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 2820 2821 2822 2823 2824 2825 2826 2827 2828 2829 2830 2831 2832 2833 2834 2835 2836 2837 2838 2839 2840 2841 2842 2843 2844 2845 2846 2847 2848 2849 2850 2851 2852 2853 2854 2855 2856 2857 2858 2859 2860 2861 2862 2863 2864 2865 2866 2867 2868 2869 2870 2871 2872 2873 2874 2875 2876 2877 2878 2879 2880 2881 2882 2883 2884 2885 2886 2887 2888 2889 2890 2891 2892 2893 2894 2895 2896 2897 2898 2899 2900 2901 2902 2903 2904 2905 2906 2907 2908 2909 2910 2911 2912 2913 2914 2915 2916 2917 2918 2919 2920 2921 2922 2923 2924 2925 2926 2927 2928 2929 2930 2931 2932 2933 2934 2935 2936 2937 2938 2939 2940 2941 2942 2943 2944 2945 2946 2947 2948 2949 2950 2951 2952 2953 2954 2955 2956 2957 2958 2959 2960 2961 2962 2963 2964 2965 2966 2967 2968 2969 2970 2971 2972 2973 2974 2975 2976 2977 2978 2979 2980 2981 2982 2983 2984 2985 2986 2987 2988 2989 2990 2991 2992 2993 2994 2995 2996 2997 2998 2999 3000 3001 3002 3003 3004 3005 3006 3007 3008 3009 3010 3011 3012 3013 3014 3015 3016 3017 3018 3019 3020 3021 3022 3023 3024 3025 3026 3027 3028 3029 3030 3031 3032 3033 3034 3035 3036 3037 3038 3039 3040 3041 3042 3043 3044 3045 3046 3047 3048 3049 3050 3051 3052 3053 3054 3055 3056 3057 3058 3059 3060 3061 3062 3063 3064 3065 3066 3067 3068 3069 3070 3071 3072 3073 3074 3075 3076 3077 3078 3079 3080 3081 3082 3083 3084 3085 3086 3087 3088 3089 3090 3091 3092 3093 3094 3095 3096 3097 3098 3099 3100 3101 3102 3103 3104 3105 3106 3107 3108 3109 3110 3111 3112 3113 3114 3115 3116 3117 3118 3119 3120 3121 3122 3123 3124 3125 3126 3127 3128 3129 3130 3131 3132 3133 3134 3135 3136 3137 3138 3139 3140 3141 3142 3143 3144 3145 3146 3147 3148 3149 3150 3151 3152 3153 3154 3155 3156 3157 3158 3159 3160 3161 3162 3163 3164 3165 3166 3167 3168 3169 3170 3171 3172 3173 3174 3175 3176 3177 3178 3179 3180 3181 3182 3183 3184 3185 3186 3187 3188 3189 3190 3191 3192 3193 3194 3195 3196 3197 3198 3199 3200 3201 3202 3203 3204 3205 3206 3207 3208 3209 3210 3211 3212 3213 3214 3215 3216 3217 3218 3219 3220 3221 3222 3223 3224 3225 3226 3227 3228 3229 3230 3231 3232 3233 3234 3235 3236 3237 3238 3239 3240 3241 3242 3243 3244 3245 3246 3247 3248 3249 3250 3251 3252 3253 3254 3255 3256 3257 3258 3259 3260 3261 3262 3263 3264 3265 3266 3267 3268 3269 3270 3271 3272 3273 3274 3275 3276 3277 3278 3279 3280 3281 3282 3283 3284 3285 3286 3287 3288 3289 3290 3291 3292 3293 3294 3295 3296 3297 3298 3299 3300 3301 3302 3303 3304 3305 3306 3307 3308 3309 3310 3311 3312 3313 3314 3315 3316 3317 3318 3319 3320 3321 3322 3323 3324 3325 3326 3327 3328 3329 3330 3331 3332 3333 3334 3335 3336 3337 3338 3339 3340 3341 3342 3343 3344 3345 3346 3347 3348 3349 3350 3351 3352 3353 3354 3355 3356 3357 3358 3359 3360 3361 3362 3363 3364 3365 3366 3367 3368 3369 3370 3371 3372 3373 3374 3375 3376 3377 3378 3379 3380 3381 3382 3383 3384 3385 3386 3387 3388 3389 3390 3391 3392 3393 3394 3395 3396 3397 3398 3399 3400 3401 3402 3403 3404 3405 3406 3407 3408 3409 3410 3411 3412 3413 3414 3415 3416 3417 3418 3419 3420 3421 3422 3423 3424 3425 3426 3427 3428 3429 3430 3431 3432 3433 3434 3435 3436 3437 3438 3439 3440 3441 3442 3443 3444 3445 3446 3447 3448 3449 3450 3451 3452 3453 3454 3455 3456 3457 3458 3459 3460 3461 3462 3463 3464 3465 3466 3467 3468 3469 3470 3471 3472 3473 3474 3475 3476 3477 3478 3479 3480 3481 3482 3483 3484 3485 3486 3487 3488 3489 3490 3491 3492 3493 3494 3495 3496 3497 3498 3499 3500 3501 3502 3503 3504 3505 3506 3507 3508 3509 3510 3511 3512 3513 3514 3515 3516 3517 3518 3519 3520 3521 3522 3523 3524 3525 3526 3527 3528 3529 3530 3531 3532 3533 3534 3535 3536 3537 3538 3539 3540 3541 3542 3543 3544 3545 3546 3547 3548 3549 3550 3551 3552 3553 3554 3555 3556 3557 3558 3559 3560 3561 3562 3563 3564 3565 3566 3567 3568 3569 3570 3571 3572 3573 3574 3575 3576 3577 3578 3579 3580 3581 3582 3583 3584 3585 3586 3587 3588 3589 3590 3591 3592 3593 3594 3595 3596 3597 3598 3599 3600 3601 3602 3603 3604 3605 3606 3607 3608 3609 3610 3611 3612 3613 3614 3615 3616 3617 3618 3619 3620 3621 3622 3623 3624 3625 3626 3627 3628 3629 3630 3631 3632 3633 3634 3635 3636 3637 3638 3639 3640 3641 3642 3643 3644 3645 3646 3647 3648 3649 3650 3651 3652 3653 3654 3655 3656 3657 3658 3659 3660 3661 3662 3663 3664 3665 3666 3667 3668 3669 3670 3671 3672 3673 3674 3675 3676 3677 3678 3679 3680 3681 3682 3683 3684 3685 3686 3687 3688 3689 3690 3691 3692 3693 3694 3695 3696 3697 3698 3699 3700 3701 3702 3703 3704 3705 3706 3707 3708 3709 3710 3711 3712 3713 3714 3715 3716 3717 3718 3719 3720 3721 3722 3723 3724 3725 3726 3727 3728 3729 3730 3731 3732 3733 3734 3735 3736 3737 3738 3739 3740 3741 3742 3743 3744 3745 3746 3747 3748 3749 3750 3751 3752 3753 3754 3755 3756 3757 3758 3759 3760 3761 3762 3763 3764 3765 3766 3767 3768 3769 3770 3771 3772 3773 3774 3775 3776 3777 3778 3779 3780 3781 3782 3783 3784 3785 3786 3787 3788 3789 3790 3791 3792 3793 3794 3795 3796 3797 3798 3799 3800 3801 3802 3803 3804 3805 3806 3807 3808 3809 3810 3811 3812 3813 3814 3815 3816 3817 3818 3819 3820 3821 3822 3823 3824 3825 3826 3827 3828 3829 3830 3831 3832 3833 3834 3835 3836 3837 3838 3839 3840 3841 3842 3843 3844 3845 3846 3847 3848 3849 3850 3851 3852 3853 3854 3855 3856 3857 3858 3859 3860 3861 3862 3863 3864 3865 3866 3867 3868 3869 3870 3871 3872 3873 3874 3875 3876 3877 3878 3879 3880 3881 3882 3883 3884 3885 3886 3887 3888 3889 3890 3891 3892 3893 3894 3895 3896 3897 3898 3899 3900 3901 3902 3903 3904 3905 3906 3907 3908 3909 3910 3911 3912 3913 3914 3915 3916 3917 3918 3919 3920 3921 3922 3923 3924 3925 3926 3927 3928 3929 3930 3931 3932 3933 3934 3935 3936 3937 3938 3939 3940 3941 3942 3943 3944 3945 3946 3947 3948 3949 3950 3951 3952 3953 3954 3955 3956 3957 3958 3959 3960 3961 3962 3963 3964 3965 3966 3967 3968 3969 3970 3971 3972 3973 3974 3975 3976 3977 3978 3979 3980 3981 3982 3983 3984 3985 3986 3987 3988 3989 3990 3991 3992 3993 3994 3995 3996 3997 3998 3999 4000 4001 4002 4003 4004 4005 4006 4007 4008 4009 4010 4011 4012 4013 4014 4015 4016 4017 4018 4019 4020 4021 4022 4023 4024 4025 4026 4027 4028 4029 4030 4031 4032 4033 4034 4035 4036 4037 4038 4039 4040 4041 4042 4043 4044 4045 4046 4047 4048 4049 4050 4051 4052 4053 4054 4055 4056 4057 4058 4059 4060 4061 4062 4063 4064 4065 4066 4067 4068 4069 4070 4071 4072 4073 4074 4075 4076 4077 4078 4079 4080 4081 4082 4083 4084 4085 4086 4087 4088
|
<?xml version="1.0" standalone="no"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"
[
]>
<article id="index">
<articleinfo>
<title>D-Bus Specification</title>
<releaseinfo>Version 0.12</releaseinfo>
<date>7 November 2006</date>
<authorgroup>
<author>
<firstname>Havoc</firstname>
<surname>Pennington</surname>
<affiliation>
<orgname>Red Hat, Inc.</orgname>
<address>
<email>hp@pobox.com</email>
</address>
</affiliation>
</author>
<author>
<firstname>Anders</firstname>
<surname>Carlsson</surname>
<affiliation>
<orgname>CodeFactory AB</orgname>
<address>
<email>andersca@codefactory.se</email>
</address>
</affiliation>
</author>
<author>
<firstname>Alexander</firstname>
<surname>Larsson</surname>
<affiliation>
<orgname>Red Hat, Inc.</orgname>
<address>
<email>alexl@redhat.com</email>
</address>
</affiliation>
</author>
</authorgroup>
</articleinfo>
<sect1 id="introduction">
<title>Introduction</title>
<para>
D-Bus is a system for low-latency, low-overhead, easy to use
interprocess communication (IPC). In more detail:
<itemizedlist>
<listitem>
<para>
D-Bus is <emphasis>low-latency</emphasis> because it is designed
to avoid round trips and allow asynchronous operation, much like
the X protocol.
</para>
</listitem>
<listitem>
<para>
D-Bus is <emphasis>low-overhead</emphasis> because it uses a
binary protocol, and does not have to convert to and from a text
format such as XML. Because D-Bus is intended for potentially
high-resolution same-machine IPC, not primarily for Internet IPC,
this is an interesting optimization.
</para>
</listitem>
<listitem>
<para>
D-Bus is <emphasis>easy to use</emphasis> because it works in terms
of <firstterm>messages</firstterm> rather than byte streams, and
automatically handles a lot of the hard IPC issues. Also, the D-Bus
library is designed to be wrapped in a way that lets developers use
their framework's existing object/type system, rather than learning
a new one specifically for IPC.
</para>
</listitem>
</itemizedlist>
</para>
<para>
The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
protocol, specified in <xref linkend="message-protocol"/>. That is, it is
a system for one application to talk to a single other
application. However, the primary intended application of the protocol is the
D-Bus <firstterm>message bus</firstterm>, specified in <xref
linkend="message-bus"/>. The message bus is a special application that
accepts connections from multiple other applications, and forwards
messages among them.
</para>
<para>
Uses of D-Bus include notification of system changes (notification of when
a camera is plugged in to a computer, or a new version of some software
has been installed), or desktop interoperability, for example a file
monitoring service or a configuration service.
</para>
<para>
D-Bus is designed for two specific use cases:
<itemizedlist>
<listitem>
<para>
A "system bus" for notifications from the system to user sessions,
and to allow the system to request input from user sessions.
</para>
</listitem>
<listitem>
<para>
A "session bus" used to implement desktop environments such as
GNOME and KDE.
</para>
</listitem>
</itemizedlist>
D-Bus is not intended to be a generic IPC system for any possible
application, and intentionally omits many features found in other
IPC systems for this reason.
</para>
<para>
At the same time, the bus daemons offer a number of features not found in
other IPC systems, such as single-owner "bus names" (similar to X
selections), on-demand startup of services, and security policies.
In many ways, these features are the primary motivation for developing
D-Bus; other systems would have sufficed if IPC were the only goal.
</para>
<para>
D-Bus may turn out to be useful in unanticipated applications, but future
versions of this spec and the reference implementation probably will not
incorporate features that interfere with the core use cases.
</para>
<para>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119. However, the
document could use a serious audit to be sure it makes sense to do
so. Also, they are not capitalized.
</para>
<sect2 id="stability">
<title>Protocol and Specification Stability</title>
<para>
The D-Bus protocol is frozen (only compatible extensions are allowed) as
of November 8, 2006. However, this specification could still use a fair
bit of work to make interoperable reimplementation possible without
reference to the D-Bus reference implementation. Thus, this
specification is not marked 1.0. To mark it 1.0, we'd like to see
someone invest significant effort in clarifying the specification
language, and growing the specification to cover more aspects of the
reference implementation's behavior.
</para>
<para>
Until this work is complete, any attempt to reimplement D-Bus will
probably require looking at the reference implementation and/or asking
questions on the D-Bus mailing list about intended behavior.
Questions on the list are very welcome.
</para>
<para>
Nonetheless, this document should be a useful starting point and is
to our knowledge accurate, though incomplete.
</para>
</sect2>
</sect1>
<sect1 id="message-protocol">
<title>Message Protocol</title>
<para>
A <firstterm>message</firstterm> consists of a
<firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
think of a message as a package, the header is the address, and the body
contains the package contents. The message delivery system uses the header
information to figure out where to send the message and how to interpret
it; the recipient interprets the body of the message.
</para>
<para>
The body of the message is made up of zero or more
<firstterm>arguments</firstterm>, which are typed values, such as an
integer or a byte array.
</para>
<para>
Both header and body use the same type system and format for
serializing data. Each type of value has a wire format.
Converting a value from some other representation into the wire
format is called <firstterm>marshaling</firstterm> and converting
it back from the wire format is <firstterm>unmarshaling</firstterm>.
</para>
<sect2 id="message-protocol-signatures">
<title>Type Signatures</title>
<para>
The D-Bus protocol does not include type tags in the marshaled data; a
block of marshaled values must have a known <firstterm>type
signature</firstterm>. The type signature is made up of <firstterm>type
codes</firstterm>. A type code is an ASCII character representing the
type of a value. Because ASCII characters are used, the type signature
will always form a valid ASCII string. A simple string compare
determines whether two type signatures are equivalent.
</para>
<para>
As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
the ASCII character 'i'. So the signature for a block of values
containing a single <literal>INT32</literal> would be:
<programlisting>
"i"
</programlisting>
A block of values containing two <literal>INT32</literal> would have this signature:
<programlisting>
"ii"
</programlisting>
</para>
<para>
All <firstterm>basic</firstterm> types work like
<literal>INT32</literal> in this example. To marshal and unmarshal
basic types, you simply read one value from the data
block corresponding to each type code in the signature.
In addition to basic types, there are four <firstterm>container</firstterm>
types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>,
and <literal>DICT_ENTRY</literal>.
</para>
<para>
<literal>STRUCT</literal> has a type code, ASCII character 'r', but this type
code does not appear in signatures. Instead, ASCII characters
'(' and ')' are used to mark the beginning and end of the struct.
So for example, a struct containing two integers would have this
signature:
<programlisting>
"(ii)"
</programlisting>
Structs can be nested, so for example a struct containing
an integer and another struct:
<programlisting>
"(i(ii))"
</programlisting>
The value block storing that struct would contain three integers; the
type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
"(iii)" or "iii".
</para>
<para>
The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
but is useful in code that implements the protocol. This type code
is specified to allow such code to interoperate in non-protocol contexts.
</para>
<para>
Empty structures are not allowed; there must be at least one
type code between the parentheses.
</para>
<para>
<literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
followed by a <firstterm>single complete type</firstterm>. The single
complete type following the array is the type of each array element. So
the simple example is:
<programlisting>
"ai"
</programlisting>
which is an array of 32-bit integers. But an array can be of any type,
such as this array-of-struct-with-two-int32-fields:
<programlisting>
"a(ii)"
</programlisting>
Or this array of array of integer:
<programlisting>
"aai"
</programlisting>
</para>
<para>
The phrase <firstterm>single complete type</firstterm> deserves some
definition. A single complete type is a basic type code, a variant type code,
an array with its element type, or a struct with its fields.
So the following signatures are not single complete types:
<programlisting>
"aa"
</programlisting>
<programlisting>
"(ii"
</programlisting>
<programlisting>
"ii)"
</programlisting>
And the following signatures contain multiple complete types:
<programlisting>
"ii"
</programlisting>
<programlisting>
"aiai"
</programlisting>
<programlisting>
"(ii)(ii)"
</programlisting>
Note however that a single complete type may <emphasis>contain</emphasis>
multiple other single complete types.
</para>
<para>
<literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
type <literal>VARIANT</literal> will have the signature of a single complete type as part
of the <emphasis>value</emphasis>. This signature will be followed by a
marshaled value of that type.
</para>
<para>
A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
than parentheses it uses curly braces, and it has more restrictions.
The restrictions are: it occurs only as an array element type; it has
exactly two single complete types inside the curly braces; the first
single complete type (the "key") must be a basic type rather than a
container type. Implementations must not accept dict entries outside of
arrays, must not accept dict entries with zero, one, or more than two
fields, and must not accept dict entries with non-basic-typed keys. A
dict entry is always a key-value pair.
</para>
<para>
The first field in the <literal>DICT_ENTRY</literal> is always the key.
A message is considered corrupt if the same key occurs twice in the same
array of <literal>DICT_ENTRY</literal>. However, for performance reasons
implementations are not required to reject dicts with duplicate keys.
</para>
<para>
In most languages, an array of dict entry would be represented as a
map, hash table, or dict object.
</para>
<para>
The following table summarizes the D-Bus types.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Code</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>0 (ASCII NUL)</entry>
<entry>Not a valid type code, used to terminate signatures</entry>
</row><row>
<entry><literal>BYTE</literal></entry>
<entry>121 (ASCII 'y')</entry>
<entry>8-bit unsigned integer</entry>
</row><row>
<entry><literal>BOOLEAN</literal></entry>
<entry>98 (ASCII 'b')</entry>
<entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
</row><row>
<entry><literal>INT16</literal></entry>
<entry>110 (ASCII 'n')</entry>
<entry>16-bit signed integer</entry>
</row><row>
<entry><literal>UINT16</literal></entry>
<entry>113 (ASCII 'q')</entry>
<entry>16-bit unsigned integer</entry>
</row><row>
<entry><literal>INT32</literal></entry>
<entry>105 (ASCII 'i')</entry>
<entry>32-bit signed integer</entry>
</row><row>
<entry><literal>UINT32</literal></entry>
<entry>117 (ASCII 'u')</entry>
<entry>32-bit unsigned integer</entry>
</row><row>
<entry><literal>INT64</literal></entry>
<entry>120 (ASCII 'x')</entry>
<entry>64-bit signed integer</entry>
</row><row>
<entry><literal>UINT64</literal></entry>
<entry>116 (ASCII 't')</entry>
<entry>64-bit unsigned integer</entry>
</row><row>
<entry><literal>DOUBLE</literal></entry>
<entry>100 (ASCII 'd')</entry>
<entry>IEEE 754 double</entry>
</row><row>
<entry><literal>STRING</literal></entry>
<entry>115 (ASCII 's')</entry>
<entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
</row><row>
<entry><literal>OBJECT_PATH</literal></entry>
<entry>111 (ASCII 'o')</entry>
<entry>Name of an object instance</entry>
</row><row>
<entry><literal>SIGNATURE</literal></entry>
<entry>103 (ASCII 'g')</entry>
<entry>A type signature</entry>
</row><row>
<entry><literal>ARRAY</literal></entry>
<entry>97 (ASCII 'a')</entry>
<entry>Array</entry>
</row><row>
<entry><literal>STRUCT</literal></entry>
<entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
<entry>Struct</entry>
</row><row>
<entry><literal>VARIANT</literal></entry>
<entry>118 (ASCII 'v') </entry>
<entry>Variant type (the type of the value is part of the value itself)</entry>
</row><row>
<entry><literal>DICT_ENTRY</literal></entry>
<entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
<entry>Entry in a dict or map (array of key-value pairs)</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect2>
<sect2 id="message-protocol-marshaling">
<title>Marshaling (Wire Format)</title>
<para>
Given a type signature, a block of bytes can be converted into typed
values. This section describes the format of the block of bytes. Byte
order and alignment issues are handled uniformly for all D-Bus types.
</para>
<para>
A block of bytes has an associated byte order. The byte order
has to be discovered in some way; for D-Bus messages, the
byte order is part of the message header as described in
<xref linkend="message-protocol-messages"/>. For now, assume
that the byte order is known to be either little endian or big
endian.
</para>
<para>
Each value in a block of bytes is aligned "naturally," for example
4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
8-byte boundary. To properly align a value, <firstterm>alignment
padding</firstterm> may be necessary. The alignment padding must always
be the minimum required padding to properly align the following value;
and it must always be made up of nul bytes. The alignment padding must
not be left uninitialized (it can't contain garbage), and more padding
than required must not be used.
</para>
<para>
Given all this, the types are marshaled on the wire as follows:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Encoding</entry>
<entry>Alignment</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>Not applicable; cannot be marshaled.</entry>
<entry>N/A</entry>
</row><row>
<entry><literal>BYTE</literal></entry>
<entry>A single 8-bit byte.</entry>
<entry>1</entry>
</row><row>
<entry><literal>BOOLEAN</literal></entry>
<entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
<entry>4</entry>
</row><row>
<entry><literal>INT16</literal></entry>
<entry>16-bit signed integer in the message's byte order.</entry>
<entry>2</entry>
</row><row>
<entry><literal>UINT16</literal></entry>
<entry>16-bit unsigned integer in the message's byte order.</entry>
<entry>2</entry>
</row><row>
<entry><literal>INT32</literal></entry>
<entry>32-bit signed integer in the message's byte order.</entry>
<entry>4</entry>
</row><row>
<entry><literal>UINT32</literal></entry>
<entry>32-bit unsigned integer in the message's byte order.</entry>
<entry>4</entry>
</row><row>
<entry><literal>INT64</literal></entry>
<entry>64-bit signed integer in the message's byte order.</entry>
<entry>8</entry>
</row><row>
<entry><literal>UINT64</literal></entry>
<entry>64-bit unsigned integer in the message's byte order.</entry>
<entry>8</entry>
</row><row>
<entry><literal>DOUBLE</literal></entry>
<entry>64-bit IEEE 754 double in the message's byte order.</entry>
<entry>8</entry>
</row><row>
<entry><literal>STRING</literal></entry>
<entry>A <literal>UINT32</literal> indicating the string's
length in bytes excluding its terminating nul, followed by
non-nul string data of the given length, followed by a terminating nul
byte.
</entry>
<entry>
4 (for the length)
</entry>
</row><row>
<entry><literal>OBJECT_PATH</literal></entry>
<entry>Exactly the same as <literal>STRING</literal> except the
content must be a valid object path (see below).
</entry>
<entry>
4 (for the length)
</entry>
</row><row>
<entry><literal>SIGNATURE</literal></entry>
<entry>The same as <literal>STRING</literal> except the length is a single
byte (thus signatures have a maximum length of 255)
and the content must be a valid signature (see below).
</entry>
<entry>
1
</entry>
</row><row>
<entry><literal>ARRAY</literal></entry>
<entry>
A <literal>UINT32</literal> giving the length of the array data in bytes, followed by
alignment padding to the alignment boundary of the array element type,
followed by each array element. The array length is from the
end of the alignment padding to the end of the last element,
i.e. it does not include the padding after the length,
or any padding after the last element.
Arrays have a maximum length defined to be 2 to the 26th power or
67108864. Implementations must not send or accept arrays exceeding this
length.
</entry>
<entry>
4 (for the length)
</entry>
</row><row>
<entry><literal>STRUCT</literal></entry>
<entry>
A struct must start on an 8-byte boundary regardless of the
type of the struct fields. The struct value consists of each
field marshaled in sequence starting from that 8-byte
alignment boundary.
</entry>
<entry>
8
</entry>
</row><row>
<entry><literal>VARIANT</literal></entry>
<entry>
A variant type has a marshaled
<literal>SIGNATURE</literal> followed by a marshaled
value with the type given in the signature. Unlike
a message signature, the variant signature can
contain only a single complete type. So "i", "ai"
or "(ii)" is OK, but "ii" is not. Use of variants may not
cause a total message depth to be larger than 64, including
other container types such as structures.
</entry>
<entry>
1 (alignment of the signature)
</entry>
</row><row>
<entry><literal>DICT_ENTRY</literal></entry>
<entry>
Identical to STRUCT.
</entry>
<entry>
8
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<sect3 id="message-protocol-marshaling-object-path">
<title>Valid Object Paths</title>
<para>
An object path is a name used to refer to an object instance.
Conceptually, each participant in a D-Bus message exchange may have
any number of object instances (think of C++ or Java objects) and each
such instance will have a path. Like a filesystem, the object
instances in an application form a hierarchical tree.
</para>
<para>
The following rules define a valid object path. Implementations must
not send or accept messages with invalid object paths.
<itemizedlist>
<listitem>
<para>
The path may be of any length.
</para>
</listitem>
<listitem>
<para>
The path must begin with an ASCII '/' (integer 47) character,
and must consist of elements separated by slash characters.
</para>
</listitem>
<listitem>
<para>
Each element must only contain the ASCII characters
"[A-Z][a-z][0-9]_"
</para>
</listitem>
<listitem>
<para>
No element may be the empty string.
</para>
</listitem>
<listitem>
<para>
Multiple '/' characters cannot occur in sequence.
</para>
</listitem>
<listitem>
<para>
A trailing '/' character is not allowed unless the
path is the root path (a single '/' character).
</para>
</listitem>
</itemizedlist>
</para>
</sect3>
<sect3 id="message-protocol-marshaling-signature">
<title>Valid Signatures</title>
<para>
An implementation must not send or accept invalid signatures.
Valid signatures will conform to the following rules:
<itemizedlist>
<listitem>
<para>
The signature ends with a nul byte.
</para>
</listitem>
<listitem>
<para>
The signature is a list of single complete types.
Arrays must have element types, and structs must
have both open and close parentheses.
</para>
</listitem>
<listitem>
<para>
Only type codes and open and close parentheses are
allowed in the signature. The <literal>STRUCT</literal> type code
is not allowed in signatures, because parentheses
are used instead.
</para>
</listitem>
<listitem>
<para>
The maximum depth of container type nesting is 32 array type
codes and 32 open parentheses. This implies that the maximum
total depth of recursion is 64, for an "array of array of array
of ... struct of struct of struct of ..." where there are 32
array and 32 struct.
</para>
</listitem>
<listitem>
<para>
The maximum length of a signature is 255.
</para>
</listitem>
<listitem>
<para>
Signatures must be nul-terminated.
</para>
</listitem>
</itemizedlist>
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-messages">
<title>Message Format</title>
<para>
A message consists of a header and a body. The header is a block of
values with a fixed signature and meaning. The body is a separate block
of values, with a signature specified in the header.
</para>
<para>
The length of the header must be a multiple of 8, allowing the body to
begin on an 8-byte boundary when storing the entire message in a single
buffer. If the header does not naturally end on an 8-byte boundary
up to 7 bytes of nul-initialized alignment padding must be added.
</para>
<para>
The message body need not end on an 8-byte boundary.
</para>
<para>
The maximum length of a message, including header, header alignment padding,
and body is 2 to the 27th power or 134217728. Implementations must not
send or accept messages exceeding this size.
</para>
<para>
The signature of the header is:
<programlisting>
"yyyyuua(yv)"
</programlisting>
Written out more readably, this is:
<programlisting>
BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
</programlisting>
</para>
<para>
These values have the following meanings:
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>1st <literal>BYTE</literal></entry>
<entry>Endianness flag; ASCII 'l' for little-endian
or ASCII 'B' for big-endian. Both header and body are
in this endianness.</entry>
</row>
<row>
<entry>2nd <literal>BYTE</literal></entry>
<entry><firstterm>Message type</firstterm>. Unknown types must be ignored.
Currently-defined types are described below.
</entry>
</row>
<row>
<entry>3rd <literal>BYTE</literal></entry>
<entry>Bitwise OR of flags. Unknown flags
must be ignored. Currently-defined flags are described below.
</entry>
</row>
<row>
<entry>4th <literal>BYTE</literal></entry>
<entry>Major protocol version of the sending application. If
the major protocol version of the receiving application does not
match, the applications will not be able to communicate and the
D-Bus connection must be disconnected. The major protocol
version for this version of the specification is 1.
</entry>
</row>
<row>
<entry>1st <literal>UINT32</literal></entry>
<entry>Length in bytes of the message body, starting
from the end of the header. The header ends after
its alignment padding to an 8-boundary.
</entry>
</row>
<row>
<entry>2nd <literal>UINT32</literal></entry>
<entry>The serial of this message, used as a cookie
by the sender to identify the reply corresponding
to this request. This must not be zero.
</entry>
</row>
<row>
<entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
<entry>An array of zero or more <firstterm>header
fields</firstterm> where the byte is the field code, and the
variant is the field value. The message type determines
which fields are required.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
<firstterm>Message types</firstterm> that can appear in the second byte
of the header are:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional name</entry>
<entry>Decimal value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>0</entry>
<entry>This is an invalid type.</entry>
</row>
<row>
<entry><literal>METHOD_CALL</literal></entry>
<entry>1</entry>
<entry>Method call.</entry>
</row>
<row>
<entry><literal>METHOD_RETURN</literal></entry>
<entry>2</entry>
<entry>Method reply with returned data.</entry>
</row>
<row>
<entry><literal>ERROR</literal></entry>
<entry>3</entry>
<entry>Error reply. If the first argument exists and is a
string, it is an error message.</entry>
</row>
<row>
<entry><literal>SIGNAL</literal></entry>
<entry>4</entry>
<entry>Signal emission.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Flags that can appear in the third byte of the header:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional name</entry>
<entry>Hex value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>NO_REPLY_EXPECTED</literal></entry>
<entry>0x1</entry>
<entry>This message does not expect method return replies or
error replies; the reply can be omitted as an
optimization. However, it is compliant with this specification
to return the reply despite this flag and the only harm
from doing so is extra network traffic.
</entry>
</row>
<row>
<entry><literal>NO_AUTO_START</literal></entry>
<entry>0x2</entry>
<entry>The bus must not launch an owner
for the destination name in response to this message.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<sect3 id="message-protocol-header-fields">
<title>Header Fields</title>
<para>
The array at the end of the header contains <firstterm>header
fields</firstterm>, where each field is a 1-byte field code followed
by a field value. A header must contain the required header fields for
its message type, and zero or more of any optional header
fields. Future versions of this protocol specification may add new
fields. Implementations must ignore fields they do not
understand. Implementations must not invent their own header fields;
only changes to this specification may introduce new header fields.
</para>
<para>
Again, if an implementation sees a header field code that it does not
expect, it must ignore that field, as it will be part of a new
(but compatible) version of this specification. This also applies
to known header fields appearing in unexpected messages, for
example: if a signal has a reply serial it must be ignored
even though it has no meaning as of this version of the spec.
</para>
<para>
However, implementations must not send or accept known header fields
with the wrong type stored in the field value. So for example a
message with an <literal>INTERFACE</literal> field of type
<literal>UINT32</literal> would be considered corrupt.
</para>
<para>
Here are the currently-defined header fields:
<informaltable>
<tgroup cols="5">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Decimal Code</entry>
<entry>Type</entry>
<entry>Required In</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>INVALID</literal></entry>
<entry>0</entry>
<entry>N/A</entry>
<entry>not allowed</entry>
<entry>Not a valid field name (error if it appears in a message)</entry>
</row>
<row>
<entry><literal>PATH</literal></entry>
<entry>1</entry>
<entry><literal>OBJECT_PATH</literal></entry>
<entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
<entry>The object to send a call to,
or the object a signal is emitted from.
The special path
<literal>/org/freedesktop/DBus/Local</literal> is reserved;
implementations should not send messages with this path,
and the reference implementation of the bus daemon will
disconnect any application that attempts to do so.
</entry>
</row>
<row>
<entry><literal>INTERFACE</literal></entry>
<entry>2</entry>
<entry><literal>STRING</literal></entry>
<entry><literal>SIGNAL</literal></entry>
<entry>
The interface to invoke a method call on, or
that a signal is emitted from. Optional for
method calls, required for signals.
The special interface
<literal>org.freedesktop.DBus.Local</literal> is reserved;
implementations should not send messages with this
interface, and the reference implementation of the bus
daemon will disconnect any application that attempts to
do so.
</entry>
</row>
<row>
<entry><literal>MEMBER</literal></entry>
<entry>3</entry>
<entry><literal>STRING</literal></entry>
<entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
<entry>The member, either the method name or signal name.</entry>
</row>
<row>
<entry><literal>ERROR_NAME</literal></entry>
<entry>4</entry>
<entry><literal>STRING</literal></entry>
<entry><literal>ERROR</literal></entry>
<entry>The name of the error that occurred, for errors</entry>
</row>
<row>
<entry><literal>REPLY_SERIAL</literal></entry>
<entry>5</entry>
<entry><literal>UINT32</literal></entry>
<entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
<entry>The serial number of the message this message is a reply
to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
</row>
<row>
<entry><literal>DESTINATION</literal></entry>
<entry>6</entry>
<entry><literal>STRING</literal></entry>
<entry>optional</entry>
<entry>The name of the connection this message is intended for.
Only used in combination with the message bus, see
<xref linkend="message-bus"/>.</entry>
</row>
<row>
<entry><literal>SENDER</literal></entry>
<entry>7</entry>
<entry><literal>STRING</literal></entry>
<entry>optional</entry>
<entry>Unique name of the sending connection.
The message bus fills in this field so it is reliable; the field is
only meaningful in combination with the message bus.</entry>
</row>
<row>
<entry><literal>SIGNATURE</literal></entry>
<entry>8</entry>
<entry><literal>SIGNATURE</literal></entry>
<entry>optional</entry>
<entry>The signature of the message body.
If omitted, it is assumed to be the
empty signature "" (i.e. the body must be 0-length).</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-names">
<title>Valid Names</title>
<para>
The various names in D-Bus messages have some restrictions.
</para>
<para>
There is a <firstterm>maximum name length</firstterm>
of 255 which applies to bus names, interfaces, and members.
</para>
<sect3 id="message-protocol-names-interface">
<title>Interface names</title>
<para>
Interfaces have names with type <literal>STRING</literal>, meaning that
they must be valid UTF-8. However, there are also some
additional restrictions that apply to interface names
specifically:
<itemizedlist>
<listitem><para>Interface names are composed of 1 or more elements separated by
a period ('.') character. All elements must contain at least
one character.
</para>
</listitem>
<listitem><para>Each element must only contain the ASCII characters
"[A-Z][a-z][0-9]_" and must not begin with a digit.
</para>
</listitem>
<listitem><para>Interface names must contain at least one '.' (period)
character (and thus at least two elements).
</para></listitem>
<listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
<listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
</itemizedlist>
</para>
</sect3>
<sect3 id="message-protocol-names-bus">
<title>Bus names</title>
<para>
Connections have one or more bus names associated with them.
A connection has exactly one bus name that is a unique connection
name. The unique connection name remains with the connection for
its entire lifetime.
A bus name is of type <literal>STRING</literal>,
meaning that it must be valid UTF-8. However, there are also
some additional restrictions that apply to bus names
specifically:
<itemizedlist>
<listitem><para>Bus names that start with a colon (':')
character are unique connection names.
</para>
</listitem>
<listitem><para>Bus names are composed of 1 or more elements separated by
a period ('.') character. All elements must contain at least
one character.
</para>
</listitem>
<listitem><para>Each element must only contain the ASCII characters
"[A-Z][a-z][0-9]_-". Only elements that are part of a unique
connection name may begin with a digit, elements in
other bus names must not begin with a digit.
</para>
</listitem>
<listitem><para>Bus names must contain at least one '.' (period)
character (and thus at least two elements).
</para></listitem>
<listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
<listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
</itemizedlist>
</para>
<para>
Note that the hyphen ('-') character is allowed in bus names but
not in interface names.
</para>
</sect3>
<sect3 id="message-protocol-names-member">
<title>Member names</title>
<para>
Member (i.e. method or signal) names:
<itemizedlist>
<listitem><para>Must only contain the ASCII characters
"[A-Z][a-z][0-9]_" and may not begin with a
digit.</para></listitem>
<listitem><para>Must not contain the '.' (period) character.</para></listitem>
<listitem><para>Must not exceed the maximum name length.</para></listitem>
<listitem><para>Must be at least 1 byte in length.</para></listitem>
</itemizedlist>
</para>
</sect3>
<sect3 id="message-protocol-names-error">
<title>Error names</title>
<para>
Error names have the same restrictions as interface names.
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-types">
<title>Message Types</title>
<para>
Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
<literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
This section describes these conventions.
</para>
<sect3 id="message-protocol-types-method">
<title>Method Calls</title>
<para>
Some messages invoke an operation on a remote object. These are
called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
messages map naturally to methods on objects in a typical program.
</para>
<para>
A method call message is required to have a <literal>MEMBER</literal> header field
indicating the name of the method. Optionally, the message has an
<literal>INTERFACE</literal> field giving the interface the method is a part of. In the
absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
a method with the same name, it is undefined which of the two methods
will be invoked. Implementations may also choose to return an error in
this ambiguous case. However, if a method name is unique
implementations must not require an interface field.
</para>
<para>
Method call messages also include a <literal>PATH</literal> field
indicating the object to invoke the method on. If the call is passing
through a message bus, the message will also have a
<literal>DESTINATION</literal> field giving the name of the connection
to receive the message.
</para>
<para>
When an application handles a method call message, it is required to
return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
</para>
<para>
If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message
are the return value(s) or "out parameters" of the method call.
If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown,
and the call fails; no return value will be provided. It makes
no sense to send multiple replies to the same method call.
</para>
<para>
Even if a method call has no return values, a <literal>METHOD_RETURN</literal>
reply is required, so the caller will know the method
was successfully processed.
</para>
<para>
The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal>
header field.
</para>
<para>
If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>,
then as an optimization the application receiving the method
call may choose to omit the reply message (regardless of
whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>).
However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
flag and reply anyway.
</para>
<para>
Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
destination name does not exist then a program to own the destination
name will be started before the message is delivered. The message
will be held until the new program is successfully started or has
failed to start; in case of failure, an error will be returned. This
flag is only relevant in the context of a message bus, it is ignored
during one-to-one communication with no intermediate bus.
</para>
<sect4 id="message-protocol-types-method-apis">
<title>Mapping method calls to native APIs</title>
<para>
APIs for D-Bus may map method calls to a method call in a specific
programming language, such as C++, or may map a method call written
in an IDL to a D-Bus message.
</para>
<para>
In APIs of this nature, arguments to a method are often termed "in"
(which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
"inout" arguments, which are both sent and received, i.e. the caller
passes in a value which is modified. Mapped to D-Bus, an "inout"
argument is equivalent to an "in" argument, followed by an "out"
argument. You can't pass things "by reference" over the wire, so
"inout" is purely an illusion of the in-process API.
</para>
<para>
Given a method with zero or one return values, followed by zero or more
arguments, where each argument may be "in", "out", or "inout", the
caller constructs a message by appending each "in" or "inout" argument,
in order. "out" arguments are not represented in the caller's message.
</para>
<para>
The recipient constructs a reply by appending first the return value
if any, then each "out" or "inout" argument, in order.
"in" arguments are not represented in the reply message.
</para>
<para>
Error replies are normally mapped to exceptions in languages that have
exceptions.
</para>
<para>
In converting from native APIs to D-Bus, it is perhaps nice to
map D-Bus naming conventions ("FooBar") to native conventions
such as "fooBar" or "foo_bar" automatically. This is OK
as long as you can say that the native API is one that
was specifically written for D-Bus. It makes the most sense
when writing object implementations that will be exported
over the bus. Object proxies used to invoke remote D-Bus
objects probably need the ability to call any D-Bus method,
and thus a magic name mapping like this could be a problem.
</para>
<para>
This specification doesn't require anything of native API bindings;
the preceding is only a suggested convention for consistency
among bindings.
</para>
</sect4>
</sect3>
<sect3 id="message-protocol-types-signal">
<title>Signal Emission</title>
<para>
Unlike method calls, signal emissions have no replies.
A signal emission is simply a single message of type <literal>SIGNAL</literal>.
It must have three header fields: <literal>PATH</literal> giving the object
the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
for signals, though it is optional for method calls.
</para>
</sect3>
<sect3 id="message-protocol-types-errors">
<title>Errors</title>
<para>
Messages of type <literal>ERROR</literal> are most commonly replies
to a <literal>METHOD_CALL</literal>, but may be returned in reply
to any kind of message. The message bus for example
will return an <literal>ERROR</literal> in reply to a signal emission if
the bus does not have enough memory to send the signal.
</para>
<para>
An <literal>ERROR</literal> may have any arguments, but if the first
argument is a <literal>STRING</literal>, it must be an error message.
The error message may be logged or shown to the user
in some way.
</para>
</sect3>
<sect3 id="message-protocol-types-notation">
<title>Notation in this document</title>
<para>
This document uses a simple pseudo-IDL to describe particular method
calls and signals. Here is an example of a method call:
<programlisting>
org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
out UINT32 resultcode)
</programlisting>
This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName,
<literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
characters so it's known that the last part of the name in
the "IDL" is the member name.
</para>
<para>
In C++ that might end up looking like this:
<programlisting>
unsigned int org::freedesktop::DBus::StartServiceByName (const char *name,
unsigned int flags);
</programlisting>
or equally valid, the return value could be done as an argument:
<programlisting>
void org::freedesktop::DBus::StartServiceByName (const char *name,
unsigned int flags,
unsigned int *resultcode);
</programlisting>
It's really up to the API designer how they want to make
this look. You could design an API where the namespace wasn't used
in C++, using STL or Qt, using varargs, or whatever you wanted.
</para>
<para>
Signals are written as follows:
<programlisting>
org.freedesktop.DBus.NameLost (STRING name)
</programlisting>
Signals don't specify "in" vs. "out" because only
a single direction is possible.
</para>
<para>
It isn't especially encouraged to use this lame pseudo-IDL in actual
API implementations; you might use the native notation for the
language you're using, or you might use COM or CORBA IDL, for example.
</para>
</sect3>
</sect2>
<sect2 id="message-protocol-handling-invalid">
<title>Invalid Protocol and Spec Extensions</title>
<para>
For security reasons, the D-Bus protocol should be strictly parsed and
validated, with the exception of defined extension points. Any invalid
protocol or spec violations should result in immediately dropping the
connection without notice to the other end. Exceptions should be
carefully considered, e.g. an exception may be warranted for a
well-understood idiosyncrasy of a widely-deployed implementation. In
cases where the other end of a connection is 100% trusted and known to
be friendly, skipping validation for performance reasons could also make
sense in certain cases.
</para>
<para>
Generally speaking violations of the "must" requirements in this spec
should be considered possible attempts to exploit security, and violations
of the "should" suggestions should be considered legitimate (though perhaps
they should generate an error in some cases).
</para>
<para>
The following extension points are built in to D-Bus on purpose and must
not be treated as invalid protocol. The extension points are intended
for use by future versions of this spec, they are not intended for third
parties. At the moment, the only way a third party could extend D-Bus
without breaking interoperability would be to introduce a way to negotiate new
feature support as part of the auth protocol, using EXTENSION_-prefixed
commands. There is not yet a standard way to negotiate features.
<itemizedlist>
<listitem>
<para>
In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown
commands result in an ERROR rather than a disconnect. This enables
future extensions to the protocol. Commands starting with EXTENSION_ are
reserved for third parties.
</para>
</listitem>
<listitem>
<para>
The authentication protocol supports pluggable auth mechanisms.
</para>
</listitem>
<listitem>
<para>
The address format (see <xref linkend="addresses"/>) supports new
kinds of transport.
</para>
</listitem>
<listitem>
<para>
Messages with an unknown type (something other than
<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
<literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored.
Unknown-type messages must still be well-formed in the same way
as the known messages, however. They still have the normal
header and body.
</para>
</listitem>
<listitem>
<para>
Header fields with an unknown or unexpected field code must be ignored,
though again they must still be well-formed.
</para>
</listitem>
<listitem>
<para>
New standard interfaces (with new methods and signals) can of course be added.
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
<sect1 id="auth-protocol">
<title>Authentication Protocol</title>
<para>
Before the flow of messages begins, two applications must
authenticate. A simple plain-text protocol is used for
authentication; this protocol is a SASL profile, and maps fairly
directly from the SASL specification. The message encoding is
NOT used here, only plain text messages.
</para>
<para>
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
</para>
<sect2 id="auth-protocol-overview">
<title>Protocol Overview</title>
<para>
The protocol is a line-based protocol, where each line ends with
\r\n. Each line begins with an all-caps ASCII command name containing
only the character range [A-Z_], a space, then any arguments for the
command, then the \r\n ending the line. The protocol is
case-sensitive. All bytes must be in the ASCII character set.
Commands from the client to the server are as follows:
<itemizedlist>
<listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
<listitem><para>CANCEL</para></listitem>
<listitem><para>BEGIN</para></listitem>
<listitem><para>DATA <data in hex encoding></para></listitem>
<listitem><para>ERROR [human-readable error explanation]</para></listitem>
</itemizedlist>
From server to client are as follows:
<itemizedlist>
<listitem><para>REJECTED <space-separated list of mechanism names></para></listitem>
<listitem><para>OK <GUID in hex></para></listitem>
<listitem><para>DATA <data in hex encoding></para></listitem>
<listitem><para>ERROR</para></listitem>
</itemizedlist>
</para>
<para>
Unofficial extensions to the command set must begin with the letters
"EXTENSION_", to avoid conflicts with future official commands.
For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
</para>
</sect2>
<sect2 id="auth-nul-byte">
<title>Special credentials-passing nul byte</title>
<para>
Immediately after connecting to the server, the client must send a
single nul byte. This byte may be accompanied by credentials
information on some operating systems that use sendmsg() with
SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
sockets. However, the nul byte must be sent even on other kinds of
socket, and even on operating systems that do not require a byte to be
sent in order to transmit credentials. The text protocol described in
this document begins after the single nul byte. If the first byte
received from the client is not a nul byte, the server may disconnect
that client.
</para>
<para>
A nul byte in any context other than the initial byte is an error;
the protocol is ASCII-only.
</para>
<para>
The credentials sent along with the nul byte may be used with the
SASL mechanism EXTERNAL.
</para>
</sect2>
<sect2 id="auth-command-auth">
<title>AUTH command</title>
<para>
If an AUTH command has no arguments, it is a request to list
available mechanisms. The server must respond with a REJECTED
command listing the mechanisms it understands, or with an error.
</para>
<para>
If an AUTH command specifies a mechanism, and the server supports
said mechanism, the server should begin exchanging SASL
challenge-response data with the client using DATA commands.
</para>
<para>
If the server does not support the mechanism given in the AUTH
command, it must send either a REJECTED command listing the mechanisms
it does support, or an error.
</para>
<para>
If the [initial-response] argument is provided, it is intended for use
with mechanisms that have no initial challenge (or an empty initial
challenge), as if it were the argument to an initial DATA command. If
the selected mechanism has an initial challenge and [initial-response]
was provided, the server should reject authentication by sending
REJECTED.
</para>
<para>
If authentication succeeds after exchanging DATA commands,
an OK command must be sent to the client.
</para>
<para>
The first octet received by the client after the \r\n of the OK
command must be the first octet of the authenticated/encrypted
stream of D-Bus messages.
</para>
<para>
The first octet received by the server after the \r\n of the BEGIN
command from the client must be the first octet of the
authenticated/encrypted stream of D-Bus messages.
</para>
</sect2>
<sect2 id="auth-command-cancel">
<title>CANCEL Command</title>
<para>
At any time up to sending the BEGIN command, the client may send a
CANCEL command. On receiving the CANCEL command, the server must
send a REJECTED command and abort the current authentication
exchange.
</para>
</sect2>
<sect2 id="auth-command-data">
<title>DATA Command</title>
<para>
The DATA command may come from either client or server, and simply
contains a hex-encoded block of data to be interpreted
according to the SASL mechanism in use.
</para>
<para>
Some SASL mechanisms support sending an "empty string";
FIXME we need some way to do this.
</para>
</sect2>
<sect2 id="auth-command-begin">
<title>BEGIN Command</title>
<para>
The BEGIN command acknowledges that the client has received an
OK command from the server, and that the stream of messages
is about to begin.
</para>
<para>
The first octet received by the server after the \r\n of the BEGIN
command from the client must be the first octet of the
authenticated/encrypted stream of D-Bus messages.
</para>
</sect2>
<sect2 id="auth-command-rejected">
<title>REJECTED Command</title>
<para>
The REJECTED command indicates that the current authentication
exchange has failed, and further exchange of DATA is inappropriate.
The client would normally try another mechanism, or try providing
different responses to challenges.
</para><para>
Optionally, the REJECTED command has a space-separated list of
available auth mechanisms as arguments. If a server ever provides
a list of supported mechanisms, it must provide the same list
each time it sends a REJECTED message. Clients are free to
ignore all lists received after the first.
</para>
</sect2>
<sect2 id="auth-command-ok">
<title>OK Command</title>
<para>
The OK command indicates that the client has been authenticated,
and that further communication will be a stream of D-Bus messages
(optionally encrypted, as negotiated) rather than this protocol.
</para>
<para>
The first octet received by the client after the \r\n of the OK
command must be the first octet of the authenticated/encrypted
stream of D-Bus messages.
</para>
<para>
The client must respond to the OK command by sending a BEGIN
command, followed by its stream of messages, or by disconnecting.
The server must not accept additional commands using this protocol
after the OK command has been sent.
</para>
<para>
The OK command has one argument, which is the GUID of the server.
See <xref linkend="addresses"/> for more on server GUIDs.
</para>
</sect2>
<sect2 id="auth-command-error">
<title>ERROR Command</title>
<para>
The ERROR command indicates that either server or client did not
know a command, does not accept the given command in the current
context, or did not understand the arguments to the command. This
allows the protocol to be extended; a client or server can send a
command present or permitted only in new protocol versions, and if
an ERROR is received instead of an appropriate response, fall back
to using some other technique.
</para>
<para>
If an ERROR is sent, the server or client that sent the
error must continue as if the command causing the ERROR had never been
received. However, the the server or client receiving the error
should try something other than whatever caused the error;
if only canceling/rejecting the authentication.
</para>
<para>
If the D-Bus protocol changes incompatibly at some future time,
applications implementing the new protocol would probably be able to
check for support of the new protocol by sending a new command and
receiving an ERROR from applications that don't understand it. Thus the
ERROR feature of the auth protocol is an escape hatch that lets us
negotiate extensions or changes to the D-Bus protocol in the future.
</para>
</sect2>
<sect2 id="auth-examples">
<title>Authentication examples</title>
<para>
<figure>
<title>Example of successful magic cookie authentication</title>
<programlisting>
(MAGIC_COOKIE is a made up mechanism)
C: AUTH MAGIC_COOKIE 3138363935333137393635383634
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of finding out mechanisms then picking one</title>
<programlisting>
C: AUTH
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of client sends unknown command then falls back to regular auth</title>
<programlisting>
C: FOOBAR
S: ERROR
C: AUTH MAGIC_COOKIE 3736343435313230333039
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of server doesn't support initial auth mechanism</title>
<programlisting>
C: AUTH MAGIC_COOKIE 3736343435313230333039
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of wrong password or the like followed by successful retry</title>
<programlisting>
C: AUTH MAGIC_COOKIE 3736343435313230333039
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: REJECTED
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
<figure>
<title>Example of skey cancelled and restarted</title>
<programlisting>
C: AUTH MAGIC_COOKIE 3736343435313230333039
S: REJECTED KERBEROS_V4 SKEY
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: CANCEL
S: REJECTED
C: AUTH SKEY 7ab83f32ee
S: DATA 8799cabb2ea93e
C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
S: OK 1234deadbeef
C: BEGIN
</programlisting>
</figure>
</para>
</sect2>
<sect2 id="auth-states">
<title>Authentication state diagrams</title>
<para>
This section documents the auth protocol in terms of
a state machine for the client and the server. This is
probably the most robust way to implement the protocol.
</para>
<sect3 id="auth-states-client">
<title>Client states</title>
<para>
To more precisely describe the interaction between the
protocol state machine and the authentication mechanisms the
following notation is used: MECH(CHALL) means that the
server challenge CHALL was fed to the mechanism MECH, which
returns one of
<itemizedlist>
<listitem>
<para>
CONTINUE(RESP) means continue the auth conversation
and send RESP as the response to the server;
</para>
</listitem>
<listitem>
<para>
OK(RESP) means that after sending RESP to the server
the client side of the auth conversation is finished
and the server should return "OK";
</para>
</listitem>
<listitem>
<para>
ERROR means that CHALL was invalid and could not be
processed.
</para>
</listitem>
</itemizedlist>
Both RESP and CHALL may be empty.
</para>
<para>
The Client starts by getting an initial response from the
default mechanism and sends AUTH MECH RESP, or AUTH MECH if
the mechanism did not provide an initial response. If the
mechanism returns CONTINUE, the client starts in state
<emphasis>WaitingForData</emphasis>, if the mechanism
returns OK the client starts in state
<emphasis>WaitingForOK</emphasis>.
</para>
<para>
The client should keep track of available mechanisms and
which it mechanisms it has already attempted. This list is
used to decide which AUTH command to send. When the list is
exhausted, the client should give up and close the
connection.
</para>
<formalpara>
<title><emphasis>WaitingForData</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive DATA CHALL
<simplelist>
<member>
MECH(CHALL) returns CONTINUE(RESP) → send
DATA RESP, goto
<emphasis>WaitingForData</emphasis>
</member>
<member>
MECH(CHALL) returns OK(RESP) → send DATA
RESP, goto <emphasis>WaitingForOK</emphasis>
</member>
<member>
MECH(CHALL) returns ERROR → send ERROR
[msg], goto <emphasis>WaitingForData</emphasis>
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive REJECTED [mechs] →
send AUTH [next mech], goto
WaitingForData or <emphasis>WaitingForOK</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send
CANCEL, goto
<emphasis>WaitingForReject</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive OK → send
BEGIN, terminate auth
conversation, authenticated
</para>
</listitem>
<listitem>
<para>
Receive anything else → send
ERROR, goto
<emphasis>WaitingForData</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForOK</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive OK → send BEGIN, terminate auth
conversation, <emphasis>authenticated</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive REJECT [mechs] → send AUTH [next mech],
goto <emphasis>WaitingForData</emphasis> or
<emphasis>WaitingForOK</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive DATA → send CANCEL, goto
<emphasis>WaitingForReject</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send CANCEL, goto
<emphasis>WaitingForReject</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → send ERROR, goto
<emphasis>WaitingForOK</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForReject</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive REJECT [mechs] → send AUTH [next mech],
goto <emphasis>WaitingForData</emphasis> or
<emphasis>WaitingForOK</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → terminate auth
conversation, disconnect
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
</sect3>
<sect3 id="auth-states-server">
<title>Server states</title>
<para>
For the server MECH(RESP) means that the client response
RESP was fed to the the mechanism MECH, which returns one of
<itemizedlist>
<listitem>
<para>
CONTINUE(CHALL) means continue the auth conversation and
send CHALL as the challenge to the client;
</para>
</listitem>
<listitem>
<para>
OK means that the client has been successfully
authenticated;
</para>
</listitem>
<listitem>
<para>
REJECT means that the client failed to authenticate or
there was an error in RESP.
</para>
</listitem>
</itemizedlist>
The server starts out in state
<emphasis>WaitingForAuth</emphasis>. If the client is
rejected too many times the server must disconnect the
client.
</para>
<formalpara>
<title><emphasis>WaitingForAuth</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive AUTH → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive AUTH MECH RESP
<simplelist>
<member>
MECH not valid mechanism → send REJECTED
[mechs], goto
<emphasis>WaitingForAuth</emphasis>
</member>
<member>
MECH(RESP) returns CONTINUE(CHALL) → send
DATA CHALL, goto
<emphasis>WaitingForData</emphasis>
</member>
<member>
MECH(RESP) returns OK → send OK, goto
<emphasis>WaitingForBegin</emphasis>
</member>
<member>
MECH(RESP) returns REJECT → send REJECTED
[mechs], goto
<emphasis>WaitingForAuth</emphasis>
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive BEGIN → terminate
auth conversation, disconnect
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → send
ERROR, goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForData</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive DATA RESP
<simplelist>
<member>
MECH(RESP) returns CONTINUE(CHALL) → send
DATA CHALL, goto
<emphasis>WaitingForData</emphasis>
</member>
<member>
MECH(RESP) returns OK → send OK, goto
<emphasis>WaitingForBegin</emphasis>
</member>
<member>
MECH(RESP) returns REJECT → send REJECTED
[mechs], goto
<emphasis>WaitingForAuth</emphasis>
</member>
</simplelist>
</para>
</listitem>
<listitem>
<para>
Receive BEGIN → terminate auth conversation,
disconnect
</para>
</listitem>
<listitem>
<para>
Receive CANCEL → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → send ERROR, goto
<emphasis>WaitingForData</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
<formalpara>
<title><emphasis>WaitingForBegin</emphasis></title>
<para>
<itemizedlist>
<listitem>
<para>
Receive BEGIN → terminate auth conversation,
client authenticated
</para>
</listitem>
<listitem>
<para>
Receive CANCEL → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive ERROR → send REJECTED [mechs], goto
<emphasis>WaitingForAuth</emphasis>
</para>
</listitem>
<listitem>
<para>
Receive anything else → send ERROR, goto
<emphasis>WaitingForBegin</emphasis>
</para>
</listitem>
</itemizedlist>
</para>
</formalpara>
</sect3>
</sect2>
<sect2 id="auth-mechanisms">
<title>Authentication mechanisms</title>
<para>
This section describes some new authentication mechanisms.
D-Bus also allows any standard SASL mechanism of course.
</para>
<sect3 id="auth-mechanisms-sha">
<title>DBUS_COOKIE_SHA1</title>
<para>
The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
has the ability to read a private file owned by the user being
authenticated. If the client can prove that it has access to a secret
cookie stored in this file, then the client is authenticated.
Thus the security of DBUS_COOKIE_SHA1 depends on a secure home
directory.
</para>
<para>
Throughout this description, "hex encoding" must output the digits
from a to f in lower-case; the digits A to F must not be used
in the DBUS_COOKIE_SHA1 mechanism.
</para>
<para>
Authentication proceeds as follows:
<itemizedlist>
<listitem>
<para>
The client sends the username it would like to authenticate
as, hex-encoded.
</para>
</listitem>
<listitem>
<para>
The server sends the name of its "cookie context" (see below); a
space character; the integer ID of the secret cookie the client
must demonstrate knowledge of; a space character; then a
randomly-generated challenge string, all of this hex-encoded into
one, single string.
</para>
</listitem>
<listitem>
<para>
The client locates the cookie and generates its own
randomly-generated challenge string. The client then concatenates
the server's decoded challenge, a ":" character, its own challenge,
another ":" character, and the cookie. It computes the SHA-1 hash
of this composite string as a hex digest. It concatenates the
client's challenge string, a space character, and the SHA-1 hex
digest, hex-encodes the result and sends it back to the server.
</para>
</listitem>
<listitem>
<para>
The server generates the same concatenated string used by the
client and computes its SHA-1 hash. It compares the hash with
the hash received from the client; if the two hashes match, the
client is authenticated.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Each server has a "cookie context," which is a name that identifies a
set of cookies that apply to that server. A sample context might be
"org_freedesktop_session_bus". Context names must be valid ASCII,
nonzero length, and may not contain the characters slash ("/"),
backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
tab ("\t"), or period ("."). There is a default context,
"org_freedesktop_general" that's used by servers that do not specify
otherwise.
</para>
<para>
Cookies are stored in a user's home directory, in the directory
<filename>~/.dbus-keyrings/</filename>. This directory must
not be readable or writable by other users. If it is,
clients and servers must ignore it. The directory
contains cookie files named after the cookie context.
</para>
<para>
A cookie file contains one cookie per line. Each line
has three space-separated fields:
<itemizedlist>
<listitem>
<para>
The cookie ID number, which must be a non-negative integer and
may not be used twice in the same file.
</para>
</listitem>
<listitem>
<para>
The cookie's creation time, in UNIX seconds-since-the-epoch
format.
</para>
</listitem>
<listitem>
<para>
The cookie itself, a hex-encoded random block of bytes. The cookie
may be of any length, though obviously security increases
as the length increases.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Only server processes modify the cookie file.
They must do so with this procedure:
<itemizedlist>
<listitem>
<para>
Create a lockfile name by appending ".lock" to the name of the
cookie file. The server should attempt to create this file
using <literal>O_CREAT | O_EXCL</literal>. If file creation
fails, the lock fails. Servers should retry for a reasonable
period of time, then they may choose to delete an existing lock
to keep users from having to manually delete a stale
lock. <footnote><para>Lockfiles are used instead of real file
locking <literal>fcntl()</literal> because real locking
implementations are still flaky on network
filesystems.</para></footnote>
</para>
</listitem>
<listitem>
<para>
Once the lockfile has been created, the server loads the cookie
file. It should then delete any cookies that are old (the
timeout can be fairly short), or more than a reasonable
time in the future (so that cookies never accidentally
become permanent, if the clock was set far into the future
at some point). If no recent keys remain, the
server may generate a new key.
</para>
</listitem>
<listitem>
<para>
The pruned and possibly added-to cookie file
must be resaved atomically (using a temporary
file which is rename()'d).
</para>
</listitem>
<listitem>
<para>
The lock must be dropped by deleting the lockfile.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Clients need not lock the file in order to load it,
because servers are required to save the file atomically.
</para>
</sect3>
</sect2>
</sect1>
<sect1 id="addresses">
<title>Server Addresses</title>
<para>
Server addresses consist of a transport name followed by a colon, and
then an optional, comma-separated list of keys and values in the form key=value.
Each value is escaped.
</para>
<para>
For example:
<programlisting>unix:path=/tmp/dbus-test</programlisting>
Which is the address to a unix socket with the path /tmp/dbus-test.
</para>
<para>
Value escaping is similar to URI escaping but simpler.
<itemizedlist>
<listitem>
<para>
The set of optionally-escaped bytes is:
<literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
<emphasis>byte</emphasis> (note, not character) which is not in the
set of optionally-escaped bytes must be replaced with an ASCII
percent (<literal>%</literal>) and the value of the byte in hex.
The hex value must always be two digits, even if the first digit is
zero. The optionally-escaped bytes may be escaped if desired.
</para>
</listitem>
<listitem>
<para>
To unescape, append each byte in the value; if a byte is an ASCII
percent (<literal>%</literal>) character then append the following
hex value instead. It is an error if a <literal>%</literal> byte
does not have two hex digits following. It is an error if a
non-optionally-escaped byte is seen unescaped.
</para>
</listitem>
</itemizedlist>
The set of optionally-escaped bytes is intended to preserve address
readability and convenience.
</para>
<para>
A server may specify a key-value pair with the key <literal>guid</literal>
and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
describes the format of the <literal>guid</literal> field. If present,
this UUID may be used to distinguish one server address from another. A
server should use a different UUID for each address it listens on. For
example, if a message bus daemon offers both UNIX domain socket and TCP
connections, but treats clients the same regardless of how they connect,
those two connections are equivalent post-connection but should have
distinct UUIDs to distinguish the kinds of connection.
</para>
<para>
The intent of the address UUID feature is to allow a client to avoid
opening multiple identical connections to the same server, by allowing the
client to check whether an address corresponds to an already-existing
connection. Comparing two addresses is insufficient, because addresses
can be recycled by distinct servers, and equivalent addresses may look
different if simply compared as strings (for example, the host in a TCP
address can be given as an IP address or as a hostname).
</para>
<para>
Note that the address key is <literal>guid</literal> even though the
rest of the API and documentation says "UUID," for historical reasons.
</para>
<para>
[FIXME clarify if attempting to connect to each is a requirement
or just a suggestion]
When connecting to a server, multiple server addresses can be
separated by a semi-colon. The library will then try to connect
to the first address and if that fails, it'll try to connect to
the next one specified, and so forth. For example
<programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
</para>
</sect1>
<sect1 id="transports">
<title>Transports</title>
<para>
[FIXME we need to specify in detail each transport and its possible arguments]
Current transports include: unix domain sockets (including
abstract namespace on linux), TCP/IP, and a debug/testing transport using
in-process pipes. Future possible transports include one that
tunnels over X11 protocol.
</para>
<sect2 id="transports-unix-domain-sockets">
<title>Unix Domain Sockets</title>
<para>
Unix domain sockets can be either paths in the file system or on Linux
kernels, they can be abstract which are similar to paths but
do not show up in the file system.
</para>
<para>
When a socket is opened by the D-Bus library it truncates the path
name right before the first trailing Nul byte. This is true for both
normal paths and abstract paths. Note that this is a departure from
previous versions of D-Bus that would create sockets with a fixed
length path name. Names which were shorter than the fixed length
would be padded by Nul bytes.
</para>
</sect2>
</sect1>
<sect1 id="naming-conventions">
<title>Naming Conventions</title>
<para>
D-Bus namespaces are all lowercase and correspond to reversed domain
names, as with Java. e.g. "org.freedesktop"
</para>
<para>
Interface, signal, method, and property names are "WindowsStyleCaps", note
that the first letter is capitalized, unlike Java.
</para>
<para>
Object paths are normally all lowercase with underscores used rather than
hyphens.
</para>
</sect1>
<sect1 id="uuids">
<title>UUIDs</title>
<para>
A working D-Bus implementation uses universally-unique IDs in two places.
First, each server address has a UUID identifying the address,
as described in <xref linkend="addresses"/>. Second, each operating
system kernel instance running a D-Bus client or server has a UUID
identifying that kernel, retrieved by invoking the method
org.freedesktop.DBus.Peer.GetMachineId() (see <xref
linkend="standard-interfaces-peer"/>).
</para>
<para>
The term "UUID" in this document is intended literally, i.e. an
identifier that is universally unique. It is not intended to refer to
RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
</para>
<para>
The UUID must contain 128 bits of data and be hex-encoded. The
hex-encoded string may not contain hyphens or other non-hex-digit
characters, and it must be exactly 32 characters long. To generate a
UUID, the current reference implementation concatenates 96 bits of random
data followed by the 32-bit time in seconds since the UNIX epoch (in big
endian byte order).
</para>
<para>
It would also be acceptable and probably better to simply generate 128
bits of random data, as long as the random number generator is of high
quality. The timestamp could conceivably help if the random bits are not
very random. With a quality random number generator, collisions are
extremely unlikely even with only 96 bits, so it's somewhat academic.
</para>
<para>
Implementations should, however, stick to random data for the first 96 bits
of the UUID.
</para>
</sect1>
<sect1 id="standard-interfaces">
<title>Standard Interfaces</title>
<para>
See <xref linkend="message-protocol-types-notation"/> for details on
the notation used in this section. There are some standard interfaces
that may be useful across various D-Bus applications.
</para>
<sect2 id="standard-interfaces-peer">
<title><literal>org.freedesktop.DBus.Peer</literal></title>
<para>
The <literal>org.freedesktop.DBus.Peer</literal> interface
has two methods:
<programlisting>
org.freedesktop.DBus.Peer.Ping ()
org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
</programlisting>
</para>
<para>
On receipt of the <literal>METHOD_CALL</literal> message
<literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
nothing other than reply with a <literal>METHOD_RETURN</literal> as
usual. It does not matter which object path a ping is sent to. The
reference implementation handles this method automatically.
</para>
<para>
On receipt of the <literal>METHOD_CALL</literal> message
<literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should
reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded
UUID representing the identity of the machine the process is running on.
This UUID must be the same for all processes on a single system at least
until that system next reboots. It should be the same across reboots
if possible, but this is not always possible to implement and is not
guaranteed.
It does not matter which object path a GetMachineId is sent to. The
reference implementation handles this method automatically.
</para>
<para>
The UUID is intended to be per-instance-of-the-operating-system, so may represent
a virtual machine running on a hypervisor, rather than a physical machine.
Basically if two processes see the same UUID, they should also see the same
shared memory, UNIX domain sockets, process IDs, and other features that require
a running OS kernel in common between the processes.
</para>
<para>
The UUID is often used where other programs might use a hostname. Hostnames
can change without rebooting, however, or just be "localhost" - so the UUID
is more robust.
</para>
<para>
<xref linkend="uuids"/> explains the format of the UUID.
</para>
</sect2>
<sect2 id="standard-interfaces-introspectable">
<title><literal>org.freedesktop.DBus.Introspectable</literal></title>
<para>
This interface has one method:
<programlisting>
org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
</programlisting>
</para>
<para>
Objects instances may implement
<literal>Introspect</literal> which returns an XML description of
the object, including its interfaces (with signals and methods), objects
below it in the object path tree, and its properties.
</para>
<para>
<xref linkend="introspection-format"/> describes the format of this XML string.
</para>
</sect2>
<sect2 id="standard-interfaces-properties">
<title><literal>org.freedesktop.DBus.Properties</literal></title>
<para>
Many native APIs will have a concept of object <firstterm>properties</firstterm>
or <firstterm>attributes</firstterm>. These can be exposed via the
<literal>org.freedesktop.DBus.Properties</literal> interface.
</para>
<para>
<programlisting>
org.freedesktop.DBus.Properties.Get (in STRING interface_name,
in STRING property_name,
out VARIANT value);
org.freedesktop.DBus.Properties.Set (in STRING interface_name,
in STRING property_name,
in VARIANT value);
org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
out DICT<STRING,VARIANT> props);
</programlisting>
</para>
<para>
The available properties and whether they are writable can be determined
by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
see <xref linkend="standard-interfaces-introspectable"/>.
</para>
<para>
An empty string may be provided for the interface name; in this case,
if there are multiple properties on an object with the same name,
the results are undefined (picking one by according to an arbitrary
deterministic rule, or returning an error, are the reasonable
possibilities).
</para>
</sect2>
</sect1>
<sect1 id="introspection-format">
<title>Introspection Data Format</title>
<para>
As described in <xref linkend="standard-interfaces-introspectable"/>,
objects may be introspected at runtime, returning an XML string
that describes the object. The same XML format may be used in
other contexts as well, for example as an "IDL" for generating
static language bindings.
</para>
<para>
Here is an example of introspection data:
<programlisting>
<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node name="/org/freedesktop/sample_object">
<interface name="org.freedesktop.SampleInterface">
<method name="Frobate">
<arg name="foo" type="i" direction="in"/>
<arg name="bar" type="s" direction="out"/>
<arg name="baz" type="a{us}" direction="out"/>
<annotation name="org.freedesktop.DBus.Deprecated" value="true"/>
</method>
<method name="Bazify">
<arg name="bar" type="(iiu)" direction="in"/>
<arg name="bar" type="v" direction="out"/>
</method>
<method name="Mogrify">
<arg name="bar" type="(iiav)" direction="in"/>
</method>
<signal name="Changed">
<arg name="new_value" type="b"/>
</signal>
<property name="Bar" type="y" access="readwrite"/>
</interface>
<node name="child_of_sample_object"/>
<node name="another_child_of_sample_object"/>
</node>
</programlisting>
</para>
<para>
A more formal DTD and spec needs writing, but here are some quick notes.
<itemizedlist>
<listitem>
<para>
Only the root <node> element can omit the node name, as it's
known to be the object that was introspected. If the root
<node> does have a name attribute, it must be an absolute
object path. If child <node> have object paths, they must be
relative.
</para>
</listitem>
<listitem>
<para>
If a child <node> has any sub-elements, then they
must represent a complete introspection of the child.
If a child <node> is empty, then it may or may
not have sub-elements; the child must be introspected
in order to find out. The intent is that if an object
knows that its children are "fast" to introspect
it can go ahead and return their information, but
otherwise it can omit it.
</para>
</listitem>
<listitem>
<para>
The direction element on <arg> may be omitted,
in which case it defaults to "in" for method calls
and "out" for signals. Signals only allow "out"
so while direction may be specified, it's pointless.
</para>
</listitem>
<listitem>
<para>
The possible directions are "in" and "out",
unlike CORBA there is no "inout"
</para>
</listitem>
<listitem>
<para>
The possible property access flags are
"readwrite", "read", and "write"
</para>
</listitem>
<listitem>
<para>
Multiple interfaces can of course be listed for
one <node>.
</para>
</listitem>
<listitem>
<para>
The "name" attribute on arguments is optional.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Method, interface, property, and signal elements may have
"annotations", which are generic key/value pairs of metadata.
They are similar conceptually to Java's annotations and C# attributes.
Well-known annotations:
</para>
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Values (separated by ,)</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>org.freedesktop.DBus.Deprecated</entry>
<entry>true,false</entry>
<entry>Whether or not the entity is deprecated; defaults to false</entry>
</row>
<row>
<entry>org.freedesktop.DBus.GLib.CSymbol</entry>
<entry>(string)</entry>
<entry>The C symbol; may be used for methods and interfaces</entry>
</row>
<row>
<entry>org.freedesktop.DBus.Method.NoReply</entry>
<entry>true,false</entry>
<entry>If set, don't expect a reply to the method call; defaults to false.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</sect1>
<sect1 id="message-bus">
<title>Message Bus Specification</title>
<sect2 id="message-bus-overview">
<title>Message Bus Overview</title>
<para>
The message bus accepts connections from one or more applications.
Once connected, applications can exchange messages with other
applications that are also connected to the bus.
</para>
<para>
In order to route messages among connections, the message bus keeps a
mapping from names to connections. Each connection has one
unique-for-the-lifetime-of-the-bus name automatically assigned.
Applications may request additional names for a connection. Additional
names are usually "well-known names" such as
"org.freedesktop.TextEditor". When a name is bound to a connection,
that connection is said to <firstterm>own</firstterm> the name.
</para>
<para>
The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>.
This name routes messages to the bus, allowing applications to make
administrative requests. For example, applications can ask the bus
to assign a name to a connection.
</para>
<para>
Each name may have <firstterm>queued owners</firstterm>. When an
application requests a name for a connection and the name is already in
use, the bus will optionally add the connection to a queue waiting for
the name. If the current owner of the name disconnects or releases
the name, the next connection in the queue will become the new owner.
</para>
<para>
This feature causes the right thing to happen if you start two text
editors for example; the first one may request "org.freedesktop.TextEditor",
and the second will be queued as a possible owner of that name. When
the first exits, the second will take over.
</para>
<para>
Messages may have a <literal>DESTINATION</literal> field (see <xref
linkend="message-protocol-header-fields"/>). If the
<literal>DESTINATION</literal> field is present, it specifies a message
recipient by name. Method calls and replies normally specify this field.
</para>
<para>
Signals normally do not specify a destination; they are sent to all
applications with <firstterm>message matching rules</firstterm> that
match the message.
</para>
<para>
When the message bus receives a method call, if the
<literal>DESTINATION</literal> field is absent, the call is taken to be
a standard one-to-one message and interpreted by the message bus
itself. For example, sending an
<literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
<literal>DESTINATION</literal> will cause the message bus itself to
reply to the ping immediately; the message bus will not make this
message visible to other applications.
</para>
<para>
Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
the ping message were sent with a <literal>DESTINATION</literal> name of
<literal>com.yoyodyne.Screensaver</literal>, then the ping would be
forwarded, and the Yoyodyne Corporation screensaver application would be
expected to reply to the ping.
</para>
</sect2>
<sect2 id="message-bus-names">
<title>Message Bus Names</title>
<para>
Each connection has at least one name, assigned at connection time and
returned in response to the
<literal>org.freedesktop.DBus.Hello</literal> method call. This
automatically-assigned name is called the connection's <firstterm>unique
name</firstterm>. Unique names are never reused for two different
connections to the same bus.
</para>
<para>
Ownership of a unique name is a prerequisite for interaction with
the message bus. It logically follows that the unique name is always
the first name that an application comes to own, and the last
one that it loses ownership of.
</para>
<para>
Unique connection names must begin with the character ':' (ASCII colon
character); bus names that are not unique names must not begin
with this character. (The bus must reject any attempt by an application
to manually request a name beginning with ':'.) This restriction
categorically prevents "spoofing"; messages sent to a unique name
will always go to the expected connection.
</para>
<para>
When a connection is closed, all the names that it owns are deleted (or
transferred to the next connection in the queue if any).
</para>
<para>
A connection can request additional names to be associated with it using
the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
linkend="message-protocol-names-bus"/> describes the format of a valid
name. These names can be released again using the
<literal>org.freedesktop.DBus.ReleaseName</literal> message.
</para>
<sect3 id="bus-messages-request-name">
<title><literal>org.freedesktop.DBus.RequestName</literal></title>
<para>
As a method:
<programlisting>
UINT32 RequestName (in STRING name, in UINT32 flags)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to request</entry>
</row>
<row>
<entry>1</entry>
<entry>UINT32</entry>
<entry>Flags</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Return value</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This method call should be sent to
<literal>org.freedesktop.DBus</literal> and asks the message bus to
assign the given name to the method caller. Each name maintains a
queue of possible owners, where the head of the queue is the primary
or current owner of the name. Each potential owner in the queue
maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
call. When RequestName is invoked the following occurs:
<itemizedlist>
<listitem>
<para>
If the method caller is currently the primary owner of the name,
the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
values are updated with the values from the new RequestName call,
and nothing further happens.
</para>
</listitem>
<listitem>
<para>
If the current primary owner (head of the queue) has
DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
the caller of RequestName replaces the current primary owner at
the head of the queue and the current primary owner moves to the
second position in the queue. If the caller of RequestName was
in the queue previously its flags are updated with the values from
the new RequestName in addition to moving it to the head of the queue.
</para>
</listitem>
<listitem>
<para>
If replacement is not possible, and the method caller is
currently in the queue but not the primary owner, its flags are
updated with the values from the new RequestName call.
</para>
</listitem>
<listitem>
<para>
If replacement is not possible, and the method caller is
currently not in the queue, the method caller is appended to the
queue.
</para>
</listitem>
<listitem>
<para>
If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
set and is not the primary owner, it is removed from the
queue. This can apply to the previous primary owner (if it
was replaced) or the method caller (if it updated the
DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
queue, or if it was just added to the queue with that flag set).
</para>
</listitem>
</itemizedlist>
</para>
<para>
Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
queue," even if another application already in the queue had specified
DBUS_NAME_FLAG_REPLACE_EXISTING. This comes up if a primary owner
that does not allow replacement goes away, and the next primary owner
does allow replacement. In this case, queued items that specified
DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
automatically replace the new primary owner. In other words,
DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
time RequestName is called. This is deliberate to avoid an infinite loop
anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT
and DBUS_NAME_FLAG_REPLACE_EXISTING.
</para>
<para>
The flags argument contains any of the following values logically ORed
together:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
<entry>0x1</entry>
<entry>
If an application A specifies this flag and succeeds in
becoming the owner of the name, and another application B
later calls RequestName with the
DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
will lose ownership and receive a
<literal>org.freedesktop.DBus.NameLost</literal> signal, and
application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
is not specified by application B, then application B will not replace
application A as the owner.
</entry>
</row>
<row>
<entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
<entry>0x2</entry>
<entry>
Try to replace the current owner if there is one. If this
flag is not set the application will only become the owner of
the name if there is no current owner. If this flag is set,
the application will replace the current owner if
the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.
</entry>
</row>
<row>
<entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
<entry>0x4</entry>
<entry>
Without this flag, if an application requests a name that is
already owned, the application will be placed in a queue to
own the name when the current owner gives it up. If this
flag is given, the application will not be placed in the
queue, the request for the name will simply fail. This flag
also affects behavior when an application is replaced as
name owner; by default the application moves back into the
waiting queue, unless this flag was provided when the application
became the name owner.
</entry>
</row>
</tbody>
</tgroup>
</informaltable>
The return code can be one of the following values:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
<entry>1</entry> <entry>The caller is now the primary owner of
the name, replacing any previous owner. Either the name had no
owner before, or the caller specified
DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
</row>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
<entry>2</entry>
<entry>The name already had an owner,
DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
the current owner did not specify
DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
</entry>
</row>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
<entry>The name already has an owner,
DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
specified by the requesting application.</entry>
</row>
<row>
<entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
<entry>4</entry>
<entry>The application trying to request ownership of a name is already the owner of it.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
<sect3 id="bus-messages-release-name">
<title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
<para>
As a method:
<programlisting>
UINT32 ReleaseName (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to release</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Return value</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This method call should be sent to
<literal>org.freedesktop.DBus</literal> and asks the message bus to
release the method caller's claim to the given name. If the caller is
the primary owner, a new primary owner will be selected from the
queue if any other owners are waiting. If the caller is waiting in
the queue for the name, the caller will removed from the queue and
will not be made an owner of the name if it later becomes available.
If there are no other owners in the queue for the name, it will be
removed from the bus entirely.
The return code can be one of the following values:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Conventional Name</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
<entry>1</entry> <entry>The caller has released his claim on
the given name. Either the caller was the primary owner of
the name, and the name is now unused or taken by somebody
waiting in the queue for the name, or the caller was waiting
in the queue for the name and has now been removed from the
queue.</entry>
</row>
<row>
<entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
<entry>2</entry>
<entry>The given name does not exist on this bus.</entry>
</row>
<row>
<entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
<entry>3</entry>
<entry>The caller was not the primary owner of this name,
and was also not waiting in the queue to own this name.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
</sect2>
<sect2 id="message-bus-routing">
<title>Message Bus Message Routing</title>
<para>
FIXME
</para>
<sect3 id="message-bus-routing-match-rules">
<title>Match Rules</title>
<para>
An important part of the message bus routing protocol is match
rules. Match rules describe what messages can be sent to a client
based on the contents of the message. When a message is routed
through the bus it is compared to clients' match rules. If any
of the rules match, the message is dispatched to the client.
If none of the rules match the message never leaves the bus. This
is an effective way to control traffic over the bus and to make sure
only relevant message need to be processed by the client.
</para>
<para>
Match rules are added using the AddMatch bus method
(see xref linkend="bus-messages-add-match"/>). Rules are
specified as a string of comma separated key/value pairs.
Excluding a key from the rule indicates a wildcard match.
For instance excluding the the member from a match rule but
adding a sender would let all messages from that sender through.
An example of a complete rule would be
"type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
</para>
<para>
The following table describes the keys that can be used to create
a match rule:
The following table summarizes the D-Bus types.
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Key</entry>
<entry>Possible Values</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>type</literal></entry>
<entry>'signal', 'method_call', 'method_return', 'error'</entry>
<entry>Match on the message type. An example of a type match is type='signal'</entry>
</row>
<row>
<entry><literal>sender</literal></entry>
<entry>A bus or unique name (see <xref linkend="term-bus-name"/>
and <xref linkend="term-unique-name"/> respectively)
</entry>
<entry>Match messages sent by a particular sender. An example of a sender match
is sender='org.freedesktop.Hal'</entry>
</row>
<row>
<entry><literal>interface</literal></entry>
<entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
<entry>Match messages sent over or to a particular interface. An example of an
interface match is interface='org.freedesktop.Hal.Manager'.
If a message omits the interface header, it must not match any rule
that specifies this key.</entry>
</row>
<row>
<entry><literal>member</literal></entry>
<entry>Any valid method or signal name</entry>
<entry>Matches messages which have the give method or signal name. An example of
a member match is member='NameOwnerChanged'</entry>
</row>
<row>
<entry><literal>path</literal></entry>
<entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
<entry>Matches messages which are sent from or to the given object. An example of a
path match is path='/org/freedesktop/Hal/Manager'</entry>
</row>
<row>
<entry><literal>destination</literal></entry>
<entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
<entry>Matches messages which are being sent to the given unique name. An
example of a destination match is destination=':1.0'</entry>
</row>
<row>
<entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
<entry>Any string</entry>
<entry>Arg matches are special and are used for further restricting the
match based on the arguments in the body of a message. As of this time
only string arguments can be matched. An example of an argument match
would be arg3='Foo'. Only argument indexes from 0 to 63 should be
accepted.</entry>
</row>
<row>
<entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
<entry>Any string</entry>
<entry>Argument path matches provide a specialised form of wildcard
matching for path-like namespaces. As with normal argument matches,
if the argument is exactly equal to the string given in the match
rule then the rule is satisfied. Additionally, there is also a
match when either the string given in the match rule or the
appropriate message argument ends with '/' and is a prefix of the
other. An example argument path match is arg0path='/aa/bb/'. This
would match messages with first arguments of '/', '/aa/',
'/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
</sect2>
<sect2 id="message-bus-starting-services">
<title>Message Bus Starting Services</title>
<para>
The message bus can start applications on behalf of other applications.
In CORBA terms, this would be called <firstterm>activation</firstterm>.
An application that can be started in this way is called a
<firstterm>service</firstterm>.
</para>
<para>
With D-Bus, starting a service is normally done by name. That is,
applications ask the message bus to start some program that will own a
well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
This implies a contract documented along with the name
<literal>org.freedesktop.TextEditor</literal> for which objects
the owner of that name will provide, and what interfaces those
objects will have.
</para>
<para>
To find an executable corresponding to a particular name, the bus daemon
looks for <firstterm>service description files</firstterm>. Service
description files define a mapping from names to executables. Different
kinds of message bus will look for these files in different places, see
<xref linkend="message-bus-types"/>.
</para>
<para>
[FIXME the file format should be much better specified than "similar to
.desktop entries" esp. since desktop entries are already
badly-specified. ;-)] Service description files have the ".service" file
extension. The message bus will only load service description files
ending with .service; all other files will be ignored. The file format
is similar to that of <ulink
url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
entries</ulink>. All service description files must be in UTF-8
encoding. To ensure that there will be no name collisions, service files
must be namespaced using the same mechanism as messages and service
names.
<figure>
<title>Example service description file</title>
<programlisting>
# Sample service description file
[D-BUS Service]
Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
Exec=/usr/libexec/gconfd-2
</programlisting>
</figure>
</para>
<para>
When an application asks to start a service by name, the bus daemon tries to
find a service that will own that name. It then tries to spawn the
executable associated with it. If this fails, it will report an
error. [FIXME what happens if two .service files offer the same service;
what kind of error is reported, should we have a way for the client to
choose one?]
</para>
<para>
The executable launched will have the environment variable
<literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
message bus so it can connect and request the appropriate names.
</para>
<para>
The executable being launched may want to know whether the message bus
starting it is one of the well-known message buses (see <xref
linkend="message-bus-types"/>). To facilitate this, the bus must also set
the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
of the well-known buses. The currently-defined values for this variable
are <literal>system</literal> for the systemwide message bus,
and <literal>session</literal> for the per-login-session message
bus. The new executable must still connect to the address given
in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
resulting connection is to the well-known bus.
</para>
<para>
[FIXME there should be a timeout somewhere, either specified
in the .service file, by the client, or just a global value
and if the client being activated fails to connect within that
timeout, an error should be sent back.]
</para>
<sect3 id="message-bus-starting-services-scope">
<title>Message Bus Service Scope</title>
<para>
The "scope" of a service is its "per-", such as per-session,
per-machine, per-home-directory, or per-display. The reference
implementation doesn't yet support starting services in a different
scope from the message bus itself. So e.g. if you start a service
on the session bus its scope is per-session.
</para>
<para>
We could add an optional scope to a bus name. For example, for
per-(display,session pair), we could have a unique ID for each display
generated automatically at login and set on screen 0 by executing a
special "set display ID" binary. The ID would be stored in a
<literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
random bytes. This ID would then be used to scope names.
Starting/locating a service could be done by ID-name pair rather than
only by name.
</para>
<para>
Contrast this with a per-display scope. To achieve that, we would
want a single bus spanning all sessions using a given display.
So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal>
property on screen 0 of the display, pointing to this bus.
</para>
</sect3>
</sect2>
<sect2 id="message-bus-types">
<title>Well-known Message Bus Instances</title>
<para>
Two standard message bus instances are defined here, along with how
to locate them and where their service files live.
</para>
<sect3 id="message-bus-types-login">
<title>Login session message bus</title>
<para>
Each time a user logs in, a <firstterm>login session message
bus</firstterm> may be started. All applications in the user's login
session may interact with one another using this message bus.
</para>
<para>
The address of the login session message bus is given
in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment
variable. If that variable is not set, applications may
also try to read the address from the X Window System root
window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
The root window property must have type <literal>STRING</literal>.
The environment variable should have precedence over the
root window property.
</para>
<para>
[FIXME specify location of .service files, probably using
DESKTOP_DIRS etc. from basedir specification, though login session
bus is not really desktop-specific]
</para>
</sect3>
<sect3 id="message-bus-types-system">
<title>System message bus</title>
<para>
A computer may have a <firstterm>system message bus</firstterm>,
accessible to all applications on the system. This message bus may be
used to broadcast system events, such as adding new hardware devices,
changes in the printer queue, and so forth.
</para>
<para>
The address of the system message bus is given
in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment
variable. If that variable is not set, applications should try
to connect to the well-known address
<literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
<footnote>
<para>
The D-Bus reference implementation actually honors the
<literal>$(localstatedir)</literal> configure option
for this address, on both client and server side.
</para>
</footnote>
</para>
<para>
[FIXME specify location of system bus .service files]
</para>
</sect3>
</sect2>
<sect2 id="message-bus-messages">
<title>Message Bus Messages</title>
<para>
The special message bus name <literal>org.freedesktop.DBus</literal>
responds to a number of additional messages.
</para>
<sect3 id="bus-messages-hello">
<title><literal>org.freedesktop.DBus.Hello</literal></title>
<para>
As a method:
<programlisting>
STRING Hello ()
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique name assigned to the connection</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Before an application is able to send messages to other applications
it must send the <literal>org.freedesktop.DBus.Hello</literal> message
to the message bus to obtain a unique name. If an application without
a unique name tries to send a message to another application, or a
message to the message bus itself that isn't the
<literal>org.freedesktop.DBus.Hello</literal> message, it will be
disconnected from the bus.
</para>
<para>
There is no corresponding "disconnect" request; if a client wishes to
disconnect from the bus, it simply closes the socket (or other
communication channel).
</para>
</sect3>
<sect3 id="bus-messages-list-names">
<title><literal>org.freedesktop.DBus.ListNames</literal></title>
<para>
As a method:
<programlisting>
ARRAY of STRING ListNames ()
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of STRING</entry>
<entry>Array of strings where each string is a bus name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Returns a list of all currently-owned names on the bus.
</para>
</sect3>
<sect3 id="bus-messages-list-activatable-names">
<title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
<para>
As a method:
<programlisting>
ARRAY of STRING ListActivatableNames ()
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of STRING</entry>
<entry>Array of strings where each string is a bus name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Returns a list of all names that can be activated on the bus.
</para>
</sect3>
<sect3 id="bus-messages-name-exists">
<title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
<para>
As a method:
<programlisting>
BOOLEAN NameHasOwner (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to check</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>BOOLEAN</entry>
<entry>Return value, true if the name exists</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
Checks if the specified name exists (currently has an owner).
</para>
</sect3>
<sect3 id="bus-messages-name-owner-changed">
<title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
<para>
This is a signal:
<programlisting>
NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name with a new owner</entry>
</row>
<row>
<entry>1</entry>
<entry>STRING</entry>
<entry>Old owner or empty string if none</entry>
</row>
<row>
<entry>2</entry>
<entry>STRING</entry>
<entry>New owner or empty string if none</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This signal indicates that the owner of a name has changed.
It's also the signal to use to detect the appearance of
new names on the bus.
</para>
</sect3>
<sect3 id="bus-messages-name-lost">
<title><literal>org.freedesktop.DBus.NameLost</literal></title>
<para>
This is a signal:
<programlisting>
NameLost (STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name which was lost</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This signal is sent to a specific application when it loses
ownership of a name.
</para>
</sect3>
<sect3 id="bus-messages-name-acquired">
<title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
<para>
This is a signal:
<programlisting>
NameAcquired (STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name which was acquired</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
This signal is sent to a specific application when it gains
ownership of a name.
</para>
</sect3>
<sect3 id="bus-messages-start-service-by-name">
<title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
<para>
As a method:
<programlisting>
UINT32 StartServiceByName (in STRING name, in UINT32 flags)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name of the service to start</entry>
</row>
<row>
<entry>1</entry>
<entry>UINT32</entry>
<entry>Flags (currently not used)</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>Return value</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.
</para>
<para>
The return value can be one of the following values:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Identifier</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>DBUS_START_REPLY_SUCCESS</entry>
<entry>1</entry>
<entry>The service was successfully started.</entry>
</row>
<row>
<entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
<entry>2</entry>
<entry>A connection already owns the given name.</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
</sect3>
<sect3 id="bus-messages-update-activation-environment">
<title><literal>org.freedesktop.DBus.UpdateActivationEnvironment</literal></title>
<para>
As a method:
<programlisting>
UpdateActivationEnvironment (in ARRAY of DICT<STRING,STRING> environment)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>ARRAY of DICT<STRING,STRING></entry>
<entry>Environment to add or update</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Normally, session bus activated services inherit the environment of the bus daemon. This method adds to or modifies that environment when activating services.
</para>
<para>
Some bus instances, such as the standard system bus, may disable access to this method for some or all callers.
</para>
</sect3>
<sect3 id="bus-messages-get-name-owner">
<title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
<para>
As a method:
<programlisting>
STRING GetNameOwner (in STRING name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name to get the owner of</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Return value, a unique connection name</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Returns the unique connection name of the primary owner of the name
given. If the requested name doesn't have an owner, returns a
<literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
</para>
</sect3>
<sect3 id="bus-messages-get-connection-unix-user">
<title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
<para>
As a method:
<programlisting>
UINT32 GetConnectionUnixUser (in STRING connection_name)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Name of the connection to query</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>UINT32</entry>
<entry>unix user id</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Returns the unix uid of the process connected to the server. If unable to
determine it, a <literal>org.freedesktop.DBus.Error.Failed</literal>
error is returned.
</para>
</sect3>
<sect3 id="bus-messages-add-match">
<title><literal>org.freedesktop.DBus.AddMatch</literal></title>
<para>
As a method:
<programlisting>
AddMatch (in STRING rule)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Match rule to add to the connection</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>).
If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
error is returned.
</para>
</sect3>
<sect3 id="bus-messages-remove-match">
<title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
<para>
As a method:
<programlisting>
RemoveMatch (in STRING rule)
</programlisting>
Message arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Match rule to remove from the connection</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>).
If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
error is returned.
</para>
</sect3>
<sect3 id="bus-messages-get-id">
<title><literal>org.freedesktop.DBus.GetId</literal></title>
<para>
As a method:
<programlisting>
GetId (out STRING id)
</programlisting>
Reply arguments:
<informaltable>
<tgroup cols="3">
<thead>
<row>
<entry>Argument</entry>
<entry>Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>0</entry>
<entry>STRING</entry>
<entry>Unique ID identifying the bus daemon</entry>
</row>
</tbody>
</tgroup>
</informaltable>
Gets the unique ID of the bus. The unique ID here is shared among all addresses the
bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in
<xref linkend="uuids"/>. Each address the bus is listening on also has its own unique
ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
by org.freedesktop.DBus.Peer.GetMachineId().
For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
</para>
</sect3>
</sect2>
</sect1>
<!--
<appendix id="implementation-notes">
<title>Implementation notes</title>
<sect1 id="implementation-notes-subsection">
<title></title>
<para>
</para>
</sect1>
</appendix>
-->
<glossary><title>Glossary</title>
<para>
This glossary defines some of the terms used in this specification.
</para>
<glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
<glossdef>
<para>
The message bus maintains an association between names and
connections. (Normally, there's one connection per application.) A
bus name is simply an identifier used to locate connections. For
example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
name might be used to send a message to a screensaver from Yoyodyne
Corporation. An application is said to <firstterm>own</firstterm> a
name if the message bus has associated the application's connection
with the name. Names may also have <firstterm>queued
owners</firstterm> (see <xref linkend="term-queued-owner"/>).
The bus assigns a unique name to each connection,
see <xref linkend="term-unique-name"/>. Other names
can be thought of as "well-known names" and are
used to find applications that offer specific functionality.
</para>
</glossdef>
</glossentry>
<glossentry id="term-message"><glossterm>Message</glossterm>
<glossdef>
<para>
A message is the atomic unit of communication via the D-Bus
protocol. It consists of a <firstterm>header</firstterm> and a
<firstterm>body</firstterm>; the body is made up of
<firstterm>arguments</firstterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
<glossdef>
<para>
The message bus is a special application that forwards
or routes messages between a group of applications
connected to the message bus. It also manages
<firstterm>names</firstterm> used for routing
messages.
</para>
</glossdef>
</glossentry>
<glossentry id="term-name"><glossterm>Name</glossterm>
<glossdef>
<para>
See <xref linkend="term-bus-name"/>. "Name" may
also be used to refer to some of the other names
in D-Bus, such as interface names.
</para>
</glossdef>
</glossentry>
<glossentry id="namespace"><glossterm>Namespace</glossterm>
<glossdef>
<para>
Used to prevent collisions when defining new interfaces or bus
names. The convention used is the same one Java uses for defining
classes: a reversed domain name.
</para>
</glossdef>
</glossentry>
<glossentry id="term-object"><glossterm>Object</glossterm>
<glossdef>
<para>
Each application contains <firstterm>objects</firstterm>, which have
<firstterm>interfaces</firstterm> and
<firstterm>methods</firstterm>. Objects are referred to by a name,
called a <firstterm>path</firstterm>.
</para>
</glossdef>
</glossentry>
<glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
<glossdef>
<para>
An application talking directly to another application, without going
through a message bus. One-to-one connections may be "peer to peer" or
"client to server." The D-Bus protocol has no concept of client
vs. server after a connection has authenticated; the flow of messages
is symmetrical (full duplex).
</para>
</glossdef>
</glossentry>
<glossentry id="term-path"><glossterm>Path</glossterm>
<glossdef>
<para>
Object references (object names) in D-Bus are organized into a
filesystem-style hierarchy, so each object is named by a path. As in
LDAP, there's no difference between "files" and "directories"; a path
can refer to an object, while still having child objects below it.
</para>
</glossdef>
</glossentry>
<glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
<glossdef>
<para>
Each bus name has a primary owner; messages sent to the name go to the
primary owner. However, certain names also maintain a queue of
secondary owners "waiting in the wings." If the primary owner releases
the name, then the first secondary owner in the queue automatically
becomes the new owner of the name.
</para>
</glossdef>
</glossentry>
<glossentry id="term-service"><glossterm>Service</glossterm>
<glossdef>
<para>
A service is an executable that can be launched by the bus daemon.
Services normally guarantee some particular features, for example they
may guarantee that they will request a specific name such as
"org.freedesktop.Screensaver", have a singleton object
"/org/freedesktop/Application", and that object will implement the
interface "org.freedesktop.ScreensaverControl".
</para>
</glossdef>
</glossentry>
<glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
<glossdef>
<para>
".service files" tell the bus about service applications that can be
launched (see <xref linkend="term-service"/>). Most importantly they
provide a mapping from bus names to services that will request those
names when they start up.
</para>
</glossdef>
</glossentry>
<glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
<glossdef>
<para>
The special name automatically assigned to each connection by the
message bus. This name will never change owner, and will be unique
(never reused during the lifetime of the message bus).
It will begin with a ':' character.
</para>
</glossdef>
</glossentry>
</glossary>
</article>
|