, struct timeval *tv)
{
	int r;

	USBI_GET_CONTEXT(ctx);
	if (tv == NULL) {
		usbi_cond_wait(&ctx->event_waiters_cond, &ctx->event_waiters_lock);
		return 0;
	}

	r = usbi_cond_timedwait(&ctx->event_waiters_cond,
		&ctx->event_waiters_lock, tv);

	if (r < 0)
		return r;
	else
		return (r == ETIMEDOUT);
}

static void handle_timeout(struct usbi_transfer *itransfer)
{
	struct libusb_transfer *transfer =
		USBI_TRANSFER_TO_LIBUSB_TRANSFER(itransfer);
	int r;

	itransfer->timeout_flags |= USBI_TRANSFER_TIMEOUT_HANDLED;
	r = libusb_cancel_transfer(transfer);
	if (r == LIBUSB_SUCCESS)
		itransfer->timeout_flags |= USBI_TRANSFER_TIMED_OUT;
	else
		usbi_warn(TRANSFER_CTX(transfer),
			"async cancel failed %d errno=%d", r, errno);
}

static int handle_timeouts_locked(struct libusb_context *ctx)
{
	int r;
	struct timespec systime_ts;
	struct timeval systime;
	struct usbi_transfer *transfer;

	if (list_empty(&ctx->flying_transfers))
		return 0;

	/* get current time */
	r = usbi_backend->clock_gettime(USBI_CLOCK_MONOTONIC, &systime_ts);
	if (r < 0)
		return r;

	TIMESPEC_TO_TIMEVAL(&systime, &systime_ts);

	/* iterate through flying transfers list, finding all transfers that
	 * have expired timeouts */
	list_for_each_entry(transfer, &ctx->flying_transfers, list, struct usbi_transfer) {
		struct timeval *cur_tv = &transfer->timeout;

		/* if we've reached transfers of infinite timeout, we're all done */
		if (!timerisset(cur_tv))
			return 0;

		/* ignore timeouts we've already handled */
		if (transfer->timeout_flags & (USBI_TRANSFER_TIMEOUT_HANDLED | USBI_TRANSFER_OS_HANDLES_TIMEOUT))
			continue;

		/* if transfer has non-expired timeout, nothing more to do */
		if ((cur_tv->tv_sec > systime.tv_sec) ||
				(cur_tv->tv_sec == systime.tv_sec &&
					cur_tv->tv_usec > systime.tv_usec))
			return 0;

		/* otherwise, we've got an expired timeout to handle */
		handle_timeout(transfer);
	}
	return 0;
}

static int handle_timeouts(struct libusb_context *ctx)
{
	int r;
	USBI_GET_CONTEXT(ctx);
	usbi_mutex_lock(&ctx->flying_transfers_lock);
	r = handle_timeouts_locked(ctx);
	usbi_mutex_unlock(&ctx->flying_transfers_lock);
	return r;
}

#ifdef USBI_TIMERFD_AVAILABLE
static int handle_timerfd_trigger(struct libusb_context *ctx)
{
	int r;

	usbi_mutex_lock(&ctx->flying_transfers_lock);

	/* process the timeout that just happened */
	r = handle_timeouts_locked(ctx);
	if (r < 0)
		goto out;

	/* arm for next timeout*/
	r = arm_timerfd_for_next_timeout(ctx);

out:
	usbi_mutex_unlock(&ctx->flying_transfers_lock);
	return r;
}
#endif

/* do the actual event handling. assumes that no other thread is concurrently
 * doing the same thing. */
static int handle_events(struct libusb_context *ctx, struct timeval *tv)
{
	int r;
	struct usbi_pollfd *ipollfd;
	POLL_NFDS_TYPE nfds = 0;
	POLL_NFDS_TYPE internal_nfds;
	struct pollfd *fds = NULL;
	int i = -1;
	int timeout_ms;
	int special_event;

	/* prevent attempts to recursively handle events (e.g. calling into
	 * libusb_handle_events() from within a hotplug or transfer callback) */
	if (usbi_handling_events(ctx))
		return LIBUSB_ERROR_BUSY;
	usbi_start_event_handling(ctx);

	/* there are certain fds that libusb uses internally, currently:
	 *
	 *   1) event pipe
	 *   2) timerfd
	 *
	 * the backend will never need to attempt to handle events on these fds, so
	 * we determine how many fds are in use internally for this context and when
	 * handle_events() is called in the backend, the pollfd list and count will
	 * be adjusted to skip over these internal fds */
	if (usbi_using_timerfd(ctx))
		internal_nfds = 2;
	else
		internal_nfds = 1;

	/* only reallocate the poll fds when the list of poll fds has been modified
	 * since the last poll, otherwise reuse them to save the additional overhead */
	usbi_mutex_lock(&ctx->event_data_lock);
	if (ctx->event_flags & USBI_EVENT_POLLFDS_MODIFIED) {
		usbi_dbg("poll fds modified, reallocating");

		if (ctx->pollfds) {
			free(ctx->pollfds);
			ctx->pollfds = NULL;
		}

		/* sanity check - it is invalid for a context to have fewer than the
		 * required internal fds (memory corruption?) */
		assert(ctx->pollfds_cnt >= internal_nfds);

		ctx->pollfds = calloc(ctx->pollfds_cnt, sizeof(*ctx->pollfds));
		if (!ctx->pollfds) {
			usbi_mutex_unlock(&ctx->event_data_lock);
			r = LIBUSB_ERROR_NO_MEM;
			goto done;
		}

		list_for_each_entry(ipollfd, &ctx->ipollfds, list, struct usbi_pollfd) {
			struct libusb_pollfd *pollfd = &ipollfd->pollfd;
			i++;
			ctx->pollfds[i].fd = pollfd->fd;
			ctx->pollfds[i].events = pollfd->events;
		}

		/* reset the flag now that we have the updated list */
		ctx->event_flags &= ~USBI_EVENT_POLLFDS_MODIFIED;

		/* if no further pending events, clear the event pipe so that we do
		 * not immediately return from poll */
		if (!usbi_pending_events(ctx))
			usbi_clear_event(ctx);
	}
	fds = ctx->pollfds;
	nfds = ctx->pollfds_cnt;
	usbi_mutex_unlock(&ctx->event_data_lock);

	timeout_ms = (int)(tv->tv_sec * 1000) + (tv->tv_usec / 1000);

	/* round up to next millisecond */
	if (tv->tv_usec % 1000)
		timeout_ms++;

redo_poll:
	usbi_dbg("poll() %d fds with timeout in %dms", nfds, timeout_ms);
	r = usbi_poll(fds, nfds, timeout_ms);
	usbi_dbg("poll() returned %d", r);
	if (r == 0) {
		r = handle_timeouts(ctx);
		goto done;
	}
	else if (r == -1 && errno == EINTR) {
		r = LIBUSB_ERROR_INTERRUPTED;
		goto done;
	}
	else if (r < 0) {
		usbi_err(ctx, "poll failed %d err=%d", r, errno);
		r = LIBUSB_ERROR_IO;
		goto done;
	}

	special_event = 0;

	/* fds[0] is always the event pipe */
	if (fds[0].revents) {
		libusb_hotplug_message *message = NULL;
		struct usbi_transfer *itransfer;
		int ret = 0;

		usbi_dbg("caught a fish on the event pipe");

		/* take the the event data lock while processing events */
		usbi_mutex_lock(&ctx->event_data_lock);

		/* check if someone added a new poll fd */
		if (ctx->event_flags & USBI_EVENT_POLLFDS_MODIFIED)
			usbi_dbg("someone updated the poll fds");

		if (ctx->event_flags & USBI_EVENT_USER_INTERRUPT) {
			usbi_dbg("someone purposely interrupted");
			ctx->event_flags &= ~USBI_EVENT_USER_INTERRUPT;
		}

		/* check if someone is closing a device */
		if (ctx->device_close)
			usbi_dbg("someone is closing a device");

		/* check for any pending hotplug messages */
		if (!list_empty(&ctx->hotplug_msgs)) {
			usbi_dbg("hotplug message received");
			special_event = 1;
			message = list_first_entry(&ctx->hotplug_msgs, libusb_hotplug_message, list);
			list_del(&message->list);
		}

		/* complete any pending transfers */
		while (ret == 0 && !list_empty(&ctx->completed_transfers)) {
			itransfer = list_first_entry(&ctx->completed_transfers, struct usbi_transfer, completed_list);
			list_del(&itransfer->completed_list);
			usbi_mutex_unlock(&ctx->event_data_lock);
			ret = usbi_backend->handle_transfer_completion(itransfer);
			if (ret)
				usbi_err(ctx, "backend handle_transfer_completion failed with error %d", ret);
			usbi_mutex_lock(&ctx->event_data_lock);
		}

		/* if no further pending events, clear the event pipe */
		if (!usbi_pending_events(ctx))
			usbi_clear_event(ctx);

		usbi_mutex_unlock(&ctx->event_data_lock);

		/* process the hotplug message, if any */
		if (message) {
			usbi_hotplug_match(ctx, message->device, message->event);

			/* the device left, dereference the device */
			if (LIBUSB_HOTPLUG_EVENT_DEVICE_LEFT == message->event)
				libusb_unref_device(message->device);

			free(message);
		}

		if (ret) {
			/* return error code */
			r = ret;
			goto done;
		}

		if (0 == --r)
			goto handled;
	}

#ifdef USBI_TIMERFD_AVAILABLE
	/* on timerfd configurations, fds[1] is the timerfd */
	if (usbi_using_timerfd(ctx) && fds[1].revents) {
		/* timerfd indicates that a timeout has expired */
		int ret;
		usbi_dbg("timerfd triggered");
		special_event = 1;

		ret = handle_timerfd_trigger(ctx);
		if (ret < 0) {
			/* return error code */
			r = ret;
			goto done;
		}

		if (0 == --r)
			goto handled;
	}
#endif

	r = usbi_backend->handle_events(ctx, fds + internal_nfds, nfds - internal_nfds, r);
	if (r)
		usbi_err(ctx, "backend handle_events failed with error %d", r);

handled:
	if (r == 0 && special_event) {
		timeout_ms = 0;
		goto redo_poll;
	}

done:
	usbi_end_event_handling(ctx);
	return r;
}

/* returns the smallest of:
 *  1. timeout of next URB
 *  2. user-supplied timeout
 * returns 1 if there is an already-expired timeout, otherwise returns 0
 * and populates out
 */
static int get_next_timeout(libusb_context *ctx, struct timeval *tv,
	struct timeval *out)
{
	struct timeval timeout;
	int r = libusb_get_next_timeout(ctx, &timeout);
	if (r) {
		/* timeout already expired? */
		if (!timerisset(&timeout))
			return 1;

		/* choose the smallest of next URB timeout or user specified timeout */
		if (timercmp(&timeout, tv, <))
			*out = timeout;
		else
			*out = *tv;
	} else {
		*out = *tv;
	}
	return 0;
}

/** \ingroup libusb_poll
 * Handle any pending events.
 *
 * libusb determines "pending events" by checking if any timeouts have expired
 * and by checking the set of file descriptors for activity.
 *
 * If a zero timeval is passed, this function will handle any already-pending
 * events and then immediately return in non-blocking style.
 *
 * If a non-zero timeval is passed and no events are currently pending, this
 * function will block waiting for events to handle up until the specified
 * timeout. If an event arrives or a signal is raised, this function will
 * return early.
 *
 * If the parameter completed is not NULL then <em>after obtaining the event
 * handling lock</em> this function will return immediately if the integer
 * pointed to is not 0. This allows for race free waiting for the completion
 * of a specific transfer.
 *
 * \param ctx the context to operate on, or NULL for the default context
 * \param tv the maximum time to block waiting for events, or an all zero
 * timeval struct for non-blocking mode
 * \param completed pointer to completion integer to check, or NULL
 * \returns 0 on success, or a LIBUSB_ERROR code on failure
 * \ref libusb_mtasync
 */
int API_EXPORTED libusb_handle_events_timeout_completed(libusb_context *ctx,
	struct timeval *tv, int *completed)
{
	int r;
	struct timeval poll_timeout;

	USBI_GET_CONTEXT(ctx);
	r = get_next_timeout(ctx, tv, &poll_timeout);
	if (r) {
		/* timeout already expired */
		return handle_timeouts(ctx);
	}

retry:
	if (libusb_try_lock_events(ctx) == 0) {
		if (completed == NULL || !*completed) {
			/* we obtained the event lock: do our own event handling */
			usbi_dbg("doing our own event handling");
			r = handle_events(ctx, &poll_timeout);
		}
		libusb_unlock_events(ctx);
		return r;
	}

	/* another thread is doing event handling. wait for thread events that
	 * notify event completion. */
	libusb_lock_event_waiters(ctx);

	if (completed && *completed)
		goto already_done;

	if (!libusb_event_handler_active(ctx)) {
		/* we hit a race: whoever was event handling earlier finished in the
		 * time it took us to reach this point. try the cycle again. */
		libusb_unlock_event_waiters(ctx);
		usbi_dbg("event handler was active but went away, retrying");
		goto retry;
	}

	usbi_dbg("another thread is doing event handling");
	r = libusb_wait_for_event(ctx, &poll_timeout);

already_done:
	libusb_unlock_event_waiters(ctx);

	if (r < 0)
		return r;
	else if (r ==