summaryrefslogtreecommitdiffstats
path: root/Doc/library/os.rst
blob: b98ad8057b9d173d19fc9a5a635e8958263ac0cc (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
:mod:`os` --- Miscellaneous operating system interfaces
=======================================================

.. module:: os
   :synopsis: Miscellaneous operating system interfaces.


This module provides a portable way of using operating system dependent
functionality.  If you just want to read or write a file see :func:`open`, if
you want to manipulate paths, see the :mod:`os.path` module, and if you want to
read all the lines in all the files on the command line see the :mod:`fileinput`
module.  For creating temporary files and directories see the :mod:`tempfile`
module, and for high-level file and directory handling see the :mod:`shutil`
module.

Notes on the availability of these functions:

* The design of all built-in operating system dependent modules of Python is
  such that as long as the same functionality is available, it uses the same
  interface; for example, the function ``os.stat(path)`` returns stat
  information about *path* in the same format (which happens to have originated
  with the POSIX interface).

* Extensions peculiar to a particular operating system are also available
  through the :mod:`os` module, but using them is of course a threat to
  portability.

* All functions accepting path or file names accept both bytes and string
  objects, and result in an object of the same type, if a path or file name is
  returned.

* An "Availability: Unix" note means that this function is commonly found on
  Unix systems.  It does not make any claims about its existence on a specific
  operating system.

* If not separately noted, all functions that claim "Availability: Unix" are
  supported on Mac OS X, which builds on a Unix core.

.. Availability notes get their own line and occur at the end of the function
.. documentation.

.. note::

   All functions in this module raise :exc:`OSError` in the case of invalid or
   inaccessible file names and paths, or other arguments that have the correct
   type, but are not accepted by the operating system.

.. exception:: error

   An alias for the built-in :exc:`OSError` exception.


.. data:: name

   The name of the operating system dependent module imported.  The following
   names have currently been registered: ``'posix'``, ``'nt'``, ``'mac'``,
   ``'ce'``, ``'java'``.

   .. seealso::
      :attr:`sys.platform` has a finer granularity.  :func:`os.uname` gives
      system-dependent version information.

      The :mod:`platform` module provides detailed checks for the
      system's identity.


.. _os-filenames:

File Names, Command Line Arguments, and Environment Variables
-------------------------------------------------------------

In Python, file names, command line arguments, and environment variables are
represented using the string type. On some systems, decoding these strings to
and from bytes is necessary before passing them to the operating system. Python
uses the file system encoding to perform this conversion (see
:func:`sys.getfilesystemencoding`).

.. versionchanged:: 3.1
   On some systems, conversion using the file system encoding may fail. In this
   case, Python uses the ``surrogateescape`` encoding error handler, which means
   that undecodable bytes are replaced by a Unicode character U+DCxx on
   decoding, and these are again translated to the original byte on encoding.


The file system encoding must guarantee to successfully decode all bytes
below 128. If the file system encoding fails to provide this guarantee, API
functions may raise UnicodeErrors.


.. _os-procinfo:

Process Parameters
------------------

These functions and data items provide information and operate on the current
process and user.


.. function:: ctermid()

   Return the filename corresponding to the controlling terminal of the process.

   Availability: Unix.


.. data:: environ

   A :term:`mapping` object representing the string environment. For example,
   ``environ['HOME']`` is the pathname of your home directory (on some platforms),
   and is equivalent to ``getenv("HOME")`` in C.

   This mapping is captured the first time the :mod:`os` module is imported,
   typically during Python startup as part of processing :file:`site.py`.  Changes
   to the environment made after this time are not reflected in ``os.environ``,
   except for changes made by modifying ``os.environ`` directly.

   If the platform supports the :func:`putenv` function, this mapping may be used
   to modify the environment as well as query the environment.  :func:`putenv` will
   be called automatically when the mapping is modified.

   On Unix, keys and values use :func:`sys.getfilesystemencoding` and
   ``'surrogateescape'`` error handler. Use :data:`environb` if you would like
   to use a different encoding.

   .. note::

      Calling :func:`putenv` directly does not change ``os.environ``, so it's better
      to modify ``os.environ``.

   .. note::

      On some platforms, including FreeBSD and Mac OS X, setting ``environ`` may
      cause memory leaks.  Refer to the system documentation for
      :c:func:`putenv`.

   If :func:`putenv` is not provided, a modified copy of this mapping  may be
   passed to the appropriate process-creation functions to cause  child processes
   to use a modified environment.

   If the platform supports the :func:`unsetenv` function, you can delete items in
   this mapping to unset environment variables. :func:`unsetenv` will be called
   automatically when an item is deleted from ``os.environ``, and when
   one of the :meth:`pop` or :meth:`clear` methods is called.


.. data:: environb

   Bytes version of :data:`environ`: a :term:`mapping` object representing the
   environment as byte strings. :data:`environ` and :data:`environb` are
   synchronized (modify :data:`environb` updates :data:`environ`, and vice
   versa).

   :data:`environb` is only available if :data:`supports_bytes_environ` is
   True.

   .. versionadded:: 3.2


.. function:: chdir(path)
              fchdir(fd)
              getcwd()
   :noindex:

   These functions are described in :ref:`os-file-dir`.


.. function:: fsencode(filename)

   Encode *filename* to the filesystem encoding with ``'surrogateescape'``
   error handler, or ``'strict'`` on Windows; return :class:`bytes` unchanged.

   :func:`fsdecode` is the reverse function.

   .. versionadded:: 3.2


.. function:: fsdecode(filename)

   Decode *filename* from the filesystem encoding with ``'surrogateescape'``
   error handler, or ``'strict'`` on Windows; return :class:`str` unchanged.

   :func:`fsencode` is the reverse function.

   .. versionadded:: 3.2


.. function:: getenv(key, default=None)

   Return the value of the environment variable *key* if it exists, or
   *default* if it doesn't. *key*, *default* and the result are str.

   On Unix, keys and values are decoded with :func:`sys.getfilesystemencoding`
   and ``'surrogateescape'`` error handler. Use :func:`os.getenvb` if you
   would like to use a different encoding.

   Availability: most flavors of Unix, Windows.


.. function:: getenvb(key, default=None)

   Return the value of the environment variable *key* if it exists, or
   *default* if it doesn't. *key*, *default* and the result are bytes.

   Availability: most flavors of Unix.

   .. versionadded:: 3.2


.. function:: get_exec_path(env=None)

   Returns the list of directories that will be searched for a named
   executable, similar to a shell, when launching a process.
   *env*, when specified, should be an environment variable dictionary
   to lookup the PATH in.
   By default, when *env* is None, :data:`environ` is used.

   .. versionadded:: 3.2


.. function:: getegid()

   Return the effective group id of the current process.  This corresponds to the
   "set id" bit on the file being executed in the current process.

   Availability: Unix.


.. function:: geteuid()

   .. index:: single: user; effective id

   Return the current process's effective user id.

   Availability: Unix.


.. function:: getgid()

   .. index:: single: process; group

   Return the real group id of the current process.

   Availability: Unix.


.. function:: getgrouplist(user, group)

   Return list of group ids that *user* belongs to. If *group* is not in the
   list, it is included; typically, *group* is specified as the group ID
   field from the password record for *user*.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: getgroups()

   Return list of supplemental group ids associated with the current process.

   Availability: Unix.

   .. note:: On Mac OS X, :func:`getgroups` behavior differs somewhat from
      other Unix platforms. If the Python interpreter was built with a
      deployment target of :const:`10.5` or earlier, :func:`getgroups` returns
      the list of effective group ids associated with the current user process;
      this list is limited to a system-defined number of entries, typically 16,
      and may be modified by calls to :func:`setgroups` if suitably privileged.
      If built with a deployment target greater than :const:`10.5`,
      :func:`getgroups` returns the current group access list for the user
      associated with the effective user id of the process; the group access
      list may change over the lifetime of the process, it is not affected by
      calls to :func:`setgroups`, and its length is not limited to 16.  The
      deployment target value, :const:`MACOSX_DEPLOYMENT_TARGET`, can be
      obtained with :func:`sysconfig.get_config_var`.


.. function:: getlogin()

   Return the name of the user logged in on the controlling terminal of the
   process.  For most purposes, it is more useful to use the environment variables
   :envvar:`LOGNAME` or :envvar:`USERNAME` to find out who the user is, or
   ``pwd.getpwuid(os.getuid())[0]`` to get the login name of the currently
   effective user id.

   Availability: Unix, Windows.


.. function:: getpgid(pid)

   Return the process group id of the process with process id *pid*. If *pid* is 0,
   the process group id of the current process is returned.

   Availability: Unix.

.. function:: getpgrp()

   .. index:: single: process; group

   Return the id of the current process group.

   Availability: Unix.


.. function:: getpid()

   .. index:: single: process; id

   Return the current process id.

   Availability: Unix, Windows.


.. function:: getppid()

   .. index:: single: process; id of parent

   Return the parent's process id.  When the parent process has exited, on Unix
   the id returned is the one of the init process (1), on Windows it is still
   the same id, which may be already reused by another process.

   Availability: Unix, Windows.

   .. versionchanged:: 3.2
      Added support for Windows.


.. function:: getpriority(which, who)

   .. index:: single: process; scheduling priority

   Get program scheduling priority.  The value *which* is one of
   :const:`PRIO_PROCESS`, :const:`PRIO_PGRP`, or :const:`PRIO_USER`, and *who*
   is interpreted relative to *which* (a process identifier for
   :const:`PRIO_PROCESS`, process group identifier for :const:`PRIO_PGRP`, and a
   user ID for :const:`PRIO_USER`).  A zero value for *who* denotes
   (respectively) the calling process, the process group of the calling process,
   or the real user ID of the calling process.

   Availability: Unix.

   .. versionadded:: 3.3


.. data:: PRIO_PROCESS
          PRIO_PGRP
          PRIO_USER

   Parameters for the :func:`getpriority` and :func:`setpriority` functions.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: getresuid()

   Return a tuple (ruid, euid, suid) denoting the current process's
   real, effective, and saved user ids.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: getresgid()

   Return a tuple (rgid, egid, sgid) denoting the current process's
   real, effective, and saved group ids.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: getuid()

   .. index:: single: user; id

   Return the current process's user id.

   Availability: Unix.


.. function:: initgroups(username, gid)

   Call the system initgroups() to initialize the group access list with all of
   the groups of which the specified username is a member, plus the specified
   group id.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: putenv(key, value)

   .. index:: single: environment variables; setting

   Set the environment variable named *key* to the string *value*.  Such
   changes to the environment affect subprocesses started with :func:`os.system`,
   :func:`popen` or :func:`fork` and :func:`execv`.

   Availability: most flavors of Unix, Windows.

   .. note::

      On some platforms, including FreeBSD and Mac OS X, setting ``environ`` may
      cause memory leaks. Refer to the system documentation for putenv.

   When :func:`putenv` is supported, assignments to items in ``os.environ`` are
   automatically translated into corresponding calls to :func:`putenv`; however,
   calls to :func:`putenv` don't update ``os.environ``, so it is actually
   preferable to assign to items of ``os.environ``.


.. function:: setegid(egid)

   Set the current process's effective group id.

   Availability: Unix.


.. function:: seteuid(euid)

   Set the current process's effective user id.

   Availability: Unix.


.. function:: setgid(gid)

   Set the current process' group id.

   Availability: Unix.


.. function:: setgroups(groups)

   Set the list of supplemental group ids associated with the current process to
   *groups*. *groups* must be a sequence, and each element must be an integer
   identifying a group. This operation is typically available only to the superuser.

   Availability: Unix.

   .. note:: On Mac OS X, the length of *groups* may not exceed the
      system-defined maximum number of effective group ids, typically 16.
      See the documentation for :func:`getgroups` for cases where it may not
      return the same group list set by calling setgroups().

.. function:: setpgrp()

   Call the system call :c:func:`setpgrp` or ``setpgrp(0, 0)`` depending on
   which version is implemented (if any).  See the Unix manual for the semantics.

   Availability: Unix.


.. function:: setpgid(pid, pgrp)

   Call the system call :c:func:`setpgid` to set the process group id of the
   process with id *pid* to the process group with id *pgrp*.  See the Unix manual
   for the semantics.

   Availability: Unix.


.. function:: setpriority(which, who, priority)

   .. index:: single: process; scheduling priority

   Set program scheduling priority. The value *which* is one of
   :const:`PRIO_PROCESS`, :const:`PRIO_PGRP`, or :const:`PRIO_USER`, and *who*
   is interpreted relative to *which* (a process identifier for
   :const:`PRIO_PROCESS`, process group identifier for :const:`PRIO_PGRP`, and a
   user ID for :const:`PRIO_USER`). A zero value for *who* denotes
   (respectively) the calling process, the process group of the calling process,
   or the real user ID of the calling process.
   *priority* is a value in the range -20 to 19. The default priority is 0;
   lower priorities cause more favorable scheduling.

   Availability: Unix

   .. versionadded:: 3.3


.. function:: setregid(rgid, egid)

   Set the current process's real and effective group ids.

   Availability: Unix.


.. function:: setresgid(rgid, egid, sgid)

   Set the current process's real, effective, and saved group ids.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: setresuid(ruid, euid, suid)

   Set the current process's real, effective, and saved user ids.

   Availability: Unix.

   .. versionadded:: 3.2


.. function:: setreuid(ruid, euid)

   Set the current process's real and effective user ids.

   Availability: Unix.


.. function:: getsid(pid)

   Call the system call :c:func:`getsid`.  See the Unix manual for the semantics.

   Availability: Unix.


.. function:: setsid()

   Call the system call :c:func:`setsid`.  See the Unix manual for the semantics.

   Availability: Unix.


.. function:: setuid(uid)

   .. index:: single: user; id, setting

   Set the current process's user id.

   Availability: Unix.


.. placed in this section since it relates to errno.... a little weak
.. function:: strerror(code)

   Return the error message corresponding to the error code in *code*.
   On platforms where :c:func:`strerror` returns ``NULL`` when given an unknown
   error number, :exc:`ValueError` is raised.

   Availability: Unix, Windows.


.. data:: supports_bytes_environ

   ``True`` if the native OS type of the environment is bytes (eg. ``False`` on
   Windows).

   .. versionadded:: 3.2


.. function:: umask(mask)

   Set the current numeric umask and return the previous umask.

   Availability: Unix, Windows.


.. function:: uname()

   .. index::
      single: gethostname() (in module socket)
      single: gethostbyaddr() (in module socket)

   Returns information identifying the current operating system.
   The return value is an object with five attributes:

   * :attr:`sysname` - operating system name
   * :attr:`nodename` - name of machine on network (implementation-defined)
   * :attr:`release` - operating system release
   * :attr:`version` - operating system version
   * :attr:`machine` - hardware identifier

   For backwards compatibility, this object is also iterable, behaving
   like a five-tuple containing :attr:`sysname`, :attr:`nodename`,
   :attr:`release`, :attr:`version`, and :attr:`machine`
   in that order.

   Some systems truncate :attr:`nodename` to 8 characters or to the
   leading component; a better way to get the hostname is
   :func:`socket.gethostname`  or even
   ``socket.gethostbyaddr(socket.gethostname())``.

   Availability: recent flavors of Unix.

   .. versionchanged:: 3.3
      Return type changed from a tuple to a tuple-like object
      with named attributes.


.. function:: unsetenv(key)

   .. index:: single: environment variables; deleting

   Unset (delete) the environment variable named *key*. Such changes to the
   environment affect subprocesses started with :func:`os.system`, :func:`popen` or
   :func:`fork` and :func:`execv`.

   When :func:`unsetenv` is supported, deletion of items in ``os.environ`` is
   automatically translated into a corresponding call to :func:`unsetenv`; however,
   calls to :func:`unsetenv` don't update ``os.environ``, so it is actually
   preferable to delete items of ``os.environ``.

   Availability: most flavors of Unix, Windows.


.. _os-newstreams:

File Object Creation
--------------------

This function creates new :term:`file objects <file object>`.  (See also
:func:`~os.open` for opening file descriptors.)


.. function:: fdopen(fd, *args, **kwargs)

   Return an open file object connected to the file descriptor *fd*.  This is an
   alias of the :func:`open` built-in function and accepts the same arguments.
   The only difference is that the first argument of :func:`fdopen` must always
   be an integer.


.. _os-fd-ops:

File Descriptor Operations
--------------------------

These functions operate on I/O streams referenced using file descriptors.

File descriptors are small integers corresponding to a file that has been opened
by the current process.  For example, standard input is usually file descriptor
0, standard output is 1, and standard error is 2.  Further files opened by a
process will then be assigned 3, 4, 5, and so forth.  The name "file descriptor"
is slightly deceptive; on Unix platforms, sockets and pipes are also referenced
by file descriptors.

The :meth:`~io.IOBase.fileno` method can be used to obtain the file descriptor
associated with a :term:`file object` when required.  Note that using the file
descriptor directly will bypass the file object methods, ignoring aspects such
as internal buffering of data.


.. function:: close(fd)

   Close file descriptor *fd*.

   Availability: Unix, Windows.

   .. note::

      This function is intended for low-level I/O and must be applied to a file
      descriptor as returned by :func:`os.open` or :func:`pipe`.  To close a "file
      object" returned by the built-in function :func:`open` or by :func:`popen` or
      :func:`fdopen`, use its :meth:`~io.IOBase.close` method.


.. function:: closerange(fd_low, fd_high)

   Close all file descriptors from *fd_low* (inclusive) to *fd_high* (exclusive),
   ignoring errors. Equivalent to (but much faster than)::

      for fd in range(fd_low, fd_high):
          try:
              os.close(fd)
          except OSError:
              pass

   Availability: Unix, Windows.


.. function:: device_encoding(fd)

   Return a string describing the encoding of the device associated with *fd*
   if it is connected to a terminal; else return :const:`None`.


.. function:: dup(fd)

   Return a duplicate of file descriptor *fd*. The new file descriptor is
   :ref:`non-inheritable <fd_inheritance>`.

   On Windows, when duplicating a standard stream (0: stdin, 1: stdout,
   2: stderr), the new file descriptor is :ref:`inheritable
   <fd_inheritance>`.

   Availability: Unix, Windows.

   .. versionchanged:: 3.4
      The new file descriptor is now non-inheritable.


.. function:: dup2(fd, fd2, inheritable=True)

   Duplicate file descriptor *fd* to *fd2*, closing the latter first if necessary.
   The file descriptor *fd2* is :ref:`inheritable <fd_inheritance>` by default,
   or non-inheritable if *inheritable* is ``False``.

   Availability: Unix, Windows.

   .. versionchanged:: 3.4
      Add the optional *inheritable* parameter.


.. function:: fchmod(fd, mode)

   Change the mode of the file given by *fd* to the numeric *mode*.  See the
   docs for :func:`chmod` for possible values of *mode*.  As of Python 3.3, this
   is equivalent to ``os.chmod(fd, mode)``.

   Availability: Unix.


.. function:: fchown(fd, uid, gid)

   Change the owner and group id of the file given by *fd* to the numeric *uid*
   and *gid*.  To leave one of the ids unchanged, set it to -1.  See
   :func:`chown`.  As of Python 3.3, this is equivalent to ``os.chown(fd, uid,
   gid)``.

   Availability: Unix.


.. function:: fdatasync(fd)

   Force write of file with filedescriptor *fd* to disk. Does not force update of
   metadata.

   Availability: Unix.

   .. note::
      This function is not available on MacOS.


.. function:: fpathconf(fd, name)

   Return system configuration information relevant to an open file. *name*
   specifies the configuration value to retrieve; it may be a string which is the
   name of a defined system value; these names are specified in a number of
   standards (POSIX.1, Unix 95, Unix 98, and others).  Some platforms define
   additional names as well.  The names known to the host operating system are
   given in the ``pathconf_names`` dictionary.  For configuration variables not
   included in that mapping, passing an integer for *name* is also accepted.

   If *name* is a string and is not known, :exc:`ValueError` is raised.  If a
   specific value for *name* is not supported by the host system, even if it is
   included in ``pathconf_names``, an :exc:`OSError` is raised with
   :const:`errno.EINVAL` for the error number.

   As of Python 3.3, this is equivalent to ``os.pathconf(fd, name)``.

   Availability: Unix.


.. function:: fstat(fd)

   Return status for file descriptor *fd*, like :func:`~os.stat`.  As of Python
   3.3, this is equivalent to ``os.stat(fd)``.

   Availability: Unix, Windows.


.. function:: fstatvfs(fd)

   Return information about the filesystem containing the file associated with
   file descriptor *fd*, like :func:`statvfs`.  As of Python 3.3, this is
   equivalent to ``os.statvfs(fd)``.

   Availability: Unix.


.. function:: fsync(fd)

   Force write of file with filedescriptor *fd* to disk.  On Unix, this calls the
   native :c:func:`fsync` function; on Windows, the MS :c:func:`_commit` function.

   If you're starting with a buffered Python :term:`file object` *f*, first do
   ``f.flush()``, and then do ``os.fsync(f.fileno())``, to ensure that all internal
   buffers associated with *f* are written to disk.

   Availability: Unix, Windows.


.. function:: ftruncate(fd, length)

   Truncate the file corresponding to file descriptor *fd*, so that it is at
   most *length* bytes in size.  As of Python 3.3, this is equivalent to
   ``os.truncate(fd, length)``.

   Availability: Unix.


.. function:: isatty(fd)

   Return ``True`` if the file descriptor *fd* is open and connected to a
   tty(-like) device, else ``False``.


.. function:: lockf(fd, cmd, len)

   Apply, test or remove a POSIX lock on an open file descriptor.
   *fd* is an open file descriptor.
   *cmd* specifies the command to use - one of :data:`F_LOCK`, :data:`F_TLOCK`,
   :data:`F_ULOCK` or :data:`F_TEST`.
   *len* specifies the section of the file to lock.

   Availability: Unix.

   .. versionadded:: 3.3


.. data:: F_LOCK
          F_TLOCK
          F_ULOCK
          F_TEST

   Flags that specify what action :func:`lockf` will take.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: lseek(fd, pos, how)

   Set the current position of file descriptor *fd* to position *pos*, modified
   by *how*: :const:`SEEK_SET` or ``0`` to set the position relative to the
   beginning of the file; :const:`SEEK_CUR` or ``1`` to set it relative to the
   current position; :const:`SEEK_END` or ``2`` to set it relative to the end of
   the file. Return the new cursor position in bytes, starting from the beginning.

   Availability: Unix, Windows.


.. data:: SEEK_SET
          SEEK_CUR
          SEEK_END

   Parameters to the :func:`lseek` function. Their values are 0, 1, and 2,
   respectively.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      Some operating systems could support additional values, like
      :data:`os.SEEK_HOLE` or :data:`os.SEEK_DATA`.


.. function:: open(file, flags, mode=0o777, *, dir_fd=None)

   Open the file *file* and set various flags according to *flags* and possibly
   its mode according to *mode*.  When computing *mode*, the current umask value
   is first masked out.  Return the file descriptor for the newly opened file.
   The new file descriptor is :ref:`non-inheritable <fd_inheritance>`.

   For a description of the flag and mode values, see the C run-time documentation;
   flag constants (like :const:`O_RDONLY` and :const:`O_WRONLY`) are defined in
   the :mod:`os` module.  In particular, on Windows adding
   :const:`O_BINARY` is needed to open files in binary mode.

   This function can support :ref:`paths relative to directory descriptors
   <dir_fd>` with the *dir_fd* parameter.

   Availability: Unix, Windows.

   .. versionchanged:: 3.4
      The new file descriptor is now non-inheritable.

   .. note::

      This function is intended for low-level I/O.  For normal usage, use the
      built-in function :func:`open`, which returns a :term:`file object` with
      :meth:`~file.read` and :meth:`~file.write` methods (and many more).  To
      wrap a file descriptor in a file object, use :func:`fdopen`.

   .. versionadded:: 3.3
      The *dir_fd* argument.

The following constants are options for the *flags* parameter to the
:func:`~os.open` function.  They can be combined using the bitwise OR operator
``|``.  Some of them are not available on all platforms.  For descriptions of
their availability and use, consult the :manpage:`open(2)` manual page on Unix
or `the MSDN <http://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windows.


.. data:: O_RDONLY
          O_WRONLY
          O_RDWR
          O_APPEND
          O_CREAT
          O_EXCL
          O_TRUNC

   These constants are available on Unix and Windows.


.. data:: O_DSYNC
          O_RSYNC
          O_SYNC
          O_NDELAY
          O_NONBLOCK
          O_NOCTTY
          O_SHLOCK
          O_EXLOCK
          O_CLOEXEC

   These constants are only available on Unix.

   .. versionchanged:: 3.3
      Add :data:`O_CLOEXEC` constant.

.. data:: O_BINARY
          O_NOINHERIT
          O_SHORT_LIVED
          O_TEMPORARY
          O_RANDOM
          O_SEQUENTIAL
          O_TEXT

   These constants are only available on Windows.


.. data:: O_ASYNC
          O_DIRECT
          O_DIRECTORY
          O_NOFOLLOW
          O_NOATIME
          O_PATH
          O_TMPFILE

   These constants are GNU extensions and not present if they are not defined by
   the C library.

   .. versionchanged:: 3.4
      Add :data:`O_TMPFILE` constant. It's only available on Linux Kernel 3.11
      or newer.


.. function:: openpty()

   .. index:: module: pty

   Open a new pseudo-terminal pair. Return a pair of file descriptors
   ``(master, slave)`` for the pty and the tty, respectively. The new file
   descriptors are :ref:`non-inheritable <fd_inheritance>`. For a (slightly) more
   portable approach, use the :mod:`pty` module.

   Availability: some flavors of Unix.

   .. versionchanged:: 3.4
      The new file descriptors are now non-inheritable.


.. function:: pipe()

   Create a pipe.  Return a pair of file descriptors ``(r, w)`` usable for
   reading and writing, respectively. The new file descriptor is
   :ref:`non-inheritable <fd_inheritance>`.

   Availability: Unix, Windows.

   .. versionchanged:: 3.4
      The new file descriptors are now non-inheritable.


.. function:: pipe2(flags)

   Create a pipe with *flags* set atomically.
   *flags* can be constructed by ORing together one or more of these values:
   :data:`O_NONBLOCK`, :data:`O_CLOEXEC`.
   Return a pair of file descriptors ``(r, w)`` usable for reading and writing,
   respectively.

   Availability: some flavors of Unix.

   .. versionadded:: 3.3


.. function:: posix_fallocate(fd, offset, len)

   Ensures that enough disk space is allocated for the file specified by *fd*
   starting from *offset* and continuing for *len* bytes.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: posix_fadvise(fd, offset, len, advice)

   Announces an intention to access data in a specific pattern thus allowing
   the kernel to make optimizations.
   The advice applies to the region of the file specified by *fd* starting at
   *offset* and continuing for *len* bytes.
   *advice* is one of :data:`POSIX_FADV_NORMAL`, :data:`POSIX_FADV_SEQUENTIAL`,
   :data:`POSIX_FADV_RANDOM`, :data:`POSIX_FADV_NOREUSE`,
   :data:`POSIX_FADV_WILLNEED` or :data:`POSIX_FADV_DONTNEED`.

   Availability: Unix.

   .. versionadded:: 3.3


.. data:: POSIX_FADV_NORMAL
          POSIX_FADV_SEQUENTIAL
          POSIX_FADV_RANDOM
          POSIX_FADV_NOREUSE
          POSIX_FADV_WILLNEED
          POSIX_FADV_DONTNEED

   Flags that can be used in *advice* in :func:`posix_fadvise` that specify
   the access pattern that is likely to be used.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: pread(fd, buffersize, offset)

   Read from a file descriptor, *fd*, at a position of *offset*. It will read up
   to *buffersize* number of bytes. The file offset remains unchanged.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: pwrite(fd, string, offset)

   Write *string* to a file descriptor, *fd*, from *offset*, leaving the file
   offset unchanged.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: read(fd, n)

   Read at most *n* bytes from file descriptor *fd*. Return a bytestring containing the
   bytes read.  If the end of the file referred to by *fd* has been reached, an
   empty bytes object is returned.

   Availability: Unix, Windows.

   .. note::

      This function is intended for low-level I/O and must be applied to a file
      descriptor as returned by :func:`os.open` or :func:`pipe`.  To read a
      "file object" returned by the built-in function :func:`open` or by
      :func:`popen` or :func:`fdopen`, or :data:`sys.stdin`, use its
      :meth:`~file.read` or :meth:`~file.readline` methods.


.. function:: sendfile(out, in, offset, nbytes)
              sendfile(out, in, offset, nbytes, headers=None, trailers=None, flags=0)

   Copy *nbytes* bytes from file descriptor *in* to file descriptor *out*
   starting at *offset*.
   Return the number of bytes sent. When EOF is reached return 0.

   The first function notation is supported by all platforms that define
   :func:`sendfile`.

   On Linux, if *offset* is given as ``None``, the bytes are read from the
   current position of *in* and the position of *in* is updated.

   The second case may be used on Mac OS X and FreeBSD where *headers* and
   *trailers* are arbitrary sequences of buffers that are written before and
   after the data from *in* is written. It returns the same as the first case.

   On Mac OS X and FreeBSD, a value of 0 for *nbytes* specifies to send until
   the end of *in* is reached.

   All platforms support sockets as *out* file descriptor, and some platforms
   allow other types (e.g. regular file, pipe) as well.

   Availability: Unix.

   .. versionadded:: 3.3


.. data:: SF_NODISKIO
          SF_MNOWAIT
          SF_SYNC

   Parameters to the :func:`sendfile` function, if the implementation supports
   them.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: readv(fd, buffers)

   Read from a file descriptor *fd* into a number of mutable :term:`bytes-like
   objects <bytes-like object>` *buffers*. :func:`~os.readv` will transfer data
   into each buffer until it is full and then move on to the next buffer in the
   sequence to hold the rest of the data. :func:`~os.readv` returns the total
   number of bytes read (which may be less than the total capacity of all the
   objects).

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: tcgetpgrp(fd)

   Return the process group associated with the terminal given by *fd* (an open
   file descriptor as returned by :func:`os.open`).

   Availability: Unix.


.. function:: tcsetpgrp(fd, pg)

   Set the process group associated with the terminal given by *fd* (an open file
   descriptor as returned by :func:`os.open`) to *pg*.

   Availability: Unix.


.. function:: ttyname(fd)

   Return a string which specifies the terminal device associated with
   file descriptor *fd*.  If *fd* is not associated with a terminal device, an
   exception is raised.

   Availability: Unix.


.. function:: write(fd, str)

   Write the bytestring in *str* to file descriptor *fd*. Return the number of
   bytes actually written.

   Availability: Unix, Windows.

   .. note::

      This function is intended for low-level I/O and must be applied to a file
      descriptor as returned by :func:`os.open` or :func:`pipe`.  To write a "file
      object" returned by the built-in function :func:`open` or by :func:`popen` or
      :func:`fdopen`, or :data:`sys.stdout` or :data:`sys.stderr`, use its
      :meth:`~file.write` method.


.. function:: writev(fd, buffers)

   Write the contents of *buffers* to file descriptor *fd*. *buffers* must be a
   sequence of :term:`bytes-like objects <bytes-like object>`.
   :func:`~os.writev` writes the contents of each object to the file descriptor
   and returns the total number of bytes written.

   Availability: Unix.

   .. versionadded:: 3.3


.. _terminal-size:

Querying the size of a terminal
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 3.3

.. function:: get_terminal_size(fd=STDOUT_FILENO)

   Return the size of the terminal window as ``(columns, lines)``,
   tuple of type :class:`terminal_size`.

   The optional argument ``fd`` (default ``STDOUT_FILENO``, or standard
   output) specifies which file descriptor should be queried.

   If the file descriptor is not connected to a terminal, an :exc:`OSError`
   is raised.

   :func:`shutil.get_terminal_size` is the high-level function which
   should normally be used, ``os.get_terminal_size`` is the low-level
   implementation.

   Availability: Unix, Windows.

.. class:: terminal_size

   A subclass of tuple, holding ``(columns, lines)`` of the terminal window size.

   .. attribute:: columns

      Width of the terminal window in characters.

   .. attribute:: lines

      Height of the terminal window in characters.


.. _fd_inheritance:

Inheritance of File Descriptors
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 3.4

A file descriptor has an "inheritable" flag which indicates if the file descriptor
can be inherited by child processes.  Since Python 3.4, file descriptors
created by Python are non-inheritable by default.

On UNIX, non-inheritable file descriptors are closed in child processes at the
execution of a new program, other file descriptors are inherited.

On Windows, non-inheritable handles and file descriptors are closed in child
processes, except for standard streams (file descriptors 0, 1 and 2: stdin, stdout
and stderr), which are always inherited.  Using :func:`spawn\* <spawnl>` functions,
all inheritable handles and all inheritable file descriptors are inherited.
Using the :mod:`subprocess` module, all file descriptors except standard
streams are closed, and inheritable handles are only inherited if the
*close_fds* parameter is ``False``.

.. function:: get_inheritable(fd)

   Get the "inheritable" flag of the specified file descriptor (a boolean).

.. function:: set_inheritable(fd, inheritable)

   Set the "inheritable" flag of the specified file descriptor.

.. function:: get_handle_inheritable(handle)

   Get the "inheritable" flag of the specified handle (a boolean).

   Availability: Windows.

.. function:: set_handle_inheritable(handle, inheritable)

   Set the "inheritable" flag of the specified handle.

   Availability: Windows.


.. _os-file-dir:

Files and Directories
---------------------

On some Unix platforms, many of these functions support one or more of these
features:

.. _path_fd:

* **specifying a file descriptor:**
  For some functions, the *path* argument can be not only a string giving a path
  name, but also a file descriptor.  The function will then operate on the file
  referred to by the descriptor.  (For POSIX systems, Python will call the
  ``f...`` version of the function.)

  You can check whether or not *path* can be specified as a file descriptor on
  your platform using :data:`os.supports_fd`.  If it is unavailable, using it
  will raise a :exc:`NotImplementedError`.

  If the function also supports *dir_fd* or *follow_symlinks* arguments, it is
  an error to specify one of those when supplying *path* as a file descriptor.

.. _dir_fd:

* **paths relative to directory descriptors:** If *dir_fd* is not ``None``, it
  should be a file descriptor referring to a directory, and the path to operate
  on should be relative; path will then be relative to that directory.  If the
  path is absolute, *dir_fd* is ignored.  (For POSIX systems, Python will call
  the ``...at`` or ``f...at`` version of the function.)

  You can check whether or not *dir_fd* is supported on your platform using
  :data:`os.supports_dir_fd`.  If it is unavailable, using it will raise a
  :exc:`NotImplementedError`.

.. _follow_symlinks:

* **not following symlinks:** If *follow_symlinks* is
  ``False``, and the last element of the path to operate on is a symbolic link,
  the function will operate on the symbolic link itself instead of the file the
  link points to.  (For POSIX systems, Python will call the ``l...`` version of
  the function.)

  You can check whether or not *follow_symlinks* is supported on your platform
  using :data:`os.supports_follow_symlinks`.  If it is unavailable, using it
  will raise a :exc:`NotImplementedError`.



.. function:: access(path, mode, *, dir_fd=None, effective_ids=False, follow_symlinks=True)

   Use the real uid/gid to test for access to *path*.  Note that most operations
   will use the effective uid/gid, therefore this routine can be used in a
   suid/sgid environment to test if the invoking user has the specified access to
   *path*.  *mode* should be :const:`F_OK` to test the existence of *path*, or it
   can be the inclusive OR of one or more of :const:`R_OK`, :const:`W_OK`, and
   :const:`X_OK` to test permissions.  Return :const:`True` if access is allowed,
   :const:`False` if not. See the Unix man page :manpage:`access(2)` for more
   information.

   This function can support specifying :ref:`paths relative to directory
   descriptors <dir_fd>` and :ref:`not following symlinks <follow_symlinks>`.

   If *effective_ids* is ``True``, :func:`access` will perform its access
   checks using the effective uid/gid instead of the real uid/gid.
   *effective_ids* may not be supported on your platform; you can check whether
   or not it is available using :data:`os.supports_effective_ids`.  If it is
   unavailable, using it will raise a :exc:`NotImplementedError`.

   Availability: Unix, Windows.

   .. note::

      Using :func:`access` to check if a user is authorized to e.g. open a file
      before actually doing so using :func:`open` creates a security hole,
      because the user might exploit the short time interval between checking
      and opening the file to manipulate it. It's preferable to use :term:`EAFP`
      techniques. For example::

         if os.access("myfile", os.R_OK):
             with open("myfile") as fp:
                 return fp.read()
         return "some default data"

      is better written as::

         try:
             fp = open("myfile")
         except PermissionError:
             return "some default data"
         else:
             with fp:
                 return fp.read()

   .. note::

      I/O operations may fail even when :func:`access` indicates that they would
      succeed, particularly for operations on network filesystems which may have
      permissions semantics beyond the usual POSIX permission-bit model.

   .. versionchanged:: 3.3
      Added the *dir_fd*, *effective_ids*, and *follow_symlinks* parameters.


.. data:: F_OK
          R_OK
          W_OK
          X_OK

   Values to pass as the *mode* parameter of :func:`access` to test the
   existence, readability, writability and executability of *path*,
   respectively.


.. function:: chdir(path)

   .. index:: single: directory; changing

   Change the current working directory to *path*.

   This function can support :ref:`specifying a file descriptor <path_fd>`.  The
   descriptor must refer to an opened directory, not an open file.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      Added support for specifying *path* as a file descriptor
      on some platforms.


.. function:: chflags(path, flags, *, follow_symlinks=True)

   Set the flags of *path* to the numeric *flags*. *flags* may take a combination
   (bitwise OR) of the following values (as defined in the :mod:`stat` module):

   * :data:`stat.UF_NODUMP`
   * :data:`stat.UF_IMMUTABLE`
   * :data:`stat.UF_APPEND`
   * :data:`stat.UF_OPAQUE`
   * :data:`stat.UF_NOUNLINK`
   * :data:`stat.UF_COMPRESSED`
   * :data:`stat.UF_HIDDEN`
   * :data:`stat.SF_ARCHIVED`
   * :data:`stat.SF_IMMUTABLE`
   * :data:`stat.SF_APPEND`
   * :data:`stat.SF_NOUNLINK`
   * :data:`stat.SF_SNAPSHOT`

   This function can support :ref:`not following symlinks <follow_symlinks>`.

   Availability: Unix.

   .. versionadded:: 3.3
      The *follow_symlinks* argument.


.. function:: chmod(path, mode, *, dir_fd=None, follow_symlinks=True)

   Change the mode of *path* to the numeric *mode*. *mode* may take one of the
   following values (as defined in the :mod:`stat` module) or bitwise ORed
   combinations of them:

   * :data:`stat.S_ISUID`
   * :data:`stat.S_ISGID`
   * :data:`stat.S_ENFMT`
   * :data:`stat.S_ISVTX`
   * :data:`stat.S_IREAD`
   * :data:`stat.S_IWRITE`
   * :data:`stat.S_IEXEC`
   * :data:`stat.S_IRWXU`
   * :data:`stat.S_IRUSR`
   * :data:`stat.S_IWUSR`
   * :data:`stat.S_IXUSR`
   * :data:`stat.S_IRWXG`
   * :data:`stat.S_IRGRP`
   * :data:`stat.S_IWGRP`
   * :data:`stat.S_IXGRP`
   * :data:`stat.S_IRWXO`
   * :data:`stat.S_IROTH`
   * :data:`stat.S_IWOTH`
   * :data:`stat.S_IXOTH`

   This function can support :ref:`specifying a file descriptor <path_fd>`,
   :ref:`paths relative to directory descriptors <dir_fd>` and :ref:`not
   following symlinks <follow_symlinks>`.

   Availability: Unix, Windows.

   .. note::

      Although Windows supports :func:`chmod`, you can only set the file's
      read-only flag with it (via the ``stat.S_IWRITE`` and ``stat.S_IREAD``
      constants or a corresponding integer value).  All other bits are ignored.

   .. versionadded:: 3.3
      Added support for specifying *path* as an open file descriptor,
      and the *dir_fd* and *follow_symlinks* arguments.


.. function:: chown(path, uid, gid, *, dir_fd=None, follow_symlinks=True)

   Change the owner and group id of *path* to the numeric *uid* and *gid*.  To
   leave one of the ids unchanged, set it to -1.

   This function can support :ref:`specifying a file descriptor <path_fd>`,
   :ref:`paths relative to directory descriptors <dir_fd>` and :ref:`not
   following symlinks <follow_symlinks>`.

   See :func:`shutil.chown` for a higher-level function that accepts names in
   addition to numeric ids.

   Availability: Unix.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*,
      and the *dir_fd* and *follow_symlinks* arguments.


.. function:: chroot(path)

   Change the root directory of the current process to *path*.

   Availability: Unix.


.. function:: fchdir(fd)

   Change the current working directory to the directory represented by the file
   descriptor *fd*.  The descriptor must refer to an opened directory, not an
   open file.  As of Python 3.3, this is equivalent to ``os.chdir(fd)``.

   Availability: Unix.


.. function:: getcwd()

   Return a string representing the current working directory.

   Availability: Unix, Windows.


.. function:: getcwdb()

   Return a bytestring representing the current working directory.

   Availability: Unix, Windows.


.. function:: lchflags(path, flags)

   Set the flags of *path* to the numeric *flags*, like :func:`chflags`, but do
   not follow symbolic links.  As of Python 3.3, this is equivalent to
   ``os.chflags(path, flags, follow_symlinks=False)``.

   Availability: Unix.


.. function:: lchmod(path, mode)

   Change the mode of *path* to the numeric *mode*. If path is a symlink, this
   affects the symlink rather than the target.  See the docs for :func:`chmod`
   for possible values of *mode*.  As of Python 3.3, this is equivalent to
   ``os.chmod(path, mode, follow_symlinks=False)``.

   Availability: Unix.


.. function:: lchown(path, uid, gid)

   Change the owner and group id of *path* to the numeric *uid* and *gid*.  This
   function will not follow symbolic links.  As of Python 3.3, this is equivalent
   to ``os.chown(path, uid, gid, follow_symlinks=False)``.

   Availability: Unix.


.. function:: link(src, dst, *, src_dir_fd=None, dst_dir_fd=None, follow_symlinks=True)

   Create a hard link pointing to *src* named *dst*.

   This function can support specifying *src_dir_fd* and/or *dst_dir_fd* to
   supply :ref:`paths relative to directory descriptors <dir_fd>`, and :ref:`not
   following symlinks <follow_symlinks>`.

   Availability: Unix, Windows.

   .. versionchanged:: 3.2
      Added Windows support.

   .. versionadded:: 3.3
      Added the *src_dir_fd*, *dst_dir_fd*, and *follow_symlinks* arguments.


.. function:: listdir(path='.')

   Return a list containing the names of the entries in the directory given by
   *path*.  The list is in arbitrary order, and does not include the special
   entries ``'.'`` and ``'..'`` even if they are present in the directory.

   *path* may be either of type ``str`` or of type ``bytes``.  If *path*
   is of type ``bytes``, the filenames returned will also be of type ``bytes``;
   in all other circumstances, they will be of type ``str``.

   This function can also support :ref:`specifying a file descriptor
   <path_fd>`; the file descriptor must refer to a directory.

   .. note::
      To encode ``str`` filenames to ``bytes``, use :func:`~os.fsencode`.

   Availability: Unix, Windows.

   .. versionchanged:: 3.2
      The *path* parameter became optional.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*.


.. function:: lstat(path, *, dir_fd=None)

   Perform the equivalent of an :c:func:`lstat` system call on the given path.
   Similar to :func:`~os.stat`, but does not follow symbolic links.  On
   platforms that do not support symbolic links, this is an alias for
   :func:`~os.stat`.  As of Python 3.3, this is equivalent to ``os.stat(path,
   dir_fd=dir_fd, follow_symlinks=False)``.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   .. versionchanged:: 3.2
      Added support for Windows 6.0 (Vista) symbolic links.

   .. versionchanged:: 3.3
      Added the *dir_fd* parameter.


.. function:: mkdir(path, mode=0o777, *, dir_fd=None)

   Create a directory named *path* with numeric mode *mode*.

   On some systems, *mode* is ignored.  Where it is used, the current umask
   value is first masked out.  If the directory already exists, :exc:`OSError`
   is raised.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   It is also possible to create temporary directories; see the
   :mod:`tempfile` module's :func:`tempfile.mkdtemp` function.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      The *dir_fd* argument.


.. function:: makedirs(path, mode=0o777, exist_ok=False)

   .. index::
      single: directory; creating
      single: UNC paths; and os.makedirs()

   Recursive directory creation function.  Like :func:`mkdir`, but makes all
   intermediate-level directories needed to contain the leaf directory.

   The default *mode* is ``0o777`` (octal).  On some systems, *mode* is
   ignored.  Where it is used, the current umask value is first masked out.

   If *exist_ok* is ``False`` (the default), an :exc:`OSError` is raised if
   the target directory already exists.  If *exist_ok* is ``True`` an
   :exc:`OSError` is still raised if the umask-masked *mode* is different from
   the existing mode, on systems where the mode is used.  :exc:`OSError` will
   also be raised if the directory creation fails.

   .. note::

      :func:`makedirs` will become confused if the path elements to create
      include :data:`pardir` (eg. ".." on UNIX systems).

   This function handles UNC paths correctly.

   .. versionadded:: 3.2
      The *exist_ok* parameter.


.. function:: mkfifo(path, mode=0o666, *, dir_fd=None)

   Create a FIFO (a named pipe) named *path* with numeric mode *mode*.
   The current umask value is first masked out from the mode.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   FIFOs are pipes that can be accessed like regular files.  FIFOs exist until they
   are deleted (for example with :func:`os.unlink`). Generally, FIFOs are used as
   rendezvous between "client" and "server" type processes: the server opens the
   FIFO for reading, and the client opens it for writing.  Note that :func:`mkfifo`
   doesn't open the FIFO --- it just creates the rendezvous point.

   Availability: Unix.

   .. versionadded:: 3.3
      The *dir_fd* argument.


.. function:: mknod(filename, mode=0o600, device=0, *, dir_fd=None)

   Create a filesystem node (file, device special file or named pipe) named
   *filename*. *mode* specifies both the permissions to use and the type of node
   to be created, being combined (bitwise OR) with one of ``stat.S_IFREG``,
   ``stat.S_IFCHR``, ``stat.S_IFBLK``, and ``stat.S_IFIFO`` (those constants are
   available in :mod:`stat`).  For ``stat.S_IFCHR`` and ``stat.S_IFBLK``,
   *device* defines the newly created device special file (probably using
   :func:`os.makedev`), otherwise it is ignored.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   .. versionadded:: 3.3
      The *dir_fd* argument.


.. function:: major(device)

   Extract the device major number from a raw device number (usually the
   :attr:`st_dev` or :attr:`st_rdev` field from :c:type:`stat`).


.. function:: minor(device)

   Extract the device minor number from a raw device number (usually the
   :attr:`st_dev` or :attr:`st_rdev` field from :c:type:`stat`).


.. function:: makedev(major, minor)

   Compose a raw device number from the major and minor device numbers.


.. function:: pathconf(path, name)

   Return system configuration information relevant to a named file. *name*
   specifies the configuration value to retrieve; it may be a string which is the
   name of a defined system value; these names are specified in a number of
   standards (POSIX.1, Unix 95, Unix 98, and others).  Some platforms define
   additional names as well.  The names known to the host operating system are
   given in the ``pathconf_names`` dictionary.  For configuration variables not
   included in that mapping, passing an integer for *name* is also accepted.

   If *name* is a string and is not known, :exc:`ValueError` is raised.  If a
   specific value for *name* is not supported by the host system, even if it is
   included in ``pathconf_names``, an :exc:`OSError` is raised with
   :const:`errno.EINVAL` for the error number.

   This function can support :ref:`specifying a file descriptor
   <path_fd>`.

   Availability: Unix.


.. data:: pathconf_names

   Dictionary mapping names accepted by :func:`pathconf` and :func:`fpathconf` to
   the integer values defined for those names by the host operating system.  This
   can be used to determine the set of names known to the system.

   Availability: Unix.


.. function:: readlink(path, *, dir_fd=None)

   Return a string representing the path to which the symbolic link points.  The
   result may be either an absolute or relative pathname; if it is relative, it
   may be converted to an absolute pathname using
   ``os.path.join(os.path.dirname(path), result)``.

   If the *path* is a string object, the result will also be a string object,
   and the call may raise an UnicodeDecodeError. If the *path* is a bytes
   object, the result will be a bytes object.

   This function can also support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   Availability: Unix, Windows

   .. versionchanged:: 3.2
      Added support for Windows 6.0 (Vista) symbolic links.

   .. versionadded:: 3.3
      The *dir_fd* argument.


.. function:: remove(path, *, dir_fd=None)

   Remove (delete) the file *path*.  If *path* is a directory, :exc:`OSError` is
   raised.  Use :func:`rmdir` to remove directories.

   This function can support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   On Windows, attempting to remove a file that is in use causes an exception to
   be raised; on Unix, the directory entry is removed but the storage allocated
   to the file is not made available until the original file is no longer in use.

   This function is identical to :func:`unlink`.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      The *dir_fd* argument.


.. function:: removedirs(path)

   .. index:: single: directory; deleting

   Remove directories recursively.  Works like :func:`rmdir` except that, if the
   leaf directory is successfully removed, :func:`removedirs`  tries to
   successively remove every parent directory mentioned in  *path* until an error
   is raised (which is ignored, because it generally means that a parent directory
   is not empty). For example, ``os.removedirs('foo/bar/baz')`` will first remove
   the directory ``'foo/bar/baz'``, and then remove ``'foo/bar'`` and ``'foo'`` if
   they are empty. Raises :exc:`OSError` if the leaf directory could not be
   successfully removed.


.. function:: rename(src, dst, *, src_dir_fd=None, dst_dir_fd=None)

   Rename the file or directory *src* to *dst*.  If *dst* is a directory,
   :exc:`OSError` will be raised.  On Unix, if *dst* exists and is a file, it will
   be replaced silently if the user has permission.  The operation may fail on some
   Unix flavors if *src* and *dst* are on different filesystems.  If successful,
   the renaming will be an atomic operation (this is a POSIX requirement).  On
   Windows, if *dst* already exists, :exc:`OSError` will be raised even if it is a
   file.

   This function can support specifying *src_dir_fd* and/or *dst_dir_fd* to
   supply :ref:`paths relative to directory descriptors <dir_fd>`.

   If you want cross-platform overwriting of the destination, use :func:`replace`.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      The *src_dir_fd* and *dst_dir_fd* arguments.


.. function:: renames(old, new)

   Recursive directory or file renaming function. Works like :func:`rename`, except
   creation of any intermediate directories needed to make the new pathname good is
   attempted first. After the rename, directories corresponding to rightmost path
   segments of the old name will be pruned away using :func:`removedirs`.

   .. note::

      This function can fail with the new directory structure made if you lack
      permissions needed to remove the leaf directory or file.


.. function:: replace(src, dst, *, src_dir_fd=None, dst_dir_fd=None)

   Rename the file or directory *src* to *dst*.  If *dst* is a directory,
   :exc:`OSError` will be raised.  If *dst* exists and is a file, it will
   be replaced silently if the user has permission.  The operation may fail
   if *src* and *dst* are on different filesystems.  If successful,
   the renaming will be an atomic operation (this is a POSIX requirement).

   This function can support specifying *src_dir_fd* and/or *dst_dir_fd* to
   supply :ref:`paths relative to directory descriptors <dir_fd>`.

   Availability: Unix, Windows.

   .. versionadded:: 3.3


.. function:: rmdir(path, *, dir_fd=None)

   Remove (delete) the directory *path*.  Only works when the directory is
   empty, otherwise, :exc:`OSError` is raised.  In order to remove whole
   directory trees, :func:`shutil.rmtree` can be used.

   This function can support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      The *dir_fd* parameter.


.. function:: stat(path, *, dir_fd=None, follow_symlinks=True)

   Perform the equivalent of a :c:func:`stat` system call on the given path.
   *path* may be specified as either a string or as an open file descriptor.
   (This function normally follows symlinks; to stat a symlink add the argument
   ``follow_symlinks=False``, or use :func:`lstat`.)

   The return value is an object whose attributes correspond roughly
   to the members of the :c:type:`stat` structure, namely:

   * :attr:`st_mode` - protection bits,
   * :attr:`st_ino` - inode number,
   * :attr:`st_dev` - device,
   * :attr:`st_nlink` - number of hard links,
   * :attr:`st_uid` - user id of owner,
   * :attr:`st_gid` - group id of owner,
   * :attr:`st_size` - size of file, in bytes,
   * :attr:`st_atime` - time of most recent access expressed in seconds,
   * :attr:`st_mtime` - time of most recent content modification
     expressed in seconds,
   * :attr:`st_ctime` - platform dependent; time of most recent metadata
     change on Unix, or the time of creation on Windows, expressed in seconds
   * :attr:`st_atime_ns` - time of most recent access
     expressed in nanoseconds as an integer,
   * :attr:`st_mtime_ns` - time of most recent content modification
     expressed in nanoseconds as an integer,
   * :attr:`st_ctime_ns` - platform dependent; time of most recent metadata
     change on Unix, or the time of creation on Windows,
     expressed in nanoseconds as an integer

   On some Unix systems (such as Linux), the following attributes may also be
   available:

   * :attr:`st_blocks` - number of 512-byte blocks allocated for file
   * :attr:`st_blksize` - filesystem blocksize for efficient file system I/O
   * :attr:`st_rdev` - type of device if an inode device
   * :attr:`st_flags` - user defined flags for file

   On other Unix systems (such as FreeBSD), the following attributes may be
   available (but may be only filled out if root tries to use them):

   * :attr:`st_gen` - file generation number
   * :attr:`st_birthtime` - time of file creation

   On Mac OS systems, the following attributes may also be available:

   * :attr:`st_rsize`
   * :attr:`st_creator`
   * :attr:`st_type`

   .. note::

      The exact meaning and resolution of the :attr:`st_atime`,
      :attr:`st_mtime`, and :attr:`st_ctime` attributes depend on the operating
      system and the file system. For example, on Windows systems using the FAT
      or FAT32 file systems, :attr:`st_mtime` has 2-second resolution, and
      :attr:`st_atime` has only 1-day resolution.  See your operating system
      documentation for details.
      Similarly, although :attr:`st_atime_ns`, :attr:`st_mtime_ns`,
      and :attr:`st_ctime_ns` are always expressed in nanoseconds, many
      systems do not provide nanosecond precision.  On systems that do
      provide nanosecond precision, the floating-point object used to
      store :attr:`st_atime`, :attr:`st_mtime`, and :attr:`st_ctime`
      cannot preserve all of it, and as such will be slightly inexact.
      If you need the exact timestamps you should always use
      :attr:`st_atime_ns`, :attr:`st_mtime_ns`, and :attr:`st_ctime_ns`.

   For backward compatibility, the return value of :func:`~os.stat` is also
   accessible as a tuple of at least 10 integers giving the most important (and
   portable) members of the :c:type:`stat` structure, in the order
   :attr:`st_mode`, :attr:`st_ino`, :attr:`st_dev`, :attr:`st_nlink`,
   :attr:`st_uid`, :attr:`st_gid`, :attr:`st_size`, :attr:`st_atime`,
   :attr:`st_mtime`, :attr:`st_ctime`. More items may be added at the end by
   some implementations.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.

   .. index:: module: stat

   The standard module :mod:`stat` defines functions and constants that are useful
   for extracting information from a :c:type:`stat` structure. (On Windows, some
   items are filled with dummy values.)

   Example::

      >>> import os
      >>> statinfo = os.stat('somefile.txt')
      >>> statinfo
      posix.stat_result(st_mode=33188, st_ino=7876932, st_dev=234881026,
      st_nlink=1, st_uid=501, st_gid=501, st_size=264, st_atime=1297230295,
      st_mtime=1297230027, st_ctime=1297230027)
      >>> statinfo.st_size
      264

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      Added the *dir_fd* and *follow_symlinks* arguments,
      specifying a file descriptor instead of a path,
      and the :attr:`st_atime_ns`, :attr:`st_mtime_ns`,
      and :attr:`st_ctime_ns` members.


.. function:: stat_float_times([newvalue])

   Determine whether :class:`stat_result` represents time stamps as float objects.
   If *newvalue* is ``True``, future calls to :func:`~os.stat` return floats, if it is
   ``False``, future calls return ints. If *newvalue* is omitted, return the
   current setting.

   For compatibility with older Python versions, accessing :class:`stat_result` as
   a tuple always returns integers.

   Python now returns float values by default. Applications which do not work
   correctly with floating point time stamps can use this function to restore the
   old behaviour.

   The resolution of the timestamps (that is the smallest possible fraction)
   depends on the system. Some systems only support second resolution; on these
   systems, the fraction will always be zero.

   It is recommended that this setting is only changed at program startup time in
   the *__main__* module; libraries should never change this setting. If an
   application uses a library that works incorrectly if floating point time stamps
   are processed, this application should turn the feature off until the library
   has been corrected.

   .. deprecated:: 3.3


.. function:: statvfs(path)

   Perform a :c:func:`statvfs` system call on the given path.  The return value is
   an object whose attributes describe the filesystem on the given path, and
   correspond to the members of the :c:type:`statvfs` structure, namely:
   :attr:`f_bsize`, :attr:`f_frsize`, :attr:`f_blocks`, :attr:`f_bfree`,
   :attr:`f_bavail`, :attr:`f_files`, :attr:`f_ffree`, :attr:`f_favail`,
   :attr:`f_flag`, :attr:`f_namemax`.

   Two module-level constants are defined for the :attr:`f_flag` attribute's
   bit-flags: if :const:`ST_RDONLY` is set, the filesystem is mounted
   read-only, and if :const:`ST_NOSUID` is set, the semantics of
   setuid/setgid bits are disabled or not supported.

   Additional module-level constants are defined for GNU/glibc based systems.
   These are :const:`ST_NODEV` (disallow access to device special files),
   :const:`ST_NOEXEC` (disallow program execution), :const:`ST_SYNCHRONOUS`
   (writes are synced at once), :const:`ST_MANDLOCK` (allow mandatory locks on an FS),
   :const:`ST_WRITE` (write on file/directory/symlink), :const:`ST_APPEND`
   (append-only file), :const:`ST_IMMUTABLE` (immutable file), :const:`ST_NOATIME`
   (do not update access times), :const:`ST_NODIRATIME` (do not update directory access
   times), :const:`ST_RELATIME` (update atime relative to mtime/ctime).

   This function can support :ref:`specifying a file descriptor <path_fd>`.

   .. versionchanged:: 3.2
      The :const:`ST_RDONLY` and :const:`ST_NOSUID` constants were added.

   .. versionchanged:: 3.4
      The :const:`ST_NODEV`, :const:`ST_NOEXEC`, :const:`ST_SYNCHRONOUS`,
      :const:`ST_MANDLOCK`, :const:`ST_WRITE`, :const:`ST_APPEND`,
      :const:`ST_IMMUTABLE`, :const:`ST_NOATIME`, :const:`ST_NODIRATIME`,
      and :const:`ST_RELATIME` constants were added.

   Availability: Unix.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*.


.. data:: supports_dir_fd

   A :class:`~collections.abc.Set` object indicating which functions in the
   :mod:`os` module permit use of their *dir_fd* parameter.  Different platforms
   provide different functionality, and an option that might work on one might
   be unsupported on another.  For consistency's sakes, functions that support
   *dir_fd* always allow specifying the parameter, but will raise an exception
   if the functionality is not actually available.

   To check whether a particular function permits use of its *dir_fd*
   parameter, use the ``in`` operator on ``supports_dir_fd``.  As an example,
   this expression determines whether the *dir_fd* parameter of :func:`os.stat`
   is locally available::

       os.stat in os.supports_dir_fd

   Currently *dir_fd* parameters only work on Unix platforms; none of them work
   on Windows.

   .. versionadded:: 3.3


.. data:: supports_effective_ids

   A :class:`~collections.abc.Set` object indicating which functions in the
   :mod:`os` module permit use of the *effective_ids* parameter for
   :func:`os.access`.  If the local platform supports it, the collection will
   contain :func:`os.access`, otherwise it will be empty.

   To check whether you can use the *effective_ids* parameter for
   :func:`os.access`, use the ``in`` operator on ``supports_dir_fd``, like so::

       os.access in os.supports_effective_ids

   Currently *effective_ids* only works on Unix platforms; it does not work on
   Windows.

   .. versionadded:: 3.3


.. data:: supports_fd

   A :class:`~collections.abc.Set` object indicating which functions in the
   :mod:`os` module permit specifying their *path* parameter as an open file
   descriptor.  Different platforms provide different functionality, and an
   option that might work on one might be unsupported on another.  For
   consistency's sakes, functions that support *fd* always allow specifying
   the parameter, but will raise an exception if the functionality is not
   actually available.

   To check whether a particular function permits specifying an open file
   descriptor for its *path* parameter, use the ``in`` operator on
   ``supports_fd``. As an example, this expression determines whether
   :func:`os.chdir` accepts open file descriptors when called on your local
   platform::

       os.chdir in os.supports_fd

   .. versionadded:: 3.3


.. data:: supports_follow_symlinks

   A :class:`~collections.abc.Set` object indicating which functions in the
   :mod:`os` module permit use of their *follow_symlinks* parameter.  Different
   platforms provide different functionality, and an option that might work on
   one might be unsupported on another.  For consistency's sakes, functions that
   support *follow_symlinks* always allow specifying the parameter, but will
   raise an exception if the functionality is not actually available.

   To check whether a particular function permits use of its *follow_symlinks*
   parameter, use the ``in`` operator on ``supports_follow_symlinks``.  As an
   example, this expression determines whether the *follow_symlinks* parameter
   of :func:`os.stat` is locally available::

       os.stat in os.supports_follow_symlinks

   .. versionadded:: 3.3


.. function:: symlink(source, link_name, target_is_directory=False, *, dir_fd=None)

   Create a symbolic link pointing to *source* named *link_name*.

   On Windows, a symlink represents either a file or a directory, and does not
   morph to the target dynamically.  If the target is present, the type of the
   symlink will be created to match. Otherwise, the symlink will be created
   as a directory if *target_is_directory* is ``True`` or a file symlink (the
   default) otherwise.  On non-Window platforms, *target_is_directory* is ignored.

   Symbolic link support was introduced in Windows 6.0 (Vista).  :func:`symlink`
   will raise a :exc:`NotImplementedError` on Windows versions earlier than 6.0.

   This function can support :ref:`paths relative to directory descriptors
   <dir_fd>`.

   .. note::

      On Windows, the *SeCreateSymbolicLinkPrivilege* is required in order to
      successfully create symlinks. This privilege is not typically granted to
      regular users but is available to accounts which can escalate privileges
      to the administrator level. Either obtaining the privilege or running your
      application as an administrator are ways to successfully create symlinks.


      :exc:`OSError` is raised when the function is called by an unprivileged
      user.

   Availability: Unix, Windows.

   .. versionchanged:: 3.2
      Added support for Windows 6.0 (Vista) symbolic links.

   .. versionadded:: 3.3
      Added the *dir_fd* argument, and now allow *target_is_directory*
      on non-Windows platforms.


.. function:: sync()

   Force write of everything to disk.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: truncate(path, length)

   Truncate the file corresponding to *path*, so that it is at most
   *length* bytes in size.

   This function can support :ref:`specifying a file descriptor <path_fd>`.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: unlink(path, *, dir_fd=None)

   Remove (delete) the file *path*.  This function is identical to
   :func:`remove`; the ``unlink`` name is its traditional Unix
   name.  Please see the documentation for :func:`remove` for
   further information.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      The *dir_fd* parameter.


.. function:: utime(path, times=None, *, ns=None, dir_fd=None, follow_symlinks=True)

   Set the access and modified times of the file specified by *path*.

   :func:`utime` takes two optional parameters, *times* and *ns*.
   These specify the times set on *path* and are used as follows:

   - If *ns* is not ``None``,
     it must be a 2-tuple of the form ``(atime_ns, mtime_ns)``
     where each member is an int expressing nanoseconds.
   - If *times* is not ``None``,
     it must be a 2-tuple of the form ``(atime, mtime)``
     where each member is an int or float expressing seconds.
   - If *times* and *ns* are both ``None``,
     this is equivalent to specifying ``ns=(atime_ns, mtime_ns)``
     where both times are the current time.

   It is an error to specify tuples for both *times* and *ns*.

   Whether a directory can be given for *path*
   depends on whether the operating system implements directories as files
   (for example, Windows does not).  Note that the exact times you set here may
   not be returned by a subsequent :func:`~os.stat` call, depending on the
   resolution with which your operating system records access and modification
   times; see :func:`~os.stat`.  The best way to preserve exact times is to
   use the *st_atime_ns* and *st_mtime_ns* fields from the :func:`os.stat`
   result object with the *ns* parameter to `utime`.

   This function can support :ref:`specifying a file descriptor <path_fd>`,
   :ref:`paths relative to directory descriptors <dir_fd>` and :ref:`not
   following symlinks <follow_symlinks>`.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*,
      and the *dir_fd*, *follow_symlinks*, and *ns* parameters.


.. function:: walk(top, topdown=True, onerror=None, followlinks=False)

   .. index::
      single: directory; walking
      single: directory; traversal

   Generate the file names in a directory tree by walking the tree
   either top-down or bottom-up. For each directory in the tree rooted at directory
   *top* (including *top* itself), it yields a 3-tuple ``(dirpath, dirnames,
   filenames)``.

   *dirpath* is a string, the path to the directory.  *dirnames* is a list of the
   names of the subdirectories in *dirpath* (excluding ``'.'`` and ``'..'``).
   *filenames* is a list of the names of the non-directory files in *dirpath*.
   Note that the names in the lists contain no path components.  To get a full path
   (which begins with *top*) to a file or directory in *dirpath*, do
   ``os.path.join(dirpath, name)``.

   If optional argument *topdown* is ``True`` or not specified, the triple for a
   directory is generated before the triples for any of its subdirectories
   (directories are generated top-down).  If *topdown* is ``False``, the triple for a
   directory is generated after the triples for all of its subdirectories
   (directories are generated bottom-up).

   When *topdown* is ``True``, the caller can modify the *dirnames* list in-place
   (perhaps using :keyword:`del` or slice assignment), and :func:`walk` will only
   recurse into the subdirectories whose names remain in *dirnames*; this can be
   used to prune the search, impose a specific order of visiting, or even to inform
   :func:`walk` about directories the caller creates or renames before it resumes
   :func:`walk` again.  Modifying *dirnames* when *topdown* is ``False`` is
   ineffective, because in bottom-up mode the directories in *dirnames* are
   generated before *dirpath* itself is generated.

   By default, errors from the :func:`listdir` call are ignored.  If optional
   argument *onerror* is specified, it should be a function; it will be called with
   one argument, an :exc:`OSError` instance.  It can report the error to continue
   with the walk, or raise the exception to abort the walk.  Note that the filename
   is available as the ``filename`` attribute of the exception object.

   By default, :func:`walk` will not walk down into symbolic links that resolve to
   directories. Set *followlinks* to ``True`` to visit directories pointed to by
   symlinks, on systems that support them.

   .. note::

      Be aware that setting *followlinks* to ``True`` can lead to infinite
      recursion if a link points to a parent directory of itself. :func:`walk`
      does not keep track of the directories it visited already.

   .. note::

      If you pass a relative pathname, don't change the current working directory
      between resumptions of :func:`walk`.  :func:`walk` never changes the current
      directory, and assumes that its caller doesn't either.

   This example displays the number of bytes taken by non-directory files in each
   directory under the starting directory, except that it doesn't look under any
   CVS subdirectory::

      import os
      from os.path import join, getsize
      for root, dirs, files in os.walk('python/Lib/email'):
          print(root, "consumes", end=" ")
          print(sum(getsize(join(root, name)) for name in files), end=" ")
          print("bytes in", len(files), "non-directory files")
          if 'CVS' in dirs:
              dirs.remove('CVS')  # don't visit CVS directories

   In the next example, walking the tree bottom-up is essential: :func:`rmdir`
   doesn't allow deleting a directory before the directory is empty::

      # Delete everything reachable from the directory named in "top",
      # assuming there are no symbolic links.
      # CAUTION:  This is dangerous!  For example, if top == '/', it
      # could delete all your disk files.
      import os
      for root, dirs, files in os.walk(top, topdown=False):
          for name in files:
              os.remove(os.path.join(root, name))
          for name in dirs:
              os.rmdir(os.path.join(root, name))


.. function:: fwalk(top='.', topdown=True, onerror=None, *, follow_symlinks=False, dir_fd=None)

   .. index::
      single: directory; walking
      single: directory; traversal

   This behaves exactly like :func:`walk`, except that it yields a 4-tuple
   ``(dirpath, dirnames, filenames, dirfd)``, and it supports ``dir_fd``.

   *dirpath*, *dirnames* and *filenames* are identical to :func:`walk` output,
   and *dirfd* is a file descriptor referring to the directory *dirpath*.

   This function always supports :ref:`paths relative to directory descriptors
   <dir_fd>` and :ref:`not following symlinks <follow_symlinks>`.  Note however
   that, unlike other functions, the :func:`fwalk` default value for
   *follow_symlinks* is ``False``.

   .. note::

      Since :func:`fwalk` yields file descriptors, those are only valid until
      the next iteration step, so you should duplicate them (e.g. with
      :func:`dup`) if you want to keep them longer.

   This example displays the number of bytes taken by non-directory files in each
   directory under the starting directory, except that it doesn't look under any
   CVS subdirectory::

      import os
      for root, dirs, files, rootfd in os.fwalk('python/Lib/email'):
          print(root, "consumes", end="")
          print(sum([os.stat(name, dir_fd=rootfd).st_size for name in files]),
                end="")
          print("bytes in", len(files), "non-directory files")
          if 'CVS' in dirs:
              dirs.remove('CVS')  # don't visit CVS directories

   In the next example, walking the tree bottom-up is essential:
   :func:`rmdir` doesn't allow deleting a directory before the directory is
   empty::

      # Delete everything reachable from the directory named in "top",
      # assuming there are no symbolic links.
      # CAUTION:  This is dangerous!  For example, if top == '/', it
      # could delete all your disk files.
      import os
      for root, dirs, files, rootfd in os.fwalk(top, topdown=False):
          for name in files:
              os.unlink(name, dir_fd=rootfd)
          for name in dirs:
              os.rmdir(name, dir_fd=rootfd)

   Availability: Unix.

   .. versionadded:: 3.3


Linux extended attributes
~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 3.3

These functions are all available on Linux only.

.. function:: getxattr(path, attribute, *, follow_symlinks=True)

   Return the value of the extended filesystem attribute *attribute* for
   *path*. *attribute* can be bytes or str. If it is str, it is encoded
   with the filesystem encoding.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.


.. function:: listxattr(path=None, *, follow_symlinks=True)

   Return a list of the extended filesystem attributes on *path*.  The
   attributes in the list are represented as strings decoded with the filesystem
   encoding.  If *path* is ``None``, :func:`listxattr` will examine the current
   directory.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.


.. function:: removexattr(path, attribute, *, follow_symlinks=True)

   Removes the extended filesystem attribute *attribute* from *path*.
   *attribute* should be bytes or str. If it is a string, it is encoded
   with the filesystem encoding.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.


.. function:: setxattr(path, attribute, value, flags=0, *, follow_symlinks=True)

   Set the extended filesystem attribute *attribute* on *path* to *value*.
   *attribute* must be a bytes or str with no embedded NULs. If it is a str,
   it is encoded with the filesystem encoding.  *flags* may be
   :data:`XATTR_REPLACE` or :data:`XATTR_CREATE`. If :data:`XATTR_REPLACE` is
   given and the attribute does not exist, ``EEXISTS`` will be raised.
   If :data:`XATTR_CREATE` is given and the attribute already exists, the
   attribute will not be created and ``ENODATA`` will be raised.

   This function can support :ref:`specifying a file descriptor <path_fd>` and
   :ref:`not following symlinks <follow_symlinks>`.

   .. note::

      A bug in Linux kernel versions less than 2.6.39 caused the flags argument
      to be ignored on some filesystems.


.. data:: XATTR_SIZE_MAX

   The maximum size the value of an extended attribute can be. Currently, this
   is 64 KiB on Linux.


.. data:: XATTR_CREATE

   This is a possible value for the flags argument in :func:`setxattr`. It
   indicates the operation must create an attribute.


.. data:: XATTR_REPLACE

   This is a possible value for the flags argument in :func:`setxattr`. It
   indicates the operation must replace an existing attribute.


.. _os-process:

Process Management
------------------

These functions may be used to create and manage processes.

The various :func:`exec\* <execl>` functions take a list of arguments for the new
program loaded into the process.  In each case, the first of these arguments is
passed to the new program as its own name rather than as an argument a user may
have typed on a command line.  For the C programmer, this is the ``argv[0]``
passed to a program's :c:func:`main`.  For example, ``os.execv('/bin/echo',
['foo', 'bar'])`` will only print ``bar`` on standard output; ``foo`` will seem
to be ignored.


.. function:: abort()

   Generate a :const:`SIGABRT` signal to the current process.  On Unix, the default
   behavior is to produce a core dump; on Windows, the process immediately returns
   an exit code of ``3``.  Be aware that calling this function will not call the
   Python signal handler registered for :const:`SIGABRT` with
   :func:`signal.signal`.

   Availability: Unix, Windows.


.. function:: execl(path, arg0, arg1, ...)
              execle(path, arg0, arg1, ..., env)
              execlp(file, arg0, arg1, ...)
              execlpe(file, arg0, arg1, ..., env)
              execv(path, args)
              execve(path, args, env)
              execvp(file, args)
              execvpe(file, args, env)

   These functions all execute a new program, replacing the current process; they
   do not return.  On Unix, the new executable is loaded into the current process,
   and will have the same process id as the caller.  Errors will be reported as
   :exc:`OSError` exceptions.

   The current process is replaced immediately. Open file objects and
   descriptors are not flushed, so if there may be data buffered
   on these open files, you should flush them using
   :func:`sys.stdout.flush` or :func:`os.fsync` before calling an
   :func:`exec\* <execl>` function.

   The "l" and "v" variants of the :func:`exec\* <execl>` functions differ in how
   command-line arguments are passed.  The "l" variants are perhaps the easiest
   to work with if the number of parameters is fixed when the code is written; the
   individual parameters simply become additional parameters to the :func:`execl\*`
   functions.  The "v" variants are good when the number of parameters is
   variable, with the arguments being passed in a list or tuple as the *args*
   parameter.  In either case, the arguments to the child process should start with
   the name of the command being run, but this is not enforced.

   The variants which include a "p" near the end (:func:`execlp`,
   :func:`execlpe`, :func:`execvp`, and :func:`execvpe`) will use the
   :envvar:`PATH` environment variable to locate the program *file*.  When the
   environment is being replaced (using one of the :func:`exec\*e <execl>` variants,
   discussed in the next paragraph), the new environment is used as the source of
   the :envvar:`PATH` variable. The other variants, :func:`execl`, :func:`execle`,
   :func:`execv`, and :func:`execve`, will not use the :envvar:`PATH` variable to
   locate the executable; *path* must contain an appropriate absolute or relative
   path.

   For :func:`execle`, :func:`execlpe`, :func:`execve`, and :func:`execvpe` (note
   that these all end in "e"), the *env* parameter must be a mapping which is
   used to define the environment variables for the new process (these are used
   instead of the current process' environment); the functions :func:`execl`,
   :func:`execlp`, :func:`execv`, and :func:`execvp` all cause the new process to
   inherit the environment of the current process.

   For :func:`execve` on some platforms, *path* may also be specified as an open
   file descriptor.  This functionality may not be supported on your platform;
   you can check whether or not it is available using :data:`os.supports_fd`.
   If it is unavailable, using it will raise a :exc:`NotImplementedError`.

   Availability: Unix, Windows.

   .. versionadded:: 3.3
      Added support for specifying an open file descriptor for *path*
      for :func:`execve`.

.. function:: _exit(n)

   Exit the process with status *n*, without calling cleanup handlers, flushing
   stdio buffers, etc.

   Availability: Unix, Windows.

   .. note::

      The standard way to exit is ``sys.exit(n)``.  :func:`_exit` should
      normally only be used in the child process after a :func:`fork`.

The following exit codes are defined and can be used with :func:`_exit`,
although they are not required.  These are typically used for system programs
written in Python, such as a mail server's external command delivery program.

.. note::

   Some of these may not be available on all Unix platforms, since there is some
   variation.  These constants are defined where they are defined by the underlying
   platform.


.. data:: EX_OK

   Exit code that means no error occurred.

   Availability: Unix.


.. data:: EX_USAGE

   Exit code that means the command was used incorrectly, such as when the wrong
   number of arguments are given.

   Availability: Unix.


.. data:: EX_DATAERR

   Exit code that means the input data was incorrect.

   Availability: Unix.


.. data:: EX_NOINPUT

   Exit code that means an input file did not exist or was not readable.

   Availability: Unix.


.. data:: EX_NOUSER

   Exit code that means a specified user did not exist.

   Availability: Unix.


.. data:: EX_NOHOST

   Exit code that means a specified host did not exist.

   Availability: Unix.


.. data:: EX_UNAVAILABLE

   Exit code that means that a required service is unavailable.

   Availability: Unix.


.. data:: EX_SOFTWARE

   Exit code that means an internal software error was detected.

   Availability: Unix.


.. data:: EX_OSERR

   Exit code that means an operating system error was detected, such as the
   inability to fork or create a pipe.

   Availability: Unix.


.. data:: EX_OSFILE

   Exit code that means some system file did not exist, could not be opened, or had
   some other kind of error.

   Availability: Unix.


.. data:: EX_CANTCREAT

   Exit code that means a user specified output file could not be created.

   Availability: Unix.


.. data:: EX_IOERR

   Exit code that means that an error occurred while doing I/O on some file.

   Availability: Unix.


.. data:: EX_TEMPFAIL

   Exit code that means a temporary failure occurred.  This indicates something
   that may not really be an error, such as a network connection that couldn't be
   made during a retryable operation.

   Availability: Unix.


.. data:: EX_PROTOCOL

   Exit code that means that a protocol exchange was illegal, invalid, or not
   understood.

   Availability: Unix.


.. data:: EX_NOPERM

   Exit code that means that there were insufficient permissions to perform the
   operation (but not intended for file system problems).

   Availability: Unix.


.. data:: EX_CONFIG

   Exit code that means that some kind of configuration error occurred.

   Availability: Unix.


.. data:: EX_NOTFOUND

   Exit code that means something like "an entry was not found".

   Availability: Unix.


.. function:: fork()

   Fork a child process.  Return ``0`` in the child and the child's process id in the
   parent.  If an error occurs :exc:`OSError` is raised.

   Note that some platforms including FreeBSD <= 6.3 and Cygwin have
   known issues when using fork() from a thread.

   .. warning::

      See :mod:`ssl` for applications that use the SSL module with fork().

   Availability: Unix.


.. function:: forkpty()

   Fork a child process, using a new pseudo-terminal as the child's controlling
   terminal. Return a pair of ``(pid, fd)``, where *pid* is ``0`` in the child, the
   new child's process id in the parent, and *fd* is the file descriptor of the
   master end of the pseudo-terminal.  For a more portable approach, use the
   :mod:`pty` module.  If an error occurs :exc:`OSError` is raised.

   Availability: some flavors of Unix.


.. function:: kill(pid, sig)

   .. index::
      single: process; killing
      single: process; signalling

   Send signal *sig* to the process *pid*.  Constants for the specific signals
   available on the host platform are defined in the :mod:`signal` module.

   Windows: The :data:`signal.CTRL_C_EVENT` and
   :data:`signal.CTRL_BREAK_EVENT` signals are special signals which can
   only be sent to console processes which share a common console window,
   e.g., some subprocesses. Any other value for *sig* will cause the process
   to be unconditionally killed by the TerminateProcess API, and the exit code
   will be set to *sig*. The Windows version of :func:`kill` additionally takes
   process handles to be killed.

   See also :func:`signal.pthread_kill`.

   .. versionadded:: 3.2
      Windows support.


.. function:: killpg(pgid, sig)

   .. index::
      single: process; killing
      single: process; signalling

   Send the signal *sig* to the process group *pgid*.

   Availability: Unix.


.. function:: nice(increment)

   Add *increment* to the process's "niceness".  Return the new niceness.

   Availability: Unix.


.. function:: plock(op)

   Lock program segments into memory.  The value of *op* (defined in
   ``<sys/lock.h>``) determines which segments are locked.

   Availability: Unix.


.. function:: popen(...)

   Run child processes, returning opened pipes for communications.  These functions
   are described in section :ref:`os-newstreams`.


.. function:: spawnl(mode, path, ...)
              spawnle(mode, path, ..., env)
              spawnlp(mode, file, ...)
              spawnlpe(mode, file, ..., env)
              spawnv(mode, path, args)
              spawnve(mode, path, args, env)
              spawnvp(mode, file, args)
              spawnvpe(mode, file, args, env)

   Execute the program *path* in a new process.

   (Note that the :mod:`subprocess` module provides more powerful facilities for
   spawning new processes and retrieving their results; using that module is
   preferable to using these functions.  Check especially the
   :ref:`subprocess-replacements` section.)

   If *mode* is :const:`P_NOWAIT`, this function returns the process id of the new
   process; if *mode* is :const:`P_WAIT`, returns the process's exit code if it
   exits normally, or ``-signal``, where *signal* is the signal that killed the
   process.  On Windows, the process id will actually be the process handle, so can
   be used with the :func:`waitpid` function.

   The "l" and "v" variants of the :func:`spawn\* <spawnl>` functions differ in how
   command-line arguments are passed.  The "l" variants are perhaps the easiest
   to work with if the number of parameters is fixed when the code is written; the
   individual parameters simply become additional parameters to the
   :func:`spawnl\*` functions.  The "v" variants are good when the number of
   parameters is variable, with the arguments being passed in a list or tuple as
   the *args* parameter.  In either case, the arguments to the child process must
   start with the name of the command being run.

   The variants which include a second "p" near the end (:func:`spawnlp`,
   :func:`spawnlpe`, :func:`spawnvp`, and :func:`spawnvpe`) will use the
   :envvar:`PATH` environment variable to locate the program *file*.  When the
   environment is being replaced (using one of the :func:`spawn\*e <spawnl>` variants,
   discussed in the next paragraph), the new environment is used as the source of
   the :envvar:`PATH` variable.  The other variants, :func:`spawnl`,
   :func:`spawnle`, :func:`spawnv`, and :func:`spawnve`, will not use the
   :envvar:`PATH` variable to locate the executable; *path* must contain an
   appropriate absolute or relative path.

   For :func:`spawnle`, :func:`spawnlpe`, :func:`spawnve`, and :func:`spawnvpe`
   (note that these all end in "e"), the *env* parameter must be a mapping
   which is used to define the environment variables for the new process (they are
   used instead of the current process' environment); the functions
   :func:`spawnl`, :func:`spawnlp`, :func:`spawnv`, and :func:`spawnvp` all cause
   the new process to inherit the environment of the current process.  Note that
   keys and values in the *env* dictionary must be strings; invalid keys or
   values will cause the function to fail, with a return value of ``127``.

   As an example, the following calls to :func:`spawnlp` and :func:`spawnvpe` are
   equivalent::

      import os
      os.spawnlp(os.P_WAIT, 'cp', 'cp', 'index.html', '/dev/null')

      L = ['cp', 'index.html', '/dev/null']
      os.spawnvpe(os.P_WAIT, 'cp', L, os.environ)

   Availability: Unix, Windows.  :func:`spawnlp`, :func:`spawnlpe`, :func:`spawnvp`
   and :func:`spawnvpe` are not available on Windows.  :func:`spawnle` and
   :func:`spawnve` are not thread-safe on Windows; we advise you to use the
   :mod:`subprocess` module instead.


.. data:: P_NOWAIT
          P_NOWAITO

   Possible values for the *mode* parameter to the :func:`spawn\* <spawnl>` family of
   functions.  If either of these values is given, the :func:`spawn\*` functions
   will return as soon as the new process has been created, with the process id as
   the return value.

   Availability: Unix, Windows.


.. data:: P_WAIT

   Possible value for the *mode* parameter to the :func:`spawn\* <spawnl>` family of
   functions.  If this is given as *mode*, the :func:`spawn\*` functions will not
   return until the new process has run to completion and will return the exit code
   of the process the run is successful, or ``-signal`` if a signal kills the
   process.

   Availability: Unix, Windows.


.. data:: P_DETACH
          P_OVERLAY

   Possible values for the *mode* parameter to the :func:`spawn\* <spawnl>` family of
   functions.  These are less portable than those listed above. :const:`P_DETACH`
   is similar to :const:`P_NOWAIT`, but the new process is detached from the
   console of the calling process. If :const:`P_OVERLAY` is used, the current
   process will be replaced; the :func:`spawn\* <spawnl>` function will not return.

   Availability: Windows.


.. function:: startfile(path[, operation])

   Start a file with its associated application.

   When *operation* is not specified or ``'open'``, this acts like double-clicking
   the file in Windows Explorer, or giving the file name as an argument to the
   :program:`start` command from the interactive command shell: the file is opened
   with whatever application (if any) its extension is associated.

   When another *operation* is given, it must be a "command verb" that specifies
   what should be done with the file. Common verbs documented by Microsoft are
   ``'print'`` and  ``'edit'`` (to be used on files) as well as ``'explore'`` and
   ``'find'`` (to be used on directories).

   :func:`startfile` returns as soon as the associated application is launched.
   There is no option to wait for the application to close, and no way to retrieve
   the application's exit status.  The *path* parameter is relative to the current
   directory.  If you want to use an absolute path, make sure the first character
   is not a slash (``'/'``); the underlying Win32 :c:func:`ShellExecute` function
   doesn't work if it is.  Use the :func:`os.path.normpath` function to ensure that
   the path is properly encoded for Win32.

   Availability: Windows.


.. function:: system(command)

   Execute the command (a string) in a subshell.  This is implemented by calling
   the Standard C function :c:func:`system`, and has the same limitations.
   Changes to :data:`sys.stdin`, etc. are not reflected in the environment of
   the executed command. If *command* generates any output, it will be sent to
   the interpreter standard output stream.

   On Unix, the return value is the exit status of the process encoded in the
   format specified for :func:`wait`.  Note that POSIX does not specify the
   meaning of the return value of the C :c:func:`system` function, so the return
   value of the Python function is system-dependent.

   On Windows, the return value is that returned by the system shell after
   running *command*.  The shell is given by the Windows environment variable
   :envvar:`COMSPEC`: it is usually :program:`cmd.exe`, which returns the exit
   status of the command run; on systems using a non-native shell, consult your
   shell documentation.

   The :mod:`subprocess` module provides more powerful facilities for spawning
   new processes and retrieving their results; using that module is preferable
   to using this function.  See the :ref:`subprocess-replacements` section in
   the :mod:`subprocess` documentation for some helpful recipes.

   Availability: Unix, Windows.


.. function:: times()

   Returns the current global process times.
   The return value is an object with five attributes:

   * :attr:`user` - user time
   * :attr:`system` - system time
   * :attr:`children_user` - user time of all child processes
   * :attr:`children_system` - system time of all child processes
   * :attr:`elapsed` - elapsed real time since a fixed point in the past

   For backwards compatibility, this object also behaves like a five-tuple
   containing :attr:`user`, :attr:`system`, :attr:`children_user`,
   :attr:`children_system`, and :attr:`elapsed` in that order.

   See the Unix manual page
   :manpage:`times(2)` or the corresponding Windows Platform API documentation.
   On Windows, only :attr:`user` and :attr:`system` are known; the other
   attributes are zero.

   Availability: Unix, Windows.

   .. versionchanged:: 3.3
      Return type changed from a tuple to a tuple-like object
      with named attributes.


.. function:: wait()

   Wait for completion of a child process, and return a tuple containing its pid
   and exit status indication: a 16-bit number, whose low byte is the signal number
   that killed the process, and whose high byte is the exit status (if the signal
   number is zero); the high bit of the low byte is set if a core file was
   produced.

   Availability: Unix.

.. function:: waitid(idtype, id, options)

   Wait for the completion of one or more child processes.
   *idtype* can be :data:`P_PID`, :data:`P_PGID` or :data:`P_ALL`.
   *id* specifies the pid to wait on.
   *options* is constructed from the ORing of one or more of :data:`WEXITED`,
   :data:`WSTOPPED` or :data:`WCONTINUED` and additionally may be ORed with
   :data:`WNOHANG` or :data:`WNOWAIT`. The return value is an object
   representing the data contained in the :c:type:`siginfo_t` structure, namely:
   :attr:`si_pid`, :attr:`si_uid`, :attr:`si_signo`, :attr:`si_status`,
   :attr:`si_code` or ``None`` if :data:`WNOHANG` is specified and there are no
   children in a waitable state.

   Availability: Unix.

   .. versionadded:: 3.3

.. data:: P_PID
          P_PGID
          P_ALL

   These are the possible values for *idtype* in :func:`waitid`. They affect
   how *id* is interpreted.

   Availability: Unix.

   .. versionadded:: 3.3

.. data:: WEXITED
          WSTOPPED
          WNOWAIT

   Flags that can be used in *options* in :func:`waitid` that specify what
   child signal to wait for.

   Availability: Unix.

   .. versionadded:: 3.3


.. data:: CLD_EXITED
          CLD_DUMPED
          CLD_TRAPPED
          CLD_CONTINUED

   These are the possible values for :attr:`si_code` in the result returned by
   :func:`waitid`.

   Availability: Unix.

   .. versionadded:: 3.3


.. function:: waitpid(pid, options)

   The details of this function differ on Unix and Windows.

   On Unix: Wait for completion of a child process given by process id *pid*, and
   return a tuple containing its process id and exit status indication (encoded as
   for :func:`wait`).  The semantics of the call are affected by the value of the
   integer *options*, which should be ``0`` for normal operation.

   If *pid* is greater than ``0``, :func:`waitpid` requests status information for
   that specific process.  If *pid* is ``0``, the request is for the status of any
   child in the process group of the current process.  If *pid* is ``-1``, the
   request pertains to any child of the current process.  If *pid* is less than
   ``-1``, status is requested for any process in the process group ``-pid`` (the
   absolute value of *pid*).

   An :exc:`OSError` is raised with the value of errno when the syscall
   returns -1.

   On Windows: Wait for completion of a process given by process handle *pid*, and
   return a tuple containing *pid*, and its exit status shifted left by 8 bits
   (shifting makes cross-platform use of the function easier). A *pid* less than or
   equal to ``0`` has no special meaning on Windows, and raises an exception. The
   value of integer *options* has no effect. *pid* can refer to any process whose
   id is known, not necessarily a child process. The :func:`spawn\* <spawnl>`
   functions called with :const:`P_NOWAIT` return suitable process handles.


.. function:: wait3(options)

   Similar to :func:`waitpid`, except no process id argument is given and a
   3-element tuple containing the child's process id, exit status indication, and
   resource usage information is returned.  Refer to :mod:`resource`.\
   :func:`~resource.getrusage` for details on resource usage information.  The
   option argument is the same as that provided to :func:`waitpid` and
   :func:`wait4`.

   Availability: Unix.


.. function:: wait4(pid, options)

   Similar to :func:`waitpid`, except a 3-element tuple, containing the child's
   process id, exit status indication, and resource usage information is returned.
   Refer to :mod:`resource`.\ :func:`~resource.getrusage` for details on
   resource usage information.  The arguments to :func:`wait4` are the same
   as those provided to :func:`waitpid`.

   Availability: Unix.


.. data:: WNOHANG

   The option for :func:`waitpid` to return immediately if no child process status
   is available immediately. The function returns ``(0, 0)`` in this case.

   Availability: Unix.


.. data:: WCONTINUED

   This option causes child processes to be reported if they have been continued
   from a job control stop since their status was last reported.

   Availability: some Unix systems.


.. data:: WUNTRACED

   This option causes child processes to be reported if they have been stopped but
   their current state has not been reported since they were stopped.

   Availability: Unix.


The following functions take a process status code as returned by
:func:`system`, :func:`wait`, or :func:`waitpid` as a parameter.  They may be
used to determine the disposition of a process.

.. function:: WCOREDUMP(status)

   Return ``True`` if a core dump was generated for the process, otherwise
   return ``False``.

   Availability: Unix.


.. function:: WIFCONTINUED(status)

   Return ``True`` if the process has been continued from a job control stop,
   otherwise return ``False``.

   Availability: Unix.


.. function:: WIFSTOPPED(status)

   Return ``True`` if the process has been stopped, otherwise return
   ``False``.

   Availability: Unix.


.. function:: WIFSIGNALED(status)

   Return ``True`` if the process exited due to a signal, otherwise return
   ``False``.

   Availability: Unix.


.. function:: WIFEXITED(status)

   Return ``True`` if the process exited using the :manpage:`exit(2)` system call,
   otherwise return ``False``.

   Availability: Unix.


.. function:: WEXITSTATUS(status)

   If ``WIFEXITED(status)`` is true, return the integer parameter to the
   :manpage:`exit(2)` system call.  Otherwise, the return value is meaningless.

   Availability: Unix.


.. function:: WSTOPSIG(status)

   Return the signal which caused the process to stop.

   Availability: Unix.


.. function:: WTERMSIG(status)

   Return the signal which caused the process to exit.

   Availability: Unix.


Interface to the scheduler
--------------------------

These functions control how a process is allocated CPU time by the operating
system. They are only available on some Unix platforms. For more detailed
information, consult your Unix manpages.

.. versionadded:: 3.3

The following scheduling policies are exposed if they are supported by the
operating system.

.. data:: SCHED_OTHER

   The default scheduling policy.

.. data:: SCHED_BATCH

   Scheduling policy for CPU-intensive processes that tries to preserve
   interactivity on the rest of the computer.

.. data:: SCHED_IDLE

   Scheduling policy for extremely low priority background tasks.

.. data:: SCHED_SPORADIC

   Scheduling policy for sporadic server programs.

.. data:: SCHED_FIFO

   A First In First Out scheduling policy.

.. data:: SCHED_RR

   A round-robin scheduling policy.

.. data:: SCHED_RESET_ON_FORK

   This flag can OR'ed with any other scheduling policy. When a process with
   this flag set forks, its child's scheduling policy and priority are reset to
   the default.


.. class:: sched_param(sched_priority)

   This class represents tunable scheduling parameters used in
   :func:`sched_setparam`, :func:`sched_setscheduler`, and
   :func:`sched_getparam`. It is immutable.

   At the moment, there is only one possible parameter:

   .. attribute:: sched_priority

      The scheduling priority for a scheduling policy.


.. function:: sched_get_priority_min(policy)

   Get the minimum priority value for *policy*. *policy* is one of the
   scheduling policy constants above.


.. function:: sched_get_priority_max(policy)

   Get the maximum priority value for *policy*. *policy* is one of the
   scheduling policy constants above.


.. function:: sched_setscheduler(pid, policy, param)

   Set the scheduling policy for the process with PID *pid*. A *pid* of 0 means
   the calling process. *policy* is one of the scheduling policy constants
   above. *param* is a :class:`sched_param` instance.


.. function:: sched_getscheduler(pid)

   Return the scheduling policy for the process with PID *pid*. A *pid* of 0
   means the calling process. The result is one of the scheduling policy
   constants above.


.. function:: sched_setparam(pid, param)

   Set a scheduling parameters for the process with PID *pid*. A *pid* of 0 means
   the calling process. *param* is a :class:`sched_param` instance.


.. function:: sched_getparam(pid)

   Return the scheduling parameters as a :class:`sched_param` instance for the
   process with PID *pid*. A *pid* of 0 means the calling process.


.. function:: sched_rr_get_interval(pid)

   Return the round-robin quantum in seconds for the process with PID *pid*. A
   *pid* of 0 means the calling process.


.. function:: sched_yield()

   Voluntarily relinquish the CPU.


.. function:: sched_setaffinity(pid, mask)

   Restrict the process with PID *pid* (or the current process if zero) to a
   set of CPUs.  *mask* is an iterable of integers representing the set of
   CPUs to which the process should be restricted.


.. function:: sched_getaffinity(pid)

   Return the set of CPUs the process with PID *pid* (or the current process
   if zero) is restricted to.


.. _os-path:

Miscellaneous System Information
--------------------------------


.. function:: confstr(name)

   Return string-valued system configuration values. *name* specifies the
   configuration value to retrieve; it may be a string which is the name of a
   defined system value; these names are specified in a number of standards (POSIX,
   Unix 95, Unix 98, and others).  Some platforms define additional names as well.
   The names known to the host operating system are given as the keys of the
   ``confstr_names`` dictionary.  For configuration variables not included in that
   mapping, passing an integer for *name* is also accepted.

   If the configuration value specified by *name* isn't defined, ``None`` is
   returned.

   If *name* is a string and is not known, :exc:`ValueError` is raised.  If a
   specific value for *name* is not supported by the host system, even if it is
   included in ``confstr_names``, an :exc:`OSError` is raised with
   :const:`errno.EINVAL` for the error number.

   Availability: Unix.


.. data:: confstr_names

   Dictionary mapping names accepted by :func:`confstr` to the integer values
   defined for those names by the host operating system. This can be used to
   determine the set of names known to the system.

   Availability: Unix.


.. function:: cpu_count()

   Return the number of CPUs in the system. Returns None if undetermined.

   .. versionadded:: 3.4


.. function:: getloadavg()

   Return the number of processes in the system run queue averaged over the last
   1, 5, and 15 minutes or raises :exc:`OSError` if the load average was
   unobtainable.

   Availability: Unix.


.. function:: sysconf(name)

   Return integer-valued system configuration values. If the configuration value
   specified by *name* isn't defined, ``-1`` is returned.  The comments regarding
   the *name* parameter for :func:`confstr` apply here as well; the dictionary that
   provides information on the known names is given by ``sysconf_names``.

   Availability: Unix.


.. data:: sysconf_names

   Dictionary mapping names accepted by :func:`sysconf` to the integer values
   defined for those names by the host operating system. This can be used to
   determine the set of names known to the system.

   Availability: Unix.

The following data values are used to support path manipulation operations.  These
are defined for all platforms.

Higher-level operations on pathnames are defined in the :mod:`os.path` module.


.. data:: curdir

   The constant string used by the operating system to refer to the current
   directory. This is ``'.'`` for Windows and POSIX. Also available via
   :mod:`os.path`.


.. data:: pardir

   The constant string used by the operating system to refer to the parent
   directory. This is ``'..'`` for Windows and POSIX. Also available via
   :mod:`os.path`.


.. data:: sep

   The character used by the operating system to separate pathname components.
   This is ``'/'`` for POSIX and ``'\\'`` for Windows.  Note that knowing this
   is not sufficient to be able to parse or concatenate pathnames --- use
   :func:`os.path.split` and :func:`os.path.join` --- but it is occasionally
   useful. Also available via :mod:`os.path`.


.. data:: altsep

   An alternative character used by the operating system to separate pathname
   components, or ``None`` if only one separator character exists.  This is set to
   ``'/'`` on Windows systems where ``sep`` is a backslash. Also available via
   :mod:`os.path`.


.. data:: extsep

   The character which separates the base filename from the extension; for example,
   the ``'.'`` in :file:`os.py`. Also available via :mod:`os.path`.


.. data:: pathsep

   The character conventionally used by the operating system to separate search
   path components (as in :envvar:`PATH`), such as ``':'`` for POSIX or ``';'`` for
   Windows. Also available via :mod:`os.path`.


.. data:: defpath

   The default search path used by :func:`exec\*p\* <execl>` and
   :func:`spawn\*p\* <spawnl>` if the environment doesn't have a ``'PATH'``
   key. Also available via :mod:`os.path`.


.. data:: linesep

   The string used to separate (or, rather, terminate) lines on the current
   platform.  This may be a single character, such as ``'\n'`` for POSIX, or
   multiple characters, for example, ``'\r\n'`` for Windows. Do not use
   *os.linesep* as a line terminator when writing files opened in text mode (the
   default); use a single ``'\n'`` instead, on all platforms.


.. data:: devnull

   The file path of the null device. For example: ``'/dev/null'`` for
   POSIX, ``'nul'`` for Windows.  Also available via :mod:`os.path`.

.. data:: RTLD_LAZY
          RTLD_NOW
          RTLD_GLOBAL
          RTLD_LOCAL
          RTLD_NODELETE
          RTLD_NOLOAD
          RTLD_DEEPBIND

   Flags for use with the :func:`~sys.setdlopenflags` and
   :func:`~sys.getdlopenflags` functions.  See the Unix manual page
   :manpage:`dlopen(3)` for what the different flags mean.

   .. versionadded:: 3.3

.. _os-miscfunc:

Miscellaneous Functions
-----------------------


.. function:: urandom(n)

   Return a string of *n* random bytes suitable for cryptographic use.

   This function returns random bytes from an OS-specific randomness source.  The
   returned data should be unpredictable enough for cryptographic applications,
   though its exact quality depends on the OS implementation.  On a Unix-like
   system this will query ``/dev/urandom``, and on Windows it will use
   ``CryptGenRandom()``.  If a randomness source is not found,
   :exc:`NotImplementedError` will be raised.

   For an easy-to-use interface to the random number generator
   provided by your platform, please see :class:`random.SystemRandom`.