summaryrefslogtreecommitdiff
path: root/pod/perlfunc.pod
blob: df8d23fc1eddfea8e5c06d9ff258c5e854d939dc (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
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
3360
3361
3362
3363
3364
3365
3366
3367
3368
3369
3370
3371
3372
3373
3374
3375
3376
3377
3378
3379
3380
3381
3382
3383
3384
3385
3386
3387
3388
3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401
3402
3403
3404
3405
3406
3407
3408
3409
3410
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
3433
3434
3435
3436
3437
3438
3439
3440
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
3452
3453
3454
3455
3456
3457
3458
3459
3460
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3478
3479
3480
3481
3482
3483
3484
3485
3486
3487
3488
3489
3490
3491
3492
3493
3494
3495
3496
3497
=head1 NAME

perlfunc - Perl builtin functions

=head1 DESCRIPTION

The functions in this section can serve as terms in an expression.
They fall into two major categories: list operators and named unary
operators.  These differ in their precedence relationship with a
following comma.  (See the precedence table in L<perlop>.)  List
operators take more than one argument, while unary operators can never
take more than one argument.  Thus, a comma terminates the argument of
a unary operator, but merely separates the arguments of a list
operator.  A unary operator generally provides a scalar context to its
argument, while a list operator may provide either scalar and list
contexts for its arguments.  If it does both, the scalar arguments will
be first, and the list argument will follow.  (Note that there can only
ever be one list argument.)  For instance, splice() has three scalar
arguments followed by a list.

In the syntax descriptions that follow, list operators that expect a
list (and provide list context for the elements of the list) are shown
with LIST as an argument.  Such a list may consist of any combination
of scalar arguments or list values; the list values will be included
in the list as if each individual element were interpolated at that
point in the list, forming a longer single-dimensional list value.
Elements of the LIST should be separated by commas.

Any function in the list below may be used either with or without
parentheses around its arguments.  (The syntax descriptions omit the
parens.)  If you use the parens, the simple (but occasionally
surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
function, and precedence doesn't matter.  Otherwise it's a list
operator or unary operator, and precedence does matter.  And whitespace
between the function and left parenthesis doesn't count--so you need to
be careful sometimes:

    print 1+2+3;	# Prints 6.
    print(1+2) + 3;	# Prints 3.
    print (1+2)+3;	# Also prints 3!
    print +(1+2)+3;	# Prints 6.
    print ((1+2)+3);	# Prints 6.

If you run Perl with the B<-w> switch it can warn you about this.  For
example, the third line above produces:

    print (...) interpreted as function at - line 1.
    Useless use of integer addition in void context at - line 1.

For functions that can be used in either a scalar or list context,
non-abortive failure is generally indicated in a scalar context by
returning the undefined value, and in a list context by returning the
null list.

Remember the following rule:

=over 8

=item  

I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>

=back

Each operator and function decides which sort of value it would be most
appropriate to return in a scalar context.  Some operators return the
length of the list that would have been returned in a list context.  Some
operators return the first value in the list.  Some operators return the
last value in the list.  Some operators return a count of successful
operations.  In general, they do what you want, unless you want
consistency.

=head2 Perl Functions by Category

Here are Perl's functions (including things that look like
functions, like some of the keywords and named operators)
arranged by category.  Some functions appear in more
than one place.

=over

=item Functions for SCALARs or strings

chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
sprintf, substr, tr///, uc, ucfirst, y///

=item Regular expressions and pattern matching

m//, pos, quotemeta, s///, split, study

=item Numeric functions

abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
srand

=item Functions for real @ARRAYs

pop, push, shift, splice, unshift

=item Functions for list data

grep, join, map, qw/STRING/, reverse, sort, unpack

=item Functions for real %HASHes

delete, each, exists, keys, values

=item Input and output functions

binmode, close, closedir, dbmclose, dbmopen, die, eof,
fileno, flock, format, getc, print, printf, read, readdir,
rewinddir, seek, seekdir, select, syscall, sysread,
syswrite, tell, telldir, truncate, warn, write

=item Functions for fixed length data or records

pack, read, syscall, sysread, syswrite, unpack, vec

=item Functions for filehandles, files, or directories

I<-X>, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
lstat, mkdir, open, opendir, readlink, rename, rmdir,
stat, symlink, umask, unlink, utime

=item Keywords related to the control flow of your perl program

caller, continue, die, do, dump, eval, exit, goto, last,
next, redo, return, sub, wantarray

=item Keywords related to scoping 

caller, import, local, my, package, use

=item Miscellaneous functions

defined, dump, eval, formline, local, my, reset, scalar,
undef, wantarray

=item Functions for processes and process groups

alarm, exec, fork, getpgrp, getppid, getpriority, kill,
pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
times, wait, waitpid

=item Keywords related to perl modules

do, import, no, package, require, use

=item Keywords related to classes and object-orientedness

bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use

=item Low-level socket functions

accept, bind, connect, getpeername, getsockname,
getsockopt, listen, recv, send, setsockopt, shutdown,
socket, socketpair

=item System V interprocess communication functions

msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
shmctl, shmget, shmread, shmwrite

=item Fetching user and group info

endgrent, endhostent, endnetent, endpwent, getgrent,
getgrgid, getgrnam, getlogin, getpwent, getpwnam,
getpwuid, setgrent, setpwent

=item Fetching network info

endprotoent, endservent, gethostbyaddr, gethostbyname,
gethostent, getnetbyaddr, getnetbyname, getnetent,
getprotobyname, getprotobynumber, getprotoent,
getservbyname, getservbyport, getservent, sethostent,
setnetent, setprotoent, setservent

=item Time-related functions

gmtime, localtime, time, times

=item Functions new in perl5

abs, bless, chomp, chr, exists, formline, glob, import, lc,
lcfirst, map, my, no, prototype, qx, qw, readline, readpipe,
ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use

* - C<sub> was a keyword in perl4, but in perl5 it is an
operator which can be used in expressions.

=item Functions obsoleted in perl5

dbmclose, dbmopen


=back

=head2 Alphabetical Listing of Perl Functions


=over 8

=item -X FILEHANDLE

=item -X EXPR

=item -X

A file test, where X is one of the letters listed below.  This unary
operator takes one argument, either a filename or a filehandle, and
tests the associated file to see if something is true about it.  If the
argument is omitted, tests $_, except for C<-t>, which tests STDIN.
Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
the undefined value if the file doesn't exist.  Despite the funny
names, precedence is the same as any other named unary operator, and
the argument may be parenthesized like any other unary operator.  The
operator may be any of:

    -r	File is readable by effective uid/gid.
    -w	File is writable by effective uid/gid.
    -x	File is executable by effective uid/gid.
    -o	File is owned by effective uid.

    -R	File is readable by real uid/gid.
    -W	File is writable by real uid/gid.
    -X	File is executable by real uid/gid.
    -O	File is owned by real uid.

    -e	File exists.
    -z	File has zero size.
    -s	File has non-zero size (returns size).

    -f	File is a plain file.
    -d	File is a directory.
    -l	File is a symbolic link.
    -p	File is a named pipe (FIFO).
    -S	File is a socket.
    -b	File is a block special file.
    -c	File is a character special file.
    -t	Filehandle is opened to a tty.

    -u	File has setuid bit set.
    -g	File has setgid bit set.
    -k	File has sticky bit set.

    -T	File is a text file.
    -B	File is a binary file (opposite of -T).

    -M	Age of file in days when script started.
    -A	Same for access time.
    -C	Same for inode change time.

The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
uids and gids of the user.  There may be other reasons you can't actually
read, write or execute the file.  Also note that, for the superuser,
C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
1 if any execute bit is set in the mode.  Scripts run by the superuser may
thus need to do a stat() in order to determine the actual mode of the
file, or temporarily set the uid to something else.

Example:

    while (<>) {
	chop;
	next unless -f $_;	# ignore specials
	...
    }

Note that C<-s/a/b/> does not do a negated substitution.  Saying
C<-exp($foo)> still works as expected, however--only single letters
following a minus are interpreted as file tests.

The C<-T> and C<-B> switches work as follows.  The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set.  If too many odd characters (E<gt>30%)
are found, it's a C<-B> file, otherwise it's a C<-T> file.  Also, any file
containing null in the first block is considered a binary file.  If C<-T>
or C<-B> is used on a filehandle, the current stdio buffer is examined
rather than the first block.  Both C<-T> and C<-B> return TRUE on a null
file, or a file at EOF when testing a filehandle.  Because you have to 
read a file to do the C<-T> test, on most occasions you want to use a C<-f>
against the file first, as in C<next unless -f $file && -T $file>.

If any of the file tests (or either the stat() or lstat() operators) are given the
special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call.  (This doesn't work with C<-t>, and you need to remember
that lstat() and C<-l> will leave values in the stat structure for the
symbolic link, not the real file.)  Example:

    print "Can do.\n" if -r $a || -w _ || -x _;

    stat($filename);
    print "Readable\n" if -r _;
    print "Writable\n" if -w _;
    print "Executable\n" if -x _;
    print "Setuid\n" if -u _;
    print "Setgid\n" if -g _;
    print "Sticky\n" if -k _;
    print "Text\n" if -T _;
    print "Binary\n" if -B _;

=item abs VALUE

=item abs 

Returns the absolute value of its argument.
If VALUE is omitted, uses $_.

=item accept NEWSOCKET,GENERICSOCKET

Accepts an incoming socket connect, just as the accept(2) system call
does.  Returns the packed address if it succeeded, FALSE otherwise.
See example in L<perlipc/"Sockets: Client/Server Communication">.

=item alarm SECONDS

=item alarm 

Arranges to have a SIGALRM delivered to this process after the
specified number of seconds have elapsed.  If SECONDS is not specified,
the value stored in $_ is used. (On some machines,
unfortunately, the elapsed time may be up to one second less than you
specified because of how seconds are counted.)  Only one timer may be
counting at once.  Each call disables the previous timer, and an
argument of 0 may be supplied to cancel the previous timer without
starting a new one.  The returned value is the amount of time remaining
on the previous timer.

For delays of finer granularity than one second, you may use Perl's
syscall() interface to access setitimer(2) if your system supports it, 
or else see L</select()> below.  It is not advised to intermix alarm() 
and sleep() calls.

=item atan2 Y,X

Returns the arctangent of Y/X in the range -PI to PI.

=item bind SOCKET,NAME

Binds a network address to a socket, just as the bind system call
does.  Returns TRUE if it succeeded, FALSE otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item binmode FILEHANDLE

Arranges for the file to be read or written in "binary" mode in operating
systems that distinguish between binary and text files.  Files that are
not in binary mode have CR LF sequences translated to LF on input and LF
translated to CR LF on output.  Binmode has no effect under Unix; in DOS
and similarly archaic systems, it may be imperative--otherwise your
DOS-damaged C library may mangle your file.  The key distinction between
systems that need binmode and those that don't is their text file
formats.  Systems like Unix and Plan9 that delimit lines with a single
character, and that encode that character in C as '\n', do not need
C<binmode>.  The rest need it.  If FILEHANDLE is an expression, the value
is taken as the name of the filehandle.

=item bless REF,CLASSNAME

=item bless REF

This function tells the referenced object (passed as REF) that it is now
an object in the CLASSNAME package--or the current package if no CLASSNAME
is specified, which is often the case.  It returns the reference for
convenience, since a bless() is often the last thing in a constructor.
Always use the two-argument version if the function doing the blessing
might be inherited by a derived class.  See L<perlobj> for more about the
blessing (and blessings) of objects.

=item caller EXPR

=item caller

Returns the context of the current subroutine call.  In a scalar context,
returns TRUE if there is a caller, that is, if we're in a subroutine or
eval() or require(), and FALSE otherwise.  In a list context, returns

    ($package, $filename, $line) = caller;

With EXPR, it returns some extra information that the debugger uses to
print a stack trace.  The value of EXPR indicates how many call frames
to go back before the current one.

    ($package, $filename, $line,
     $subroutine, $hasargs, $wantargs) = caller($i);

Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable @DB::args to be the
arguments with which that subroutine was invoked.

=item chdir EXPR

Changes the working directory to EXPR, if possible.  If EXPR is
omitted, changes to home directory.  Returns TRUE upon success, FALSE
otherwise.  See example under die().

=item chmod LIST

Changes the permissions of a list of files.  The first element of the
list must be the numerical mode, which should probably be an octal
number.  Returns the number of files successfully changed.

    $cnt = chmod 0755, 'foo', 'bar';
    chmod 0755, @executables;

=item chomp VARIABLE

=item chomp LIST

=item chomp

This is a slightly safer version of chop (see below).  It removes any
line ending that corresponds to the current value of C<$/> (also known as
$INPUT_RECORD_SEPARATOR in the C<English> module).  It returns the number
of characters removed.  It's often used to remove the newline from the
end of an input record when you're worried that the final record may be
missing its newline.  When in paragraph mode (C<$/ = "">), it removes all
trailing newlines from the string.  If VARIABLE is omitted, it chomps
$_.  Example:

    while (<>) {
	chomp;	# avoid \n on last field
	@array = split(/:/);
	...
    }

You can actually chomp anything that's an lvalue, including an assignment:

    chomp($cwd = `pwd`);
    chomp($answer = <STDIN>);

If you chomp a list, each element is chomped, and the total number of
characters removed is returned.

=item chop VARIABLE

=item chop LIST

=item chop

Chops off the last character of a string and returns the character
chopped.  It's used primarily to remove the newline from the end of an
input record, but is much more efficient than C<s/\n//> because it neither
scans nor copies the string.  If VARIABLE is omitted, chops $_.
Example:

    while (<>) {
	chop;	# avoid \n on last field
	@array = split(/:/);
	...
    }

You can actually chop anything that's an lvalue, including an assignment:

    chop($cwd = `pwd`);
    chop($answer = <STDIN>);

If you chop a list, each element is chopped.  Only the value of the
last chop is returned.

Note that chop returns the last character.  To return all but the last
character, use C<substr($string, 0, -1)>.

=item chown LIST

Changes the owner (and group) of a list of files.  The first two
elements of the list must be the I<NUMERICAL> uid and gid, in that order.
Returns the number of files successfully changed.

    $cnt = chown $uid, $gid, 'foo', 'bar';
    chown $uid, $gid, @filenames;

Here's an example that looks up non-numeric uids in the passwd file:

    print "User: ";
    chop($user = <STDIN>);
    print "Files: "
    chop($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)
	or die "$user not in passwd file";

    @ary = <${pattern}>;	# expand filenames
    chown $uid, $gid, @ary;

On most systems, you are not allowed to change the ownership of the 
file unless you're the superuser, although you should be able to change
the group to any of your secondary groups.  On insecure systems, these
restrictions may be relaxed, but this is not a portable assumption.

=item chr NUMBER

=item chr 

Returns the character represented by that NUMBER in the character set.
For example, C<chr(65)> is "A" in ASCII.

If NUMBER is omitted, uses $_.

=item chroot FILENAME

=item chroot 

This function works as the system call by the same name: it makes the
named directory the new root directory for all further pathnames that
begin with a "/" by your process and all of its children.  (It doesn't
change your current working directory is unaffected.)  For security
reasons, this call is restricted to the superuser.  If FILENAME is
omitted, does chroot to $_.

=item close FILEHANDLE

Closes the file or pipe associated with the file handle, returning TRUE
only if stdio successfully flushes buffers and closes the system file
descriptor.  You don't have to close FILEHANDLE if you are immediately
going to do another open() on it, since open() will close it for you.  (See
open().)  However, an explicit close on an input file resets the line
counter ($.), while the implicit close done by open() does not.  Also,
closing a pipe will wait for the process executing on the pipe to
complete, in case you want to look at the output of the pipe
afterwards.  Closing a pipe explicitly also puts the status value of
the command into C<$?>.  Example:

    open(OUTPUT, '|sort >foo');	# pipe to sort
    ...				# print stuff to output
    close OUTPUT;		# wait for sort to finish
    open(INPUT, 'foo');		# get sort's results

FILEHANDLE may be an expression whose value gives the real filehandle name.

=item closedir DIRHANDLE

Closes a directory opened by opendir().

=item connect SOCKET,NAME

Attempts to connect to a remote socket, just as the connect system call
does.  Returns TRUE if it succeeded, FALSE otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item continue BLOCK

Actually a flow control statement rather than a function.  If there is a
C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
C<foreach>), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a C<for> loop in C.  Thus
it can be used to increment a loop variable, even when the loop has been
continued via the C<next> statement (which is similar to the C C<continue>
statement).

=item cos EXPR

Returns the cosine of EXPR (expressed in radians).  If EXPR is omitted
takes cosine of $_.

=item crypt PLAINTEXT,SALT

Encrypts a string exactly like the crypt(3) function in the C library
(assuming that you actually have a version there that has not been
extirpated as a potential munition).  This can prove useful for checking
the password file for lousy passwords, amongst other things.  Only the
guys wearing white hats should do this.

Here's an example that makes sure that whoever runs this program knows
their own password:

    $pwd = (getpwuid($<))[1];
    $salt = substr($pwd, 0, 2);

    system "stty -echo";
    print "Password: ";
    chop($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $salt) ne $pwd) {
	die "Sorry...\n";
    } else {
	print "ok\n";
    } 

Of course, typing in your own password to whoever asks you 
for it is unwise.

=item dbmclose ASSOC_ARRAY

[This function has been superseded by the untie() function.]

Breaks the binding between a DBM file and an associative array.

=item dbmopen ASSOC,DBNAME,MODE

[This function has been superseded by the tie() function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
associative array.  ASSOC is the name of the associative array.  (Unlike
normal open, the first argument is I<NOT> a filehandle, even though it
looks like one).  DBNAME is the name of the database (without the F<.dir>
or F<.pag> extension if any).  If the database does not exist, it is
created with protection specified by MODE (as modified by the umask()).
If your system only supports the older DBM functions, you may perform only
one dbmopen() in your program.  In older versions of Perl, if your system
had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
falls back to sdbm(3).

If you don't have write access to the DBM file, you can only read
associative array variables, not set them.  If you want to test whether
you can write, either use file tests or try setting a dummy array entry
inside an eval(), which will trap the error.

Note that functions such as keys() and values() may return huge array
values when used on large DBM files.  You may prefer to use the each()
function to iterate over large DBM files.  Example:

    # print out history file offsets
    dbmopen(%HIST,'/usr/lib/news/history',0666);
    while (($key,$val) = each %HIST) {
	print $key, ' = ', unpack('L',$val), "\n";
    }
    dbmclose(%HIST);

See also L<AnyDBM_File> for a more general description of the pros and
cons of the various dbm approaches, as well as L<DB_File> for a particularly
rich implementation.

=item defined EXPR

=item defined 

Returns a boolean value saying whether EXPR has a real value
or not. If EXPR is not present, $_ will be checked. Many operations
return the undefined value under exceptional conditions, such as end of
file, uninitialized variable, system error and such.  This function
allows you to distinguish between an undefined
null scalar and a defined null scalar with operations that might return
a real null string, such as referencing elements of an array.  You may
also check to see if arrays or subroutines exist.  Use of defined on
predefined variables is not guaranteed to produce intuitive results.

When used on a hash array element, it tells you whether the value
is defined, not whether the key exists in the hash.  Use exists() for that.

Examples:

    print if defined $switch{'D'};
    print "$val\n" while defined($val = pop(@ary));
    die "Can't readlink $sym: $!"
	unless defined($value = readlink $sym);
    eval '@foo = ()' if defined(@foo);
    die "No XYZ package defined" unless defined %_XYZ;
    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }

See also undef().

Note: many folks tend to overuse defined(), and then are surprised to
discover that the number 0 and the null string are, in fact, defined
concepts.  For example, if you say

    "ab" =~ /a(.*)b/;

the pattern match succeeds, and $1 is defined, despite the fact that it
matched "nothing".  But it didn't really match nothing--rather, it
matched something that happened to be 0 characters long.  This is all
very above-board and honest.  When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer.  So
you should only use defined() when you're questioning the integrity
of what you're trying to do.  At other times, a simple comparison to
0 or "" is what you want.

=item delete EXPR

Deletes the specified value from its hash array.  Returns the deleted
value, or the undefined value if nothing was deleted.  Deleting from
C<$ENV{}> modifies the environment.  Deleting from an array tied to a DBM
file deletes the entry from the DBM file.  (But deleting from a tie()d
hash doesn't necessarily return anything.)

The following deletes all the values of an associative array:

    foreach $key (keys %ARRAY) {
	delete $ARRAY{$key};
    }

(But it would be faster to use the undef() command.)  Note that the
EXPR can be arbitrarily complicated as long as the final operation is
a hash key lookup:

    delete $ref->[$x][$y]{$key};

=item die LIST

Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
the current value of C<$!> (errno).  If C<$!> is 0, exits with the value of
C<($? E<gt>E<gt> 8)> (backtick `command` status).  If C<($? E<gt>E<gt> 8)> is 0,
exits with 255.  Inside an eval(), the error message is stuffed into C<$@>,
and the eval() is terminated with the undefined value; this makes die()
the way to raise an exception.

Equivalent examples:

    die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
    chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" 

If the value of EXPR does not end in a newline, the current script line
number and input line number (if any) are also printed, and a newline
is supplied.  Hint: sometimes appending ", stopped" to your message
will cause it to make better sense when the string "at foo line 123" is
appended.  Suppose you are running script "canasta".

    die "/etc/games is no good";
    die "/etc/games is no good, stopped";

produce, respectively

    /etc/games is no good at canasta line 123.
    /etc/games is no good, stopped at canasta line 123.

See also exit() and warn().

=item do BLOCK

Not really a function.  Returns the value of the last command in the
sequence of commands indicated by BLOCK.  When modified by a loop
modifier, executes the BLOCK once before testing the loop condition.
(On other statements the loop modifiers test the conditional first.)

=item do SUBROUTINE(LIST)

A deprecated form of subroutine call.  See L<perlsub>.

=item do EXPR

Uses the value of EXPR as a filename and executes the contents of the
file as a Perl script.  Its primary use is to include subroutines
from a Perl subroutine library.

    do 'stat.pl';

is just like

    eval `cat stat.pl`;

except that it's more efficient, more concise, keeps track of the
current filename for error messages, and searches all the B<-I>
libraries if the file isn't in the current directory (see also the @INC
array in L<perlvar/Predefined Names>).  It's the same, however, in that it does
reparse the file every time you call it, so you probably don't want to
do this inside a loop.

Note that inclusion of library modules is better done with the
use() and require() operators, which also do error checking
and raise an exception if there's a problem.

=item dump LABEL

This causes an immediate core dump.  Primarily this is so that you can
use the B<undump> program to turn your core dump into an executable binary
after having initialized all your variables at the beginning of the
program.  When the new binary is executed it will begin by executing a
C<goto LABEL> (with all the restrictions that C<goto> suffers).  Think of
it as a goto with an intervening core dump and reincarnation.  If LABEL
is omitted, restarts the program from the top.  WARNING: any files
opened at the time of the dump will NOT be open any more when the
program is reincarnated, with possible resulting confusion on the part
of Perl.  See also B<-u> option in L<perlrun>.

Example:

    #!/usr/bin/perl
    require 'getopt.pl';
    require 'stat.pl';
    %days = (
	'Sun' => 1,
	'Mon' => 2,
	'Tue' => 3,
	'Wed' => 4,
	'Thu' => 5,
	'Fri' => 6,
	'Sat' => 7,
    );

    dump QUICKSTART if $ARGV[0] eq '-d';

    QUICKSTART:
    Getopt('f');

=item each ASSOC_ARRAY

When called in a list context, returns a 2-element array consisting
of the key and value for the next element of an associative array,
so that you can iterate over it.  When called in a scalar context,
returns the key only for the next element in the associative array.
Entries are returned in an apparently random order.  When the array is
entirely read, a null array is returned in list context (which when
assigned produces a FALSE (0) value), and C<undef> is returned in a
scalar context.  The next call to each() after that will start
iterating again.  The iterator can be reset only by reading all the
elements from the array.  You should not add elements to an array while
you're iterating over it.  There is a single iterator for each
associative array, shared by all each(), keys() and values() function
calls in the program.  The following prints out your environment like
the printenv(1) program, only in a different order:

    while (($key,$value) = each %ENV) {
	print "$key=$value\n";
    }

See also keys() and values().

=item eof FILEHANDLE

=item eof ()

=item eof

Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open.  FILEHANDLE may be an expression whose value
gives the real filehandle name.  (Note that this function actually
reads a character and then ungetc()s it, so it is not very useful in an
interactive context.)  Do not read from a terminal file (or call
C<eof(FILEHANDLE)> on it) after end-of-file is reached.  Filetypes such
as terminals may lose the end-of-file condition if you do.

An C<eof> without an argument uses the last file read as argument.
Empty parentheses () may be used to indicate
the pseudofile formed of the files listed on the command line, i.e.
C<eof()> is reasonable to use inside a while (E<lt>E<gt>) loop to detect the end
of only the last file.  Use C<eof(ARGV)> or eof without the parentheses to
test I<EACH> file in a while (E<lt>E<gt>) loop.  Examples:

    # reset line numbering on each input file
    while (<>) {
	print "$.\t$_";
	close(ARGV) if (eof);	# Not eof().
    }

    # insert dashes just before last line of last file
    while (<>) {
	if (eof()) {
	    print "--------------\n";
	    close(ARGV);	# close or break; is needed if we
				# are reading from the terminal
	}
	print;
    }

Practical hint: you almost never need to use C<eof> in Perl, because the
input operators return undef when they run out of data.  

=item eval EXPR

=item eval BLOCK

EXPR is parsed and executed as if it were a little Perl program.  It
is executed in the context of the current Perl program, so that any
variable settings, subroutine or format definitions remain afterwards.
The value returned is the value of the last expression evaluated, or a
return statement may be used, just as with subroutines.  The last
expression is evaluated in scalar or array context, depending on the
context of the eval.

If there is a syntax error or runtime error, or a die() statement is
executed, an undefined value is returned by eval(), and C<$@> is set to the
error message.  If there was no error, C<$@> is guaranteed to be a null
string.  If EXPR is omitted, evaluates $_.  The final semicolon, if
any, may be omitted from the expression.

Note that, since eval() traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as socket() or symlink())
is implemented.  It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.

If the code to be executed doesn't vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
recompiling each time.  The error, if any, is still returned in C<$@>.
Examples:

    # make divide-by-zero non-fatal
    eval { $answer = $a / $b; }; warn $@ if $@;

    # same thing, but less efficient
    eval '$answer = $a / $b'; warn $@ if $@;

    # a compile-time error
    eval { $answer = };

    # a run-time error
    eval '$answer =';	# sets $@

With an eval(), you should be especially careful to remember what's 
being looked at when:

    eval $x;		# CASE 1
    eval "$x";		# CASE 2

    eval '$x';		# CASE 3
    eval { $x };	# CASE 4

    eval "\$$x++"	# CASE 5
    $$x++;		# CASE 6

Cases 1 and 2 above behave identically: they run the code contained in the
variable $x.  (Although case 2 has misleading double quotes making the
reader wonder what else might be happening (nothing is).) Cases 3 and 4
likewise behave in the same way: they run the code E<lt>$xE<gt>, which does
nothing at all.  (Case 4 is preferred for purely visual reasons.) Case 5
is a place where normally you I<WOULD> like to use double quotes, except
that in that particular situation, you can just use symbolic references
instead, as in case 6.

=item exec LIST

The exec() function executes a system command I<AND NEVER RETURNS>,
unless the command does not exist and is executed directly instead of
via C</bin/sh -c> (see below).  Use system() instead of exec() if you
want it to return.

If there is more than one argument in LIST, or if LIST is an array with
more than one value, calls execvp(3) with the arguments in LIST.  If
there is only one scalar argument, the argument is checked for shell
metacharacters.  If there are any, the entire argument is passed to
C</bin/sh -c> for parsing.  If there are none, the argument is split
into words and passed directly to execvp(), which is more efficient.
Note: exec() and system() do not flush your output buffer, so you may
need to set C<$|> to avoid lost output.  Examples:

    exec '/bin/echo', 'Your arguments are: ', @ARGV;
    exec "sort $outfile | uniq";

If you don't really want to execute the first argument, but want to lie
to the program you are executing about its own name, you can specify
the program you actually want to run as an "indirect object" (without a
comma) in front of the LIST.  (This always forces interpretation of the
LIST as a multi-valued list, even if there is only a single scalar in
the list.)  Example:

    $shell = '/bin/csh';
    exec $shell '-sh';		# pretend it's a login shell

or, more directly,

    exec {'/bin/csh'} '-sh';	# pretend it's a login shell

=item exists EXPR

Returns TRUE if the specified hash key exists in its hash array, even
if the corresponding value is undefined.

    print "Exists\n" if exists $array{$key};
    print "Defined\n" if defined $array{$key};
    print "True\n" if $array{$key};

A hash element can only be TRUE if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.

Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:

    if (exists $ref->[$x][$y]{$key}) { ... }

=item exit EXPR

Evaluates EXPR and exits immediately with that value.  (Actually, it
calls any defined C<END> routines first, but the C<END> routines may not
abort the exit.  Likewise any object destructors that need to be called
are called before exit.)  Example:

    $ans = <STDIN>;
    exit 0 if $ans =~ /^[Xx]/;

See also die().  If EXPR is omitted, exits with 0 status.

=item exp EXPR

=item exp 

Returns I<e> (the natural logarithm base) to the power of EXPR.  
If EXPR is omitted, gives C<exp($_)>.

=item fcntl FILEHANDLE,FUNCTION,SCALAR

Implements the fcntl(2) function.  You'll probably have to say

    use Fcntl;

first to get the correct function definitions.  Argument processing and
value return works just like ioctl() below.  Note that fcntl() will produce
a fatal error if used on a machine that doesn't implement fcntl(2).
For example:

    use Fcntl;
    fcntl($filehandle, F_GETLK, $packed_return_buffer);

=item fileno FILEHANDLE

Returns the file descriptor for a filehandle.  This is useful for
constructing bitmaps for select().  If FILEHANDLE is an expression, the
value is taken as the name of the filehandle.

=item flock FILEHANDLE,OPERATION

Calls flock(2) on FILEHANDLE.  See L<flock(2)> for definition of
OPERATION.  Returns TRUE for success, FALSE on failure.  Will produce a
fatal error if used on a machine that doesn't implement either flock(2) or
fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
is missing from your system.  This makes flock() the portable file locking
strategy, although it will only lock entire files, not records.  Note also
that some versions of flock() cannot lock things over the network; you
would need to use the more system-specific fcntl() for that.

Here's a mailbox appender for BSD systems.

    $LOCK_SH = 1;
    $LOCK_EX = 2;
    $LOCK_NB = 4;
    $LOCK_UN = 8;

    sub lock {
	flock(MBOX,$LOCK_EX);
	# and, in case someone appended
	# while we were waiting...
	seek(MBOX, 0, 2);
    }

    sub unlock {
	flock(MBOX,$LOCK_UN);
    }

    open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
	    or die "Can't open mailbox: $!";

    lock();
    print MBOX $msg,"\n\n";
    unlock();

See also L<DB_File> for other flock() examples.

=item fork

Does a fork(2) system call.  Returns the child pid to the parent process
and 0 to the child process, or C<undef> if the fork is unsuccessful.
Note: unflushed buffers remain unflushed in both processes, which means
you may need to set C<$|> ($AUTOFLUSH in English) or call the 
autoflush() FileHandle method to avoid duplicate output.

If you fork() without ever waiting on your children, you will accumulate
zombies:

    $SIG{CHLD} = sub { wait };

There's also the double-fork trick (error checking on 
fork() returns omitted);

    unless ($pid = fork) {
	unless (fork) {
	    exec "what you really wanna do";
	    die "no exec";
	    # ... or ...
	    ## (some_perl_code_here)
	    exit 0;
	}
	exit 0;
    }
    waitpid($pid,0);

See also L<perlipc> for more examples of forking and reaping
moribund children.

=item format

Declare a picture format with use by the write() function.  For
example:

    format Something = 
	Test: @<<<<<<<< @||||| @>>>>>
	      $str,     $%,    '$' . int($num)
    .

    $str = "widget";
    $num = $cost/$quantity;
    $~ = 'Something';
    write;

See L<perlform> for many details and examples.


=item formline PICTURE, LIST

This is an internal function used by C<format>s, though you may call it
too.  It formats (see L<perlform>) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, C<$^A> (or $ACCUMULATOR in English).
Eventually, when a write() is done, the contents of
C<$^A> are written to some filehandle, but you could also read C<$^A>
yourself and then set C<$^A> back to "".  Note that a format typically
does one formline() per line of form, but the formline() function itself
doesn't care how many newlines are embedded in the PICTURE.  This means
that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.

Be careful if you put double quotes around the picture, since an "C<@>"
character may be taken to mean the beginning of an array name.
formline() always returns TRUE.  See L<perlform> for other examples.

=item getc FILEHANDLE

=item getc

Returns the next character from the input file attached to FILEHANDLE,
or a null string at end of file.  If FILEHANDLE is omitted, reads from STDIN.
This is not particularly efficient.  It cannot be used to get unbuffered
single-characters, however.  For that, try something more like:

    if ($BSD_STYLE) {
	system "stty cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
	system "stty", '-icanon', 'eol', "\001"; 
    }

    $key = getc(STDIN);

    if ($BSD_STYLE) {
	system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
	system "stty", 'icanon', 'eol', '^@'; # ascii null
    }
    print "\n";

Determination of whether to whether $BSD_STYLE should be set 
is left as an exercise to the reader.  

See also the C<Term::ReadKey> module from your nearest CPAN site;
details on CPAN can be found on L<perlmod/CPAN> 

=item getlogin

Returns the current login from F</etc/utmp>, if any.  If null, use
getpwuid().  

    $login = getlogin || (getpwuid($<))[0] || "Kilroy";

Do not consider getlogin() for authentication: it is not as
secure as getpwuid().

=item getpeername SOCKET

Returns the packed sockaddr address of other end of the SOCKET connection.

    use Socket;
    $hersockaddr    = getpeername(SOCK);
    ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
    $herhostname    = gethostbyaddr($iaddr, AF_INET);
    $herstraddr     = inet_ntoa($iaddr);

=item getpgrp PID

Returns the current process group for the specified PID.  Use
a PID of 0 to get the current process group for the
current process.  Will raise an exception if used on a machine that
doesn't implement getpgrp(2).  If PID is omitted, returns process
group of current process.  Note that the POSIX version of getpgrp()
does not accept a PID argument, so only PID==0 is truly portable.

=item getppid

Returns the process id of the parent process.

=item getpriority WHICH,WHO

Returns the current priority for a process, a process group, or a user.
(See L<getpriority(2)>.)  Will raise a fatal exception if used on a
machine that doesn't implement getpriority(2).

=item getpwnam NAME

=item getgrnam NAME

=item gethostbyname NAME

=item getnetbyname NAME

=item getprotobyname NAME

=item getpwuid UID

=item getgrgid GID

=item getservbyname NAME,PROTO

=item gethostbyaddr ADDR,ADDRTYPE

=item getnetbyaddr ADDR,ADDRTYPE

=item getprotobynumber NUMBER

=item getservbyport PORT,PROTO

=item getpwent

=item getgrent

=item gethostent

=item getnetent

=item getprotoent

=item getservent

=item setpwent

=item setgrent

=item sethostent STAYOPEN

=item setnetent STAYOPEN

=item setprotoent STAYOPEN

=item setservent STAYOPEN

=item endpwent

=item endgrent

=item endhostent

=item endnetent

=item endprotoent

=item endservent

These routines perform the same functions as their counterparts in the
system library.  Within a list context, the return values from the
various get routines are as follows:

    ($name,$passwd,$uid,$gid,
       $quota,$comment,$gcos,$dir,$shell) = getpw*
    ($name,$passwd,$gid,$members) = getgr*
    ($name,$aliases,$addrtype,$length,@addrs) = gethost*
    ($name,$aliases,$addrtype,$net) = getnet*
    ($name,$aliases,$proto) = getproto*
    ($name,$aliases,$port,$proto) = getserv*

(If the entry doesn't exist you get a null list.)

Within a scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
(If the entry doesn't exist you get the undefined value.)  For example:

    $uid = getpwnam
    $name = getpwuid
    $name = getpwent
    $gid = getgrnam
    $name = getgrgid
    $name = getgrent
    etc.

The $members value returned by I<getgr*()> is a space separated list of
the login names of the members of the group.

For the I<gethost*()> functions, if the C<h_errno> variable is supported in
C, it will be returned to you via C<$?> if the function call fails.  The
@addrs value returned by a successful call is a list of the raw
addresses returned by the corresponding system library call.  In the
Internet domain, each address is four bytes long and you can unpack it
by saying something like:

    ($a,$b,$c,$d) = unpack('C4',$addr[0]);

=item getsockname SOCKET

Returns the packed sockaddr address of this end of the SOCKET connection.

    use Socket;
    $mysockaddr = getsockname(SOCK);
    ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);

=item getsockopt SOCKET,LEVEL,OPTNAME

Returns the socket option requested, or undefined if there is an error.

=item glob EXPR

Returns the value of EXPR with filename expansions such as a shell
would do.  This is the internal function implementing the E<lt>*.*E<gt>
operator, except it's easier to use.

=item gmtime EXPR

Converts a time as returned by the time function to a 9-element array
with the time localized for the standard Greenwich timezone.  
Typically used as follows:


    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
					    gmtime(time);

All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6.  If EXPR is omitted, does C<gmtime(time())>.

=item goto LABEL

=item goto EXPR

=item goto &NAME

The goto-LABEL form finds the statement labeled with LABEL and resumes
execution there.  It may not be used to go into any construct that
requires initialization, such as a subroutine or a foreach loop.  It
also can't be used to go into a construct that is optimized away.  It
can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
construct such as last or die.  The author of Perl has never felt the
need to use this form of goto (in Perl, that is--C is another matter).

The goto-EXPR form expects a label name, whose scope will be resolved
dynamically.  This allows for computed gotos per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:

    goto ("FOO", "BAR", "GLARCH")[$i];

The goto-&NAME form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine.  This is used by
AUTOLOAD subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place
(except that any modifications to @_ in the current subroutine are
propagated to the other subroutine.)  After the goto, not even caller()
will be able to tell that this routine was called first.

=item grep BLOCK LIST

=item grep EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value consisting of those
elements for which the expression evaluated to TRUE.  In a scalar
context, returns the number of times the expression was TRUE.

    @foo = grep(!/^#/, @bar);    # weed out comments

or equivalently,

    @foo = grep {!/^#/} @bar;    # weed out comments

Note that, since $_ is a reference into the list value, it can be used
to modify the elements of the array.  While this is useful and
supported, it can cause bizarre results if the LIST is not a named
array.

=item hex EXPR

=item hex 

Interprets EXPR as a hex string and returns the corresponding decimal
value.  (To convert strings that might start with 0 or 0x see
oct().)  If EXPR is omitted, uses $_.

=item import

There is no built-in import() function.  It is merely an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
names to another module.  The use() function calls the import() method
for the package used.  See also L</use>, L<perlmod>, and L<Exporter>.

=item index STR,SUBSTR,POSITION

=item index STR,SUBSTR

Returns the position of the first occurrence of SUBSTR in STR at or after
POSITION.  If POSITION is omitted, starts searching from the beginning of
the string.  The return value is based at 0 (or whatever you've set the C<$[>
variable to--but don't do that).  If the substring is not found, returns
one less than the base, ordinarily -1.

=item int EXPR

=item int 

Returns the integer portion of EXPR.  If EXPR is omitted, uses $_.

=item ioctl FILEHANDLE,FUNCTION,SCALAR

Implements the ioctl(2) function.  You'll probably have to say

    require "ioctl.ph";	# probably in /usr/local/lib/perl/ioctl.ph

first to get the correct function definitions.  If F<ioctl.ph> doesn't
exist or doesn't have the correct definitions you'll have to roll your
own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
(There is a Perl script called B<h2ph> that comes with the Perl kit which
may help you in this, but it's non-trivial.)  SCALAR will be read and/or
written depending on the FUNCTION--a pointer to the string value of SCALAR
will be passed as the third argument of the actual ioctl call.  (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value.  To guarantee this to be
TRUE, add a 0 to the scalar before using it.)  The pack() and unpack()
functions are useful for manipulating the values of structures used by
ioctl().  The following example sets the erase character to DEL.

    require 'ioctl.ph';
    $getp = &TIOCGETP;
    die "NO TIOCGETP" if $@ || !$getp;
    $sgttyb_t = "ccccs";		# 4 chars and a short
    if (ioctl(STDIN,$getp,$sgttyb)) {
	@ary = unpack($sgttyb_t,$sgttyb);
	$ary[2] = 127;
	$sgttyb = pack($sgttyb_t,@ary);
	ioctl(STDIN,&TIOCSETP,$sgttyb)
	    || die "Can't ioctl: $!";
    }

The return value of ioctl (and fcntl) is as follows:

	if OS returns:		then Perl returns:
	    -1	  		  undefined value
	     0	 		string "0 but true"
	anything else		    that number

Thus Perl returns TRUE on success and FALSE on failure, yet you can
still easily determine the actual value returned by the operating
system:

    ($retval = ioctl(...)) || ($retval = -1);
    printf "System returned %d\n", $retval;

=item join EXPR,LIST

Joins the separate strings of LIST or ARRAY into a single string with
fields separated by the value of EXPR, and returns the string.
Example:

    $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

See L<perlfunc/split>.

=item keys ASSOC_ARRAY

Returns a normal array consisting of all the keys of the named
associative array.  (In a scalar context, returns the number of keys.)
The keys are returned in an apparently random order, but it is the same
order as either the values() or each() function produces (given that
the associative array has not been modified).  Here is yet another way
to print your environment:

    @keys = keys %ENV;
    @values = values %ENV;
    while ($#keys >= 0) {
	print pop(@keys), '=', pop(@values), "\n";
    }

or how about sorted by key:

    foreach $key (sort(keys %ENV)) {
	print $key, '=', $ENV{$key}, "\n";
    }

To sort an array by value, you'll need to use a C<sort{}>
function.  Here's a descending numeric sort of a hash by its values:

    foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
	printf "%4d %s\n", $hash{$key}, $key;
    }

As an lvalue C<keys> allows you to increase the number of hash buckets
allocated for the given associative array.  This can gain you a measure
of efficiency if you know the hash is going to get big.  (This is
similar to pre-extending an array by assigning a larger number to
$#array.)  If you say

    keys %hash = 200;

then C<%hash> will have at least 200 buckets allocated for it.  These
buckets will be retained even if you do C<%hash = ()>, use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
C<keys> in this way (but you needn't worry about doing this by accident,
as trying has no effect).

=item kill LIST

Sends a signal to a list of processes.  The first element of 
the list must be the signal to send.  Returns the number of 
processes successfully signaled.

    $cnt = kill 1, $child1, $child2;
    kill 9, @goners;

Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
process groups instead of processes.  (On System V, a negative I<PROCESS>
number will also kill process groups, but that's not portable.)  That
means you usually want to use positive not negative signals.  You may also
use a signal name in quotes.  See L<perlipc/"Signals"> for details.

=item last LABEL

=item last

The C<last> command is like the C<break> statement in C (as used in
loops); it immediately exits the loop in question.  If the LABEL is
omitted, the command refers to the innermost enclosing loop.  The
C<continue> block, if any, is not executed:

    LINE: while (<STDIN>) {
	last LINE if /^$/;	# exit when done with header
	...
    }

=item lc EXPR

=item lc 

Returns an lowercased version of EXPR.  This is the internal function
implementing the \L escape in double-quoted strings.  
Should respect any POSIX setlocale() settings.

If EXPR is omitted, uses $_.

=item lcfirst EXPR

=item lcfirst 

Returns the value of EXPR with the first character lowercased.  This is
the internal function implementing the \l escape in double-quoted strings.
Should respect any POSIX setlocale() settings.

If EXPR is omitted, uses $_.

=item length EXPR

=item length 

Returns the length in characters of the value of EXPR.  If EXPR is
omitted, returns length of $_.

=item link OLDFILE,NEWFILE

Creates a new filename linked to the old filename.  Returns 1 for
success, 0 otherwise.

=item listen SOCKET,QUEUESIZE

Does the same thing that the listen system call does.  Returns TRUE if
it succeeded, FALSE otherwise.  See example in L<perlipc/"Sockets: Client/Server Communication">.

=item local EXPR

A local modifies the listed variables to be local to the enclosing block,
subroutine, C<eval{}> or C<do>.  If more than one value is listed, the
list must be placed in parens.  See L<perlsub/"Temporary Values via
local()"> for details.

But you really probably want to be using my() instead, because local() isn't
what most people think of as "local").  See L<perlsub/"Private Variables
via my()"> for details.

=item localtime EXPR

Converts a time as returned by the time function to a 9-element array
with the time analyzed for the local timezone.  Typically used as
follows:

    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
						localtime(time);

All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6.  If EXPR is omitted, does localtime(time).

In a scalar context, prints out the ctime(3) value:

    $now_string = localtime;  # e.g. "Thu Oct 13 04:54:34 1994"

Also see the F<timelocal.pl> library, and the strftime(3) function available
via the POSIX module.

=item log EXPR

=item log 

Returns logarithm (base I<e>) of EXPR.  If EXPR is omitted, returns log
of $_.

=item lstat FILEHANDLE

=item lstat EXPR

=item lstat 

Does the same thing as the stat() function, but stats a symbolic link
instead of the file the symbolic link points to.  If symbolic links are
unimplemented on your system, a normal stat() is done.

If EXPR is omitted, stats $_.

=item m//

The match operator.  See L<perlop>.

=item map BLOCK LIST

=item map EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
element) and returns the list value composed of the results of each such
evaluation.  Evaluates BLOCK or EXPR in a list context, so each element of LIST
may produce zero, one, or more elements in the returned value.

    @chars = map(chr, @nums);

translates a list of numbers to the corresponding characters.  And

    %hash = map { getkey($_) => $_ } @array;

is just a funny way to write

    %hash = ();
    foreach $_ (@array) {
	$hash{getkey($_)} = $_;
    }

=item mkdir FILENAME,MODE

Creates the directory specified by FILENAME, with permissions specified
by MODE (as modified by umask).  If it succeeds it returns 1, otherwise
it returns 0 and sets C<$!> (errno).

=item msgctl ID,CMD,ARG

Calls the System V IPC function msgctl(2).  If CMD is &IPC_STAT, then ARG
must be a variable which will hold the returned msqid_ds structure.
Returns like ioctl: the undefined value for error, "0 but true" for
zero, or the actual return value otherwise.

=item msgget KEY,FLAGS

Calls the System V IPC function msgget(2).  Returns the message queue id,
or the undefined value if there is an error.

=item msgsnd ID,MSG,FLAGS

Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID.  MSG must begin with the long integer message type,
which may be created with C<pack("l", $type)>.  Returns TRUE if
successful, or FALSE if there is an error.

=item msgrcv ID,VAR,SIZE,TYPE,FLAGS

Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
SIZE.  Note that if a message is received, the message type will be the
first thing in VAR, and the maximum length of VAR is SIZE plus the size
of the message type.  Returns TRUE if successful, or FALSE if there is
an error.

=item my EXPR

A "my" declares the listed variables to be local (lexically) to the
enclosing block, subroutine, C<eval>, or C<do/require/use>'d file.  If
more than one value is listed, the list must be placed in parens.  See
L<perlsub/"Private Variables via my()"> for details.

=item next LABEL

=item next

The C<next> command is like the C<continue> statement in C; it starts
the next iteration of the loop:

    LINE: while (<STDIN>) {
	next LINE if /^#/;	# discard comments
	...
    }

Note that if there were a C<continue> block on the above, it would get
executed even on discarded lines.  If the LABEL is omitted, the command
refers to the innermost enclosing loop.

=item no Module LIST

See the "use" function, which "no" is the opposite of.

=item oct EXPR

=item oct 

Interprets EXPR as an octal string and returns the corresponding
decimal value.  (If EXPR happens to start off with 0x, interprets it as
a hex string instead.)  The following will handle decimal, octal, and
hex in the standard Perl or C notation:

    $val = oct($val) if $val =~ /^0/;

If EXPR is omitted, uses $_.

=item open FILEHANDLE,EXPR

=item open FILEHANDLE

Opens the file whose filename is given by EXPR, and associates it with
FILEHANDLE.  If FILEHANDLE is an expression, its value is used as the name
of the real filehandle wanted.  If EXPR is omitted, the scalar variable of
the same name as the FILEHANDLE contains the filename.  If the filename
begins with "E<lt>" or nothing, the file is opened for input.  If the filename
begins with "E<gt>", the file is opened for output.  If the filename begins
with "E<gt>E<gt>", the file is opened for appending.  You can put a '+' in
front of the 'E<gt>' or 'E<lt>' to indicate that you want both read and write
access to the file; thus '+E<lt>' is usually preferred for read/write
updates--the '+E<gt>' mode would clobber the file first.  These correspond to
the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.

If the filename begins with "|", the filename is interpreted
as a command to which output is to be piped, and if the filename ends with
a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
for more examples of this.  as command which pipes input to us.  (You may
not have a raw open() to a command that pipes both in I<and> out, but see L<open2>,
L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)

Opening '-' opens STDIN and opening 'E<gt>-' opens STDOUT.  Open returns
non-zero upon success, the undefined value otherwise.  If the open
involved a pipe, the return value happens to be the pid of the
subprocess.  

If you're unfortunate enough to be running Perl on a system that
distinguishes between text files and binary files (modern operating
systems don't care), then you should check out L</binmode> for tips for
dealing with this.  The key distinction between systems that need binmode
and those that don't is their text file formats.  Systems like Unix and
Plan9 that delimit lines with a single character, and that encode that
character in C as '\n', do not need C<binmode>.  The rest need it.

Examples:

    $ARTICLE = 100;
    open ARTICLE or die "Can't find article $ARTICLE: $!\n";
    while (<ARTICLE>) {...

    open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)

    open(DBASE, '+<dbase.mine');	    # open for update

    open(ARTICLE, "caesar <$article |");    # decrypt article

    open(EXTRACT, "|sort >/tmp/Tmp$$");     # $$ is our process id

    # process argument list of files along with any includes

    foreach $file (@ARGV) {
	process($file, 'fh00');
    }

    sub process {
	local($filename, $input) = @_;
	$input++;		# this is a string increment
	unless (open($input, $filename)) {
	    print STDERR "Can't open $filename: $!\n";
	    return;
	}

	while (<$input>) {		# note use of indirection
	    if (/^#include "(.*)"/) {
		process($1, $input);
		next;
	    }
	    ...		# whatever
	}
    }

You may also, in the Bourne shell tradition, specify an EXPR beginning
with "E<gt>&", in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) which is to be
duped and opened.  You may use & after E<gt>, E<gt>E<gt>, E<lt>, +E<gt>,
+E<gt>E<gt> and +E<lt>.  The
mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents of
stdio buffers.)
Here is a script that saves, redirects, and restores STDOUT and
STDERR:

    #!/usr/bin/perl
    open(SAVEOUT, ">&STDOUT");
    open(SAVEERR, ">&STDERR");

    open(STDOUT, ">foo.out") || die "Can't redirect stdout";
    open(STDERR, ">&STDOUT") || die "Can't dup stdout";

    select(STDERR); $| = 1;	# make unbuffered
    select(STDOUT); $| = 1;	# make unbuffered

    print STDOUT "stdout 1\n";	# this works for
    print STDERR "stderr 1\n"; 	# subprocesses too

    close(STDOUT);
    close(STDERR);

    open(STDOUT, ">&SAVEOUT");
    open(STDERR, ">&SAVEERR");

    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";


If you specify "E<lt>&=N", where N is a number, then Perl will do an
equivalent of C's fdopen() of that file descriptor; this is more
parsimonious of file descriptors.  For example:

    open(FILEHANDLE, "<&=$fd")

If you open a pipe on the command "-", i.e. either "|-" or "-|", then
there is an implicit fork done, and the return value of open is the pid
of the child within the parent process, and 0 within the child
process.  (Use C<defined($pid)> to determine whether the open was successful.)
The filehandle behaves normally for the parent, but i/o to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
In the child process the filehandle isn't opened--i/o happens from/to
the new STDOUT or STDIN.  Typically this is used like the normal
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when you are running setuid, and
don't want to have to scan shell commands for metacharacters.  
The following pairs are more or less equivalent:

    open(FOO, "|tr '[a-z]' '[A-Z]'");
    open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';

    open(FOO, "cat -n '$file'|");
    open(FOO, "-|") || exec 'cat', '-n', $file;

See L<perlipc/"Safe Pipe Opens"> for more examples of this.

Explicitly closing any piped filehandle causes the parent process to
wait for the child to finish, and returns the status value in C<$?>.
Note: on any operation which may do a fork, unflushed buffers remain
unflushed in both processes, which means you may need to set C<$|> to
avoid duplicate output.

Using the FileHandle constructor from the FileHandle package,
you can generate anonymous filehandles which have the scope of whatever
variables hold references to them, and automatically close whenever
and however you leave that scope:

    use FileHandle;
    ...
    sub read_myfile_munged {
	my $ALL = shift;
	my $handle = new FileHandle;
	open($handle, "myfile") or die "myfile: $!";
	$first = <$handle>
	    or return ();     # Automatically closed here.
	mung $first or die "mung failed";	# Or here.
	return $first, <$handle> if $ALL;	# Or here.
	$first;					# Or here.
    }

The filename that is passed to open will have leading and trailing
whitespace deleted.  In order to open a file with arbitrary weird
characters in it, it's necessary to protect any leading and trailing
whitespace thusly:

    $file =~ s#^(\s)#./$1#;
    open(FOO, "< $file\0");

If you want a "real" C open() (see L<open(2)> on your system), then
you should use the sysopen() function.  This is another way to
protect your filenames from interpretation.  For example:

    use FileHandle;
    sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
	or die "sysopen $path: $!";
    HANDLE->autoflush(1);
    HANDLE->print("stuff $$\n");
    seek(HANDLE, 0, 0);
    print "File contains: ", <HANDLE>;

See L</seek()> for some details about mixing reading and writing.

=item opendir DIRHANDLE,EXPR

Opens a directory named EXPR for processing by readdir(), telldir(),
seekdir(), rewinddir() and closedir().  Returns TRUE if successful.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.

=item ord EXPR

=item ord 

Returns the numeric ascii value of the first character of EXPR.  If
EXPR is omitted, uses $_.

=item pack TEMPLATE,LIST

Takes an array or list of values and packs it into a binary structure,
returning the string containing the structure.  The TEMPLATE is a
sequence of characters that give the order and type of values, as
follows:

    A	An ascii string, will be space padded.
    a	An ascii string, will be null padded.
    b	A bit string (ascending bit order, like vec()).
    B	A bit string (descending bit order).
    h	A hex string (low nybble first).
    H	A hex string (high nybble first).

    c	A signed char value.
    C	An unsigned char value.
    s	A signed short value.
    S	An unsigned short value.
    i	A signed integer value.
    I	An unsigned integer value.
    l	A signed long value.
    L	An unsigned long value.

    n	A short in "network" order.
    N	A long in "network" order.
    v	A short in "VAX" (little-endian) order.
    V	A long in "VAX" (little-endian) order.

    f	A single-precision float in the native format.
    d	A double-precision float in the native format.

    p	A pointer to a null-terminated string.
    P	A pointer to a structure (fixed-length string).

    u	A uuencoded string.

    w A BER compressed integer. Bytes give an unsigned integer base
      128, most significant digit first, with as few digits as
      possible, and with the bit 8 of each byte except the last set
      to "1."

    x	A null byte.
    X	Back up a byte.
    @	Null fill to absolute position.

Each letter may optionally be followed by a number which gives a repeat
count.  With all types except "a", "A", "b", "B", "h" and "H", and "P" the
pack function will gobble up that many values from the LIST.  A * for the
repeat count means to use however many items are left.  The "a" and "A"
types gobble just one value, but pack it as a string of length count,
padding with nulls or spaces as necessary.  (When unpacking, "A" strips
trailing spaces and nulls, but "a" does not.)  Likewise, the "b" and "B"
fields pack a string that many bits long.  The "h" and "H" fields pack a
string that many nybbles long.  The "P" packs a pointer to a structure of
the size indicated by the length.  Real numbers (floats and doubles) are
in the native machine format only; due to the multiplicity of floating
formats around, and the lack of a standard "network" representation, no
facility for interchange has been made.  This means that packed floating
point data written on one machine may not be readable on another - even if
both use IEEE floating point arithmetic (as the endian-ness of the memory
representation is not part of the IEEE spec).  Note that Perl uses doubles
internally for all numeric calculation, and converting from double into
float and thence back to double again will lose precision (i.e.
C<unpack("f", pack("f", $foo)>) will not in general equal $foo).

Examples:

    $foo = pack("cccc",65,66,67,68);
    # foo eq "ABCD"
    $foo = pack("c4",65,66,67,68);
    # same thing

    $foo = pack("ccxxcc",65,66,67,68);
    # foo eq "AB\0\0CD"

    $foo = pack("s2",1,2);
    # "\1\0\2\0" on little-endian
    # "\0\1\0\2" on big-endian

    $foo = pack("a4","abcd","x","y","z");
    # "abcd"

    $foo = pack("aaaa","abcd","x","y","z");
    # "axyz"

    $foo = pack("a14","abcdefg");
    # "abcdefg\0\0\0\0\0\0\0"

    $foo = pack("i9pl", gmtime);
    # a real struct tm (on my system anyway)

    sub bintodec {
	unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
    }

The same template may generally also be used in the unpack function.

=item package NAMESPACE

Declares the compilation unit as being in the given namespace.  The scope
of the package declaration is from the declaration itself through the end of
the enclosing block (the same scope as the local() operator).  All further
unqualified dynamic identifiers will be in this namespace.  A package
statement only affects dynamic variables--including those you've used
local() on--but I<not> lexical variables created with my().  Typically it
would be the first declaration in a file to be included by the C<require>
or C<use> operator.  You can switch into a package in more than one place;
it merely influences which symbol table is used by the compiler for the
rest of that block.  You can refer to variables and filehandles in other
packages by prefixing the identifier with the package name and a double
colon:  C<$Package::Variable>.  If the package name is null, the C<main>
package as assumed.  That is, C<$::sail> is equivalent to C<$main::sail>.

See L<perlmod/"Packages"> for more information about packages, modules,
and classes.  See L<perlsub> for other scoping issues.

=item pipe READHANDLE,WRITEHANDLE

Opens a pair of connected pipes like the corresponding system call.
Note that if you set up a loop of piped processes, deadlock can occur
unless you are very careful.  In addition, note that Perl's pipes use
stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
after each command, depending on the application.

See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
for examples of such things.

=item pop ARRAY

Pops and returns the last value of the array, shortening the array by
1.  Has a similar effect to

    $tmp = $ARRAY[$#ARRAY--];

If there are no elements in the array, returns the undefined value.
If ARRAY is omitted, pops the
@ARGV array in the main program, and the @_ array in subroutines, just
like shift().

=item pos SCALAR

=item pos 

Returns the offset of where the last C<m//g> search left off for the variable
is in question ($_ is used when the variable is not specified). May be
modified to change that offset.

=item print FILEHANDLE LIST

=item print LIST

=item print

Prints a string or a comma-separated list of strings.  Returns TRUE
if successful.  FILEHANDLE may be a scalar variable name, in which case
the variable contains the name of or a reference to the filehandle, thus introducing one
level of indirection.  (NOTE: If FILEHANDLE is a variable and the next
token is a term, it may be misinterpreted as an operator unless you
interpose a + or put parens around the arguments.)  If FILEHANDLE is
omitted, prints by default to standard output (or to the last selected
output channel--see L</select>).  If LIST is also omitted, prints $_ to
STDOUT.  To set the default output channel to something other than
STDOUT use the select operation.  Note that, because print takes a
LIST, anything in the LIST is evaluated in a list context, and any
subroutine that you call will have one or more of its expressions
evaluated in a list context.  Also be careful not to follow the print
keyword with a left parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print--interpose a + or
put parens around all the arguments.

Note that if you're storing FILEHANDLES in an array or other expression,
you will have to use a block returning its value instead:

    print { $files[$i] } "stuff\n";
    print { $OK ? STDOUT : STDERR } "stuff\n";

=item printf FILEHANDLE LIST

=item printf LIST

Equivalent to a "print FILEHANDLE sprintf(LIST)".  The first argument
of the list will be interpreted as the printf format.

=item prototype FUNCTION

Returns the prototype of a function as a string (or C<undef> if the
function has no prototype).  FUNCTION is a reference to the the
function whose prototype you want to retrieve.

=item push ARRAY,LIST

Treats ARRAY as a stack, and pushes the values of LIST
onto the end of ARRAY.  The length of ARRAY increases by the length of
LIST.  Has the same effect as

    for $value (LIST) {
	$ARRAY[++$#ARRAY] = $value;
    }

but is more efficient.  Returns the new number of elements in the array.

=item q/STRING/

=item qq/STRING/

=item qx/STRING/

=item qw/STRING/

Generalized quotes.  See L<perlop>.

=item quotemeta EXPR

=item quotemeta 

Returns the value of EXPR with with all regular expression
metacharacters backslashed.  This is the internal function implementing
the \Q escape in double-quoted strings.

If EXPR is omitted, uses $_.

=item rand EXPR

=item rand

Returns a random fractional number between 0 and the value of EXPR.
(EXPR should be positive.)  If EXPR is omitted, returns a value between 
0 and 1.  This function produces repeatable sequences unless srand() 
is invoked.  See also srand().

(Note: if your rand function consistently returns numbers that are too
large or too small, then your version of Perl was probably compiled
with the wrong number of RANDBITS.  As a workaround, you can usually
multiply EXPR by the correct power of 2 to get the range you want.
This will make your script unportable, however.  It's better to recompile
if you can.)

=item read FILEHANDLE,SCALAR,LENGTH,OFFSET

=item read FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE.  Returns the number of bytes actually read, or
undef if there was an error.  SCALAR will be grown or shrunk to the
length actually read.  An OFFSET may be specified to place the read
data at some other place than the beginning of the string.  This call
is actually implemented in terms of stdio's fread call.  To get a true
read system call, see sysread().

=item readdir DIRHANDLE

Returns the next directory entry for a directory opened by opendir().
If used in a list context, returns all the rest of the entries in the
directory.  If there are no more entries, returns an undefined value in
a scalar context or a null list in a list context.

If you're planning to filetest the return values out of a readdir(), you'd
better prepend the directory in question.  Otherwise, since we didn't
chdir() there, it would have been testing the wrong file.

    opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
    @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
    closedir DIR;

=item readlink EXPR

=item readlink 

Returns the value of a symbolic link, if symbolic links are
implemented.  If not, gives a fatal error.  If there is some system
error, returns the undefined value and sets C<$!> (errno).  If EXPR is
omitted, uses $_.

=item recv SOCKET,SCALAR,LEN,FLAGS

Receives a message on a socket.  Attempts to receive LENGTH bytes of
data into variable SCALAR from the specified SOCKET filehandle.
Actually does a C recvfrom(), so that it can returns the address of the
sender.  Returns the undefined value if there's an error.  SCALAR will
be grown or shrunk to the length actually read.  Takes the same flags
as the system call of the same name.  
See L<perlipc/"UDP: Message Passing"> for examples.

=item redo LABEL

=item redo

The C<redo> command restarts the loop block without evaluating the
conditional again.  The C<continue> block, if any, is not executed.  If
the LABEL is omitted, the command refers to the innermost enclosing
loop.  This command is normally used by programs that want to lie to
themselves about what was just input:

    # a simpleminded Pascal comment stripper
    # (warning: assumes no { or } in strings)
    LINE: while (<STDIN>) {
	while (s|({.*}.*){.*}|$1 |) {}
	s|{.*}| |;
	if (s|{.*| |) {
	    $front = $_;
	    while (<STDIN>) {
		if (/}/) {	# end of comment?
		    s|^|$front{|;
		    redo LINE;
		}
	    }
	}
	print;
    }

=item ref EXPR

=item ref 

Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
is not specified, $_ will be used. The value returned depends on the
type of thing the reference is a reference to.
Builtin types include:

    REF
    SCALAR
    ARRAY
    HASH
    CODE
    GLOB

If the referenced object has been blessed into a package, then that package 
name is returned instead.  You can think of ref() as a typeof() operator.

    if (ref($r) eq "HASH") {
	print "r is a reference to an associative array.\n";
    } 
    if (!ref ($r) {
	print "r is not a reference at all.\n";
    } 

See also L<perlref>.

=item rename OLDNAME,NEWNAME

Changes the name of a file.  Returns 1 for success, 0 otherwise.  Will
not work across filesystem boundaries.

=item require EXPR

=item require

Demands some semantics specified by EXPR, or by $_ if EXPR is not
supplied.  If EXPR is numeric, demands that the current version of Perl
(C<$]> or $PERL_VERSION) be equal or greater than EXPR.

Otherwise, demands that a library file be included if it hasn't already
been included.  The file is included via the do-FILE mechanism, which is
essentially just a variety of eval().  Has semantics similar to the following
subroutine:

    sub require {
	local($filename) = @_;
	return 1 if $INC{$filename};
	local($realfilename,$result);
	ITER: {
	    foreach $prefix (@INC) {
		$realfilename = "$prefix/$filename";
		if (-f $realfilename) {
		    $result = do $realfilename;
		    last ITER;
		}
	    }
	    die "Can't find $filename in \@INC";
	}
	die $@ if $@;
	die "$filename did not return true value" unless $result;
	$INC{$filename} = $realfilename;
	$result;
    }

Note that the file will not be included twice under the same specified
name.  The file must return TRUE as the last statement to indicate
successful execution of any initialization code, so it's customary to
end such a file with "1;" unless you're sure it'll return TRUE
otherwise.  But it's better just to put the "C<1;>", in case you add more
statements.

If EXPR is a bare word, the require assumes a "F<.pm>" extension and
replaces "F<::>" with "F</>" in the filename for you,
to make it easy to load standard modules.  This form of loading of 
modules does not risk altering your namespace.

For a yet-more-powerful import facility, see L</use> and 
L<perlmod>.

=item reset EXPR

=item reset

Generally used in a C<continue> block at the end of a loop to clear
variables and reset ?? searches so that they work again.  The
expression is interpreted as a list of single characters (hyphens
allowed for ranges).  All variables and arrays beginning with one of
those letters are reset to their pristine state.  If the expression is
omitted, one-match searches (?pattern?) are reset to match again.  Only
resets variables or searches in the current package.  Always returns
1.  Examples:

    reset 'X';		# reset all X variables
    reset 'a-z';	# reset lower case variables
    reset;		# just reset ?? searches

Resetting "A-Z" is not recommended since you'll wipe out your
ARGV and ENV arrays.  Only resets package variables--lexical variables
are unaffected, but they clean themselves up on scope exit anyway,
so you'll probably want to use them instead.  See L</my>.

=item return LIST

Returns from a subroutine or eval with the value specified.  (Note that
in the absence of a return a subroutine or eval() will automatically
return the value of the last expression evaluated.)

=item reverse LIST

In a list context, returns a list value consisting of the elements
of LIST in the opposite order.  In a scalar context, returns a string
value consisting of the bytes of the first element of LIST in the
opposite order.   

    print reverse <>;			# line tac 

    undef $/;
    print scalar reverse scalar <>;	# byte tac

=item rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the
readdir() routine on DIRHANDLE.

=item rindex STR,SUBSTR,POSITION

=item rindex STR,SUBSTR

Works just like index except that it returns the position of the LAST
occurrence of SUBSTR in STR.  If POSITION is specified, returns the
last occurrence at or before that position.

=item rmdir FILENAME

=item rmdir 

Deletes the directory specified by FILENAME if it is empty.  If it
succeeds it returns 1, otherwise it returns 0 and sets C<$!> (errno).  If
FILENAME is omitted, uses $_.

=item s///

The substitution operator.  See L<perlop>.

=item scalar EXPR

Forces EXPR to be interpreted in a scalar context and returns the value
of EXPR.  

    @counts = ( scalar @a, scalar @b, scalar @c );

There is no equivalent operator to force an expression to 
be interpolated in a list context because it's in practice never
needed.  If you really wanted to do so, however, you could use
the construction C<@{[ (some expression) ]}>, but usually a simple
C<(some expression)> suffices.

=item seek FILEHANDLE,POSITION,WHENCE

Randomly positions the file pointer for FILEHANDLE, just like the fseek()
call of stdio.  FILEHANDLE may be an expression whose value gives the name
of the filehandle.  The values for WHENCE are 0 to set the file pointer to
POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
plus offset.  You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
this from POSIX module.  Returns 1 upon success, 0 otherwise.

On some systems you have to do a seek whenever you switch between reading
and writing.  Amongst other things, this may have the effect of calling
stdio's clearerr(3).  A "whence" of 1 (SEEK_CUR) is useful for not moving
the file pointer:

    seek(TEST,0,1);

This is also useful for applications emulating C<tail -f>.  Once you hit
EOF on your read, and then sleep for a while, you might have to stick in a
seek() to reset things.  First the simple trick listed above to clear the
filepointer.  The seek() doesn't change the current position, but it
I<does> clear the end-of-file condition on the handle, so that the next
C<E<lt>FILEE<gt>> makes Perl try again to read something.  Hopefully.

If that doesn't work (some stdios are particularly cantankerous), then
you may need something more like this:

    for (;;) {
	for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
	    # search for some stuff and put it into files
	}
	sleep($for_a_while);
	seek(FILE, $curpos, 0);
    }

=item seekdir DIRHANDLE,POS

Sets the current position for the readdir() routine on DIRHANDLE.  POS
must be a value returned by telldir().  Has the same caveats about
possible directory compaction as the corresponding system library
routine.

=item select FILEHANDLE

=item select

Returns the currently selected filehandle.  Sets the current default
filehandle for output, if FILEHANDLE is supplied.  This has two
effects: first, a C<write> or a C<print> without a filehandle will
default to this FILEHANDLE.  Second, references to variables related to
output will refer to this output channel.  For example, if you have to
set the top of form format for more than one output channel, you might
do the following:

    select(REPORT1);
    $^ = 'report1_top';
    select(REPORT2);
    $^ = 'report2_top';

FILEHANDLE may be an expression whose value gives the name of the
actual filehandle.  Thus:

    $oldfh = select(STDERR); $| = 1; select($oldfh);

Some programmers may prefer to think of filehandles as objects with
methods, preferring to write the last example as:

    use FileHandle;
    STDERR->autoflush(1);

=item select RBITS,WBITS,EBITS,TIMEOUT

This calls the select(2) system call with the bitmasks specified, which
can be constructed using fileno() and vec(), along these lines:

    $rin = $win = $ein = '';
    vec($rin,fileno(STDIN),1) = 1;
    vec($win,fileno(STDOUT),1) = 1;
    $ein = $rin | $win;

If you want to select on many filehandles you might wish to write a
subroutine:

    sub fhbits {
	local(@fhlist) = split(' ',$_[0]);
	local($bits);
	for (@fhlist) {
	    vec($bits,fileno($_),1) = 1;
	}
	$bits;
    }
    $rin = fhbits('STDIN TTY SOCK');

The usual idiom is:

    ($nfound,$timeleft) =
      select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

or to block until something becomes ready just do this 

    $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

Most systems do not both to return anything useful in $timeleft, so
calling select() in a scalar context just returns $nfound.

Any of the bitmasks can also be undef.  The timeout, if specified, is
in seconds, which may be fractional.  Note: not all implementations are
capable of returning the $timeleft.  If not, they always return
$timeleft equal to the supplied $timeout.

You can effect a 250-millisecond sleep this way:

    select(undef, undef, undef, 0.25);

B<WARNING>: Do not attempt to mix buffered I/O (like read() or E<lt>FHE<gt>)
with select().  You have to use sysread() instead.

=item semctl ID,SEMNUM,CMD,ARG

Calls the System V IPC function semctl.  If CMD is &IPC_STAT or
&GETALL, then ARG must be a variable which will hold the returned
semid_ds structure or semaphore value array.  Returns like ioctl: the
undefined value for error, "0 but true" for zero, or the actual return
value otherwise.

=item semget KEY,NSEMS,FLAGS

Calls the System V IPC function semget.  Returns the semaphore id, or
the undefined value if there is an error.

=item semop KEY,OPSTRING

Calls the System V IPC function semop to perform semaphore operations
such as signaling and waiting.  OPSTRING must be a packed array of
semop structures.  Each semop structure can be generated with
C<pack("sss", $semnum, $semop, $semflag)>.  The number of semaphore
operations is implied by the length of OPSTRING.  Returns TRUE if
successful, or FALSE if there is an error.  As an example, the
following code waits on semaphore $semnum of semaphore id $semid:

    $semop = pack("sss", $semnum, -1, 0);
    die "Semaphore trouble: $!\n" unless semop($semid, $semop);

To signal the semaphore, replace "-1" with "1".

=item send SOCKET,MSG,FLAGS,TO

=item send SOCKET,MSG,FLAGS

Sends a message on a socket.  Takes the same flags as the system call
of the same name.  On unconnected sockets you must specify a
destination to send TO, in which case it does a C sendto().  Returns
the number of characters sent, or the undefined value if there is an
error.
See L<perlipc/"UDP: Message Passing"> for examples.

=item setpgrp PID,PGRP

Sets the current process group for the specified PID, 0 for the current
process.  Will produce a fatal error if used on a machine that doesn't
implement setpgrp(2).  If the arguments are ommitted, it defaults to
0,0.  Note that the POSIX version of setpgrp() does not accept any
arguments, so only setpgrp 0,0 is portable.

=item setpriority WHICH,WHO,PRIORITY

Sets the current priority for a process, a process group, or a user.
(See setpriority(2).)  Will produce a fatal error if used on a machine
that doesn't implement setpriority(2).

=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

Sets the socket option requested.  Returns undefined if there is an
error.  OPTVAL may be specified as undef if you don't want to pass an
argument.

=item shift ARRAY

=item shift

Shifts the first value of the array off and returns it, shortening the
array by 1 and moving everything down.  If there are no elements in the
array, returns the undefined value.  If ARRAY is omitted, shifts the
@ARGV array in the main program, and the @_ array in subroutines.
(This is determined lexically.)  See also unshift(), push(), and pop().
Shift() and unshift() do the same thing to the left end of an array
that push() and pop() do to the right end.

=item shmctl ID,CMD,ARG

Calls the System V IPC function shmctl.  If CMD is &IPC_STAT, then ARG
must be a variable which will hold the returned shmid_ds structure.
Returns like ioctl: the undefined value for error, "0 but true" for
zero, or the actual return value otherwise.

=item shmget KEY,SIZE,FLAGS

Calls the System V IPC function shmget.  Returns the shared memory
segment id, or the undefined value if there is an error.

=item shmread ID,VAR,POS,SIZE

=item shmwrite ID,STRING,POS,SIZE

Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out, and
detaching from it.  When reading, VAR must be a variable which will
hold the data read.  When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
SIZE bytes.  Return TRUE if successful, or FALSE if there is an error.

=item shutdown SOCKET,HOW

Shuts down a socket connection in the manner indicated by HOW, which
has the same interpretation as in the system call of the same name.

=item sin EXPR

=item sin 

Returns the sine of EXPR (expressed in radians).  If EXPR is omitted,
returns sine of $_.

=item sleep EXPR

=item sleep

Causes the script to sleep for EXPR seconds, or forever if no EXPR.
May be interrupted by sending the process a SIGALRM.  Returns the
number of seconds actually slept.  You probably cannot mix alarm() and
sleep() calls, since sleep() is often implemented using alarm().

On some older systems, it may sleep up to a full second less than what
you requested, depending on how it counts seconds.  Most modern systems
always sleep the full amount.

For delays of finer granularity than one second, you may use Perl's
syscall() interface to access setitimer(2) if your system supports it, 
or else see L</select()> below.  

=item socket SOCKET,DOMAIN,TYPE,PROTOCOL

Opens a socket of the specified kind and attaches it to filehandle
SOCKET.  DOMAIN, TYPE and PROTOCOL are specified the same as for the
system call of the same name.  You should "use Socket;" first to get
the proper definitions imported.  See the example in L<perlipc/"Sockets: Client/Server Communication">.

=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL

Creates an unnamed pair of sockets in the specified domain, of the
specified type.  DOMAIN, TYPE and PROTOCOL are specified the same as
for the system call of the same name.  If unimplemented, yields a fatal
error.  Returns TRUE if successful.

=item sort SUBNAME LIST

=item sort BLOCK LIST

=item sort LIST

Sorts the LIST and returns the sorted list value.  Nonexistent values
of arrays are stripped out.  If SUBNAME or BLOCK is omitted, sorts
in standard string comparison order.  If SUBNAME is specified, it
gives the name of a subroutine that returns an integer less than, equal
to, or greater than 0, depending on how the elements of the array are
to be ordered.  (The E<lt>=E<gt> and cmp operators are extremely useful in such
routines.)  SUBNAME may be a scalar variable name, in which case the
value provides the name of the subroutine to use.  In place of a
SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
subroutine.

In the interests of efficiency the normal calling code for subroutines is
bypassed, with the following effects: the subroutine may not be a
recursive subroutine, and the two elements to be compared are passed into
the subroutine not via @_ but as the package global variables $a and
$b (see example below).  They are passed by reference, so don't
modify $a and $b.  And don't try to declare them as lexicals either.

Examples:

    # sort lexically
    @articles = sort @files;

    # same thing, but with explicit sort routine
    @articles = sort {$a cmp $b} @files;

    # now case-insensitively
    @articles = sort { uc($a) cmp uc($b)} @files;

    # same thing in reversed order
    @articles = sort {$b cmp $a} @files;

    # sort numerically ascending
    @articles = sort {$a <=> $b} @files;

    # sort numerically descending
    @articles = sort {$b <=> $a} @files;

    # sort using explicit subroutine name
    sub byage {
	$age{$a} <=> $age{$b};	# presuming integers
    }
    @sortedclass = sort byage @class;

    # this sorts the %age associative arrays by value 
    # instead of key using an inline function
    @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

    sub backwards { $b cmp $a; }
    @harry = ('dog','cat','x','Cain','Abel');
    @george = ('gone','chased','yz','Punished','Axed');
    print sort @harry;
	    # prints AbelCaincatdogx
    print sort backwards @harry;
	    # prints xdogcatCainAbel
    print sort @george, 'to', @harry;
	    # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

    # inefficiently sort by descending numeric compare using 
    # the first integer after the first = sign, or the 
    # whole record case-insensitively otherwise

    @new = sort {
	($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
			    ||
	            uc($a)  cmp  uc($b)
    } @old;

    # same thing, but much more efficiently;
    # we'll build auxiliary indices instead
    # for speed
    @nums = @caps = ();
    for (@old) { 
	push @nums, /=(\d+)/;
	push @caps, uc($_);
    } 

    @new = @old[ sort {
			$nums[$b] <=> $nums[$a]
				 ||
			$caps[$a] cmp $caps[$b]
		       } 0..$#old
	       ];

    # same thing using a Schwartzian Transform (no temps)
    @new = map { $_->[0] }
        sort { $b->[1] <=> $a->[1]
                        ||
               $a->[2] cmp $b->[2]
        } map { [$_, /=(\d+)/, uc($_)] } @old;

If you're using strict, you I<MUST NOT> declare $a
and $b as lexicals.  They are package globals.  That means
if you're in the C<main> package, it's

    @articles = sort {$main::b <=> $main::a} @files;

or just

    @articles = sort {$::b <=> $::a} @files;

but if you're in the C<FooPack> package, it's

    @articles = sort {$FooPack::b <=> $FooPack::a} @files;

The comparison function is required to behave.  If it returns
inconsistent results (sometimes saying $x[1] is less than $x[2] and
sometimes saying the opposite, for example) the Perl interpreter will
probably crash and dump core.  This is entirely due to and dependent
upon your system's qsort(3) library routine; this routine often avoids
sanity checks in the interest of speed.

=item splice ARRAY,OFFSET,LENGTH,LIST

=item splice ARRAY,OFFSET,LENGTH

=item splice ARRAY,OFFSET

Removes the elements designated by OFFSET and LENGTH from an array, and
replaces them with the elements of LIST, if any.  Returns the elements
removed from the array.  The array grows or shrinks as necessary.  If
LENGTH is omitted, removes everything from OFFSET onward.  The
following equivalencies hold (assuming C<$[ == 0>):

    push(@a,$x,$y)	splice(@a,$#a+1,0,$x,$y)
    pop(@a)		splice(@a,-1)
    shift(@a)		splice(@a,0,1)
    unshift(@a,$x,$y)	splice(@a,0,0,$x,$y)
    $a[$x] = $y		splice(@a,$x,1,$y);

Example, assuming array lengths are passed before arrays:

    sub aeq {	# compare two list values
	local(@a) = splice(@_,0,shift);
	local(@b) = splice(@_,0,shift);
	return 0 unless @a == @b;	# same len?
	while (@a) {
	    return 0 if pop(@a) ne pop(@b);
	}
	return 1;
    }
    if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }

=item split /PATTERN/,EXPR,LIMIT

=item split /PATTERN/,EXPR

=item split /PATTERN/

=item split

Splits a string into an array of strings, and returns it.

If not in a list context, returns the number of fields found and splits into
the @_ array.  (In a list context, you can force the split into @_ by
using C<??> as the pattern delimiters, but it still returns the array
value.)  The use of implicit split to @_ is deprecated, however.

If EXPR is omitted, splits the $_ string.  If PATTERN is also omitted,
splits on whitespace (after skipping any leading whitespace).  Anything
matching PATTERN is taken to be a delimiter separating the fields.  (Note
that the delimiter may be longer than one character.)  If LIMIT is
specified and is not negative, splits into no more than that many fields
(though it may split into fewer).  If LIMIT is unspecified, trailing null
fields are stripped (which potential users of pop() would do well to
remember).  If LIMIT is negative, it is treated as if an arbitrarily large
LIMIT had been specified.

A pattern matching the null string (not to be confused with
a null pattern C<//>, which is just one member of the set of patterns
matching a null string) will split the value of EXPR into separate
characters at each point it matches that way.  For example:

    print join(':', split(/ */, 'hi there'));

produces the output 'h:i:t:h:e:r:e'.

The LIMIT parameter can be used to partially split a line

    ($login, $passwd, $remainder) = split(/:/, $_, 3);

When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
one larger than the number of variables in the list, to avoid
unnecessary work.  For the list above LIMIT would have been 4 by
default.  In time critical applications it behooves you not to split
into more fields than you really need.

If the PATTERN contains parentheses, additional array elements are
created from each matching substring in the delimiter.

    split(/([,-])/, "1-10,20", 3);

produces the list value

    (1, '-', 10, ',', 20)

If you had the entire header of a normal Unix email message in $header, 
you could split it up into fields and their values this way:

    $header =~ s/\n\s+/ /g;  # fix continuation lines
    %hdrs   =  (UNIX_FROM => split /^(.*?):\s*/m, $header);

The pattern C</PATTERN/> may be replaced with an expression to specify
patterns that vary at runtime.  (To do runtime compilation only once,
use C</$variable/o>.)

As a special case, specifying a PATTERN of space (C<' '>) will split on
white space just as split with no arguments does.  Thus, split(' ') can
be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
will give you as many null initial fields as there are leading spaces.
A split on /\s+/ is like a split(' ') except that any leading
whitespace produces a null first field.  A split with no arguments
really does a C<split(' ', $_)> internally.

Example:

    open(passwd, '/etc/passwd');
    while (<passwd>) {
	($login, $passwd, $uid, $gid, $gcos, 
	    $home, $shell) = split(/:/);
	...
    }

(Note that $shell above will still have a newline on it.  See L</chop>, 
L</chomp>, and L</join>.)

=item sprintf FORMAT,LIST

Returns a string formatted by the usual printf conventions of the C
language.  See L<sprintf(3)> or L<printf(3)> on your system for details.
(The * character for an indirectly specified length is not
supported, but you can get the same effect by interpolating a variable
into the pattern.)  Some C libraries' implementations of sprintf() can
dump core when fed ludicrous arguments.

=item sqrt EXPR

=item sqrt 

Return the square root of EXPR.  If EXPR is omitted, returns square
root of $_.

=item srand EXPR

Sets the random number seed for the C<rand> operator.  If EXPR is omitted,
uses a semirandom value based on the current time and process ID, among
other things.  Of course, you'd need something much more random than that for
cryptographic purposes, since it's easy to guess the current time.
Checksumming the compressed output of rapidly changing operating system
status programs is the usual method.  Examples are posted regularly to
the comp.security.unix newsgroup.

=item stat FILEHANDLE

=item stat EXPR

=item stat 

Returns a 13-element array giving the status info for a file, either the
file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it
stats $_.  Returns a null list if the stat fails.  Typically used as
follows:


    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
       $atime,$mtime,$ctime,$blksize,$blocks)
           = stat($filename);

Not all fields are supported on all filesystem types.  Here are the 
meaning of the fields:

  dev	    device number of filesystem 
  ino	    inode number 
  mode	    file mode  (type and permissions)
  nlink	    number of (hard) links to the file 
  uid	    numeric user ID of file's owner 
  gid	    numer group ID of file's owner 
  rdev	    the device identifier (special files only)
  size	    total size of file, in bytes 
  atime	    last access time since the epoch
  mtime	    last modify time since the epoch
  ctime	    inode change time (NOT creation type!) since the epoch
  blksize   preferred blocksize for file system I/O
  blocks    actual number of blocks allocated

(The epoch was at 00:00 January 1, 1970 GMT.)

If stat is passed the special filehandle consisting of an underline, no
stat is done, but the current contents of the stat structure from the
last stat or filetest are returned.  Example:

    if (-x $file && (($d) = stat(_)) && $d < 0) {
	print "$file is executable NFS file\n";
    }

(This only works on machines for which the device number is negative under NFS.)

=item study SCALAR

=item study

Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching on, and on the distribution of character
frequencies in the string to be searched--you probably want to compare
runtimes with and without it to see which runs faster.  Those loops
which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most.  You may have only
one study active at a time--if you study a different scalar the first
is "unstudied".  (The way study works is this: a linked list of every
character in the string to be searched is made, so we know, for
example, where all the 'k' characters are.  From each search string,
the rarest character is selected, based on some static frequency tables
constructed from some C programs and English text.  Only those places
that contain this "rarest" character are examined.)

For example, here is a loop which inserts index producing entries
before any line containing a certain pattern:

    while (<>) {
	study;
	print ".IX foo\n" if /\bfoo\b/;
	print ".IX bar\n" if /\bbar\b/;
	print ".IX blurfl\n" if /\bblurfl\b/;
	...
	print;
    }

In searching for /\bfoo\b/, only those locations in $_ that contain "f"
will be looked at, because "f" is rarer than "o".  In general, this is
a big win except in pathological cases.  The only question is whether
it saves you more time than it took to build the linked list in the
first place.

Note that if you have to look for strings that you don't know till
runtime, you can build an entire loop as a string and eval that to
avoid recompiling all your patterns all the time.  Together with
undefining $/ to input entire files as one record, this can be very
fast, often faster than specialized programs like fgrep(1).  The following
scans a list of files (C<@files>) for a list of words (C<@words>), and prints
out the names of those files that contain a match:

    $search = 'while (<>) { study;';
    foreach $word (@words) {
	$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
    }
    $search .= "}";
    @ARGV = @files;
    undef $/;
    eval $search;		# this screams
    $/ = "\n";		# put back to normal input delim
    foreach $file (sort keys(%seen)) {
	print $file, "\n";
    }

=item sub BLOCK

=item sub NAME

=item sub NAME BLOCK

This is subroutine definition, not a real function I<per se>.  With just a
NAME (and possibly prototypes), it's just a forward declaration.  Without
a NAME, it's an anonymous function declaration, and does actually return a
value: the CODE ref of the closure you just created. See L<perlsub> and
L<perlref> for details.

=item substr EXPR,OFFSET,LEN

=item substr EXPR,OFFSET

Extracts a substring out of EXPR and returns it.  First character is at
offset 0, or whatever you've set $[ to.  If OFFSET is negative, starts
that far from the end of the string.  If LEN is omitted, returns
everything to the end of the string.  If LEN is negative, leaves that
many characters off the end of the string.

You can use the substr() function
as an lvalue, in which case EXPR must be an lvalue.  If you assign
something shorter than LEN, the string will shrink, and if you assign
something longer than LEN, the string will grow to accommodate it.  To
keep the string the same length you may need to pad or chop your value
using sprintf().

=item symlink OLDFILE,NEWFILE

Creates a new filename symbolically linked to the old filename.
Returns 1 for success, 0 otherwise.  On systems that don't support
symbolic links, produces a fatal error at run time.  To check for that,
use eval:

    $symlink_exists = (eval 'symlink("","");', $@ eq '');

=item syscall LIST

Calls the system call specified as the first element of the list,
passing the remaining elements as arguments to the system call.  If
unimplemented, produces a fatal error.  The arguments are interpreted
as follows: if a given argument is numeric, the argument is passed as
an int.  If not, the pointer to the string value is passed.  You are
responsible to make sure a string is pre-extended long enough to
receive any result that might be written into a string.  If your
integer arguments are not literals and have never been interpreted in a
numeric context, you may need to add 0 to them to force them to look
like numbers.

    require 'syscall.ph';		# may need to run h2ph
    syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);

Note that Perl only supports passing of up to 14 arguments to your system call,
which in practice should usually suffice.

=item sysopen FILEHANDLE,FILENAME,MODE

=item sysopen FILEHANDLE,FILENAME,MODE,PERMS

Opens the file whose filename is given by FILENAME, and associates it
with FILEHANDLE.  If FILEHANDLE is an expression, its value is used as
the name of the real filehandle wanted.  This function calls the
underlying operating system's C<open> function with the parameters
FILENAME, MODE, PERMS.

The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module C<Fcntl>.
However, for historical reasons, some values are universal: zero means
read-only, one means write-only, and two means read/write.

If the file named by FILENAME does not exist and the C<open> call
creates it (typically because MODE includes the O_CREAT flag), then
the value of PERMS specifies the permissions of the newly created
file.  If PERMS is omitted, the default value is 0666, which allows
read and write for all.  This default is reasonable: see C<umask>.

=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET

=item sysread FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE, using the system call read(2).  It bypasses
stdio, so mixing this with other kinds of reads may cause confusion.
Returns the number of bytes actually read, or undef if there was an
error.  SCALAR will be grown or shrunk to the length actually read.
In the case of growing the new data area will be padded with "\0" bytes.
An OFFSET may be specified to place the read data at some other
place than the beginning of the string. A negative OFFSET means
placing the read data at that many bytes counting backwards from the end
of the string.

=item system LIST

Does exactly the same thing as "exec LIST" except that a fork is done
first, and the parent process waits for the child process to complete.
Note that argument processing varies depending on the number of
arguments.  The return value is the exit status of the program as
returned by the wait() call.  To get the actual exit value divide by
256.  See also L</exec>.  This is I<NOT> what you want to use to capture 
the output from a command, for that you should merely use backticks, as
described in L<perlop/"`STRING`">.

=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET

=item syswrite FILEHANDLE,SCALAR,LENGTH

Attempts to write LENGTH bytes of data from variable SCALAR to the
specified FILEHANDLE, using the system call write(2).  It bypasses
stdio, so mixing this with prints may cause confusion.  Returns the
number of bytes actually written, or undef if there was an error.
If the length is greater than the available data, only as much data as
is available will be written.  An OFFSET may be specified to write the
data from some other place than the beginning of the string.
A negative OFFSET means starting the writing from that many bytes
counting backwards from the end of the string.

=item tell FILEHANDLE

=item tell

Returns the current file position for FILEHANDLE.  FILEHANDLE may be an
expression whose value gives the name of the actual filehandle.  If
FILEHANDLE is omitted, assumes the file last read.

=item telldir DIRHANDLE

Returns the current position of the readdir() routines on DIRHANDLE.
Value may be given to seekdir() to access a particular location in a
directory.  Has the same caveats about possible directory compaction as
the corresponding system library routine.

=item tie VARIABLE,CLASSNAME,LIST

This function binds a variable to a package class that will provide the
implementation for the variable.  VARIABLE is the name of the variable
to be enchanted.  CLASSNAME is the name of a class implementing objects
of correct type.  Any additional arguments are passed to the "new"
method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
Typically these are arguments such as might be passed to the dbm_open()
function of C.  The object returned by the "new" method is also
returned by the tie() function, which would be useful if you want to
access other methods in CLASSNAME.

Note that functions such as keys() and values() may return huge array
values when used on large objects, like DBM files.  You may prefer to
use the each() function to iterate over such.  Example:

    # print out history file offsets
    use NDBM_File;
    tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
    while (($key,$val) = each %HIST) {
	print $key, ' = ', unpack('L',$val), "\n";
    }
    untie(%HIST);

A class implementing an associative array should have the following
methods:

    TIEHASH classname, LIST
    DESTROY this
    FETCH this, key
    STORE this, key, value
    DELETE this, key
    EXISTS this, key
    FIRSTKEY this
    NEXTKEY this, lastkey

A class implementing an ordinary array should have the following methods:

    TIEARRAY classname, LIST
    DESTROY this
    FETCH this, key
    STORE this, key, value
    [others TBD]

A class implementing a scalar should have the following methods:

    TIESCALAR classname, LIST
    DESTROY this
    FETCH this, 
    STORE this, value

Unlike dbmopen(), the tie() function will not use or require a module
for you--you need to do that explicitly yourself.  See L<DB_File>
or the F<Config> module for interesting tie() implementations.

=item tied VARIABLE

Returns a reference to the object underlying VARIABLE (the same value
that was originally returned by the tie() call which bound the variable
to a package.)  Returns the undefined value if VARIABLE isn't tied to a
package.

=item time

Returns the number of non-leap seconds since whatever time the system
considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
and 00:00:00 UTC, January 1, 1970 for most other systems).
Suitable for feeding to gmtime() and localtime().

=item times

Returns a four-element array giving the user and system times, in
seconds, for this process and the children of this process.

    ($user,$system,$cuser,$csystem) = times;

=item tr///

The translation operator.  See L<perlop>.

=item truncate FILEHANDLE,LENGTH

=item truncate EXPR,LENGTH

Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length.  Produces a fatal error if truncate isn't implemented
on your system.

=item uc EXPR

=item uc 

Returns an uppercased version of EXPR.  This is the internal function
implementing the \U escape in double-quoted strings.
Should respect any POSIX setlocale() settings.

If EXPR is omitted, uses $_.

=item ucfirst EXPR

=item ucfirst 

Returns the value of EXPR with the first character uppercased.  This is
the internal function implementing the \u escape in double-quoted strings.
Should respect any POSIX setlocale() settings.

If EXPR is omitted, uses $_.

=item umask EXPR

=item umask

Sets the umask for the process and returns the old one.  If EXPR is
omitted, merely returns current umask.

=item undef EXPR

=item undef

Undefines the value of EXPR, which must be an lvalue.  Use only on a
scalar value, an entire array, or a subroutine name (using "&").  (Using undef()
will probably not do what you expect on most predefined variables or
DBM list values, so don't do that.)  Always returns the undefined value.  You can omit
the EXPR, in which case nothing is undefined, but you still get an
undefined value that you could, for instance, return from a
subroutine.  Examples:

    undef $foo;
    undef $bar{'blurfl'};
    undef @ary;
    undef %assoc;
    undef &mysub;
    return (wantarray ? () : undef) if $they_blew_it;

=item unlink LIST

=item unlink 

Deletes a list of files.  Returns the number of files successfully
deleted.

    $cnt = unlink 'a', 'b', 'c';
    unlink @goners;
    unlink <*.bak>;

Note: unlink will not delete directories unless you are superuser and
the B<-U> flag is supplied to Perl.  Even if these conditions are
met, be warned that unlinking a directory can inflict damage on your
filesystem.  Use rmdir instead.

If LIST is omitted, uses $_.

=item unpack TEMPLATE,EXPR

Unpack does the reverse of pack: it takes a string representing a
structure and expands it out into a list value, returning the array
value.  (In a scalar context, it merely returns the first value
produced.)  The TEMPLATE has the same format as in the pack function.
Here's a subroutine that does substring:

    sub substr {
	local($what,$where,$howmuch) = @_;
	unpack("x$where a$howmuch", $what);
    }

and then there's

    sub ordinal { unpack("c",$_[0]); } # same as ord()

In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
themselves.  Default is a 16-bit checksum.  For example, the following
computes the same number as the System V sum program:

    while (<>) {
	$checksum += unpack("%16C*", $_);
    }
    $checksum %= 65536;

The following efficiently counts the number of set bits in a bit vector:

    $setbits = unpack("%32b*", $selectmask);

=item untie VARIABLE

Breaks the binding between a variable and a package.  (See tie().)

=item unshift ARRAY,LIST

Does the opposite of a C<shift>.  Or the opposite of a C<push>,
depending on how you look at it.  Prepends list to the front of the
array, and returns the new number of elements in the array.

    unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;

Note the LIST is prepended whole, not one element at a time, so the
prepended elements stay in the same order.  Use reverse to do the
reverse.

=item use Module LIST

=item use Module

=item use Module VERSION LIST

=item use VERSION

Imports some semantics into the current package from the named module,
generally by aliasing certain subroutine or variable names into your
package.  It is exactly equivalent to

    BEGIN { require Module; import Module LIST; }

except that Module I<must> be a bare word.

If the first argument to C<use> is a number, it is treated as a version
number instead of a module name.  If the version of the Perl interpreter
is less than VERSION, then an error message is printed and Perl exits
immediately.  This is often useful if you need to check the current
Perl version before C<use>ing library modules which have changed in
incompatible ways from older versions of Perl.  (We try not to do
this more than we have to.)

The BEGIN forces the require and import to happen at compile time.  The
require makes sure the module is loaded into memory if it hasn't been
yet.  The import is not a builtin--it's just an ordinary static method
call into the "Module" package to tell the module to import the list of
features back into the current package.  The module can implement its
import method any way it likes, though most modules just choose to
derive their import method via inheritance from the Exporter class that
is defined in the Exporter module. See L<Exporter>.  If no import
method can be found then the error is currently silently ignored. This
may change to a fatal error in a future version.

If you don't want your namespace altered, explicitly supply an empty list:

    use Module ();

That is exactly equivalent to

    BEGIN { require Module; }

If the VERSION argument is present between Module and LIST, then the
C<use> will fail if the C<$VERSION> variable in package Module is
less than VERSION.

Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way.  Currently implemented pragmas are:

    use integer;
    use diagnostics;
    use sigtrap qw(SEGV BUS);
    use strict  qw(subs vars refs);
    use subs    qw(afunc blurfl);

These pseudomodules import semantics into the current block scope, unlike
ordinary modules, which import symbols into the current package (which are
effective through the end of the file).

There's a corresponding "no" command that unimports meanings imported
by use, i.e. it calls C<unimport Module LIST> instead of C<import>.

    no integer;
    no strict 'refs';

If no unimport method can be found the call fails with a fatal error.

See L<perlmod> for a list of standard modules and pragmas.

=item utime LIST

Changes the access and modification times on each file of a list of
files.  The first two elements of the list must be the NUMERICAL access
and modification times, in that order.  Returns the number of files
successfully changed.  The inode modification time of each file is set
to the current time.  Example of a "touch" command:

    #!/usr/bin/perl
    $now = time;
    utime $now, $now, @ARGV;

=item values ASSOC_ARRAY

Returns a normal array consisting of all the values of the named
associative array.  (In a scalar context, returns the number of
values.)  The values are returned in an apparently random order, but it
is the same order as either the keys() or each() function would produce
on the same array.  See also keys(), each(), and sort().

=item vec EXPR,OFFSET,BITS

Treats the string in EXPR as a vector of unsigned integers, and
returns the value of the bitfield specified by OFFSET.  BITS specifies
the number of bits that are reserved for each entry in the bit
vector. This must be a power of two from 1 to 32. vec() may also be
assigned to, in which case parens are needed to give the expression
the correct precedence as in

    vec($image, $max_x * $x + $y, 8) = 3;

Vectors created with vec() can also be manipulated with the logical
operators |, & and ^, which will assume a bit vector operation is
desired when both operands are strings.

To transform a bit vector into a string or array of 0's and 1's, use these:

    $bits = unpack("b*", $vector);
    @bits = split(//, unpack("b*", $vector));

If you know the exact length in bits, it can be used in place of the *.

=item wait

Waits for a child process to terminate and returns the pid of the
deceased process, or -1 if there are no child processes.  The status is
returned in C<$?>.

=item waitpid PID,FLAGS

Waits for a particular child process to terminate and returns the pid
of the deceased process, or -1 if there is no such child process.  The
status is returned in C<$?>.  If you say

    use POSIX ":wait_h";
    ...
    waitpid(-1,&WNOHANG);

then you can do a non-blocking wait for any process.  Non-blocking wait
is only available on machines supporting either the waitpid(2) or
wait4(2) system calls.  However, waiting for a particular pid with
FLAGS of 0 is implemented everywhere.  (Perl emulates the system call
by remembering the status values of processes that have exited but have
not been harvested by the Perl script yet.)

=item wantarray

Returns TRUE if the context of the currently executing subroutine is
looking for a list value.  Returns FALSE if the context is looking
for a scalar.

    return wantarray ? () : undef;

=item warn LIST

Produces a message on STDERR just like die(), but doesn't exit or
on an exception.

=item write FILEHANDLE

=item write EXPR

=item write

Writes a formatted record (possibly multi-line) to the specified file,
using the format associated with that file.  By default the format for
a file is the one having the same name is the filehandle, but the
format for the current output channel (see the select() function) may be set
explicitly by assigning the name of the format to the C<$~> variable.

Top of form processing is handled automatically:  if there is
insufficient room on the current page for the formatted record, the
page is advanced by writing a form feed, a special top-of-page format
is used to format the new page header, and then the record is written.
By default the top-of-page format is the name of the filehandle with
"_TOP" appended, but it may be dynamically set to the format of your
choice by assigning the name to the C<$^> variable while the filehandle is
selected.  The number of lines remaining on the current page is in
variable C<$->, which can be set to 0 to force a new page.

If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
C<select> operator.  If the FILEHANDLE is an EXPR, then the expression
is evaluated and the resulting string is used to look up the name of
the FILEHANDLE at run time.  For more on formats, see L<perlform>.

Note that write is I<NOT> the opposite of read.  Unfortunately.

=item y///

The translation operator.  See L<perlop>.

=back