aboutsummaryrefslogtreecommitdiff
path: root/libbutl/fdstream.cxx
blob: 8098e507138b169adbf47c37ddb119089defa335 (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
// file      : libbutl/fdstream.cxx -*- C++ -*-
// license   : MIT; see accompanying LICENSE file

#include <libbutl/fdstream.hxx>

#include <errno.h> // errno, E*

#ifndef _WIN32
#  include <fcntl.h>      // open(), O_*, fcntl()
#  include <unistd.h>     // close(), read(), write(), lseek(), dup(), pipe(),
                          // ftruncate(), isatty(), ssize_t, STD*_FILENO
#  include <sys/uio.h>    // writev(), iovec
#  include <sys/stat.h>   // stat(), fstat(), S_I*
#  include <sys/time.h>   // timeval
#  include <sys/types.h>  // stat, off_t
#  include <sys/select.h>
#else
#  include <libbutl/win32-utility.hxx>

#  include <io.h>        // _close(), _read(), _write(), _setmode(), _sopen(),
                         // _lseek(), _dup(), _pipe(), _chsize_s,
                         // _get_osfhandle()
#  include <share.h>     // _SH_DENYNO
#  include <stdio.h>     // _fileno(), stdin, stdout, stderr, SEEK_*
#  include <fcntl.h>     // _O_*
#  include <sys/types.h> // _stat
#  include <sys/stat.h>  // fstat(), S_I*

#  ifdef _MSC_VER // Unlikely to be fixed in newer versions.
#    define S_ISREG(m) (((m) & S_IFMT) == S_IFREG)
#    define S_ISDIR(m) (((m) & S_IFMT) == S_IFDIR)
#    define S_ISCHR(m) (((m) & S_IFMT) == S_IFCHR)
#  endif

#  include <wchar.h> // wcsncmp(), wcsstr()

#  include <thread>    // this_thread::yield()
#  include <algorithm> // count()
#endif

#include <ios>          // ios_base::openmode, ios_base::failure
#include <new>          // bad_alloc
#include <limits>       // numeric_limits
#include <cassert>
#include <cstring>      // memcpy(), memmove(), memchr()
#include <iostream>     // cin, cout
#include <exception>    // uncaught_exception[s]()
#include <stdexcept>    // invalid_argument
#include <system_error>

#include <libbutl/ft/exception.hxx>    // uncaught_exceptions
#include <libbutl/process-details.hxx>

#include <libbutl/utility.hxx>   // throw_*_ios_failure(), function_cast()
#include <libbutl/timestamp.hxx>

using namespace std;

#ifdef _WIN32
using namespace butl::win32;
#endif

namespace butl
{
  // auto_fd
  //
  void auto_fd::
  close ()
  {
    if (fd_ >= 0)
    {
      bool r (fdclose (fd_));

      // If fdclose() failed then no reason to expect it to succeed the next
      // time.
      //
      fd_ = -1;

      if (!r)
        throw_generic_ios_failure (errno);
    }
  }

#ifdef _WIN32
  // Resolve a file descriptor to HANDLE. On the underlying OS error throw
  // ios::failure or return INVALID_HANDLE_VALUE if ignore_error is true.
  //
  // Note that such a handle is closed either when CloseHandle() is called on
  // the HANDLE or when _close() is called on the associated file descriptor.
  // So make sure that either the original file descriptor or the resulting
  // HANDLE is closed but not both of them.
  //
  static HANDLE
  fd_to_handle (int fd, bool ignore_error = false)
  {
    HANDLE h (reinterpret_cast<HANDLE> (_get_osfhandle (fd)));
    if (h == INVALID_HANDLE_VALUE && !ignore_error)
      throw_generic_ios_failure (errno); // EBADF (POSIX value).

    return h;
  }

  // Return the pipe handle and blocking mode or nullopt if the file
  // descriptor is not a pipe. On the underlying OS error throw ios::failure
  // or return nullopt if ignore_error is true.
  //
  struct handle_mode
  {
    HANDLE handle;
    fdstream_mode mode;
  };

  static optional<handle_mode>
  pipe_mode (int fd, bool ignore_error = false)
  {
    // We don't need to close it (see fd_to_handle()).
    //
    HANDLE h (fd_to_handle (fd, ignore_error));
    if (h == INVALID_HANDLE_VALUE)
      return nullopt;

    // If we fail to obtain a pipe state for a valid handle, then we assume
    // that this is not a pipe.
    //
    // Note that for other handle types GetLastError() may return different
    // error codes depending on the type and if we run natively or under Wine.
    //
    DWORD m;
    if (!GetNamedPipeHandleState (h,
                                  &m,
                                  nullptr /* lpCurInstances */,
                                  nullptr /* lpMaxCollectionCount */,
                                  nullptr /* lpCollectDataTimeout */,
                                  nullptr /* lpUserName */,
                                  0       /* nMaxUserNameSize */))
      return nullopt;

    return handle_mode {h,
                        (m & PIPE_NOWAIT) != 0
                        ? fdstream_mode::non_blocking
                        : fdstream_mode::blocking};
  }
#endif

  // fdstreambuf
  //
  // Return true if the file descriptor is in the non-blocking mode. Throw
  // ios::failure on the underlying OS error.
  //
  static bool
  non_blocking (int fd)
  {
#ifndef _WIN32
    int flags (fcntl (fd, F_GETFL));

    if (flags == -1)
      throw_generic_ios_failure (errno);

    return (flags & O_NONBLOCK) == O_NONBLOCK;
#else
    optional<handle_mode> m (pipe_mode (fd));
    return m && m->mode == fdstream_mode::non_blocking;
#endif
  }

  void fdstreambuf::
  open (auto_fd&& fd, uint64_t pos)
  {
    close ();

    non_blocking_ = non_blocking (fd.get ());

    setg (buf_, buf_, buf_);
    setp (buf_, buf_ + sizeof (buf_) - 1); // Keep space for overflow's char.
    off_ = pos;
    fd_ = move (fd);
  }

  bool fdstreambuf::
  blocking (bool m)
  {
    // Verify that the file descriptor is open.
    //
    if (!is_open ())
      throw_generic_ios_failure (EBADF); // POSIX value.

    // Bail out if we are already in the requested mode.
    //
    if (m != non_blocking_)
      return m;

    // Change the mode.
    //
    fdmode (fd (), m ? fdstream_mode::blocking : fdstream_mode::non_blocking);

    // Get the effective file descriptor blocking mode (see fdmode() for
    // details).
    //
    non_blocking_ = non_blocking (fd ());
    return !m;
  }

  streamsize fdstreambuf::
  showmanyc ()
  {
    if (!is_open ())
      return -1;

    streamsize n (egptr () - gptr ());

    if (n > 0)
      return n;

    if (non_blocking_)
    {
      streamsize n (fdread (fd_.get (), buf_, sizeof (buf_)));

      if (n == -1)
      {
        if (errno == EAGAIN || errno == EINTR)
          return 0;

        throw_generic_ios_failure (errno);
      }

      if (n == 0) // EOF.
        return -1;

      setg (buf_, buf_, buf_ + n);
      off_ += n;

      return n;
    }

    return 0;
  }

  fdstreambuf::int_type fdstreambuf::
  underflow ()
  {
    int_type r (traits_type::eof ());

    if (is_open ())
    {
      // The underflow() function interface doesn't support the non-blocking
      // semantics as it must return either the next character or EOF. In the
      // future we may implement the blocking behavior for a non-blocking file
      // descriptor.
      //
      if (non_blocking_)
        throw_generic_ios_failure (ENOTSUP);

      if (gptr () < egptr () || load ())
        r = traits_type::to_int_type (*gptr ());
    }

    return r;
  }

  bool fdstreambuf::
  load ()
  {
    // Doesn't handle blocking mode and so should not be called.
    //
    assert (!non_blocking_);

    streamsize n (fdread (fd_.get (), buf_, sizeof (buf_)));

    if (n == -1)
      throw_generic_ios_failure (errno);

    setg (buf_, buf_, buf_ + n);
    off_ += n;
    return n != 0;
  }

  void fdstreambuf::
  seekg (uint64_t off)
  {
    // In the future we may implement the blocking behavior for a non-blocking
    // file descriptor.
    //
    if (non_blocking_)
      throw_generic_ios_failure (ENOTSUP);

    // The plan is to rewind to the beginning of the stream, read the
    // requested number of characters and reset the get area, so it will be
    // filled from scratch on the next read from the stream.
    //
    fdseek (fd_.get (), 0, fdseek_mode::set);

    for (uint64_t n (off); n != 0; )
    {
      size_t m (n > sizeof (buf_) ? sizeof (buf_) : static_cast<size_t> (n));
      streamsize r (fdread (fd_.get (), buf_, m));

      if (r == -1)
        throw_generic_ios_failure (errno);

      // Fail if trying to seek beyond the end of the stream.
      //
      if (r == 0)
        throw_generic_ios_failure (EINVAL);

      n -= r;
    }

    off_ = off;
    setg (buf_, buf_, buf_);
  }

  fdstreambuf::int_type fdstreambuf::
  overflow (int_type c)
  {
    int_type r (traits_type::eof ());

    if (is_open () && c != traits_type::eof ())
    {
      // The overflow() function interface doesn't support the non-blocking
      // semantics since being unable to serialize the character is supposed
      // to be an error. In the future we may implement the blocking behavior
      // for a non-blocking file descriptor.
      //
      if (non_blocking_)
        throw_generic_ios_failure (ENOTSUP);

      // Store last character in the space we reserved in open(). Note
      // that pbump() doesn't do any checks.
      //
      *pptr () = traits_type::to_char_type (c);
      pbump (1);

      if (save ())
        r = c;
    }

    return r;
  }

  int fdstreambuf::
  sync ()
  {
    if (!is_open ())
      return -1;

    // The sync() function interface doesn't support the non-blocking
    // semantics since it should either completely sync the data or fail. In
    // the future we may implement the blocking behavior for a non-blocking
    // file descriptor.
    //
    if (non_blocking_)
      throw_generic_ios_failure (ENOTSUP);

    return save () ? 0 : -1;
  }

  bool fdstreambuf::
  save ()
  {
    size_t n (pptr () - pbase ());

    if (n != 0)
    {
      // Note that for MinGW GCC (5.2.0) _write() returns 0 for a file
      // descriptor opened for read-only access (while -1 with errno EBADF is
      // expected). This is in contrast with VC's _write() and POSIX's write().
      //
      auto m (fdwrite (fd_.get (), buf_, n));

      if (m == -1)
        throw_generic_ios_failure (errno);

      off_ += m;

      if (n != static_cast<size_t> (m))
        return false;

      setp (buf_, buf_ + sizeof (buf_) - 1);
    }

    return true;
  }

  streamsize fdstreambuf::
  xsputn (const char_type* s, streamsize sn)
  {
    // The xsputn() function interface doesn't support the non-blocking
    // semantics since the only excuse not to fully serialize the data is
    // encountering EOF (the default behaviour is defined as a sequence of
    // sputc() calls which stops when either sn characters are written or a
    // call would have returned EOF). In the future we may implement the
    // blocking behavior for a non-blocking file descriptor.
    //
    if (non_blocking_)
      throw_generic_ios_failure (ENOTSUP);

    // To avoid futher 'signed/unsigned comparison' compiler warnings.
    //
    size_t n (static_cast<size_t> (sn));

    auto advance = [this] (size_t n) {pbump (static_cast<int> (n));};

    // Buffer the data if there is enough space.
    //
    size_t an (epptr () - pptr ()); // Amount of free space in the buffer.
    if (n <= an)
    {
      assert (s != nullptr || n == 0);

      // Note that the memcpy() function behavior is undefined if either of
      // pointers is NULL, even if the bytes count is zero.
      //
      if (s != nullptr)
        memcpy (pptr (), s, n);

      advance (n);
      return n;
    }

    size_t bn (pptr () - pbase ()); // Buffered data size.

#ifndef _WIN32

    ssize_t r;
    if (bn > 0)
    {
      // Write both buffered and new data with a single system call.
      //
      iovec iov[2] = {{pbase (), bn}, {const_cast<char*> (s), n}};
      r = writev (fd_.get (), iov, 2);
    }
    else
      r = write (fd_.get (), s, n);

    if (r == -1)
      throw_generic_ios_failure (errno);

    size_t m (static_cast<size_t> (r));
    off_ += m;

    // If the buffered data wasn't fully written then move the unwritten part
    // to the beginning of the buffer.
    //
    if (m < bn)
    {
      memmove (pbase (), pbase () + m, bn - m);
      pbump (-static_cast<int> (m)); // Note that pbump() accepts negatives.
      return 0;
    }

    setp (buf_, buf_ + sizeof (buf_) - 1);
    return m - bn;

#else

    // On Windows there is no writev() available so sometimes we will make two
    // system calls. Fill and flush the buffer, then try to fit the data tail
    // into the empty buffer. If the data tail is too long then just write it
    // to the file and keep the buffer empty.
    //
    // We will end up with two _write() calls if the total data size to be
    // written exceeds double the buffer size. In this case the buffer filling
    // is redundant so let's pretend there is no free space in the buffer, and
    // so buffered and new data will be written separatelly.
    //
    if (bn + n > 2 * (bn + an))
      an = 0;
    else
    {
      assert (s != nullptr || an == 0);

      // The source can not be NULL (see above for details).
      //
      if (s != nullptr)
        memcpy (pptr (), s, an);

      advance (an);
    }

    // Flush the buffer.
    //
    size_t wn (bn + an);
    streamsize r (wn > 0 ? fdwrite (fd_.get (), buf_, wn) : 0);

    if (r == -1)
      throw_generic_ios_failure (errno);

    size_t m (static_cast<size_t> (r));
    off_ += m;

    // If the buffered data wasn't fully written then move the unwritten part
    // to the beginning of the buffer.
    //
    if (m < wn)
    {
      memmove (pbase (), pbase () + m, wn - m);
      pbump (-static_cast<int> (m)); // Note that pbump() accepts negatives.
      return m < bn ? 0 : m - bn;
    }

    setp (buf_, buf_ + sizeof (buf_) - 1);

    // Now 'an' holds the size of the data portion written as a part of the
    // buffer flush.
    //
    s += an;
    n -= an;

    // Buffer the data tail if it fits the buffer.
    //
    if (n <= static_cast<size_t> (epptr () - pbase ()))
    {
      assert (s != nullptr || n == 0);

      // The source can not be NULL (see above for details).
      //
      if (s != nullptr)
        memcpy (pbase (), s, n);

      advance (n);
      return sn;
    }

    // The data tail doesn't fit the buffer so write it to the file.
    //
    r = fdwrite (fd_.get (), s, n);

    if (r == -1)
      throw_generic_ios_failure (errno);

    off_ += r;

    return an + r;
#endif
  }

  // Common call chains:
  //
  // - basic_ostream::seekp(pos)                    ->
  //     basic_streambuf::pubseekpos(pos, ios::out) ->
  //       fdstreambuf::seekpos(pos, ios::out)
  //
  // - basic_istream::seekg(pos)                   ->
  //     basic_streambuf::pubseekpos(pos, ios::in) ->
  //       fdstreambuf::seekpos(pos, ios::in)
  //
  fdstreambuf::pos_type fdstreambuf::
  seekpos (pos_type pos, ios_base::openmode which)
  {
    // Note that the position type provides an explicit conversion to the
    // numeric offset type (see std::fpos for details). The position state is
    // disregarded in this case, which is ok since we don't mess with the
    // multibyte character conversions.
    //
    return seekoff (static_cast<off_type> (pos), ios_base::beg, which);
  }

  // Common call chains:
  //
  // - basic_ostream::seekp(off, dir)                    ->
  //     basic_streambuf::pubseekoff(off, dir, ios::out) ->
  //       fdstreambuf::seekoff(off, dir, ios::out)
  //
  // - basic_ostream::tellp()                               ->
  //     basic_streambuf::pubseekoff(0, ios::cur, ios::out) ->
  //       fdstreambuf::seekoff(0, ios::cur, ios::out)
  //
  // - basic_istream::seekg(off, dir)                   ->
  //     basic_streambuf::pubseekoff(off, dir, ios::in) ->
  //       fdstreambuf::seekoff(off, dir, ios::in)
  //
  // - basic_istream::tellg()                              ->
  //     basic_streambuf::pubseekoff(0, ios::cur, ios::in) ->
  //       fdstreambuf::seekoff(0, ios::cur, ios::in)
  //
  fdstreambuf::pos_type fdstreambuf::
  seekoff (off_type off, ios_base::seekdir dir, ios_base::openmode which)
  {
    // The seekoff() function interface doesn't support the non-blocking
    // semantics since being unable to serialize the character in write mode
    // is supposed to be an error. Also the non-blocking mode is likely to be
    // used for non-seekable file descriptors (pipes, etc.). In the future we
    // may implement the blocking behavior for a non-blocking file descriptor.
    //
    if (non_blocking_)
      throw_generic_ios_failure (ENOTSUP);

    // Translate ios_base::seekdir value to fdseek_mode. Note:
    // ios_base::seekdir is an implementation-defined type and is not
    // necessarily a enum.
    //
    fdseek_mode m (fdseek_mode::set);
    switch (dir)
    {
    case ios_base::beg: m = fdseek_mode::set; break;
    case ios_base::cur: m = fdseek_mode::cur; break;
    case ios_base::end: m = fdseek_mode::end; break;
    default: assert (false);
    }

    // Prior to fdseek() call we will flush the buffer for the write mode,
    // reset the get area for the read mode, and fail otherwise. Note that we
    // don't support the read/write mode.
    //
    // Note that the return (position) type is implicitly constructible from
    // the numeric offset type (see std::fpos for details).
    //
    switch (which)
    {
    case ios_base::out:
      {
        // Fail if unable to fully flush the buffer (for example, because the
        // device is full).
        //
        if (!save ())
          return static_cast<off_type> (-1);

        break;
      }
    case ios_base::in:
      {
        // We may have unread data in the get area and need to subtract its
        // size from the offset if we seek from the current position.
        //
        if (dir == ios_base::cur)
        {
          off_type n (egptr () - gptr ()); // Get area size.

#ifdef _WIN32
          // Note that on Windows, when reading in the text mode, newline
          // characters are translated from the CRLF character sequences.
          // Thus, in this mode, we also need to subtract the number of
          // newlines in the get area from the offset.
          //
          // Note that this approach only works for "canonical" Windows text
          // files. Specifically, if there are newlines not preceded with the
          // CR character then we may end up in the wrong place. It seems that
          // there is no reasonable solution for this problem, and neither of
          // the MSVC's or MinGW's std::ifstream implementations handle this
          // case properly.
          //

          // The only way to query the current file descriptor mode is to
          // reset it and use the result (see fdmode() for details).
          //
          fdstream_mode fm (fdmode (fd_.get (), fdstream_mode::text));

          // Note: the fdstream_mode::blocking flag is also set.
          //
          if ((fm & fdstream_mode::text) == fdstream_mode::text)
            n += count (gptr (), egptr (), '\n');
          else
            fdmode (fd_.get (), fm); // Restore the mode if it was changed.
#endif

          // Note that ifdstream::tellg() implicitly calls seekoff(0,ios::cur)
          // (see above). Let's not reset the get area for such noop seeks.
          //
          if (off == 0)
            return static_cast<off_type> (
              fdseek (fd_.get (), 0, fdseek_mode::cur) - n);

          off -= n;
        }

        // Reset the get area.
        //
        setg (buf_, buf_, buf_);
        break;
      }
    default: return static_cast<off_type> (-1);
    }

    // Note that on Windows in the text mode the logical offset (number of
    // read/written bytes) is likely to be screwed up due to newlines
    // translation (see above).
    //
    off_ = fdseek (fd_.get (), off, m);

    return static_cast<off_type> (off_);
  }

  inline static bool
  flag (fdstream_mode m, fdstream_mode flag)
  {
    return (m & flag) == flag;
  }

  inline static auto_fd
  mode (auto_fd fd, fdstream_mode m)
  {
    if (fd.get () >= 0 &&
        (flag (m, fdstream_mode::text)     ||
         flag (m, fdstream_mode::binary)   ||
         flag (m, fdstream_mode::blocking) ||
         flag (m, fdstream_mode::non_blocking)))
      fdmode (fd.get (), m);

    return fd;
  }

  // fdstream_base
  //
  fdstream_base::
  fdstream_base (auto_fd&& fd, fdstream_mode m, std::uint64_t pos)
      : fdstream_base (mode (move (fd), m), pos)
  {
  }

  static fdopen_mode
  translate_mode (ios_base::openmode m)
  {
    using openmode = ios_base::openmode;

    const openmode in    (ios_base::in);
    const openmode out   (ios_base::out);
    const openmode app   (ios_base::app);
    const openmode bin   (ios_base::binary);
    const openmode trunc (ios_base::trunc);
    const openmode ate   (ios_base::ate);

    const fdopen_mode fd_in     (fdopen_mode::in);
    const fdopen_mode fd_out    (fdopen_mode::out);
    const fdopen_mode fd_inout  (fdopen_mode::in | fdopen_mode::out);
    const fdopen_mode fd_app    (fdopen_mode::append);
    const fdopen_mode fd_trunc  (fdopen_mode::truncate);
    const fdopen_mode fd_create (fdopen_mode::create);
    const fdopen_mode fd_bin    (fdopen_mode::binary);
    const fdopen_mode fd_ate    (fdopen_mode::at_end);

    // Note: cannot use switch due to warnings (value no in enumerator).
    //
    openmode sm (m & ~(ate | bin));
    fdopen_mode r (
      sm == (in)                                ? fd_in                       :
      sm == (out)          || sm == (out|trunc) ? fd_out|fd_trunc|fd_create   :
      sm == (app)          || sm == (out|app)   ? fd_out|fd_app|fd_create     :
      sm == (out|in)                            ? fd_inout                    :
      sm == (out|in|trunc)                      ? fd_inout|fd_trunc|fd_create :
      sm == (out|in|app)   || sm == (in|app)    ? fd_inout|fd_app|fd_create   :
      fdopen_mode::none);

    if (r == fdopen_mode::none)
      throw invalid_argument ("invalid open mode");

    if (m & ate)
      r |= fd_ate;

    if (m & bin)
      r |= fd_bin;

    return r;
  }

  // ifdstream
  //
  ifdstream::
  ifdstream (const char* f, openmode m, iostate e)
      : ifdstream (f, translate_mode (m | in), e)
  {
  }

  ifdstream::
  ifdstream (const char* f, fdopen_mode m, iostate e)
      : ifdstream (fdopen (f,
                           // If fdopen_mode::in is not specified, then
                           // emulate the ios::in semantics.
                           //
                           (m & fdopen_mode::in) == fdopen_mode::in
                           ? m
                           : m | translate_mode (in)),
                   e)
  {
  }

  ifdstream::
  ~ifdstream ()
  {
    if (skip_ && is_open () && good ())
    {
      // Clear the exception mask to prevent ignore() from throwing.
      //
      exceptions (goodbit);

      // The ignore() function doesn't support the non-blocking semantics as
      // it extracts and discards characters until the limit is reached or EOF
      // is encountered. That's why we switch to the blocking mode.
      //
      // It's highly unlikely to be unable to set the blocking mode for a
      // valid fd. If, however, that happens, we can't do much about it.
      //
      try
      {
        buf_.blocking (true);
        ignore (numeric_limits<streamsize>::max ());
      }
      catch (const ios_base::failure&) {}
    }

    // Underlying file descriptor is closed by fdstreambuf dtor with errors
    // (if any) being ignored.
  }

  void ifdstream::
  open (const char* f, openmode m)
  {
    open (f, translate_mode (m | in));
  }

  void ifdstream::
  open (const char* f, fdopen_mode m)
  {
    open (fdopen (f,
                  (m & fdopen_mode::in) == fdopen_mode::in
                  ? m
                  : m | translate_mode (in)));
  }

  void ifdstream::
  open (auto_fd&& fd, fdstream_mode m, std::uint64_t pos)
  {
    open (mode (std::move (fd), m), pos);
    skip_ = (m & fdstream_mode::skip) == fdstream_mode::skip;
  }

  void ifdstream::
  close ()
  {
    if (skip_ && is_open () && good ())
    {
      // The ignore() function doesn't support the non-blocking semantics (see
      // above).
      //
      buf_.blocking (true);
      ignore (numeric_limits<streamsize>::max ());
    }

    buf_.close ();
  }

  ifdstream&
  getline (ifdstream& is, string& l, char delim)
  {
    ifdstream::iostate eb (is.exceptions ());
    assert (eb & ifdstream::badbit);

    // Amend the exception mask to prevent exceptions being thrown by the C++
    // IO runtime to avoid incompatibility issues due to ios_base::failure ABI
    // fiasco (#66145). We will not restore the mask when ios_base::failure is
    // thrown by fdstreambuf since there is no way to "silently" restore it if
    // the corresponding bits are in the error state without the exceptions()
    // call throwing ios_base::failure. Not restoring exception mask on
    // throwing because of badbit should probably be ok since the stream is no
    // longer usable.
    //
    if (eb != ifdstream::badbit)
      is.exceptions (ifdstream::badbit);

    std::getline (is, l, delim);

    // Throw if any of the newly set bits are present in the exception mask.
    //
    if ((is.rdstate () & eb) != ifdstream::goodbit)
      throw_generic_ios_failure (EIO, "getline failure");

    if (eb != ifdstream::badbit)
      is.exceptions (eb); // Restore exception mask.

    return is;
  }

  bool
  getline_non_blocking (ifdstream& is, string& l, char delim)
  {
    assert (!is.blocking () && (is.exceptions () & ifdstream::badbit) != 0);

    fdstreambuf& sb (*static_cast<fdstreambuf*> (is.rdbuf ()));

    // Read until blocked (0), EOF (-1) or encounter the delimiter.
    //
    // Note that here we reasonably assume that any failure in in_avail()
    // will lead to badbit and thus an exception (see showmanyc()).
    //
    streamsize s;
    while ((s = sb.in_avail ()) > 0)
    {
      const char* p (sb.gptr ());
      size_t n (sb.egptr () - p);

      const char* e (static_cast<const char*> (memchr (p, delim, n)));
      if (e != nullptr)
        n = e - p;

      l.append (p, n);

      // Note: consume the delimiter if found.
      //
      sb.gbump (static_cast<int> (n + (e != nullptr ? 1 : 0)));

      if (e != nullptr)
        break;
    }

    // Here s can be:
    //
    // -1 -- EOF.
    //  0 -- blocked before encountering delimiter/EOF.
    // >0 -- encountered the delimiter.
    //
    if (s == -1)
    {
      is.setstate (ifdstream::eofbit);

      // If we couldn't extract anything, not even the delimiter, then this is
      // a failure per the getline() interface.
      //
      if (l.empty ())
        is.setstate (ifdstream::failbit);
    }

    return s != 0;
  }

  // ofdstream
  //
  ofdstream::
  ofdstream (const char* f, openmode m, iostate e)
      : ofdstream (f, translate_mode (m | out), e)
  {
  }

  ofdstream::
  ofdstream (const char* f, fdopen_mode m, iostate e)
      : ofdstream (fdopen (f,
                           // If fdopen_mode::out is not specified, then
                           // emulate the ios::out semantics.
                           //
                           (m & fdopen_mode::out) == fdopen_mode::out
                           ? m
                           : m | translate_mode (out)),
                   e)
  {
  }

  ofdstream::
  ~ofdstream ()
  {
    // Enforce explicit close(). Note that we may have false negatives but not
    // false positives. Specifically, we will fail to enforce if someone is
    // using ofdstream in a dtor being called while unwinding the stack due to
    // an exception.
    //
    // C++17 deprecated uncaught_exception() so use uncaught_exceptions() if
    // available.
    //
#ifdef __cpp_lib_uncaught_exceptions
    assert (!is_open () || !good () || uncaught_exceptions () != 0);
#else
    assert (!is_open () || !good () || uncaught_exception ());
#endif
  }

  void ofdstream::
  open (const char* f, openmode m)
  {
    open (f, translate_mode (m | out));
  }

  void ofdstream::
  open (const char* f, fdopen_mode m)
  {
    open (fdopen (f,
                  (m & fdopen_mode::out) == fdopen_mode::out
                  ? m
                  : m | translate_mode (out)));
  }

  istream&
  open_file_or_stdin (path_name& pn, ifdstream& ifs)
  {
    assert (pn.path != nullptr);

    if (pn.path->string () != "-")
    {
      ifs.open (*pn.path);
      return ifs;
    }
    else
    {
      cin.exceptions (ifs.exceptions ());
      if (!pn.name)
        pn.name = "<stdin>";
      return cin;
    }
  }

  ostream&
  open_file_or_stdout (path_name& pn, ofdstream& ofs)
  {
    assert (pn.path != nullptr);

    if (pn.path->string () != "-")
    {
      ofs.open (*pn.path);
      return ofs;
    }
    else
    {
      cout.exceptions (ofs.exceptions ());
      if (!pn.name)
        pn.name = "<stdout>";
      return cout;
    }
  }

  // fd*() functions
  //

  auto_fd
  fdopen (const char* f, fdopen_mode m, permissions p)
  {
    mode_t pf (S_IREAD | S_IWRITE | S_IEXEC);

#ifdef S_IRWXG
    pf |= S_IRWXG;
#endif

#ifdef S_IRWXO
    pf |= S_IRWXO;
#endif

    pf &= static_cast<mode_t> (p);

    // Return true if the open mode contains a specific flag.
    //
    auto mode = [m](fdopen_mode flag) -> bool {return (m & flag) == flag;};

    int of (0);
    bool in (mode (fdopen_mode::in));
    bool out (mode (fdopen_mode::out));

#ifndef _WIN32

    if (in && out)
      of |= O_RDWR;
    else if (in)
      of |= O_RDONLY;
    else if (out)
      of |= O_WRONLY;

    if (out)
    {
      if (mode (fdopen_mode::append))
        of |= O_APPEND;

      if (mode (fdopen_mode::truncate))
        of |= O_TRUNC;
    }

    if (mode (fdopen_mode::create))
    {
      of |= O_CREAT;

      if (mode (fdopen_mode::exclusive))
        of |= O_EXCL;
    }

#ifdef O_LARGEFILE
    of |= O_LARGEFILE;
#endif

    // Unlike other platforms, *BSD allows opening a directory as a file which
    // will cause all kinds of problems upstream (e.g., cpfile()). So we
    // detect and diagnose this. Note: not certain this is the case for NetBSD
    // and OpenBSD.
    //
#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__)
    {
      struct stat s;
      if (stat (f, &s) == 0 && S_ISDIR (s.st_mode))
        throw_generic_ios_failure (EISDIR);
    }
#endif

    int fd (open (f, of | O_CLOEXEC, pf));

#else

    if (in && out)
      of |= _O_RDWR;
    else if (in)
      of |= _O_RDONLY;
    else if (out)
      of |= _O_WRONLY;

    if (out)
    {
      if (mode (fdopen_mode::append))
        of |= _O_APPEND;

      if (mode (fdopen_mode::truncate))
        of |= _O_TRUNC;
    }

    if (mode (fdopen_mode::create))
    {
      of |= _O_CREAT;

      if (mode (fdopen_mode::exclusive))
        of |= _O_EXCL;
    }

    of |= mode (fdopen_mode::binary) ? _O_BINARY : _O_TEXT;

    // According to Microsoft _sopen() should not change the permissions of an
    // existing file. However it does if we pass them (reproduced on Windows
    // XP, 7, and 8). And we must pass them if we have _O_CREATE. So we need
    // to take care of preserving the permissions ourselves. Note that Wine's
    // implementation of _sopen() works properly.
    //
    // Also note that _sopen() creates a dangling symlink target if _O_EXCL is
    // set. Thus, we need to prevent such a creation manually.
    //
    bool pass_perm (of & _O_CREAT);

    if (pass_perm)
    {
      if (file_exists (f)) // Follows symlink.
      {
        // If the _O_CREAT flag is set then we need to clear it so that we can
        // omit the permissions. But if the _O_EXCL flag is set as well we can't
        // do that as fdopen() wouldn't fail as expected.
        //
        if (of & _O_EXCL)
          throw_generic_ios_failure (EEXIST);

        of &= ~_O_CREAT;
        pass_perm = false;
      }
      else if (of & _O_EXCL)
      {
        pair<bool, entry_stat> pe (path_entry (f)); // Doesn't follow symlink.

        // Fail for a dangling symlink.
        //
        if (pe.first && pe.second.type == entry_type::symlink)
          throw_generic_ios_failure (EEXIST);
      }
    }

    // Make sure the file descriptor is not inheritable by default.
    //
    of |= _O_NOINHERIT;

    int fd;

    // For reasons unknown an attempt to open an existing file for writing
    // sometimes ends up with the EACCES POSIX error which is presumably a
    // translation of the ERROR_SHARING_VIOLATION system error returned by the
    // underlying CreateFile() function call (see mventry() for details). If
    // that's the case, we will keep trying to open the file for two seconds.
    //
    // Also, it turns out, if someone memory-maps a file, it takes Windows
    // some time to realize it's been unmapped and until then any attempt to
    // open it results in EINVAL POSIX error, ERROR_USER_MAPPED_FILE system
    // error. So we retry those as well.
    //
    for (size_t i (0); i < 41; ++i)
    {
      // Sleep 50 milliseconds before the open retry.
      //
      if (i != 0)
        Sleep (50);

      fd = pass_perm
           ? _sopen (f, of, _SH_DENYNO, pf)
           : _sopen (f, of, _SH_DENYNO);

      // Note that MSVCRT's _sopen() calls CreateFile() underneath,
      // translating the system error to POSIX on failure and bailing out
      // afterwords. Thus, we can query the original error code on _sopen()
      // failure.
      //
      // Note that MinGW's _sopen() is just a stub forwarding the call to the
      // (publicly available) MSVCRT's implementation.
      //
      if (!(fd == -1                             &&
            out                                  &&
            (errno == EACCES || errno == EINVAL) &&
            (GetLastError () == ERROR_SHARING_VIOLATION ||
             GetLastError () == ERROR_USER_MAPPED_FILE)))
        break;
    }

#endif

    if (fd == -1)
      throw_generic_ios_failure (errno);

    if (mode (fdopen_mode::at_end))
    {
#ifndef _WIN32
      bool r (lseek (fd, 0, SEEK_END) != static_cast<off_t> (-1));
#else
      bool r (_lseek (fd, 0, SEEK_END) != -1);
#endif

      // Note that in the case of an error we don't delete the newly created
      // file as we have no indication if it is a new one.
      //
      if (!r)
      {
        int e (errno);
        fdclose (fd); // Doesn't throw, but can change errno.
        throw_generic_ios_failure (e);
      }
    }

    return auto_fd (fd);
  }

  uint64_t
  fdseek (int fd, int64_t o, fdseek_mode fdm)
  {
    int m (-1);

    switch (fdm)
    {
    case fdseek_mode::set: m = SEEK_SET; break;
    case fdseek_mode::cur: m = SEEK_CUR; break;
    case fdseek_mode::end: m = SEEK_END; break;
    }

#ifndef _WIN32
    off_t r (lseek (fd, static_cast<off_t> (o), m));
    if (r == static_cast<off_t> (-1))
      throw_generic_ios_failure (errno);
#else
    __int64 r (_lseeki64 (fd, o, m));
    if (r == -1)
      throw_generic_ios_failure (errno);
#endif

    return static_cast<uint64_t> (r);
  }

  // The rest is platform-specific.
  //
#ifndef _WIN32

  auto_fd
  fddup (int fd)
  {
    // dup() doesn't copy FD_CLOEXEC flag, so we need to do it ourselves. Note
    // that the new descriptor can leak into child processes before we copy the
    // flag. To prevent this we will acquire the process_spawn_mutex (see
    // process-details header) prior to duplicating the descriptor. Also note
    // there is dup3() (available on Linux and FreeBSD but not on Max OS) that
    // takes flags, but it's usage tends to be hairy (need to preopen a dummy
    // file descriptor to pass as a second argument).
    //
    auto dup = [fd] () -> auto_fd
    {
      auto_fd r (::dup (fd));
      if (r.get () == -1)
        throw_generic_ios_failure (errno);

      return r;
    };

    int f (fcntl (fd, F_GETFD));
    if (f == -1)
      throw_generic_ios_failure (errno);

    // If the source descriptor has no FD_CLOEXEC flag set then no flag copy is
    // required (as the duplicate will have no flag by default).
    //
    if ((f & FD_CLOEXEC) == 0)
      return dup ();

    slock l (process_spawn_mutex);
    auto_fd r (dup ());

    f = fcntl (r.get (), F_GETFD);
    if (f == -1 || fcntl (r.get (), F_SETFD, f | FD_CLOEXEC) == -1)
      throw_generic_ios_failure (errno);

    return r;
  }

  bool
  fdclose (int fd) noexcept
  {
    return close (fd) == 0;
  }

  auto_fd
  fdopen_null ()
  {
    int fd (open ("/dev/null", O_RDWR | O_CLOEXEC));

    if (fd == -1)
      throw_generic_ios_failure (errno);

    return auto_fd (fd);
  }

  fdstream_mode
  fdmode (int fd, fdstream_mode m)
  {
    int flags (fcntl (fd, F_GETFL));

    if (flags == -1)
      throw_generic_ios_failure (errno);

    fdstream_mode r ((flags & O_NONBLOCK) == O_NONBLOCK
                     ? fdstream_mode::non_blocking
                     : fdstream_mode::blocking);

    if (flag (m, fdstream_mode::blocking) ||
        flag (m, fdstream_mode::non_blocking))
    {
      m &= fdstream_mode::blocking | fdstream_mode::non_blocking;

      // Should be exactly one blocking mode flag specified.
      //
      if (m != fdstream_mode::blocking && m != fdstream_mode::non_blocking)
        throw invalid_argument ("invalid blocking mode");

      // Set the new blocking mode if it differs from the current one.
      //
      if (m != r)
      {
        int new_flags (m == fdstream_mode::non_blocking
                       ? flags | O_NONBLOCK
                       : flags & ~O_NONBLOCK);

        if (fcntl (fd, F_SETFL, new_flags) == -1)
          throw_generic_ios_failure (errno);
      }
    }

    return r | fdstream_mode::binary;
  }

  int
  stdin_fd ()
  {
    return STDIN_FILENO;
  }

  int
  stdout_fd ()
  {
    return STDOUT_FILENO;
  }

  int
  stderr_fd ()
  {
    return STDERR_FILENO;
  }

  fdpipe
  fdopen_pipe (fdopen_mode m)
  {
    assert (m == fdopen_mode::none || m == fdopen_mode::binary);

    // Note that the pipe file descriptors can leak into child processes before
    // we set FD_CLOEXEC flag for them. To prevent this we will acquire the
    // process_spawn_mutex (see process-details header) prior to creating the
    // pipe. Also note there is pipe2() (available on Linux and FreeBSD but not
    // on Max OS) that takes flags.
    //
    slock l (process_spawn_mutex);

    int pd[2];
    if (pipe (pd) == -1)
      throw_generic_ios_failure (errno);

    fdpipe r {auto_fd (pd[0]), auto_fd (pd[1])};

    for (size_t i (0); i < 2; ++i)
    {
      int f (fcntl (pd[i], F_GETFD));
      if (f == -1 || fcntl (pd[i], F_SETFD, f | FD_CLOEXEC) == -1)
        throw_generic_ios_failure (errno);
    }

    return r;
  }

  void
  fdtruncate (int fd, uint64_t n)
  {
    if (ftruncate (fd, static_cast<off_t> (n)) != 0)
      throw_generic_ios_failure (errno);
  }

  entry_stat
  fdstat (int fd)
  {
    struct stat s;
    if (fstat (fd, &s) != 0)
      throw_generic_error (errno);

    auto m (s.st_mode);
    entry_type t (entry_type::unknown);

    // Note: cannot be a symlink.
    //
    if (S_ISREG (m))
      t = entry_type::regular;
    else if (S_ISDIR (m))
      t = entry_type::directory;
    else if (S_ISBLK (m) || S_ISCHR (m) || S_ISFIFO (m) || S_ISSOCK (m))
      t = entry_type::other;

    return entry_stat {t, static_cast<uint64_t> (s.st_size)};
  }

  bool
  fdterm (int fd)
  {
    int r (isatty (fd));

    if (r == 1)
      return true;

    // POSIX specifies ENOTTY errno code for an indication that the descriptor
    // doesn't refer to a terminal. However, both Linux and FreeBSD man pages
    // also mention EINVAL for this case.
    //
    assert (r == 0);

    if (errno == ENOTTY || errno == EINVAL)
      return false;

    throw_generic_ios_failure (errno);
  }

  static pair<size_t, size_t>
  fdselect (fdselect_set& read,
            fdselect_set& write,
            const chrono::milliseconds* timeout)
  {
    using namespace chrono;

    // Copy fdselect_set into the native fd_set, updating max_fd. Also clear
    // the ready flag in the source set.
    //
    int max_fd (-1);

    auto copy_set = [&max_fd] (fdselect_set& from, fd_set& to)
    {
      FD_ZERO (&to);

      for (fdselect_state& s: from)
      {
        s.ready = false;

        if (s.fd == nullfd)
          continue;

        if (s.fd < 0)
          throw invalid_argument ("invalid file descriptor");

        FD_SET (s.fd, &to);

        if (max_fd < s.fd)
          max_fd = s.fd;
      }
    };

    fd_set rds;
    fd_set wds;
    copy_set (read,  rds);
    copy_set (write, wds);

    if (max_fd == -1)
      throw invalid_argument ("empty file descriptor set");

    ++max_fd;

    // Note that if the timeout is not NULL, then the select timeout needs to
    // be recalculated for each select() call (of which we can potentially
    // have multiple due to EINTR). So the timeout can be used as bool.
    //
    timestamp now;
    timestamp deadline;

    if (timeout)
    {
      now = system_clock::now ();
      deadline = now + *timeout;
    }

    // Repeat the select() call while getting the EINTR error and throw on
    // any other error.
    //
    // Note that select() doesn't modify the sets on failure (according to
    // POSIX standard as well as to the Linux, FreeBSD and MacOS man pages).
    //
    for (timeval tv;;)
    {
      if (timeout)
      {
        if (now < deadline)
        {
          microseconds t (duration_cast<microseconds> (deadline - now));
          tv.tv_sec  = t.count () / 1000000;
          tv.tv_usec = t.count () % 1000000;
        }
        else
        {
          tv.tv_sec  = 0;
          tv.tv_usec = 0;
        }
      }

      int r (select (max_fd,
                     &rds,
                     &wds,
                     nullptr /* exceptfds */,
                     timeout ? &tv : nullptr));

      if (r == -1)
      {
        if (errno == EINTR)
        {
          if (timeout)
            now = system_clock::now ();

          continue;
        }

        throw_system_ios_failure (errno);
      }

      if (!timeout)
        assert (r != 0); // We don't expect the timeout to occur.

      break;
    }

    // Set the resulting ready states.
    //
    auto copy_states = [] (const fd_set& from, fdselect_set& to)
    {
      size_t r (0);
      for (fdselect_state& s: to)
      {
        if (s.fd == nullfd)
          continue;

        if (FD_ISSET (s.fd, &from))
        {
          ++r;
          s.ready = true;
        }
      }

      return r;
    };

    return make_pair (copy_states (rds, read), copy_states (wds, write));
  }

  streamsize
  fdread (int fd, void* buf, size_t n)
  {
    return read (fd, buf, n);
  }

  streamsize
  fdwrite (int fd, const void* buf, size_t n)
  {
    return write (fd, buf, n);
  }

#else

  auto_fd
  fddup (int fd)
  {
    // _dup() doesn't copy _O_NOINHERIT flag, so we need to do it ourselves.
    // Note that the new descriptor can leak into child processes before we
    // copy the flag. To prevent this we will acquire the process_spawn_mutex
    // (see process-details header) prior to duplicating the descriptor.
    //
    auto dup = [fd] () -> auto_fd
    {
      auto_fd r (_dup (fd));
      if (r.get () == -1)
        throw_generic_ios_failure (errno);

      return r;
    };

    // We can not ammend file descriptors directly (nor obtain the flag value),
    // so need to resolve them to Windows HANDLE first. Note that we don't
    // need to close them (see fd_to_handle()).
    //
    DWORD f;
    if (!GetHandleInformation (fd_to_handle (fd), &f))
      throw_system_ios_failure (GetLastError ());

    // If the source handle is inheritable then no flag copy is required (as
    // the duplicate handle will be inheritable by default).
    //
    if (f & HANDLE_FLAG_INHERIT)
      return dup ();

    slock l (process_spawn_mutex);

    auto_fd r (dup ());
    if (!SetHandleInformation (fd_to_handle (r.get ()),
                               HANDLE_FLAG_INHERIT,
                               0 /* dwFlags */))
      throw_system_ios_failure (GetLastError ());

    return r;
  }

  bool
  fdclose (int fd) noexcept
  {
    return _close (fd) == 0;
  }

  auto_fd
  fdopen_null (bool temp)
  {
    // No need to translate \r\n before sending it to void.
    //
    if (!temp)
    {
      int fd (_sopen ("nul", _O_RDWR | _O_BINARY | _O_NOINHERIT, _SH_DENYNO));

      if (fd == -1)
        throw_generic_ios_failure (errno);

      return auto_fd (fd);
    }

    try
    {
      // We could probably implement a Windows-specific version of getting
      // the temporary file that avoid any allocations and exceptions.
      //
      path p (path::temp_path ("null")); // Can throw.
      int fd (_sopen (p.string ().c_str (),
                      (_O_CREAT       |
                       _O_RDWR        |
                       _O_BINARY      | // Don't translate.
                       _O_TEMPORARY   | // Remove on close.
                       _O_SHORT_LIVED | // Don't flush to disk.
                       _O_NOINHERIT),   // Don't inherit by child process.
                      _SH_DENYNO,
                      _S_IREAD | _S_IWRITE));

      if (fd == -1)
        throw_generic_ios_failure (errno);

      return auto_fd (fd);
    }
    catch (const bad_alloc&)
    {
      throw_generic_ios_failure (ENOMEM);
    }
    catch (const system_error& e)
    {
      // Re-throw system_error as ios::failure, preserving the error category
      // and description.
      //
      int v (e.code ().value ());
      const error_category& c (e.code ().category ());

      if (c == generic_category ())
        throw_generic_ios_failure (v);
      else
      {
        assert (c == system_category ());
        throw_system_ios_failure (v, e.what ());
      }
    }
  }

  fdstream_mode
  fdmode (int fd, fdstream_mode m)
  {
    fdstream_mode r;

    // Get the current and set the new translation mode, if requested.
    //
    auto translation_mode = [fd] (fdstream_mode m)
    {
      assert (m == fdstream_mode::binary || m == fdstream_mode::text);

      // Note that _setmode() preserves the _O_NOINHERIT flag value.
      //
      int r (_setmode (fd, m == fdstream_mode::binary ? _O_BINARY : _O_TEXT));
      if (r == -1)
        throw_generic_ios_failure (errno); // EBADF or EINVAL (POSIX values).

      return (r & _O_BINARY) == _O_BINARY
             ? fdstream_mode::binary
             : fdstream_mode::text;
    };

    // It would have been natural not to change translation mode if none of
    // text or binary flags are passed. Unfortunately there is no (easy) way
    // to obtain the current mode for the file descriptor without setting a
    // new one. That's why we set the text mode (as the more common) to obtain
    // the current mode and revert it back if we didn't guess it right.
    //
    if (flag (m, fdstream_mode::text) || flag (m, fdstream_mode::binary))
    {
      fdstream_mode tm (m & (fdstream_mode::text | fdstream_mode::binary));

      // Should be exactly one translation flag specified.
      //
      if (tm != fdstream_mode::binary && tm != fdstream_mode::text)
        throw invalid_argument ("invalid translation mode");

      r = translation_mode (tm);
    }
    else
    {
      r = translation_mode (fdstream_mode::text);

      // Restore the mode if misguessed.
      //
      if (r == fdstream_mode::binary)
        translation_mode (r);
    }

    // Get the current and set the new blocking mode, if requested.
    //
    // Note that we always assume the blocking (synchronous) mode for file
    // descriptors other than pipes.
    //
    optional<handle_mode> pm (pipe_mode (fd));

    if (pm)
    {
      r |= pm->mode;

      if (flag (m, fdstream_mode::blocking) ||
          flag (m, fdstream_mode::non_blocking))
      {
        fdstream_mode bm (
          m & (fdstream_mode::blocking | fdstream_mode::non_blocking));

        // Should be exactly one blocking mode flag specified.
        //
        if (bm != fdstream_mode::blocking && bm != fdstream_mode::non_blocking)
          throw invalid_argument ("invalid blocking mode");

        // Set the new blocking mode if it differs from the current one.
        //
        if (bm != pm->mode)
        {
          DWORD flags (bm == fdstream_mode::non_blocking
                       ? PIPE_NOWAIT
                       : PIPE_WAIT);

          if (!SetNamedPipeHandleState (pm->handle,
                                        &flags,
                                        nullptr /* lpMaxCollectionCount */,
                                        nullptr /* lpCollectDataTimeout */))
            throw_system_ios_failure (GetLastError ());
        }
      }
    }
    else
    {
       r |= fdstream_mode::blocking;
    }

    return r;
  }

  int
  stdin_fd ()
  {
    int fd (_fileno (stdin));
    if (fd == -1)
      throw_generic_ios_failure (errno);

    return fd;
  }

  int
  stdout_fd ()
  {
    int fd (_fileno (stdout));
    if (fd == -1)
      throw_generic_ios_failure (errno);

    return fd;
  }

  int
  stderr_fd ()
  {
    int fd (_fileno (stderr));
    if (fd == -1)
      throw_generic_ios_failure (errno);

    return fd;
  }

  fdpipe
  fdopen_pipe (fdopen_mode m)
  {
    assert (m == fdopen_mode::none || m == fdopen_mode::binary);

    int pd[2];
    if (_pipe (
          pd,
          64 * 1024, // Set buffer size to 64K.
          _O_NOINHERIT | (m == fdopen_mode::none ? _O_TEXT : _O_BINARY)) == -1)
      throw_generic_ios_failure (errno);

    return {auto_fd (pd[0]), auto_fd (pd[1])};
  }

  void
  fdtruncate (int fd, uint64_t n)
  {
    if (errno_t e = _chsize_s (fd, static_cast<__int64> (n)))
      throw_generic_ios_failure (e);
  }

  entry_stat
  fdstat (int fd)
  {
    // Since symlinks have been taken care of, we can just _fstat().
    //
    struct __stat64 s;
    if (_fstat64 (fd, &s) != 0)
      throw_generic_error (errno);

    auto m (s.st_mode);
    entry_type t (entry_type::unknown);

    if (S_ISREG (m))
      t = entry_type::regular;
    else if (S_ISDIR (m))
      t = entry_type::directory;
    else if (S_ISCHR (m))
      t = entry_type::other;

    return entry_stat {t, static_cast<uint64_t> (s.st_size)};
  }

  bool
  fdterm (int fd)
  {
    // @@ Both GCC and Clang simply call GetConsoleMode() for this check. I
    //    wonder why we don't do the same?

    // We don't need to close it (see fd_to_handle()).
    //
    HANDLE h (fd_to_handle (fd));

    // Obtain the descriptor type.
    //
    DWORD t (GetFileType (h));
    DWORD e;

    if (t == FILE_TYPE_UNKNOWN && (e = GetLastError ()) != 0)
      throw_system_ios_failure (e);

    if (t == FILE_TYPE_CHAR) // Terminal.
      return true;

    if (t != FILE_TYPE_PIPE) // Pipe still can be a terminal (see below).
      return false;

    // MSYS2 terminal file descriptor has the pipe type. To distinguish it
    // from other pipes we will try to obtain the associated file name and
    // heuristically decide if it is a terminal or not. If we fail to obtain
    // the name (for any reason), then we consider the descriptor as not
    // referring to a terminal.
    //
    // Note that the API we need to use is only available starting with
    // Windows Vista/Server 2008. To allow programs linked to libbutl to run
    // on earlier Windows versions we will link the API in run-time and will
    // fallback to the non-terminal descriptor type on failure. We also need
    // to partially reproduce the original API types.
    //
    HMODULE kh (GetModuleHandle ("kernel32.dll"));
    if (kh == nullptr)
      return false;

    // The original type is FILE_INFO_BY_HANDLE_CLASS enum.
    //
    enum file_info
    {
      file_name = 2
    };

    using func = BOOL (*) (HANDLE, file_info, LPVOID, DWORD);

    func f (function_cast<func> (
              GetProcAddress (kh, "GetFileInformationByHandleEx")));

    if (f == nullptr)
      return false;

    // The original type is FILE_NAME_INFO structure.
    //
    struct
    {
      DWORD length;
      wchar_t name[_MAX_PATH + 1];
    } fn;

    // Reserve space for the trailing NULL character.
    //
    if (!f (h, file_name, &fn, sizeof (fn) - sizeof (wchar_t)))
      return false;

    // Add the trailing NULL character. Sounds strange, but the name length is
    // expressed in bytes.
    //
    fn.name[fn.length / sizeof (wchar_t)] = L'\0';

    // The MSYS2 terminal descriptor file name looks like this:
    //
    // \msys-dd50a72ab4668b33-pty0-to-master
    //
    // We will recognize it by the '\msys-' prefix and the presence of the
    // '-ptyN' entry.
    //
    if (wcsncmp (fn.name, L"\\msys-", 6) == 0)
    {
      const wchar_t* e (wcsstr (fn.name, L"-pty"));

      return e != nullptr && e[4] >= L'0' && e[4] <= L'9' &&
        (e[5] == L'-' || e[5] == L'\0');
    }

    return false;
  }

  static pair<size_t, size_t>
  fdselect (fdselect_set& read,
            fdselect_set& write,
            const chrono::milliseconds* timeout)
  {
    using namespace chrono;

    if (!write.empty ())
      throw invalid_argument ("write file descriptor set is not supported");

    // Validate and prepare the read set.
    //
    size_t n (0);

    for (fdselect_state& s: read)
    {
      s.ready = false;

      if (s.fd == nullfd)
        continue;

      if (s.fd < 0)
        throw invalid_argument ("invalid file descriptor");

      ++n;
    }

    if (n == 0)
      throw invalid_argument ("empty file descriptor set");

    // Note that if the timeout is not NULL, then the deadline needs to be
    // checked prior to re-probing the pipe for data presence. So the timeout
    // can be used as bool.
    //
    timestamp deadline;

    if (timeout)
      deadline = system_clock::now () + *timeout;

    // Keep iterating through the set until at least one byte can be read from
    // any of the pipes or any of them get closed (so can read eof).
    //
    size_t r (0);

    for (size_t i (0);; ++i)
    {
      for (fdselect_state& s: read)
      {
        if (s.fd == nullfd)
          continue;

        // Get the pipe handle. Note that PeekNamedPipe() doesn't care about
        // the blocking mode.
        //
        optional<handle_mode> m (pipe_mode (s.fd));

        if (!m)
          throw invalid_argument ("file descriptor is not a pipe");

        // Probe the pipe for data presence.
        //
        // Note that PeekNamedPipe() can block the thread execution in a
        // multi-threaded program. It is not clear from the documentation
        // under which circumstances and for how long this may happen. It
        // seems to relate to some internal IO synchronization mechanism and
        // probably depends on the contention level. But there doesn't appear
        // to be a better way. Specifically, WaitForMultipleObjects() does not
        // work on pipes (at least not on the non-overlapped ones).
        //
        char c;
        DWORD n;
        DWORD e (0);

        if (!PeekNamedPipe (m->handle,
                            &c,
                            1,
                            &n,
                            nullptr /* lpTotalBytesAvail */,
                            nullptr /* lpBytesLeftThisMessage */))
          e = GetLastError ();

        // If we successfully peeked a byte or failed due to the broken pipe
        // (the writing end is closed), then the descriptor is ready for
        // reading.
        //
        if ((e == 0 && n == 1) || e == ERROR_BROKEN_PIPE)
        {
          s.ready = true;
          ++r;
          continue;
        }

        // If we successfully read zero bytes, then there is no data available
        // for reading.
        //
        // Note that we never get the ERROR_NO_DATA error, but let's check for
        // good measure.
        //
        // As an observation, upon a successful call that peeks zero bytes,
        // GetLastError() returns zero (ERROR_SUCCESS) if no data were read
        // yet from the pipe and ERROR_NO_DATA otherwise.
        //
        if ((e == 0 && n == 0) || e == ERROR_NO_DATA)
          continue;

        // Both successful cases (n == 1 || n == 0) are already handled.
        //
        assert (e != 0);
        throw_system_ios_failure (e);
      }

      // Bail out if some descriptors are ready for reading or the deadline
      // has been reached, if specified, and sleep a bit and repeat otherwise.
      //
      if (r != 0)
        break;

      // Use exponential backoff but not too aggressive and with 25ms max.
      //
      DWORD t (
        static_cast<DWORD> (i <= 1000  ?  0 :
                            i >= 1000 + 100 ? 25 : 1 + ((i - 1000) / 4)));

      if (timeout)
      {
        timestamp now (system_clock::now ());

        if (now < deadline)
        {
          milliseconds tm (duration_cast<milliseconds> (deadline - now));

          if (t > tm.count ())
            t = static_cast<DWORD> (tm.count ());
        }
        else
          break;
      }

      if (t == 0)
        this_thread::yield ();
      else
        Sleep (t);
    }

    return make_pair (r, 0);
  }

  streamsize
  fdread (int fd, void* buf, size_t n)
  {
    // The _read() call fails with the EINVAL errno code either because
    // something is wrong with the arguments or there is no data to read from
    // a non-blocking pipe fd. In the latter case we translate the errno code
    // to EAGAIN.
    //
    // Note that this behavior is not really documented. However, it is both
    // described on the web and is confirmed by the tests. The ERROR_NO_DATA
    // system error is referred in the ReadFile() documentation in the context
    // of non-blocking pipes and is presumably set by the underlying
    // ReadFile() call.
    //
    // Also note that this doesn't work on Wine. We can probably implement the
    // non-blocking behavior for Wine using PeekNamedPipe() that is claimed to
    // also provide the number of total bytes available. More research is
    // required.

    // Clear last error to make sure ERROR_NO_DATA is set by _read().
    //
    SetLastError (0);

    int r (_read (fd, buf, static_cast<unsigned int> (n)));

    if (r == -1 && errno == EINVAL && GetLastError () == ERROR_NO_DATA)
    {
      // Let's also check that the file descriptor is really a non-blocking
      // pipe.
      //
      optional<handle_mode> pm (pipe_mode (fd, true /* ignore_error */));
      errno = pm && pm->mode == fdstream_mode::non_blocking ? EAGAIN : EINVAL;
    }

    return r;
  }

  streamsize
  fdwrite (int fd, const void* buf, size_t n)
  {
    return _write (fd, buf, static_cast<unsigned int> (n));
  }

#endif

  pair<size_t, size_t>
  fdselect (fdselect_set& read, fdselect_set& write)
  {
    return fdselect (read, write, nullptr /* timeout */);
  }

  template <>
  pair<size_t, size_t>
  fdselect (fdselect_set& read,
            fdselect_set& write,
            const chrono::milliseconds& timeout)
  {
    return fdselect (read, write, &timeout);
  }
}