aboutsummaryrefslogtreecommitdiffstats log msg author committer range
path: root/manual/charset.texi
blob: f6a980f6cb444f1993c0a0360e6d396d6d13755f (plain)
 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  .highlight .hll { background-color: #ffffcc } .highlight .c { color: #008800; font-style: italic } /* Comment */ .highlight .err { border: 1px solid #FF0000 } /* Error */ .highlight .k { color: #AA22FF; font-weight: bold } /* Keyword */ .highlight .o { color: #666666 } /* Operator */ .highlight .ch { color: #008800; font-style: italic } /* Comment.Hashbang */ .highlight .cm { color: #008800; font-style: italic } /* Comment.Multiline */ .highlight .cp { color: #008800 } /* Comment.Preproc */ .highlight .cpf { color: #008800; font-style: italic } /* Comment.PreprocFile */ .highlight .c1 { color: #008800; font-style: italic } /* Comment.Single */ .highlight .cs { color: #008800; font-weight: bold } /* Comment.Special */ .highlight .gd { color: #A00000 } /* Generic.Deleted */ .highlight .ge { font-style: italic } /* Generic.Emph */ .highlight .gr { color: #FF0000 } /* Generic.Error */ .highlight .gh { color: #000080; font-weight: bold } /* Generic.Heading */ .highlight .gi { color: #00A000 } /* Generic.Inserted */ .highlight .go { color: #888888 } /* Generic.Output */ .highlight .gp { color: #000080; font-weight: bold } /* Generic.Prompt */ .highlight .gs { font-weight: bold } /* Generic.Strong */ .highlight .gu { color: #800080; font-weight: bold } /* Generic.Subheading */ .highlight .gt { color: #0044DD } /* Generic.Traceback */ .highlight .kc { color: #AA22FF; font-weight: bold } /* Keyword.Constant */ .highlight .kd { color: #AA22FF; font-weight: bold } /* Keyword.Declaration */ .highlight .kn { color: #AA22FF; font-weight: bold } /* Keyword.Namespace */ .highlight .kp { color: #AA22FF } /* Keyword.Pseudo */ .highlight .kr { color: #AA22FF; font-weight: bold } /* Keyword.Reserved */ .highlight .kt { color: #00BB00; font-weight: bold } /* Keyword.Type */ .highlight .m { color: #666666 } /* Literal.Number */ .highlight .s { color: #BB4444 } /* Literal.String */ .highlight .na { color: #BB4444 } /* Name.Attribute */ .highlight .nb { color: #AA22FF } /* Name.Builtin */ .highlight .nc { color: #0000FF } /* Name.Class */ .highlight .no { color: #880000 } /* Name.Constant */ .highlight .nd { color: #AA22FF } /* Name.Decorator */ .highlight .ni { color: #999999; font-weight: bold } /* Name.Entity */ .highlight .ne { color: #D2413A; font-weight: bold } /* Name.Exception */ .highlight .nf { color: #00A000 } /* Name.Function */ .highlight .nl { color: #A0A000 } /* Name.Label */ .highlight .nn { color: #0000FF; font-weight: bold } /* Name.Namespace */ .highlight .nt { color: #008000; font-weight: bold } /* Name.Tag */ .highlight .nv { color: #B8860B } /* Name.Variable */ .highlight .ow { color: #AA22FF; font-weight: bold } /* Operator.Word */ .highlight .w { color: #bbbbbb } /* Text.Whitespace */ .highlight .mb { color: #666666 } /* Literal.Number.Bin */ .highlight .mf { color: #666666 } /* Literal.Number.Float */ .highlight .mh { color: #666666 } /* Literal.Number.Hex */ .highlight .mi { color: #666666 } /* Literal.Number.Integer */ .highlight .mo { color: #666666 } /* Literal.Number.Oct */ .highlight .sa { color: #BB4444 } /* Literal.String.Affix */ .highlight .sb { color: #BB4444 } /* Literal.String.Backtick */ .highlight .sc { color: #BB4444 } /* Literal.String.Char */ .highlight .dl { color: #BB4444 } /* Literal.String.Delimiter */ .highlight .sd { color: #BB4444; font-style: italic } /* Literal.String.Doc */ .highlight .s2 { color: #BB4444 } /* Literal.String.Double */ .highlight .se { color: #BB6622; font-weight: bold } /* Literal.String.Escape */ .highlight .sh { color: #BB4444 } /* Literal.String.Heredoc */ .highlight .si { color: #BB6688; font-weight: bold } /* Literal.String.Interpol */ .highlight .sx { color: #008000 } /* Literal.String.Other */ .highlight .sr { color: #BB6688 } /* Literal.String.Regex */ .highlight .s1 { color: #BB4444 } /* Literal.String.Single */ .highlight .ss { color: #B8860B } /* Literal.String.Symbol */ .highlight .bp { color: #AA22FF } /* Name.Builtin.Pseudo */ .highlight .fm { color: #00A000 } /* Name.Function.Magic */ .highlight .vc { color: #B8860B } /* Name.Variable.Class */ .highlight .vg { color: #B8860B } /* Name.Variable.Global */ .highlight .vi { color: #B8860B } /* Name.Variable.Instance */ .highlight .vm { color: #B8860B } /* Name.Variable.Magic */ .highlight .il { color: #666666 } /* Literal.Number.Integer.Long */@node Character Set Handling, Locales, String and Array Utilities, Top @c %MENU% Support for extended character sets @chapter Character Set Handling @ifnottex @macro cal{text} \text\ @end macro @end ifnottex Character sets used in the early days of computing had only six, seven, or eight bits for each character: there was never a case where more than eight bits (one byte) were used to represent a single character. The limitations of this approach became more apparent as more people grappled with non-Roman character sets, where not all the characters that make up a language's character set can be represented by @math{2^8} choices. This chapter shows the functionality that was added to the C library to support multiple character sets. @menu * Extended Char Intro:: Introduction to Extended Characters. * Charset Function Overview:: Overview about Character Handling Functions. * Restartable multibyte conversion:: Restartable multibyte conversion Functions. * Non-reentrant Conversion:: Non-reentrant Conversion Function. * Generic Charset Conversion:: Generic Charset Conversion. @end menu @node Extended Char Intro @section Introduction to Extended Characters A variety of solutions are available to overcome the differences between character sets with a 1:1 relation between bytes and characters and character sets with ratios of 2:1 or 4:1. The remainder of this section gives a few examples to help understand the design decisions made while developing the functionality of the @w{C library}. @cindex internal representation A distinction we have to make right away is between internal and external representation. @dfn{Internal representation} means the representation used by a program while keeping the text in memory. External representations are used when text is stored or transmitted through some communication channel. Examples of external representations include files waiting in a directory to be read and parsed. Traditionally there has been no difference between the two representations. It was equally comfortable and useful to use the same single-byte representation internally and externally. This comfort level decreases with more and larger character sets. One of the problems to overcome with the internal representation is handling text that is externally encoded using different character sets. Assume a program that reads two texts and compares them using some metric. The comparison can be usefully done only if the texts are internally kept in a common format. @cindex wide character For such a common format (@math{=} character set) eight bits are certainly no longer enough. So the smallest entity will have to grow: @dfn{wide characters} will now be used. Instead of one byte per character, two or four will be used instead. (Three are not good to address in memory and more than four bytes seem not to be necessary). @cindex Unicode @cindex ISO 10646 As shown in some other part of this manual, @c !!! Ahem, wide char string functions are not yet covered -- drepper a completely new family has been created of functions that can handle wide character texts in memory. The most commonly used character sets for such internal wide character representations are Unicode and @w{ISO 10646} (also known as UCS for Universal Character Set). Unicode was originally planned as a 16-bit character set; whereas, @w{ISO 10646} was designed to be a 31-bit large code space. The two standards are practically identical. They have the same character repertoire and code table, but Unicode specifies added semantics. At the moment, only characters in the first @code{0x10000} code positions (the so-called Basic Multilingual Plane, BMP) have been assigned, but the assignment of more specialized characters outside this 16-bit space is already in progress. A number of encodings have been defined for Unicode and @w{ISO 10646} characters: @cindex UCS-2 @cindex UCS-4 @cindex UTF-8 @cindex UTF-16 UCS-2 is a 16-bit word that can only represent characters from the BMP, UCS-4 is a 32-bit word than can represent any Unicode and @w{ISO 10646} character, UTF-8 is an ASCII compatible encoding where ASCII characters are represented by ASCII bytes and non-ASCII characters by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension of UCS-2 in which pairs of certain UCS-2 words can be used to encode non-BMP characters up to @code{0x10ffff}. To represent wide characters the @code{char} type is not suitable. For this reason the @w{ISO C} standard introduces a new type that is designed to keep one character of a wide character string. To maintain the similarity there is also a type corresponding to @code{int} for those functions that take a single wide character. @deftp {Data type} wchar_t @standards{ISO, stddef.h} This data type is used as the base type for wide character strings. In other words, arrays of objects of this type are the equivalent of @code{char[]} for multibyte character strings. The type is defined in @file{stddef.h}. The @w{ISO C90} standard, where @code{wchar_t} was introduced, does not say anything specific about the representation. It only requires that this type is capable of storing all elements of the basic character set. Therefore it would be legitimate to define @code{wchar_t} as @code{char}, which might make sense for embedded systems. But in @theglibc{} @code{wchar_t} is always 32 bits wide and, therefore, capable of representing all UCS-4 values and, therefore, covering all of @w{ISO 10646}. Some Unix systems define @code{wchar_t} as a 16-bit type and thereby follow Unicode very strictly. This definition is perfectly fine with the standard, but it also means that to represent all characters from Unicode and @w{ISO 10646} one has to use UTF-16 surrogate characters, which is in fact a multi-wide-character encoding. But resorting to multi-wide-character encoding contradicts the purpose of the @code{wchar_t} type. @end deftp @deftp {Data type} wint_t @standards{ISO, wchar.h} @code{wint_t} is a data type used for parameters and variables that contain a single wide character. As the name suggests this type is the equivalent of @code{int} when using the normal @code{char} strings. The types @code{wchar_t} and @code{wint_t} often have the same representation if their size is 32 bits wide but if @code{wchar_t} is defined as @code{char} the type @code{wint_t} must be defined as @code{int} due to the parameter promotion. @pindex wchar.h This type is defined in @file{wchar.h} and was introduced in @w{Amendment 1} to @w{ISO C90}. @end deftp As there are for the @code{char} data type macros are available for specifying the minimum and maximum value representable in an object of type @code{wchar_t}. @deftypevr Macro wint_t WCHAR_MIN @standards{ISO, wchar.h} The macro @code{WCHAR_MIN} evaluates to the minimum value representable by an object of type @code{wint_t}. This macro was introduced in @w{Amendment 1} to @w{ISO C90}. @end deftypevr @deftypevr Macro wint_t WCHAR_MAX @standards{ISO, wchar.h} The macro @code{WCHAR_MAX} evaluates to the maximum value representable by an object of type @code{wint_t}. This macro was introduced in @w{Amendment 1} to @w{ISO C90}. @end deftypevr Another special wide character value is the equivalent to @code{EOF}. @deftypevr Macro wint_t WEOF @standards{ISO, wchar.h} The macro @code{WEOF} evaluates to a constant expression of type @code{wint_t} whose value is different from any member of the extended character set. @code{WEOF} need not be the same value as @code{EOF} and unlike @code{EOF} it also need @emph{not} be negative. In other words, sloppy code like @smallexample @{ int c; @dots{} while ((c = getc (fp)) < 0) @dots{} @} @end smallexample @noindent has to be rewritten to use @code{WEOF} explicitly when wide characters are used: @smallexample @{ wint_t c; @dots{} while ((c = wgetc (fp)) != WEOF) @dots{} @} @end smallexample @pindex wchar.h This macro was introduced in @w{Amendment 1} to @w{ISO C90} and is defined in @file{wchar.h}. @end deftypevr These internal representations present problems when it comes to storage and transmittal. Because each single wide character consists of more than one byte, they are affected by byte-ordering. Thus, machines with different endianesses would see different values when accessing the same data. This byte ordering concern also applies for communication protocols that are all byte-based and therefore require that the sender has to decide about splitting the wide character in bytes. A last (but not least important) point is that wide characters often require more storage space than a customized byte-oriented character set. @cindex multibyte character @cindex EBCDIC For all the above reasons, an external encoding that is different from the internal encoding is often used if the latter is UCS-2 or UCS-4. The external encoding is byte-based and can be chosen appropriately for the environment and for the texts to be handled. A variety of different character sets can be used for this external encoding (information that will not be exhaustively presented here--instead, a description of the major groups will suffice). All of the ASCII-based character sets fulfill one requirement: they are "filesystem safe." This means that the character @code{'/'} is used in the encoding @emph{only} to represent itself. Things are a bit different for character sets like EBCDIC (Extended Binary Coded Decimal Interchange Code, a character set family used by IBM), but if the operating system does not understand EBCDIC directly the parameters-to-system calls have to be converted first anyhow. @itemize @bullet @item The simplest character sets are single-byte character sets. There can be only up to 256 characters (for @w{8 bit} character sets), which is not sufficient to cover all languages but might be sufficient to handle a specific text. Handling of a @w{8 bit} character sets is simple. This is not true for other kinds presented later, and therefore, the application one uses might require the use of @w{8 bit} character sets. @cindex ISO 2022 @item The @w{ISO 2022} standard defines a mechanism for extended character sets where one character @emph{can} be represented by more than one byte. This is achieved by associating a state with the text. Characters that can be used to change the state can be embedded in the text. Each byte in the text might have a different interpretation in each state. The state might even influence whether a given byte stands for a character on its own or whether it has to be combined with some more bytes. @cindex EUC @cindex Shift_JIS @cindex SJIS In most uses of @w{ISO 2022} the defined character sets do not allow state changes that cover more than the next character. This has the big advantage that whenever one can identify the beginning of the byte sequence of a character one can interpret a text correctly. Examples of character sets using this policy are the various EUC character sets (used by Sun's operating systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN) or Shift_JIS (SJIS, a Japanese encoding). But there are also character sets using a state that is valid for more than one character and has to be changed by another byte sequence. Examples for this are ISO-2022-JP, ISO-2022-KR, and ISO-2022-CN. @item @cindex ISO 6937 Early attempts to fix 8 bit character sets for other languages using the Roman alphabet lead to character sets like @w{ISO 6937}. Here bytes representing characters like the acute accent do not produce output themselves: one has to combine them with other characters to get the desired result. For example, the byte sequence @code{0xc2 0x61} (non-spacing acute accent, followed by lower-case a') to get the small a with acute'' character. To get the acute accent character on its own, one has to write @code{0xc2 0x20} (the non-spacing acute followed by a space). Character sets like @w{ISO 6937} are used in some embedded systems such as teletex. @item @cindex UTF-8 Instead of converting the Unicode or @w{ISO 10646} text used internally, it is often also sufficient to simply use an encoding different than UCS-2/UCS-4. The Unicode and @w{ISO 10646} standards even specify such an encoding: UTF-8. This encoding is able to represent all of @w{ISO 10646} 31 bits in a byte string of length one to six. @cindex UTF-7 There were a few other attempts to encode @w{ISO 10646} such as UTF-7, but UTF-8 is today the only encoding that should be used. In fact, with any luck UTF-8 will soon be the only external encoding that has to be supported. It proves to be universally usable and its only disadvantage is that it favors Roman languages by making the byte string representation of other scripts (Cyrillic, Greek, Asian scripts) longer than necessary if using a specific character set for these scripts. Methods like the Unicode compression scheme can alleviate these problems. @end itemize The question remaining is: how to select the character set or encoding to use. The answer: you cannot decide about it yourself, it is decided by the developers of the system or the majority of the users. Since the goal is interoperability one has to use whatever the other people one works with use. If there are no constraints, the selection is based on the requirements the expected circle of users will have. In other words, if a project is expected to be used in only, say, Russia it is fine to use KOI8-R or a similar character set. But if at the same time people from, say, Greece are participating one should use a character set that allows all people to collaborate. The most widely useful solution seems to be: go with the most general character set, namely @w{ISO 10646}. Use UTF-8 as the external encoding and problems about users not being able to use their own language adequately are a thing of the past. One final comment about the choice of the wide character representation is necessary at this point. We have said above that the natural choice is using Unicode or @w{ISO 10646}. This is not required, but at least encouraged, by the @w{ISO C} standard. The standard defines at least a macro @code{__STDC_ISO_10646__} that is only defined on systems where the @code{wchar_t} type encodes @w{ISO 10646} characters. If this symbol is not defined one should avoid making assumptions about the wide character representation. If the programmer uses only the functions provided by the C library to handle wide character strings there should be no compatibility problems with other systems. @node Charset Function Overview @section Overview about Character Handling Functions A Unix @w{C library} contains three different sets of functions in two families to handle character set conversion. One of the function families (the most commonly used) is specified in the @w{ISO C90} standard and, therefore, is portable even beyond the Unix world. Unfortunately this family is the least useful one. These functions should be avoided whenever possible, especially when developing libraries (as opposed to applications). The second family of functions got introduced in the early Unix standards (XPG2) and is still part of the latest and greatest Unix standard: @w{Unix 98}. It is also the most powerful and useful set of functions. But we will start with the functions defined in @w{Amendment 1} to @w{ISO C90}. @node Restartable multibyte conversion @section Restartable Multibyte Conversion Functions The @w{ISO C} standard defines functions to convert strings from a multibyte representation to wide character strings. There are a number of peculiarities: @itemize @bullet @item The character set assumed for the multibyte encoding is not specified as an argument to the functions. Instead the character set specified by the @code{LC_CTYPE} category of the current locale is used; see @ref{Locale Categories}. @item The functions handling more than one character at a time require NUL terminated strings as the argument (i.e., converting blocks of text does not work unless one can add a NUL byte at an appropriate place). @Theglibc{} contains some extensions to the standard that allow specifying a size, but basically they also expect terminated strings. @end itemize Despite these limitations the @w{ISO C} functions can be used in many contexts. In graphical user interfaces, for instance, it is not uncommon to have functions that require text to be displayed in a wide character string if the text is not simple ASCII. The text itself might come from a file with translations and the user should decide about the current locale, which determines the translation and therefore also the external encoding used. In such a situation (and many others) the functions described here are perfect. If more freedom while performing the conversion is necessary take a look at the @code{iconv} functions (@pxref{Generic Charset Conversion}). @menu * Selecting the Conversion:: Selecting the conversion and its properties. * Keeping the state:: Representing the state of the conversion. * Converting a Character:: Converting Single Characters. * Converting Strings:: Converting Multibyte and Wide Character Strings. * Multibyte Conversion Example:: A Complete Multibyte Conversion Example. @end menu @node Selecting the Conversion @subsection Selecting the conversion and its properties We already said above that the currently selected locale for the @code{LC_CTYPE} category decides the conversion that is performed by the functions we are about to describe. Each locale uses its own character set (given as an argument to @code{localedef}) and this is the one assumed as the external multibyte encoding. The wide character set is always UCS-4 in @theglibc{}. A characteristic of each multibyte character set is the maximum number of bytes that can be necessary to represent one character. This information is quite important when writing code that uses the conversion functions (as shown in the examples below). The @w{ISO C} standard defines two macros that provide this information. @deftypevr Macro int MB_LEN_MAX @standards{ISO, limits.h} @code{MB_LEN_MAX} specifies the maximum number of bytes in the multibyte sequence for a single character in any of the supported locales. It is a compile-time constant and is defined in @file{limits.h}. @pindex limits.h @end deftypevr @deftypevr Macro int MB_CUR_MAX @standards{ISO, stdlib.h} @code{MB_CUR_MAX} expands into a positive integer expression that is the maximum number of bytes in a multibyte character in the current locale. The value is never greater than @code{MB_LEN_MAX}. Unlike @code{MB_LEN_MAX} this macro need not be a compile-time constant, and in @theglibc{} it is not. @pindex stdlib.h @code{MB_CUR_MAX} is defined in @file{stdlib.h}. @end deftypevr Two different macros are necessary since strictly @w{ISO C90} compilers do not allow variable length array definitions, but still it is desirable to avoid dynamic allocation. This incomplete piece of code shows the problem: @smallexample @{ char buf[MB_LEN_MAX]; ssize_t len = 0; while (! feof (fp)) @{ fread (&buf[len], 1, MB_CUR_MAX - len, fp); /* @r{@dots{} process} buf */ len -= used; @} @} @end smallexample The code in the inner loop is expected to have always enough bytes in the array @var{buf} to convert one multibyte character. The array @var{buf} has to be sized statically since many compilers do not allow a variable size. The @code{fread} call makes sure that @code{MB_CUR_MAX} bytes are always available in @var{buf}. Note that it isn't a problem if @code{MB_CUR_MAX} is not a compile-time constant. @node Keeping the state @subsection Representing the state of the conversion @cindex stateful In the introduction of this chapter it was said that certain character sets use a @dfn{stateful} encoding. That is, the encoded values depend in some way on the previous bytes in the text. Since the conversion functions allow converting a text in more than one step we must have a way to pass this information from one call of the functions to another. @deftp {Data type} mbstate_t @standards{ISO, wchar.h} @cindex shift state A variable of type @code{mbstate_t} can contain all the information about the @dfn{shift state} needed from one call to a conversion function to another. @pindex wchar.h @code{mbstate_t} is defined in @file{wchar.h}. It was introduced in @w{Amendment 1} to @w{ISO C90}. @end deftp To use objects of type @code{mbstate_t} the programmer has to define such objects (normally as local variables on the stack) and pass a pointer to the object to the conversion functions. This way the conversion function can update the object if the current multibyte character set is stateful. There is no specific function or initializer to put the state object in any specific state. The rules are that the object should always represent the initial state before the first use, and this is achieved by clearing the whole variable with code such as follows: @smallexample @{ mbstate_t state; memset (&state, '\0', sizeof (state)); /* @r{from now on @var{state} can be used.} */ @dots{} @} @end smallexample When using the conversion functions to generate output it is often necessary to test whether the current state corresponds to the initial state. This is necessary, for example, to decide whether to emit escape sequences to set the state to the initial state at certain sequence points. Communication protocols often require this. @deftypefun int mbsinit (const mbstate_t *@var{ps}) @standards{ISO, wchar.h} @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} @c ps is dereferenced once, unguarded. This would call for @mtsrace:ps, @c but since a single word-sized field is (atomically) accessed, any @c race here would be harmless. Other functions that take an optional @c mbstate_t* argument named ps are marked with @mtasurace:/!ps, @c to indicate that the function uses a static buffer if ps is NULL. @c These could also have been marked with @mtsrace:ps, but we'll omit @c that for brevity, for it's somewhat redundant with the @mtasurace. The @code{mbsinit} function determines whether the state object pointed to by @var{ps} is in the initial state. If @var{ps} is a null pointer or the object is in the initial state the return value is nonzero. Otherwise it is zero. @pindex wchar.h @code{mbsinit} was introduced in @w{Amendment 1} to @w{ISO C90} and is declared in @file{wchar.h}. @end deftypefun Code using @code{mbsinit} often looks similar to this: @c Fix the example to explicitly say how to generate the escape sequence @c to restore the initial state. @smallexample @{ mbstate_t state; memset (&state, '\0', sizeof (state)); /* @r{Use @var{state}.} */ @dots{} if (! mbsinit (&state)) @{ /* @r{Emit code to return to initial state.} */ const wchar_t empty[] = L""; const wchar_t *srcp = empty; wcsrtombs (outbuf, &srcp, outbuflen, &state); @} @dots{} @} @end smallexample The code to emit the escape sequence to get back to the initial state is interesting. The @code{wcsrtombs} function can be used to determine the necessary output code (@pxref{Converting Strings}). Please note that with @theglibc{} it is not necessary to perform this extra action for the conversion from multibyte text to wide character text since the wide character encoding is not stateful. But there is nothing mentioned in any standard that prohibits making @code{wchar_t} use a stateful encoding. @node Converting a Character @subsection Converting Single Characters The most fundamental of the conversion functions are those dealing with single characters. Please note that this does not always mean single bytes. But since there is very often a subset of the multibyte character set that consists of single byte sequences, there are functions to help with converting bytes. Frequently, ASCII is a subset of the multibyte character set. In such a scenario, each ASCII character stands for itself, and all other characters have at least a first byte that is beyond the range @math{0} to @math{127}. @deftypefun wint_t btowc (int @var{c}) @standards{ISO, wchar.h} @safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} @c Calls btowc_fct or __fct; reads from locale, and from the @c get_gconv_fcts result multiple times. get_gconv_fcts calls @c __wcsmbs_load_conv to initialize the ctype if it's null. @c wcsmbs_load_conv takes a non-recursive wrlock before allocating @c memory for the fcts structure, initializing it, and then storing it @c in the locale object. The initialization involves dlopening and a @c lot more. The @code{btowc} function (byte to wide character'') converts a valid single byte character @var{c} in the initial shift state into the wide character equivalent using the conversion rules from the currently selected locale of the @code{LC_CTYPE} category. If @code{(unsigned char) @var{c}} is no valid single byte multibyte character or if @var{c} is @code{EOF}, the function returns @code{WEOF}. Please note the restriction of @var{c} being tested for validity only in the initial shift state. No @code{mbstate_t} object is used from which the state information is taken, and the function also does not use any static state. @pindex wchar.h The @code{btowc} function was introduced in @w{Amendment 1} to @w{ISO C90} and is declared in @file{wchar.h}. @end deftypefun Despite the limitation that the single byte value is always interpreted in the initial state, this function is actually useful most of the time. Most characters are either entirely single-byte character sets or they are extensions to ASCII. But then it is possible to write code like this (not that this specific example is very useful): @smallexample wchar_t * itow (unsigned long int val) @{ static wchar_t buf[30]; wchar_t *wcp = &buf[29]; *wcp = L'\0'; while (val != 0) @{ *--wcp = btowc ('0' + val % 10); val /= 10; @} if (wcp == &buf[29]) *--wcp = L'0'; return wcp; @} @end smallexample Why is it necessary to use such a complicated implementation and not simply cast @code{'0' + val % 10} to a wide character? The answer is that there is no guarantee that one can perform this kind of arithmetic on the character of the character set used for @code{wchar_t} representation. In other situations the bytes are not constant at compile time and so the compiler cannot do the work. In situations like this, using @code{btowc} is required. @noindent There is also a function for the conversion in the other direction. @deftypefun int wctob (wint_t @var{c}) @standards{ISO, wchar.h} @safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{wctob} function (wide character to byte'') takes as the parameter a valid wide character. If the multibyte representation for this character in the initial state is exactly one byte long, the return value of this function is this character. Otherwise the return value is @code{EOF}. @pindex wchar.h @code{wctob} was introduced in @w{Amendment 1} to @w{ISO C90} and is declared in @file{wchar.h}. @end deftypefun There are more general functions to convert single characters from multibyte representation to wide characters and vice versa. These functions pose no limit on the length of the multibyte representation and they also do not require it to be in the initial state. @deftypefun size_t mbrtowc (wchar_t *restrict @var{pwc}, const char *restrict @var{s}, size_t @var{n}, mbstate_t *restrict @var{ps}) @standards{ISO, wchar.h} @safety{@prelim{}@mtunsafe{@mtasurace{:mbrtowc/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} @cindex stateful The @code{mbrtowc} function (multibyte restartable to wide character'') converts the next multibyte character in the string pointed to by @var{s} into a wide character and stores it in the location pointed to by @var{pwc}. The conversion is performed according to the locale currently selected for the @code{LC_CTYPE} category. If the conversion for the character set used in the locale requires a state, the multibyte string is interpreted in the state represented by the object pointed to by @var{ps}. If @var{ps} is a null pointer, a static, internal state variable used only by the @code{mbrtowc} function is used. If the next multibyte character corresponds to the null wide character, the return value of the function is @math{0} and the state object is afterwards in the initial state. If the next @var{n} or fewer bytes form a correct multibyte character, the return value is the number of bytes starting from @var{s} that form the multibyte character. The conversion state is updated according to the bytes consumed in the conversion. In both cases the wide character (either the @code{L'\0'} or the one found in the conversion) is stored in the string pointed to by @var{pwc} if @var{pwc} is not null. If the first @var{n} bytes of the multibyte string possibly form a valid multibyte character but there are more than @var{n} bytes needed to complete it, the return value of the function is @code{(size_t) -2} and no value is stored in @code{*@var{pwc}}. The conversion state is updated and all @var{n} input bytes are consumed and should not be submitted again. Please note that this can happen even if @var{n} has a value greater than or equal to @code{MB_CUR_MAX} since the input might contain redundant shift sequences. If the first @code{n} bytes of the multibyte string cannot possibly form a valid multibyte character, no value is stored, the global variable @code{errno} is set to the value @code{EILSEQ}, and the function returns @code{(size_t) -1}. The conversion state is afterwards undefined. As specified, the @code{mbrtowc} function could deal with multibyte sequences which contain embedded null bytes (which happens in Unicode encodings such as UTF-16), but @theglibc{} does not support such multibyte encodings. When encountering a null input byte, the function will either return zero, or return @code{(size_t) -1)} and report a @code{EILSEQ} error. The @code{iconv} function can be used for converting between arbitrary encodings. @xref{Generic Conversion Interface}. @pindex wchar.h @code{mbrtowc} was introduced in @w{Amendment 1} to @w{ISO C90} and is declared in @file{wchar.h}. @end deftypefun A function that copies a multibyte string into a wide character string while at the same time converting all lowercase characters into uppercase could look like this: @smallexample @include mbstouwcs.c.texi @end smallexample In the inner loop, a single wide character is stored in @code{wc}, and the number of consumed bytes is stored in the variable @code{nbytes}. If the conversion is successful, the uppercase variant of the wide character is stored in the @code{result} array and the pointer to the input string and the number of available bytes is adjusted. If the @code{mbrtowc} function returns zero, the null input byte has not been converted, so it must be stored explicitly in the result. The above code uses the fact that there can never be more wide characters in the converted result than there are bytes in the multibyte input string. This method yields a pessimistic guess about the size of the result, and if many wide character strings have to be constructed this way or if the strings are long, the extra memory required to be allocated because the input string contains multibyte characters might be significant. The allocated memory block can be resized to the correct size before returning it, but a better solution might be to allocate just the right amount of space for the result right away. Unfortunately there is no function to compute the length of the wide character string directly from the multibyte string. There is, however, a function that does part of the work. @deftypefun size_t mbrlen (const char *restrict @var{s}, size_t @var{n}, mbstate_t *@var{ps}) @standards{ISO, wchar.h} @safety{@prelim{}@mtunsafe{@mtasurace{:mbrlen/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{mbrlen} function (multibyte restartable length'') computes the number of at most @var{n} bytes starting at @var{s}, which form the next valid and complete multibyte character. If the next multibyte character corresponds to the NUL wide character, the return value is @math{0}. If the next @var{n} bytes form a valid multibyte character, the number of bytes belonging to this multibyte character byte sequence is returned. If the first @var{n} bytes possibly form a valid multibyte character but the character is incomplete, the return value is @code{(size_t) -2}. Otherwise the multibyte character sequence is invalid and the return value is @code{(size_t) -1}. The multibyte sequence is interpreted in the state represented by the object pointed to by @var{ps}. If @var{ps} is a null pointer, a state object local to @code{mbrlen} is used. @pindex wchar.h @code{mbrlen} was introduced in @w{Amendment 1} to @w{ISO C90} and is declared in @file{wchar.h}. @end deftypefun The attentive reader now will note that @code{mbrlen} can be implemented as @smallexample mbrtowc (NULL, s, n, ps != NULL ? ps : &internal) @end smallexample This is true and in fact is mentioned in the official specification. How can this function be used to determine the length of the wide character string created from a multibyte character string? It is not directly usable, but we can define a function @code{mbslen} using it: @smallexample size_t mbslen (const char *s) @{ mbstate_t state; size_t result = 0; size_t nbytes; memset (&state, '\0', sizeof (state)); while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0) @{ if (nbytes >= (size_t) -2) /* @r{Something is wrong.} */ return (size_t) -1; s += nbytes; ++result; @} return result; @} @end smallexample This function simply calls @code{mbrlen} for each multibyte character in the string and counts the number of function calls. Please note that we here use @code{MB_LEN_MAX} as the size argument in the @code{mbrlen} call. This is acceptable since a) this value is larger than the length of the longest multibyte character sequence and b) we know that the string @var{s} ends with a NUL byte, which cannot be part of any other multibyte character sequence but the one representing the NUL wide character. Therefore, the @code{mbrlen} function will never read invalid memory. Now that this function is available (just to make this clear, this function is @emph{not} part of @theglibc{}) we can compute the number of wide characters required to store the converted multibyte character string @var{s} using @smallexample wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t); @end smallexample Please note that the @code{mbslen} function is quite inefficient. The implementation of @code{mbstouwcs} with @code{mbslen} would have to perform the conversion of the multibyte character input string twice, and this conversion might be quite expensive. So it is necessary to think about the consequences of using the easier but imprecise method before doing the work twice. @deftypefun size_t wcrtomb (char *restrict @var{s}, wchar_t @var{wc}, mbstate_t *restrict @var{ps}) @standards{ISO, wchar.h} @safety{@prelim{}@mtunsafe{@mtasurace{:wcrtomb/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} @c wcrtomb uses a static, non-thread-local unguarded state variable when @c PS is NULL. When a state is passed in, and it's not used @c concurrently in other threads, this function behaves safely as long @c as gconv modules don't bring MT safety issues of their own. @c Attempting to load gconv modules or to build conversion chains in @c signal handlers may encounter gconv databases or caches in a @c partially-updated state, and asynchronous cancellation may leave them @c in such states, besides leaking the lock that guards them. @c get_gconv_fcts ok @c wcsmbs_load_conv ok @c norm_add_slashes ok @c wcsmbs_getfct ok @c gconv_find_transform ok @c gconv_read_conf (libc_once) @c gconv_lookup_cache ok @c find_module_idx ok @c find_module ok @c gconv_find_shlib (ok) @c ->init_fct (assumed ok) @c gconv_get_builtin_trans ok @c gconv_release_step ok @c do_lookup_alias ok @c find_derivation ok @c derivation_lookup ok @c increment_counter ok @c gconv_find_shlib ok @c step->init_fct (assumed ok) @c gen_steps ok @c gconv_find_shlib ok @c dlopen (presumed ok) @c dlsym (presumed ok) @c step->init_fct (assumed ok) @c step->end_fct (assumed ok) @c gconv_get_builtin_trans ok @c gconv_release_step ok @c add_derivation ok @c gconv_close_transform ok @c gconv_release_step ok @c step->end_fct (assumed ok) @c gconv_release_shlib ok @c dlclose (presumed ok) @c gconv_release_cache ok @c ->tomb->__fct (assumed ok) The @code{wcrtomb} function (wide character restartable to multibyte'') converts a single wide character into a multibyte string corresponding to that wide character. If @var{s} is a null pointer, the function resets the state stored in the object pointed to by @var{ps} (or the internal @code{mbstate_t} object) to the initial state. This can also be achieved by a call like this: @smallexample wcrtombs (temp_buf, L'\0', ps) @end smallexample @noindent since, if @var{s} is a null pointer, @code{wcrtomb} performs as if it writes into an internal buffer, which is guaranteed to be large enough. If @var{wc} is the NUL wide character, @code{wcrtomb} emits, if necessary, a shift sequence to get the state @var{ps} into the initial state followed by a single NUL byte, which is stored in the string @var{s}. Otherwise a byte sequence (possibly including shift sequences) is written into the string @var{s}. This only happens if @var{wc} is a valid wide character (i.e., it has a multibyte representation in the character set selected by locale of the @code{LC_CTYPE} category). If @var{wc} is no valid wide character, nothing is stored in the strings @var{s}, @code{errno} is set to @code{EILSEQ}, the conversion state in @var{ps} is undefined and the return value is @code{(size_t) -1}. If no error occurred the function returns the number of bytes stored in the string @var{s}. This includes all bytes representing shift sequences. One word about the interface of the function: there is no parameter specifying the length of the array @var{s}. Instead the function assumes that there are at least @code{MB_CUR_MAX} bytes available since this is the maximum length of any byte sequence representing a single character. So the caller has to make sure that there is enough space available, otherwise buffer overruns can occur. @pindex wchar.h @code{wcrtomb} was introduced in @w{Amendment 1} to @w{ISO C90} and is declared in @file{wchar.h}. @end deftypefun Using @code{wcrtomb} is as easy as using @code{mbrtowc}. The following example appends a wide character string to a multibyte character string. Again, the code is not really useful (or correct), it is simply here to demonstrate the use and some problems. @smallexample char * mbscatwcs (char *s, size_t len, const wchar_t *ws) @{ mbstate_t state; /* @r{Find the end of the existing string.} */ char *wp = strchr (s, '\0'); len -= wp - s; memset (&state, '\0', sizeof (state)); do @{ size_t nbytes; if (len < MB_CUR_LEN) @{ /* @r{We cannot guarantee that the next} @r{character fits into the buffer, so} @r{return an error.} */ errno = E2BIG; return NULL; @} nbytes = wcrtomb (wp, *ws, &state); if (nbytes == (size_t) -1) /* @r{Error in the conversion.} */ return NULL; len -= nbytes; wp += nbytes; @} while (*ws++ != L'\0'); return s; @} @end smallexample First the function has to find the end of the string currently in the array @var{s}. The @code{strchr} call does this very efficiently since a requirement for multibyte character representations is that the NUL byte is never used except to represent itself (and in this context, the end of the string). After initializing the state object the loop is entered where the first task is to make sure there is enough room in the array @var{s}. We abort if there are not at least @code{MB_CUR_LEN} bytes available. This is not always optimal but we have no other choice. We might have less than @code{MB_CUR_LEN} bytes available but the next multibyte character might also be only one byte long. At the time the @code{wcrtomb} call returns it is too late to decide whether the buffer was large enough. If this solution is unsuitable, there is a very slow but more accurate solution. @smallexample @dots{} if (len < MB_CUR_LEN) @{ mbstate_t temp_state; memcpy (&temp_state, &state, sizeof (state)); if (wcrtomb (NULL, *ws, &temp_state) > len) @{ /* @r{We cannot guarantee that the next} @r{character fits into the buffer, so} @r{return an error.} */ errno = E2BIG; return NULL; @} @} @dots{} @end smallexample Here we perform the conversion that might overflow the buffer so that we are afterwards in the position to make an exact decision about the buffer size. Please note the @code{NULL} argument for the destination buffer in the new @code{wcrtomb} call; since we are not interested in the converted text at this point, this is a nice way to express this. The most unusual thing about this piece of code certainly is the duplication of the conversion state object, but if a change of the state is necessary to emit the next multibyte character, we want to have the same shift state change performed in the real conversion. Therefore, we have to preserve the initial shift state information. There are certainly many more and even better solutions to this problem. This example is only provided for educational purposes. @node Converting Strings @subsection Converting Multibyte and Wide Character Strings The functions described in the previous section only convert a single character at a time. Most operations to be performed in real-world programs include strings and therefore the @w{ISO C} standard also defines conversions on entire strings. However, the defined set of functions is quite limited; therefore, @theglibc{} contains a few extensions that can help in some important situations. @deftypefun size_t mbsrtowcs (wchar_t *restrict @var{dst}, const char **restrict @var{src}, size_t @var{len}, mbstate_t *restrict @var{ps}) @standards{ISO, wchar.h} @safety{@prelim{}@mtunsafe{@mtasurace{:mbsrtowcs/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{mbsrtowcs} function (multibyte string restartable to wide character string'') converts the NUL-terminated multibyte character string at @code{*@var{src}} into an equivalent wide character string, including the NUL wide character at the end. The conversion is started using the state information from the object pointed to by @var{ps} or from an internal object of @code{mbsrtowcs} if @var{ps} is a null pointer. Before returning, the state object is updated to match the state after the last converted character. The state is the initial state if the terminating NUL byte is reached and converted. If @var{dst} is not a null pointer, the result is stored in the array pointed to by @var{dst}; otherwise, the conversion result is not available since it is stored in an internal buffer. If @var{len} wide characters are stored in the array @var{dst} before reaching the end of the input string, the conversion stops and @var{len} is returned. If @var{dst} is a null pointer, @var{len} is never checked. Another reason for a premature return from the function call is if the input string contains an invalid multibyte sequence. In this case the global variable @code{errno} is set to @code{EILSEQ} and the function returns @code{(size_t) -1}. @c XXX The ISO C9x draft seems to have a problem here. It says that PS @c is not updated if DST is NULL. This is not said straightforward and @c none of the other functions is described like this. It would make sense @c to define the function this way but I don't think it is meant like this. In all other cases the function returns the number of wide characters converted during this call. If @var{dst} is not null, @code{mbsrtowcs} stores in the pointer pointed to by @var{src} either a null pointer (if the NUL byte in the input string was reached) or the address of the byte following the last converted multibyte character. @pindex wchar.h @code{mbsrtowcs} was introduced in @w{Amendment 1} to @w{ISO C90} and is declared in @file{wchar.h}. @end deftypefun The definition of the @code{mbsrtowcs} function has one important limitation. The requirement that @var{dst} has to be a NUL-terminated string provides problems if one wants to convert buffers with text. A buffer is not normally a collection of NUL-terminated strings but instead a continuous collection of lines, separated by newline characters. Now assume that a function to convert one line from a buffer is needed. Since the line is not NUL-terminated, the source pointer cannot directly point into the unmodified text buffer. This means, either one inserts the NUL byte at the appropriate place for the time of the @code{mbsrtowcs} function call (which is not doable for a read-only buffer or in a multi-threaded application) or one copies the line in an extra buffer where it can be terminated by a NUL byte. Note that it is not in general possible to limit the number of characters to convert by setting the parameter @var{len} to any specific value. Since it is not known how many bytes each multibyte character sequence is in length, one can only guess. @cindex stateful There is still a problem with the method of NUL-terminating a line right after the newline character, which could lead to very strange results. As said in the description of the @code{mbsrtowcs} function above, the conversion state is guaranteed to be in the initial shift state after processing the NUL byte at the end of the input string. But this NUL byte is not really part of the text (i.e., the conversion state after the newline in the original text could be something different than the initial shift state and therefore the first character of the next line is encoded using this state). But the state in question is never accessible to the user since the conversion stops after the NUL byte (which resets the state). Most stateful character sets in use today require that the shift state after a newline be the initial state--but this is not a strict guarantee. Therefore, simply NUL-terminating a piece of a running text is not always an adequate solution and, therefore, should never be used in generally used code. The generic conversion interface (@pxref{Generic Charset Conversion}) does not have this limitation (it simply works on buffers, not strings), and @theglibc{} contains a set of functions that take additional parameters specifying the maximal number of bytes that are consumed from the input string. This way the problem of @code{mbsrtowcs}'s example above could be solved by determining the line length and passing this length to the function. @deftypefun size_t wcsrtombs (char *restrict @var{dst}, const wchar_t **restrict @var{src}, size_t @var{len}, mbstate_t *restrict @var{ps}) @standards{ISO, wchar.h} @safety{@prelim{}@mtunsafe{@mtasurace{:wcsrtombs/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{wcsrtombs} function (wide character string restartable to multibyte string'') converts the NUL-terminated wide character string at @code{*@var{src}} into an equivalent multibyte character string and stores the result in the array pointed to by @var{dst}. The NUL wide character is also converted. The conversion starts in the state described in the object pointed to by @var{ps} or by a state object local to @code{wcsrtombs} in case @var{ps} is a null pointer. If @var{dst} is a null pointer, the conversion is performed as usual but the result is not available. If all characters of the input string were successfully converted and if @var{dst} is not a null pointer, the pointer pointed to by @var{src} gets assigned a null pointer. If one of the wide characters in the input string has no valid multibyte character equivalent, the conversion stops early, sets the global variable @code{errno} to @code{EILSEQ}, and returns @code{(size_t) -1}. Another reason for a premature stop is if @var{dst} is not a null pointer and the next converted character would require more than @var{len} bytes in total to the array @var{dst}. In this case (and if @var{dst} is not a null pointer) the pointer pointed to by @var{src} is assigned a value pointing to the wide character right after the last one successfully converted. Except in the case of an encoding error the return value of the @code{wcsrtombs} function is the number of bytes in all the multibyte character sequences stored in @var{dst}. Before returning, the state in the object pointed to by @var{ps} (or the internal object in case @var{ps} is a null pointer) is updated to reflect the state after the last conversion. The state is the initial shift state in case the terminating NUL wide character was converted. @pindex wchar.h The @code{wcsrtombs} function was introduced in @w{Amendment 1} to @w{ISO C90} and is declared in @file{wchar.h}. @end deftypefun The restriction mentioned above for the @code{mbsrtowcs} function applies here also. There is no possibility of directly controlling the number of input characters. One has to place the NUL wide character at the correct place or control the consumed input indirectly via the available output array size (the @var{len} parameter). @deftypefun size_t mbsnrtowcs (wchar_t *restrict @var{dst}, const char **restrict @var{src}, size_t @var{nmc}, size_t @var{len}, mbstate_t *restrict @var{ps}) @standards{GNU, wchar.h} @safety{@prelim{}@mtunsafe{@mtasurace{:mbsnrtowcs/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{mbsnrtowcs} function is very similar to the @code{mbsrtowcs} function. All the parameters are the same except for @var{nmc}, which is new. The return value is the same as for @code{mbsrtowcs}. This new parameter specifies how many bytes at most can be used from the multibyte character string. In other words, the multibyte character string @code{*@var{src}} need not be NUL-terminated. But if a NUL byte is found within the @var{nmc} first bytes of the string, the conversion stops there. This function is a GNU extension. It is meant to work around the problems mentioned above. Now it is possible to convert a buffer with multibyte character text piece by piece without having to care about inserting NUL bytes and the effect of NUL bytes on the conversion state. @end deftypefun A function to convert a multibyte string into a wide character string and display it could be written like this (this is not a really useful example): @smallexample void showmbs (const char *src, FILE *fp) @{ mbstate_t state; int cnt = 0; memset (&state, '\0', sizeof (state)); while (1) @{ wchar_t linebuf[100]; const char *endp = strchr (src, '\n'); size_t n; /* @r{Exit if there is no more line.} */ if (endp == NULL) break; n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state); linebuf[n] = L'\0'; fprintf (fp, "line %d: \"%S\"\n", linebuf); @} @} @end smallexample There is no problem with the state after a call to @code{mbsnrtowcs}. Since we don't insert characters in the strings that were not in there right from the beginning and we use @var{state} only for the conversion of the given buffer, there is no problem with altering the state. @deftypefun size_t wcsnrtombs (char *restrict @var{dst}, const wchar_t **restrict @var{src}, size_t @var{nwc}, size_t @var{len}, mbstate_t *restrict @var{ps}) @standards{GNU, wchar.h} @safety{@prelim{}@mtunsafe{@mtasurace{:wcsnrtombs/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{wcsnrtombs} function implements the conversion from wide character strings to multibyte character strings. It is similar to @code{wcsrtombs} but, just like @code{mbsnrtowcs}, it takes an extra parameter, which specifies the length of the input string. No more than @var{nwc} wide characters from the input string @code{*@var{src}} are converted. If the input string contains a NUL wide character in the first @var{nwc} characters, the conversion stops at this place. The @code{wcsnrtombs} function is a GNU extension and just like @code{mbsnrtowcs} helps in situations where no NUL-terminated input strings are available. @end deftypefun @node Multibyte Conversion Example @subsection A Complete Multibyte Conversion Example The example programs given in the last sections are only brief and do not contain all the error checking, etc. Presented here is a complete and documented example. It features the @code{mbrtowc} function but it should be easy to derive versions using the other functions. @smallexample int file_mbsrtowcs (int input, int output) @{ /* @r{Note the use of @code{MB_LEN_MAX}.} @r{@code{MB_CUR_MAX} cannot portably be used here.} */ char buffer[BUFSIZ + MB_LEN_MAX]; mbstate_t state; int filled = 0; int eof = 0; /* @r{Initialize the state.} */ memset (&state, '\0', sizeof (state)); while (!eof) @{ ssize_t nread; ssize_t nwrite; char *inp = buffer; wchar_t outbuf[BUFSIZ]; wchar_t *outp = outbuf; /* @r{Fill up the buffer from the input file.} */ nread = read (input, buffer + filled, BUFSIZ); if (nread < 0) @{ perror ("read"); return 0; @} /* @r{If we reach end of file, make a note to read no more.} */ if (nread == 0) eof = 1; /* @r{@code{filled} is now the number of bytes in @code{buffer}.} */ filled += nread; /* @r{Convert those bytes to wide characters--as many as we can.} */ while (1) @{ size_t thislen = mbrtowc (outp, inp, filled, &state); /* @r{Stop converting at invalid character;} @r{this can mean we have read just the first part} @r{of a valid character.} */ if (thislen == (size_t) -1) break; /* @r{We want to handle embedded NUL bytes} @r{but the return value is 0. Correct this.} */ if (thislen == 0) thislen = 1; /* @r{Advance past this character.} */ inp += thislen; filled -= thislen; ++outp; @} /* @r{Write the wide characters we just made.} */ nwrite = write (output, outbuf, (outp - outbuf) * sizeof (wchar_t)); if (nwrite < 0) @{ perror ("write"); return 0; @} /* @r{See if we have a @emph{real} invalid character.} */ if ((eof && filled > 0) || filled >= MB_CUR_MAX) @{ error (0, 0, "invalid multibyte character"); return 0; @} /* @r{If any characters must be carried forward,} @r{put them at the beginning of @code{buffer}.} */ if (filled > 0) memmove (buffer, inp, filled); @} return 1; @} @end smallexample @node Non-reentrant Conversion @section Non-reentrant Conversion Function The functions described in the previous chapter are defined in @w{Amendment 1} to @w{ISO C90}, but the original @w{ISO C90} standard also contained functions for character set conversion. The reason that these original functions are not described first is that they are almost entirely useless. The problem is that all the conversion functions described in the original @w{ISO C90} use a local state. Using a local state implies that multiple conversions at the same time (not only when using threads) cannot be done, and that you cannot first convert single characters and then strings since you cannot tell the conversion functions which state to use. These original functions are therefore usable only in a very limited set of situations. One must complete converting the entire string before starting a new one, and each string/text must be converted with the same function (there is no problem with the library itself; it is guaranteed that no library function changes the state of any of these functions). @strong{For the above reasons it is highly requested that the functions described in the previous section be used in place of non-reentrant conversion functions.} @menu * Non-reentrant Character Conversion:: Non-reentrant Conversion of Single Characters. * Non-reentrant String Conversion:: Non-reentrant Conversion of Strings. * Shift State:: States in Non-reentrant Functions. @end menu @node Non-reentrant Character Conversion @subsection Non-reentrant Conversion of Single Characters @deftypefun int mbtowc (wchar_t *restrict @var{result}, const char *restrict @var{string}, size_t @var{size}) @standards{ISO, stdlib.h} @safety{@prelim{}@mtunsafe{@mtasurace{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{mbtowc} (multibyte to wide character'') function when called with non-null @var{string} converts the first multibyte character beginning at @var{string} to its corresponding wide character code. It stores the result in @code{*@var{result}}. @code{mbtowc} never examines more than @var{size} bytes. (The idea is to supply for @var{size} the number of bytes of data you have in hand.) @code{mbtowc} with non-null @var{string} distinguishes three possibilities: the first @var{size} bytes at @var{string} start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or @var{string} points to an empty string (a null character). For a valid multibyte character, @code{mbtowc} converts it to a wide character and stores that in @code{*@var{result}}, and returns the number of bytes in that character (always at least @math{1} and never more than @var{size}). For an invalid byte sequence, @code{mbtowc} returns @math{-1}. For an empty string, it returns @math{0}, also storing @code{'\0'} in @code{*@var{result}}. If the multibyte character code uses shift characters, then @code{mbtowc} maintains and updates a shift state as it scans. If you call @code{mbtowc} with a null pointer for @var{string}, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. @xref{Shift State}. @end deftypefun @deftypefun int wctomb (char *@var{string}, wchar_t @var{wchar}) @standards{ISO, stdlib.h} @safety{@prelim{}@mtunsafe{@mtasurace{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{wctomb} (wide character to multibyte'') function converts the wide character code @var{wchar} to its corresponding multibyte character sequence, and stores the result in bytes starting at @var{string}. At most @code{MB_CUR_MAX} characters are stored. @code{wctomb} with non-null @var{string} distinguishes three possibilities for @var{wchar}: a valid wide character code (one that can be translated to a multibyte character), an invalid code, and @code{L'\0'}. Given a valid code, @code{wctomb} converts it to a multibyte character, storing the bytes starting at @var{string}. Then it returns the number of bytes in that character (always at least @math{1} and never more than @code{MB_CUR_MAX}). If @var{wchar} is an invalid wide character code, @code{wctomb} returns @math{-1}. If @var{wchar} is @code{L'\0'}, it returns @code{0}, also storing @code{'\0'} in @code{*@var{string}}. If the multibyte character code uses shift characters, then @code{wctomb} maintains and updates a shift state as it scans. If you call @code{wctomb} with a null pointer for @var{string}, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. @xref{Shift State}. Calling this function with a @var{wchar} argument of zero when @var{string} is not null has the side-effect of reinitializing the stored shift state @emph{as well as} storing the multibyte character @code{'\0'} and returning @math{0}. @end deftypefun Similar to @code{mbrlen} there is also a non-reentrant function that computes the length of a multibyte character. It can be defined in terms of @code{mbtowc}. @deftypefun int mblen (const char *@var{string}, size_t @var{size}) @standards{ISO, stdlib.h} @safety{@prelim{}@mtunsafe{@mtasurace{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{mblen} function with a non-null @var{string} argument returns the number of bytes that make up the multibyte character beginning at @var{string}, never examining more than @var{size} bytes. (The idea is to supply for @var{size} the number of bytes of data you have in hand.) The return value of @code{mblen} distinguishes three possibilities: the first @var{size} bytes at @var{string} start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or @var{string} points to an empty string (a null character). For a valid multibyte character, @code{mblen} returns the number of bytes in that character (always at least @code{1} and never more than @var{size}). For an invalid byte sequence, @code{mblen} returns @math{-1}. For an empty string, it returns @math{0}. If the multibyte character code uses shift characters, then @code{mblen} maintains and updates a shift state as it scans. If you call @code{mblen} with a null pointer for @var{string}, that initializes the shift state to its standard initial value. It also returns a nonzero value if the multibyte character code in use actually has a shift state. @xref{Shift State}. @pindex stdlib.h The function @code{mblen} is declared in @file{stdlib.h}. @end deftypefun @node Non-reentrant String Conversion @subsection Non-reentrant Conversion of Strings For convenience the @w{ISO C90} standard also defines functions to convert entire strings instead of single characters. These functions suffer from the same problems as their reentrant counterparts from @w{Amendment 1} to @w{ISO C90}; see @ref{Converting Strings}. @deftypefun size_t mbstowcs (wchar_t *@var{wstring}, const char *@var{string}, size_t @var{size}) @standards{ISO, stdlib.h} @safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} @c Odd... Although this was supposed to be non-reentrant, the internal @c state is not a static buffer, but an automatic variable. The @code{mbstowcs} (multibyte string to wide character string'') function converts the null-terminated string of multibyte characters @var{string} to an array of wide character codes, storing not more than @var{size} wide characters into the array beginning at @var{wstring}. The terminating null character counts towards the size, so if @var{size} is less than the actual number of wide characters resulting from @var{string}, no terminating null character is stored. The conversion of characters from @var{string} begins in the initial shift state. If an invalid multibyte character sequence is found, the @code{mbstowcs} function returns a value of @math{-1}. Otherwise, it returns the number of wide characters stored in the array @var{wstring}. This number does not include the terminating null character, which is present if the number is less than @var{size}. Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result. @smallexample wchar_t * mbstowcs_alloc (const char *string) @{ size_t size = strlen (string) + 1; wchar_t *buf = xmalloc (size * sizeof (wchar_t)); size = mbstowcs (buf, string, size); if (size == (size_t) -1) return NULL; buf = xrealloc (buf, (size + 1) * sizeof (wchar_t)); return buf; @} @end smallexample @end deftypefun @deftypefun size_t wcstombs (char *@var{string}, const wchar_t *@var{wstring}, size_t @var{size}) @standards{ISO, stdlib.h} @safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} The @code{wcstombs} (wide character string to multibyte string'') function converts the null-terminated wide character array @var{wstring} into a string containing multibyte characters, storing not more than @var{size} bytes starting at @var{string}, followed by a terminating null character if there is room. The conversion of characters begins in the initial shift state. The terminating null character counts towards the size, so if @var{size} is less than or equal to the number of bytes needed in @var{wstring}, no terminating null character is stored. If a code that does not correspond to a valid multibyte character is found, the @code{wcstombs} function returns a value of @math{-1}. Otherwise, the return value is the number of bytes stored in the array @var{string}. This number does not include the terminating null character, which is present if the number is less than @var{size}. @end deftypefun @node Shift State @subsection States in Non-reentrant Functions In some multibyte character codes, the @emph{meaning} of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string. Typically there are just a few sequences that can change the meaning of other sequences; these few are called @dfn{shift sequences} and we say that they set the @dfn{shift state} for other sequences that follow. To illustrate shift state and shift sequences, suppose we decide that the sequence @code{0200} (just one byte) enters Japanese mode, in which pairs of bytes in the range from @code{0240} to @code{0377} are single characters, while @code{0201} enters Latin-1 mode, in which single bytes in the range from @code{0240} to @code{0377} are characters, and interpreted according to the ISO Latin-1 character set. This is a multibyte code that has two alternative shift states (Japanese mode'' and Latin-1 mode''), and two shift sequences that specify particular shift states. When the multibyte character code in use has shift states, then @code{mblen}, @code{mbtowc}, and @code{wctomb} must maintain and update the current shift state as they scan the string. To make this work properly, you must follow these rules: @itemize @bullet @item Before starting to scan a string, call the function with a null pointer for the multibyte character address---for example, @code{mblen (NULL, 0)}. This initializes the shift state to its standard initial value. @item Scan the string one character at a time, in order. Do not back up'' and rescan characters already scanned, and do not intersperse the processing of different strings. @end itemize Here is an example of using @code{mblen} following these rules: @smallexample void scan_string (char *s) @{ int length = strlen (s); /* @r{Initialize shift state.} */ mblen (NULL, 0); while (1) @{ int thischar = mblen (s, length); /* @r{Deal with end of string and invalid characters.} */ if (thischar == 0) break; if (thischar == -1) @{ error ("invalid multibyte character"); break; @} /* @r{Advance past this character.} */ s += thischar; length -= thischar; @} @} @end smallexample The functions @code{mblen}, @code{mbtowc} and @code{wctomb} are not reentrant when using a multibyte code that uses a shift state. However, no other library functions call these functions, so you don't have to worry that the shift state will be changed mysteriously. @node Generic Charset Conversion @section Generic Charset Conversion The conversion functions mentioned so far in this chapter all had in common that they operate on character sets that are not directly specified by the functions. The multibyte encoding used is specified by the currently selected locale for the @code{LC_CTYPE} category. The wide character set is fixed by the implementation (in the case of @theglibc{} it is always UCS-4 encoded @w{ISO 10646}). This has of course several problems when it comes to general character conversion: @itemize @bullet @item For every conversion where neither the source nor the destination character set is the character set of the locale for the @code{LC_CTYPE} category, one has to change the @code{LC_CTYPE} locale using @code{setlocale}. Changing the @code{LC_CTYPE} locale introduces major problems for the rest of the programs since several more functions (e.g., the character classification functions, @pxref{Classification of Characters}) use the @code{LC_CTYPE} category. @item Parallel conversions to and from different character sets are not possible since the @code{LC_CTYPE} selection is global and shared by all threads. @item If neither the source nor the destination character set is the character set used for @code{wchar_t} representation, there is at least a two-step process necessary to convert a text using the functions above. One would have to select the source character set as the multibyte encoding, convert the text into a @code{wchar_t} text, select the destination character set as the multibyte encoding, and convert the wide character text to the multibyte (@math{=} destination) character set. Even if this is possible (which is not guaranteed) it is a very tiring work. Plus it suffers from the other two raised points even more due to the steady changing of the locale. @end itemize The XPG2 standard defines a completely new set of functions, which has none of these limitations. They are not at all coupled to the selected locales, and they have no constraints on the character sets selected for source and destination. Only the set of available conversions limits them. The standard does not specify that any conversion at all must be available. Such availability is a measure of the quality of the implementation. In the following text first the interface to @code{iconv} and then the conversion function, will be described. Comparisons with other implementations will show what obstacles stand in the way of portable applications. Finally, the implementation is described in so far as might interest the advanced user who wants to extend conversion capabilities. @menu * Generic Conversion Interface:: Generic Character Set Conversion Interface. * iconv Examples:: A complete @code{iconv} example. * Other iconv Implementations:: Some Details about other @code{iconv} Implementations. * glibc iconv Implementation:: The @code{iconv} Implementation in the GNU C library. @end menu @node Generic Conversion Interface @subsection Generic Character Set Conversion Interface This set of functions follows the traditional cycle of using a resource: open--use--close. The interface consists of three functions, each of which implements one step. Before the interfaces are described it is necessary to introduce a data type. Just like other open--use--close interfaces the functions introduced here work using handles and the @file{iconv.h} header defines a special type for the handles used. @deftp {Data Type} iconv_t @standards{XPG2, iconv.h} This data type is an abstract type defined in @file{iconv.h}. The user must not assume anything about the definition of this type; it must be completely opaque. Objects of this type can be assigned handles for the conversions using the @code{iconv} functions. The objects themselves need not be freed, but the conversions for which the handles stand for have to. @end deftp @noindent The first step is the function to create a handle. @deftypefun iconv_t iconv_open (const char *@var{tocode}, const char *@var{fromcode}) @standards{XPG2, iconv.h} @safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}} @c Calls malloc if tocode and/or fromcode are too big for alloca. Calls @c strip and upstr on both, then gconv_open. strip and upstr call @c isalnum_l and toupper_l with the C locale. gconv_open may MT-safely @c tokenize toset, replace unspecified codesets with the current locale @c (possibly two different accesses), and finally it calls @c gconv_find_transform and initializes the gconv_t result with all the @c steps in the conversion sequence, running each one's initializer, @c destructing and releasing them all if anything fails. The @code{iconv_open} function has to be used before starting a conversion. The two parameters this function takes determine the source and destination character set for the conversion, and if the implementation has the possibility to perform such a conversion, the function returns a handle. If the wanted conversion is not available, the @code{iconv_open} function returns @code{(iconv_t) -1}. In this case the global variable @code{errno} can have the following values: @table @code @item EMFILE The process already has @code{OPEN_MAX} file descriptors open. @item ENFILE The system limit of open files is reached. @item ENOMEM Not enough memory to carry out the operation. @item EINVAL The conversion from @var{fromcode} to @var{tocode} is not supported. @end table It is not possible to use the same descriptor in different threads to perform independent conversions. The data structures associated with the descriptor include information about the conversion state. This must not be messed up by using it in different conversions. An @code{iconv} descriptor is like a file descriptor as for every use a new descriptor must be created. The descriptor does not stand for all of the conversions from @var{fromset} to @var{toset}. The @glibcadj{} implementation of @code{iconv_open} has one significant extension to other implementations. To ease the extension of the set of available conversions, the implementation allows storing the necessary files with data and code in an arbitrary number of directories. How this extension must be written will be explained below (@pxref{glibc iconv Implementation}). Here it is only important to say that all directories mentioned in the @code{GCONV_PATH} environment variable are considered only if they contain a file @file{gconv-modules}. These directories need not necessarily be created by the system administrator. In fact, this extension is introduced to help users writing and using their own, new conversions. Of course, this does not work for security reasons in SUID binaries; in this case only the system directory is considered and this normally is @file{@var{prefix}/lib/gconv}. The @code{GCONV_PATH} environment variable is examined exactly once at the first call of the @code{iconv_open} function. Later modifications of the variable have no effect. @pindex iconv.h The @code{iconv_open} function was introduced early in the X/Open Portability Guide, @w{version 2}. It is supported by all commercial Unices as it is required for the Unix branding. However, the quality and completeness of the implementation varies widely. The @code{iconv_open} function is declared in @file{iconv.h}. @end deftypefun The @code{iconv} implementation can associate large data structure with the handle returned by @code{iconv_open}. Therefore, it is crucial to free all the resources once all conversions are carried out and the conversion is not needed anymore. @deftypefun int iconv_close (iconv_t @var{cd}) @standards{XPG2, iconv.h} @safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{}}} @c Calls gconv_close to destruct and release each of the conversion @c steps, release the gconv_t object, then call gconv_close_transform. @c Access to the gconv_t object is not guarded, but calling iconv_close @c concurrently with any other use is undefined. The @code{iconv_close} function frees all resources associated with the handle @var{cd}, which must have been returned by a successful call to the @code{iconv_open} function. If the function call was successful the return value is @math{0}. Otherwise it is @math{-1} and @code{errno} is set appropriately. Defined errors are: @table @code @item EBADF The conversion descriptor is invalid. @end table @pindex iconv.h The @code{iconv_close} function was introduced together with the rest of the @code{iconv} functions in XPG2 and is declared in @file{iconv.h}. @end deftypefun The standard defines only one actual conversion function. This has, therefore, the most general interface: it allows conversion from one buffer to another. Conversion from a file to a buffer, vice versa, or even file to file can be implemented on top of it. @deftypefun size_t iconv (iconv_t @var{cd}, char **@var{inbuf}, size_t *@var{inbytesleft}, char **@var{outbuf}, size_t *@var{outbytesleft}) @standards{XPG2, iconv.h} @safety{@prelim{}@mtsafe{@mtsrace{:cd}}@assafe{}@acunsafe{@acucorrupt{}}} @c Without guarding access to the iconv_t object pointed to by cd, call @c the conversion function to convert inbuf or flush the internal @c conversion state. @cindex stateful The @code{iconv} function converts the text in the input buffer according to the rules associated with the descriptor @var{cd} and stores the result in the output buffer. It is possible to call the function for the same text several times in a row since for stateful character sets the necessary state information is kept in the data structures associated with the descriptor. The input buffer is specified by @code{*@var{inbuf}} and it contains @code{*@var{inbytesleft}} bytes. The extra indirection is necessary for communicating the used input back to the caller (see below). It is important to note that the buffer pointer is of type @code{char} and the length is measured in bytes even if the input text is encoded in wide characters. The output buffer is specified in a similar way. @code{*@var{outbuf}} points to the beginning of the buffer with at least @code{*@var{outbytesleft}} bytes room for the result. The buffer pointer again is of type @code{char} and the length is measured in bytes. If @var{outbuf} or @code{*@var{outbuf}} is a null pointer, the conversion is performed but no output is available. If @var{inbuf} is a null pointer, the @code{iconv} function performs the necessary action to put the state of the conversion into the initial state. This is obviously a no-op for non-stateful encodings, but if the encoding has a state, such a function call might put some byte sequences in the output buffer, which perform the necessary state changes. The next call with @var{inbuf} not being a null pointer then simply goes on from the initial state. It is important that the programmer never makes any assumption as to whether the conversion has to deal with states. Even if the input and output character sets are not stateful, the implementation might still have to keep states. This is due to the implementation chosen for @theglibc{} as it is described below. Therefore an @code{iconv} call to reset the state should always be performed if some protocol requires this for the output text. The conversion stops for one of three reasons. The first is that all characters from the input buffer are converted. This actually can mean two things: either all bytes from the input buffer are consumed or there are some bytes at the end of the buffer that possibly can form a complete character but the input is incomplete. The second reason for a stop is that the output buffer is full. And the third reason is that the input contains invalid characters. In all of these cases the buffer pointers after the last successful conversion, for the input and output buffers, are stored in @var{inbuf} and @var{outbuf}, and the available room in each buffer is stored in @var{inbytesleft} and @var{outbytesleft}. Since the character sets selected in the @code{iconv_open} call can be almost arbitrary, there can be situations where the input buffer contains valid characters, which have no identical representation in the output character set. The behavior in this situation is undefined. The @emph{current} behavior of @theglibc{} in this situation is to return with an error immediately. This certainly is not the most desirable solution; therefore, future versions will provide better ones, but they are not yet finished. If all input from the input buffer is successfully converted and stored in the output buffer, the function returns the number of non-reversible conversions performed. In all other cases the return value is @code{(size_t) -1} and @code{errno} is set appropriately. In such cases the value pointed to by @var{inbytesleft} is nonzero. @table @code @item EILSEQ The conversion stopped because of an invalid byte sequence in the input. After the call, @code{*@var{inbuf}} points at the first byte of the invalid byte sequence. @item E2BIG The conversion stopped because it ran out of space in the output buffer. @item EINVAL The conversion stopped because of an incomplete byte sequence at the end of the input buffer. @item EBADF The @var{cd} argument is invalid. @end table @pindex iconv.h The @code{iconv} function was introduced in the XPG2 standard and is declared in the @file{iconv.h} header. @end deftypefun The definition of the @code{iconv} function is quite good overall. It provides quite flexible functionality. The only problems lie in the boundary cases, which are incomplete byte sequences at the end of the input buffer and invalid input. A third problem, which is not really a design problem, is the way conversions are selected. The standard does not say anything about the legitimate names, a minimal set of available conversions. We will see how this negatively impacts other implementations, as demonstrated below. @node iconv Examples @subsection A complete @code{iconv} example The example below features a solution for a common problem. Given that one knows the internal encoding used by the system for @code{wchar_t} strings, one often is in the position to read text from a file and store it in wide character buffers. One can do this using @code{mbsrtowcs}, but then we run into the problems discussed above. @smallexample int file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail) @{ char inbuf[BUFSIZ]; size_t insize = 0; char *wrptr = (char *) outbuf; int result = 0; iconv_t cd; cd = iconv_open ("WCHAR_T", charset); if (cd == (iconv_t) -1) @{ /* @r{Something went wrong.} */ if (errno == EINVAL) error (0, 0, "conversion from '%s' to wchar_t not available", charset); else perror ("iconv_open"); /* @r{Terminate the output string.} */ *outbuf = L'\0'; return -1; @} while (avail > 0) @{ size_t nread; size_t nconv; char *inptr = inbuf; /* @r{Read more input.} */ nread = read (fd, inbuf + insize, sizeof (inbuf) - insize); if (nread == 0) @{ /* @r{When we come here the file is completely read.} @r{This still could mean there are some unused} @r{characters in the @code{inbuf}. Put them back.} */ if (lseek (fd, -insize, SEEK_CUR) == -1) result = -1; /* @r{Now write out the byte sequence to get into the} @r{initial state if this is necessary.} */ iconv (cd, NULL, NULL, &wrptr, &avail); break; @} insize += nread; /* @r{Do the conversion.} */ nconv = iconv (cd, &inptr, &insize, &wrptr, &avail); if (nconv == (size_t) -1) @{ /* @r{Not everything went right. It might only be} @r{an unfinished byte sequence at the end of the} @r{buffer. Or it is a real problem.} */ if (errno == EINVAL) /* @r{This is harmless. Simply move the unused} @r{bytes to the beginning of the buffer so that} @r{they can be used in the next round.} */ memmove (inbuf, inptr, insize); else @{ /* @r{It is a real problem. Maybe we ran out of} @r{space in the output buffer or we have invalid} @r{input. In any case back the file pointer to} @r{the position of the last processed byte.} */ lseek (fd, -insize, SEEK_CUR); result = -1; break; @} @} @} /* @r{Terminate the output string.} */ if (avail >= sizeof (wchar_t)) *((wchar_t *) wrptr) = L'\0'; if (iconv_close (cd) != 0) perror ("iconv_close"); return (wchar_t *) wrptr - outbuf; @} @end smallexample @cindex stateful This example shows the most important aspects of using the @code{iconv} functions. It shows how successive calls to @code{iconv} can be used to convert large amounts of text. The user does not have to care about stateful encodings as the functions take care of everything. An interesting point is the case where @code{iconv} returns an error and @code{errno} is set to @code{EINVAL}. This is not really an error in the transformation. It can happen whenever the input character set contains byte sequences of more than one byte for some character and texts are not processed in one piece. In this case there is a chance that a multibyte sequence is cut. The caller can then simply read the remainder of the takes and feed the offending bytes together with new character from the input to @code{iconv} and continue the work. The internal state kept in the descriptor is @emph{not} unspecified after such an event as is the case with the conversion functions from the @w{ISO C} standard. The example also shows the problem of using wide character strings with @code{iconv}. As explained in the description of the @code{iconv} function above, the function always takes a pointer to a @code{char} array and the available space is measured in bytes. In the example, the output buffer is a wide character buffer; therefore, we use a local variable @var{wrptr} of type @code{char *}, which is used in the @code{iconv} calls. This looks rather innocent but can lead to problems on platforms that have tight restriction on alignment. Therefore the caller of @code{iconv} has to make sure that the pointers passed are suitable for access of characters from the appropriate character set. Since, in the above case, the input parameter to the function is a @code{wchar_t} pointer, this is the case (unless the user violates alignment when computing the parameter). But in other situations, especially when writing generic functions where one does not know what type of character set one uses and, therefore, treats text as a sequence of bytes, it might become tricky. @node Other iconv Implementations @subsection Some Details about other @code{iconv} Implementations This is not really the place to discuss the @code{iconv} implementation of other systems but it is necessary to know a bit about them to write portable programs. The above mentioned problems with the specification of the @code{iconv} functions can lead to portability issues. The first thing to notice is that, due to the large number of character sets in use, it is certainly not practical to encode the conversions directly in the C library. Therefore, the conversion information must come from files outside the C library. This is usually done in one or both of the following ways: @itemize @bullet @item The C library contains a set of generic conversion functions that can read the needed conversion tables and other information from data files. These files get loaded when necessary. This solution is problematic as it requires a great deal of effort to apply to all character sets (potentially an infinite set). The differences in the structure of the different character sets is so large that many different variants of the table-processing functions must be developed. In addition, the generic nature of these functions make them slower than specifically implemented functions. @item The C library only contains a framework that can dynamically load object files and execute the conversion functions contained therein. This solution provides much more flexibility. The C library itself contains only very little code and therefore reduces the general memory footprint. Also, with a documented interface between the C library and the loadable modules it is possible for third parties to extend the set of available conversion modules. A drawback of this solution is that dynamic loading must be available. @end itemize Some implementations in commercial Unices implement a mixture of these possibilities; the majority implement only the second solution. Using loadable modules moves the code out of the library itself and keeps the door open for extensions and improvements, but this design is also limiting on some platforms since not many platforms support dynamic loading in statically linked programs. On platforms without this capability it is therefore not possible to use this interface in statically linked programs. @Theglibc{} has, on ELF platforms, no problems with dynamic loading in these situations; therefore, this point is moot. The danger is that one gets acquainted with this situation and forgets about the restrictions on other systems. A second thing to know about other @code{iconv} implementations is that the number of available conversions is often very limited. Some implementations provide, in the standard release (not special international or developer releases), at most 100 to 200 conversion possibilities. This does not mean 200 different character sets are supported; for example, conversions from one character set to a set of 10 others might count as 10 conversions. Together with the other direction this makes 20 conversion possibilities used up by one character set. One can imagine the thin coverage these platforms provide. Some Unix vendors even provide only a handful of conversions, which renders them useless for almost all uses. This directly leads to a third and probably the most problematic point. The way the @code{iconv} conversion functions are implemented on all known Unix systems and the availability of the conversion functions from character set @math{@cal{A}} to @math{@cal{B}} and the conversion from @math{@cal{B}} to @math{@cal{C}} does @emph{not} imply that the conversion from @math{@cal{A}} to @math{@cal{C}} is available. This might not seem unreasonable and problematic at first, but it is a quite big problem as one will notice shortly after hitting it. To show the problem we assume to write a program that has to convert from @math{@cal{A}} to @math{@cal{C}}. A call like @smallexample cd = iconv_open ("@math{@cal{C}}", "@math{@cal{A}}"); @end smallexample @noindent fails according to the assumption above. But what does the program do now? The conversion is necessary; therefore, simply giving up is not an option. This is a nuisance. The @code{iconv} function should take care of this. But how should the program proceed from here on? If it tries to convert to character set @math{@cal{B}}, first the two @code{iconv_open} calls @smallexample cd1 = iconv_open ("@math{@cal{B}}", "@math{@cal{A}}"); @end smallexample @noindent and @smallexample cd2 = iconv_open ("@math{@cal{C}}", "@math{@cal{B}}"); @end smallexample @noindent will succeed, but how to find @math{@cal{B}}? Unfortunately, the answer is: there is no general solution. On some systems guessing might help. On those systems most character sets can convert to and from UTF-8 encoded @w{ISO 10646} or Unicode text. Besides this only some very system-specific methods can help. Since the conversion functions come from loadable modules and these modules must be stored somewhere in the filesystem, one @emph{could} try to find them and determine from the available file which conversions are available and whether there is an indirect route from @math{@cal{A}} to @math{@cal{C}}. This example shows one of the design errors of @code{iconv} mentioned above. It should at least be possible to determine the list of available conversions programmatically so that if @code{iconv_open} says there is no such conversion, one could make sure this also is true for indirect routes. @node glibc iconv Implementation @subsection The @code{iconv} Implementation in @theglibc{} After reading about the problems of @code{iconv} implementations in the last section it is certainly good to note that the implementation in @theglibc{} has none of the problems mentioned above. What follows is a step-by-step analysis of the points raised above. The evaluation is based on the current state of the development (as of January 1999). The development of the @code{iconv} functions is not complete, but basic functionality has solidified. @Theglibc{}'s @code{iconv} implementation uses shared loadable modules to implement the conversions. A very small number of conversions are built into the library itself but these are only rather trivial conversions. All the benefits of loadable modules are available in the @glibcadj{} implementation. This is especially appealing since the interface is well documented (see below), and it, therefore, is easy to write new conversion modules. The drawback of using loadable objects is not a problem in @theglibc{}, at least on ELF systems. Since the library is able to load shared objects even in statically linked binaries, static linking need not be forbidden in case one wants to use @code{iconv}. The second mentioned problem is the number of supported conversions. Currently, @theglibc{} supports more than 150 character sets. The way the implementation is designed the number of supported conversions is greater than 22350 (@math{150} times @math{149}). If any conversion from or to a character set is missing, it can be added easily. Particularly impressive as it may be, this high number is due to the fact that the @glibcadj{} implementation of @code{iconv} does not have the third problem mentioned above (i.e., whenever there is a conversion from a character set @math{@cal{A}} to @math{@cal{B}} and from @math{@cal{B}} to @math{@cal{C}} it is always possible to convert from @math{@cal{A}} to @math{@cal{C}} directly). If the @code{iconv_open} returns an error and sets @code{errno} to @code{EINVAL}, there is no known way, directly or indirectly, to perform the wanted conversion. @cindex triangulation Triangulation is achieved by providing for each character set a conversion from and to UCS-4 encoded @w{ISO 10646}. Using @w{ISO 10646} as an intermediate representation it is possible to @dfn{triangulate} (i.e., convert with an intermediate representation). There is no inherent requirement to provide a conversion to @w{ISO 10646} for a new character set, and it is also possible to provide other conversions where neither source nor destination character set is @w{ISO 10646}. The existing set of conversions is simply meant to cover all conversions that might be of interest. @cindex ISO-2022-JP @cindex EUC-JP All currently available conversions use the triangulation method above, making conversion run unnecessarily slow. If, for example, somebody often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution would involve direct conversion between the two character sets, skipping the input to @w{ISO 10646} first. The two character sets of interest are much more similar to each other than to @w{ISO 10646}. In such a situation one easily can write a new conversion and provide it as a better alternative. The @glibcadj{} @code{iconv} implementation would automatically use the module implementing the conversion if it is specified to be more efficient. @subsubsection Format of @file{gconv-modules} files All information about the available conversions comes from a file named @file{gconv-modules}, which can be found in any of the directories along the @code{GCONV_PATH}. The @file{gconv-modules} files are line-oriented text files, where each of the lines has one of the following formats: @itemize @bullet @item If the first non-whitespace character is a @kbd{#} the line contains only comments and is ignored. @item Lines starting with @code{alias} define an alias name for a character set. Two more words are expected on the line. The first word defines the alias name, and the second defines the original name of the character set. The effect is that it is possible to use the alias name in the @var{fromset} or @var{toset} parameters of @code{iconv_open} and achieve the same result as when using the real character set name. This is quite important as a character set has often many different names. There is normally an official name but this need not correspond to the most popular name. Besides this many character sets have special names that are somehow constructed. For example, all character sets specified by the ISO have an alias of the form @code{ISO-IR-@var{nnn}} where @var{nnn} is the registration number. This allows programs that know about the registration number to construct character set names and use them in @code{iconv_open} calls. More on the available names and aliases follows below. @item Lines starting with @code{module} introduce an available conversion module. These lines must contain three or four more words. The first word specifies the source character set, the second word the destination character set of conversion implemented in this module, and the third word is the name of the loadable module. The filename is constructed by appending the usual shared object suffix (normally @file{.so}) and this file is then supposed to be found in the same directory the @file{gconv-modules} file is in. The last word on the line, which is optional, is a numeric value representing the cost of the conversion. If this word is missing, a cost of @math{1} is assumed. The numeric value itself does not matter that much; what counts are the relative values of the sums of costs for all possible conversion paths. Below is a more precise description of the use of the cost value. @end itemize Returning to the example above where one has written a module to directly convert from ISO-2022-JP to EUC-JP and back. All that has to be done is to put the new module, let its name be ISO2022JP-EUCJP.so, in a directory and add a file @file{gconv-modules} with the following content in the same directory: @smallexample module ISO-2022-JP// EUC-JP// ISO2022JP-EUCJP 1 module EUC-JP// ISO-2022-JP// ISO2022JP-EUCJP 1 @end smallexample To see why this is sufficient, it is necessary to understand how the conversion used by @code{iconv} (and described in the descriptor) is selected. The approach to this problem is quite simple. At the first call of the @code{iconv_open} function the program reads all available @file{gconv-modules} files and builds up two tables: one containing all the known aliases and another that contains the information about the conversions and which shared object implements them. @subsubsection Finding the conversion path in @code{iconv} The set of available conversions form a directed graph with weighted edges. The weights on the edges are the costs specified in the @file{gconv-modules} files. The @code{iconv_open} function uses an algorithm suitable for search for the best path in such a graph and so constructs a list of conversions that must be performed in succession to get the transformation from the source to the destination character set. Explaining why the above @file{gconv-modules} files allows the @code{iconv} implementation to resolve the specific ISO-2022-JP to EUC-JP conversion module instead of the conversion coming with the library itself is straightforward. Since the latter conversion takes two steps (from ISO-2022-JP to @w{ISO 10646} and then from @w{ISO 10646} to EUC-JP), the cost is @math{1+1 = 2}. The above @file{gconv-modules} file, however, specifies that the new conversion modules can perform this conversion with only the cost of @math{1}. A mysterious item about the @file{gconv-modules} file above (and also the file coming with @theglibc{}) are the names of the character sets specified in the @code{module} lines. Why do almost all the names end in @code{//}? And this is not all: the names can actually be regular expressions. At this point in time this mystery should not be revealed, unless you have the relevant spell-casting materials: ashes from an original @w{DOS 6.2} boot disk burnt in effigy, a crucifix blessed by St.@: Emacs, assorted herbal roots from Central America, sand from Cebu, etc. Sorry! @strong{The part of the implementation where this is used is not yet finished. For now please simply follow the existing examples. It'll become clearer once it is. --drepper} A last remark about the @file{gconv-modules} is about the names not ending with @code{//}. A character set named @code{INTERNAL} is often mentioned. From the discussion above and the chosen name it should have become clear that this is the name for the representation used in the intermediate step of the triangulation. We have said that this is UCS-4 but actually that is not quite right. The UCS-4 specification also includes the specification of the byte ordering used. Since a UCS-4 value consists of four bytes, a stored value is affected by byte ordering. The internal representation is @emph{not} the same as UCS-4 in case the byte ordering of the processor (or at least the running process) is not the same as the one required for UCS-4. This is done for performance reasons as one does not want to perform unnecessary byte-swapping operations if one is not interested in actually seeing the result in UCS-4. To avoid trouble with endianness, the internal representation consistently is named @code{INTERNAL} even on big-endian systems where the representations are identical. @subsubsection @code{iconv} module data structures So far this section has described how modules are located and considered to be used. What remains to be described is the interface of the modules so that one can write new ones. This section describes the interface as it is in use in January 1999. The interface will change a bit in the future but, with luck, only in an upwardly compatible way. The definitions necessary to write new modules are publicly available in the non-standard header @file{gconv.h}. The following text, therefore, describes the definitions from this header file. First, however, it is necessary to get an overview. From the perspective of the user of @code{iconv} the interface is quite simple: the @code{iconv_open} function returns a handle that can be used in calls to @code{iconv}, and finally the handle is freed with a call to @code{iconv_close}. The problem is that the handle has to be able to represent the possibly long sequences of conversion steps and also the state of each conversion since the handle is all that is passed to the @code{iconv} function. Therefore, the data structures are really the elements necessary to understanding the implementation. We need two different kinds of data structures. The first describes the conversion and the second describes the state etc. There are really two type definitions like this in @file{gconv.h}. @pindex gconv.h @deftp {Data type} {struct __gconv_step} @standards{GNU, gconv.h} This data structure describes one conversion a module can perform. For each function in a loaded module with conversion functions there is exactly one object of this type. This object is shared by all users of the conversion (i.e., this object does not contain any information corresponding to an actual conversion; it only describes the conversion itself). @table @code @item struct __gconv_loaded_object *__shlib_handle @itemx const char *__modname @itemx int __counter All these elements of the structure are used internally in the C library to coordinate loading and unloading the shared object. One must not expect any of the other elements to be available or initialized. @item const char *__from_name @itemx const char *__to_name @code{__from_name} and @code{__to_name} contain the names of the source and destination character sets. They can be used to identify the actual conversion to be carried out since one module might implement conversions for more than one character set and/or direction. @item gconv_fct __fct @itemx gconv_init_fct __init_fct @itemx gconv_end_fct __end_fct These elements contain pointers to the functions in the loadable module. The interface will be explained below. @item int __min_needed_from @itemx int __max_needed_from @itemx int __min_needed_to @itemx int __max_needed_to; These values have to be supplied in the init function of the module. The @code{__min_needed_from} value specifies how many bytes a character of the source character set at least needs. The @code{__max_needed_from} specifies the maximum value that also includes possible shift sequences. The @code{__min_needed_to} and @code{__max_needed_to} values serve the same purpose as @code{__min_needed_from} and @code{__max_needed_from} but this time for the destination character set. It is crucial that these values be accurate since otherwise the conversion functions will have problems or not work at all. @item int __stateful This element must also be initialized by the init function. @code{int __stateful} is nonzero if the source character set is stateful. Otherwise it is zero. @item void *__data This element can be used freely by the conversion functions in the module. @code{void *__data} can be used to communicate extra information from one call to another. @code{void *__data} need not be initialized if not needed at all. If @code{void *__data} element is assigned a pointer to dynamically allocated memory (presumably in the init function) it has to be made sure that the end function deallocates the memory. Otherwise the application will leak memory. It is important to be aware that this data structure is shared by all users of this specification conversion and therefore the @code{__data} element must not contain data specific to one specific use of the conversion function. @end table @end deftp @deftp {Data type} {struct __gconv_step_data} @standards{GNU, gconv.h} This is the data structure that contains the information specific to each use of the conversion functions. @table @code @item char *__outbuf @itemx char *__outbufend These elements specify the output buffer for the conversion step. The @code{__outbuf} element points to the beginning of the buffer, and @code{__outbufend} points to the byte following the last byte in the buffer. The conversion function must not assume anything about the size of the buffer but it can be safely assumed there is room for at least one complete character in the output buffer. Once the conversion is finished, if the conversion is the last step, the @code{__outbuf} element must be modified to point after the last byte written into the buffer to signal how much output is available. If this conversion step is not the last one, the element must not be modified. The @code{__outbufend} element must not be modified. @item int __is_last This element is nonzero if this conversion step is the last one. This information is necessary for the recursion. See the description of the conversion function internals below. This element must never be modified. @item int __invocation_counter The conversion function can use this element to see how many calls of the conversion function already happened. Some character sets require a certain prolog when generating output, and by comparing this value with zero, one can find out whether it is the first call and whether, therefore, the prolog should be emitted. This element must never be modified. @item int __internal_use This element is another one rarely used but needed in certain situations. It is assigned a nonzero value in case the conversion functions are used to implement @code{mbsrtowcs} et.al.@: (i.e., the function is not used directly through the @code{iconv} interface). This sometimes makes a difference as it is expected that the @code{iconv} functions are used to translate entire texts while the @code{mbsrtowcs} functions are normally used only to convert single strings and might be used multiple times to convert entire texts. But in this situation we would have problem complying with some rules of the character set specification. Some character sets require a prolog, which must appear exactly once for an entire text. If a number of @code{mbsrtowcs} calls are used to convert the text, only the first call must add the prolog. However, because there is no communication between the different calls of @code{mbsrtowcs}, the conversion functions have no possibility to find this out. The situation is different for sequences of @code{iconv} calls since the handle allows access to the needed information. The @code{int __internal_use} element is mostly used together with @code{__invocation_counter} as follows: @smallexample if (!data->__internal_use && data->__invocation_counter == 0) /* @r{Emit prolog.} */ @dots{} @end smallexample This element must never be modified. @item mbstate_t *__statep The @code{__statep} element points to an object of type @code{mbstate_t} (@pxref{Keeping the state}). The conversion of a stateful character set must use the object pointed to by @code{__statep} to store information about the conversion state. The @code{__statep} element itself must never be modified. @item mbstate_t __state This element must @emph{never} be used directly. It is only part of this structure to have the needed space allocated. @end table @end deftp @subsubsection @code{iconv} module interfaces With the knowledge about the data structures we now can describe the conversion function itself. To understand the interface a bit of knowledge is necessary about the functionality in the C library that loads the objects with the conversions. It is often the case that one conversion is used more than once (i.e., there are several @code{iconv_open} calls for the same set of character sets during one program run). The @code{mbsrtowcs} et.al.@: functions in @theglibc{} also use the @code{iconv} functionality, which increases the number of uses of the same functions even more. Because of this multiple use of conversions, the modules do not get loaded exclusively for one conversion. Instead a module once loaded can be used by an arbitrary number of @code{iconv} or @code{mbsrtowcs} calls at the same time. The splitting of the information between conversion- function-specific information and conversion data makes this possible. The last section showed the two data structures used to do this. This is of course also reflected in the interface and semantics of the functions that the modules must provide. There are three functions that must have the following names: @table @code @item gconv_init The @code{gconv_init} function initializes the conversion function specific data structure. This very same object is shared by all conversions that use this conversion and, therefore, no state information about the conversion itself must be stored in here. If a module implements more than one conversion, the @code{gconv_init} function will be called multiple times. @item gconv_end The @code{gconv_end} function is responsible for freeing all resources allocated by the @code{gconv_init} function. If there is nothing to do, this function can be missing. Special care must be taken if the module implements more than one conversion and the @code{gconv_init} function does not allocate the same resources for all conversions. @item gconv This is the actual conversion function. It is called to convert one block of text. It gets passed the conversion step information initialized by @code{gconv_init} and the conversion data, specific to this use of the conversion functions. @end table There are three data types defined for the three module interface functions and these define the interface. @deftypevr {Data type} int {(*__gconv_init_fct)} (struct __gconv_step *) @standards{GNU, gconv.h} This specifies the interface of the initialization function of the module. It is called exactly once for each conversion the module implements. As explained in the description of the @code{struct __gconv_step} data structure above the initialization function has to initialize parts of it. @table @code @item __min_needed_from @itemx __max_needed_from @itemx __min_needed_to @itemx __max_needed_to These elements must be initialized to the exact numbers of the minimum and maximum number of bytes used by one character in the source and destination character sets, respectively. If the characters all have the same size, the minimum and maximum values are the same. @item __stateful This element must be initialized to a nonzero value if the source character set is stateful. Otherwise it must be zero. @end table If the initialization function needs to communicate some information to the conversion function, this communication can happen using the @code{__data} element of the @code{__gconv_step} structure. But since this data is shared by all the conversions, it must not be modified by the conversion function. The example below shows how this can be used. @smallexample #define MIN_NEEDED_FROM 1 #define MAX_NEEDED_FROM 4 #define MIN_NEEDED_TO 4 #define MAX_NEEDED_TO 4 int gconv_init (struct __gconv_step *step) @{ /* @r{Determine which direction.} */ struct iso2022jp_data *new_data; enum direction dir = illegal_dir; enum variant var = illegal_var; int result; if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0) @{ dir = from_iso2022jp; var = iso2022jp; @} else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0) @{ dir = to_iso2022jp; var = iso2022jp; @} else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0) @{ dir = from_iso2022jp; var = iso2022jp2; @} else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0) @{ dir = to_iso2022jp; var = iso2022jp2; @} result = __GCONV_NOCONV; if (dir != illegal_dir) @{ new_data = (struct iso2022jp_data *) malloc (sizeof (struct iso2022jp_data)); result = __GCONV_NOMEM; if (new_data != NULL) @{ new_data->dir = dir; new_data->var = var; step->__data = new_data; if (dir == from_iso2022jp) @{ step->__min_needed_from = MIN_NEEDED_FROM; step->__max_needed_from = MAX_NEEDED_FROM; step->__min_needed_to = MIN_NEEDED_TO; step->__max_needed_to = MAX_NEEDED_TO; @} else @{ step->__min_needed_from = MIN_NEEDED_TO; step->__max_needed_from = MAX_NEEDED_TO; step->__min_needed_to = MIN_NEEDED_FROM; step->__max_needed_to = MAX_NEEDED_FROM + 2; @} /* @r{Yes, this is a stateful encoding.} */ step->__stateful = 1; result = __GCONV_OK; @} @} return result; @} @end smallexample The function first checks which conversion is wanted. The module from which this function is taken implements four different conversions; which one is selected can be determined by comparing the names. The comparison should always be done without paying attention to the case. Next, a data structure, which contains the necessary information about which conversion is selected, is allocated. The data structure @code{struct iso2022jp_data} is locally defined since, outside the module, this data is not used at all. Please note that if all four conversions this module supports are requested there are four data blocks. One interesting thing is the initialization of the @code{__min_} and @code{__max_} elements of the step data object. A single ISO-2022-JP character can consist of one to four bytes. Therefore the @code{MIN_NEEDED_FROM} and @code{MAX_NEEDED_FROM} macros are defined this way. The output is always the @code{INTERNAL} character set (aka UCS-4) and therefore each character consists of exactly four bytes. For the conversion from @code{INTERNAL} to ISO-2022-JP we have to take into account that escape sequences might be necessary to switch the character sets. Therefore the @code{__max_needed_to} element for this direction gets assigned @code{MAX_NEEDED_FROM + 2}. This takes into account the two bytes needed for the escape sequences to signal the switching. The asymmetry in the maximum values for the two directions can be explained easily: when reading ISO-2022-JP text, escape sequences can be handled alone (i.e., it is not necessary to process a real character since the effect of the escape sequence can be recorded in the state information). The situation is different for the other direction. Since it is in general not known which character comes next, one cannot emit escape sequences to change the state in advance. This means the escape sequences have to be emitted together with the next character. Therefore one needs more room than only for the character itself. The possible return values of the initialization function are: @table @code @item __GCONV_OK The initialization succeeded @item __GCONV_NOCONV The requested conversion is not supported in the module. This can happen if the @file{gconv-modules} file has errors. @item __GCONV_NOMEM Memory required to store additional information could not be allocated. @end table @end deftypevr The function called before the module is unloaded is significantly easier. It often has nothing at all to do; in which case it can be left out completely. @deftypevr {Data type} void {(*__gconv_end_fct)} (struct gconv_step *) @standards{GNU, gconv.h} The task of this function is to free all resources allocated in the initialization function. Therefore only the @code{__data} element of the object pointed to by the argument is of interest. Continuing the example from the initialization function, the finalization function looks like this: @smallexample void gconv_end (struct __gconv_step *data) @{ free (data->__data); @} @end smallexample @end deftypevr The most important function is the conversion function itself, which can get quite complicated for complex character sets. But since this is not of interest here, we will only describe a possible skeleton for the conversion function. @deftypevr {Data type} int {(*__gconv_fct)} (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int) @standards{GNU, gconv.h} The conversion function can be called for two basic reasons: to convert text or to reset the state. From the description of the @code{iconv} function it can be seen why the flushing mode is necessary. What mode is selected is determined by the sixth argument, an integer. This argument being nonzero means that flushing is selected. Common to both modes is where the output buffer can be found. The information about this buffer is stored in the conversion step data. A pointer to this information is passed as the second argument to this function. The description of the @code{struct __gconv_step_data} structure has more information on the conversion step data. @cindex stateful What has to be done for flushing depends on the source character set. If the source character set is not stateful, nothing has to be done. Otherwise the function has to emit a byte sequence to bring the state object into the initial state. Once this all happened the other conversion modules in the chain of conversions have to get the same chance. Whether another step follows can be determined from the @code{__is_last} element of the step data structure to which the first parameter points. The more interesting mode is when actual text has to be converted. The first step in this case is to convert as much text as possible from the input buffer and store the result in the output buffer. The start of the input buffer is determined by the third argument, which is a pointer to a pointer variable referencing the beginning of the buffer. The fourth argument is a pointer to the byte right after the last byte in the buffer. The conversion has to be performed according to the current state if the character set is stateful. The state is stored in an object pointed to by the @code{__statep} element of the step data (second argument). Once either the input buffer is empty or the output buffer is full the conversion stops. At this point, the pointer variable referenced by the third parameter must point to the byte following the last processed byte (i.e., if all of the input is consumed, this pointer and the fourth parameter have the same value). What now happens depends on whether this step is the last one. If it is the last step, the only thing that has to be done is to update the @code{__outbuf} element of the step data structure to point after the last written byte. This update gives the caller the information on how much text is available in the output buffer. In addition, the variable pointed to by the fifth parameter, which is of type @code{size_t}, must be incremented by the number of characters (@emph{not bytes}) that were converted in a non-reversible way. Then, the function can return. In case the step is not the last one, the later conversion functions have to get a chance to do their work. Therefore, the appropriate conversion function has to be called. The information about the functions is stored in the conversion data structures, passed as the first parameter. This information and the step data are stored in arrays, so the next element in both cases can be found by simple pointer arithmetic: @smallexample int gconv (struct __gconv_step *step, struct __gconv_step_data *data, const char **inbuf, const char *inbufend, size_t *written, int do_flush) @{ struct __gconv_step *next_step = step + 1; struct __gconv_step_data *next_data = data + 1; @dots{} @end smallexample The @code{next_step} pointer references the next step information and @code{next_data} the next data record. The call of the next function therefore will look similar to this: @smallexample next_step->__fct (next_step, next_data, &outerr, outbuf, written, 0) @end smallexample But this is not yet all. Once the function call returns the conversion function might have some more to do. If the return value of the function is @code{__GCONV_EMPTY_INPUT}, more room is available in the output buffer. Unless the input buffer is empty, the conversion functions start all over again and process the rest of the input buffer. If the return value is not @code{__GCONV_EMPTY_INPUT}, something went wrong and we have to recover from this. A requirement for the conversion function is that the input buffer pointer (the third argument) always point to the last character that was put in converted form into the output buffer. This is trivially true after the conversion performed in the current step, but if the conversion functions deeper downstream stop prematurely, not all characters from the output buffer are consumed and, therefore, the input buffer pointers must be backed off to the right position. Correcting the input buffers is easy to do if the input and output character sets have a fixed width for all characters. In this situation we can compute how many characters are left in the output buffer and, therefore, can correct the input buffer pointer appropriately with a similar computation. Things are getting tricky if either character set has characters represented with variable length byte sequences, and it gets even more complicated if the conversion has to take care of the state. In these cases the conversion has to be performed once again, from the known state before the initial conversion (i.e., if necessary the state of the conversion has to be reset and the conversion loop has to be executed again). The difference now is that it is known how much input must be created, and the conversion can stop before converting the first unused character. Once this is done the input buffer pointers must be updated again and the function can return. One final thing should be mentioned. If it is necessary for the conversion to know whether it is the first invocation (in case a prolog has to be emitted), the conversion function should increment the @code{__invocation_counter} element of the step data structure just before returning to the caller. See the description of the @code{struct __gconv_step_data} structure above for more information on how this can be used. The return value must be one of the following values: @table @code @item __GCONV_EMPTY_INPUT All input was consumed and there is room left in the output buffer. @item __GCONV_FULL_OUTPUT No more room in the output buffer. In case this is not the last step this value is propagated down from the call of the next conversion function in the chain. @item __GCONV_INCOMPLETE_INPUT The input buffer is not entirely empty since it contains an incomplete character sequence. @end table The following example provides a framework for a conversion function. In case a new conversion has to be written the holes in this implementation have to be filled and that is it. @smallexample int gconv (struct __gconv_step *step, struct __gconv_step_data *data, const char **inbuf, const char *inbufend, size_t *written, int do_flush) @{ struct __gconv_step *next_step = step + 1; struct __gconv_step_data *next_data = data + 1; gconv_fct fct = next_step->__fct; int status; /* @r{If the function is called with no input this means we have} @r{to reset to the initial state. The possibly partly} @r{converted input is dropped.} */ if (do_flush) @{ status = __GCONV_OK; /* @r{Possible emit a byte sequence which put the state object} @r{into the initial state.} */ /* @r{Call the steps down the chain if there are any but only} @r{if we successfully emitted the escape sequence.} */ if (status == __GCONV_OK && ! data->__is_last) status = fct (next_step, next_data, NULL, NULL, written, 1); @} else @{ /* @r{We preserve the initial values of the pointer variables.} */ const char *inptr = *inbuf; char *outbuf = data->__outbuf; char *outend = data->__outbufend; char *outptr; do @{ /* @r{Remember the start value for this round.} */ inptr = *inbuf; /* @r{The outbuf buffer is empty.} */ outptr = outbuf; /* @r{For stateful encodings the state must be safe here.} */ /* @r{Run the conversion loop. @code{status} is set} @r{appropriately afterwards.} */ /* @r{If this is the last step, leave the loop. There is} @r{nothing we can do.} */ if (data->__is_last) @{ /* @r{Store information about how many bytes are} @r{available.} */ data->__outbuf = outbuf; /* @r{If any non-reversible conversions were performed,} @r{add the number to @code{*written}.} */ break; @} /* @r{Write out all output that was produced.} */ if (outbuf > outptr) @{ const char *outerr = data->__outbuf; int result; result = fct (next_step, next_data, &outerr, outbuf, written, 0); if (result != __GCONV_EMPTY_INPUT) @{ if (outerr != outbuf) @{ /* @r{Reset the input buffer pointer. We} @r{document here the complex case.} */ size_t nstatus; /* @r{Reload the pointers.} */ *inbuf = inptr; outbuf = outptr; /* @r{Possibly reset the state.} */ /* @r{Redo the conversion, but this time} @r{the end of the output buffer is at} @r{@code{outerr}.} */ @} /* @r{Change the status.} */ status = result; @} else /* @r{All the output is consumed, we can make} @r{ another run if everything was ok.} */ if (status == __GCONV_FULL_OUTPUT) status = __GCONV_OK; @} @} while (status == __GCONV_OK); /* @r{We finished one use of this step.} */ ++data->__invocation_counter; @} return status; @} @end smallexample @end deftypevr This information should be sufficient to write new modules. Anybody doing so should also take a look at the available source code in the @glibcadj{} sources. It contains many examples of working and optimized modules. @c File charset.texi edited October 2001 by Dennis Grace, IBM Corporation `