aboutsummaryrefslogtreecommitdiff
path: root/libbutl/fdstream.cxx
blob: a2d5b0959d7fd12d85fb243f223d2ba16e076142 (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
// file      : libbutl/fdstream.cxx -*- C++ -*-
// copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
// license   : MIT; see accompanying LICENSE file

#include <libbutl/fdstream.hxx>

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

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

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

#include <ios>          // ios_base::openmode, ios_base::failure
#include <new>          // bad_alloc
#include <limits>       // numeric_limits
#include <cassert>
#include <cstring>      // memcpy(), memmove()
#include <exception>    // uncaught_exception()
#include <stdexcept>    // invalid_argument
#include <type_traits>
#include <system_error>

#include <libbutl/process-details.hxx>

using namespace std;

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

namespace butl
{
  // throw_ios_failure
  //
  template <bool v>
  [[noreturn]] static inline typename enable_if<v>::type
  throw_ios_failure (error_code e, const char* m)
  {
    // The idea here is to make an error code to be saved into failure
    // exception and to make a string returned by what() to contain the error
    // description plus an optional custom message if provided. Unfortunatelly
    // there is no way to say that the custom message is absent. Passing an
    // empty string results for libstdc++ (as of version 5.3.1) with a
    // description like this (note the ': ' prefix):
    //
    // : No such file or directory
    //
    // Note that our custom operator<<(ostream, exception) strips this prefix.
    //
    throw ios_base::failure (m != nullptr ? m : "", e);
  }

  template <bool v>
  [[noreturn]] static inline typename enable_if<!v>::type
  throw_ios_failure (error_code e, const char* m)
  {
    throw ios_base::failure (m != nullptr ? m : e.message ().c_str ());
  }

  // Throw system_error with generic_category.
  //
  [[noreturn]] static inline void
  throw_ios_failure (int errno_code, const char* m = nullptr)
  {
    error_code ec (errno_code, generic_category ());
    throw_ios_failure<is_base_of<system_error, ios_base::failure>::value> (
      ec, m);
  }

#ifdef _WIN32
  // Throw system_error with system_category.
  //
  static inline void
  throw_ios_system_failure (int system_code)
  {
    // Here we work around MinGW libstdc++ that interprets Windows system error
    // codes (for example those returned by GetLastError()) as errno codes.
    //
    // Note that the resulting system_error description will have ': Success.'
    // suffix that is stripped by our custom operator<<(ostream, exception).
    //
    error_code ec (0, system_category ());
    string m (win32::error_msg (system_code));

    throw_ios_failure<is_base_of<system_error, ios_base::failure>::value> (
      ec, m.c_str ());
  }
#endif

  // 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_ios_failure (errno);
    }
  }

  // fdbuf
  //
  fdbuf::
  fdbuf (auto_fd&& fd)
  {
    if (fd.get () >= 0)
      open (move (fd));
  }

  void fdbuf::
  open (auto_fd&& fd)
  {
    close ();

#ifndef _WIN32
    int flags (fcntl (fd.get (), F_GETFL));

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

    non_blocking_ = (flags & O_NONBLOCK) == O_NONBLOCK;
#endif

    setg (buf_, buf_, buf_);
    setp (buf_, buf_ + sizeof (buf_) - 1); // Keep space for overflow's char.

    fd_ = move (fd);
  }

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

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

    if (n > 0)
      return n;

#ifndef _WIN32
    if (non_blocking_)
    {
      ssize_t n (read (fd_.get (), buf_, sizeof (buf_)));

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

        throw_ios_failure (errno);
      }

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

      setg (buf_, buf_, buf_ + n);
      return n;
    }
#endif

    return 0;
  }

  fdbuf::int_type fdbuf::
  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_ios_failure (ENOTSUP);

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

    return r;
  }

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

#ifndef _WIN32
    ssize_t n (read (fd_.get (), buf_, sizeof (buf_)));
#else
    int n (_read (fd_.get (), buf_, sizeof (buf_)));
#endif

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

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

  fdbuf::int_type fdbuf::
  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_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 fdbuf::
  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_ios_failure (ENOTSUP);

    return save () ? 0 : -1;
  }

  bool fdbuf::
  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().
      //
#ifndef _WIN32
      ssize_t m (write (fd_.get (), buf_, n));
#else
      int m (_write (fd_.get (), buf_, n));
#endif

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

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

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

    return true;
  }

  streamsize fdbuf::
  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_ios_failure (ENOTSUP);

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

    // Buffer the data if there is enough space.
    //
    size_t an (epptr () - pptr ()); // Amount of free space in the buffer.
    if (n <= an)
    {
      memcpy (pptr (), s, n);
      pbump (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_ios_failure (errno);

    size_t m (static_cast<size_t> (r));

    // 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
    {
      memcpy (pptr (), s, an);
      pbump (an);
    }

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

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

    size_t m (static_cast<size_t> (r));

    // 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 ()))
    {
      memcpy (pbase (), s, n);
      pbump (n);
      return sn;
    }

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

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

    return an + r;
#endif
  }

  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)
      : fdstream_base (mode (move (fd), m)) // Delegate.
  {
  }

  static fdopen_mode
  translate_mode (ios_base::openmode m)
  {
    enum
    {
      in    = ios_base::in,
      out   = ios_base::out,
      app   = ios_base::app,
      bin   = ios_base::binary,
      trunc = ios_base::trunc,
      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);

    fdopen_mode r;
    switch (m & ~(ate | bin))
    {
    case in               : r = fd_in                           ; break;
    case out              :
    case out | trunc      : r = fd_out   | fd_trunc | fd_create ; break;
    case app              :
    case out | app        : r = fd_out   | fd_app   | fd_create ; break;
    case out | in         : r = fd_inout                        ; break;
    case out | in | trunc : r = fd_inout | fd_trunc | fd_create ; break;
    case out | in | app   :
    case in  | app        : r = fd_inout | fd_app   | fd_create ; break;

    default: 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) // Delegate.
  {
  }

  ifdstream::
  ifdstream (const char* f, fdopen_mode m, iostate e)
      : ifdstream (fdopen (f, m | fdopen_mode::in), e) // Delegate.
  {
  }

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

    // Underlying file descriptor is closed by fdbuf 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));
  }

  void ifdstream::
  close ()
  {
    if (skip_ && is_open () && good ())
      ignore (numeric_limits<streamsize>::max ());

    buf_.close ();
  }

  ifdstream&
  getline (ifdstream& is, string& s, 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 fdbuf 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, s, delim);

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

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

    return is;
  }

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

  ofdstream::
  ofdstream (const char* f, fdopen_mode m, iostate e)
      : ofdstream (fdopen (f, m | fdopen_mode::out), e) // Delegate.
  {
  }

  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.
    //
    assert (!is_open () || !good () || uncaught_exception ());
  }

  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));
  }

  // Utility 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, FreeBSD allows opening a directory as a file
    // which will cause all kinds of problems upstream (e.g., cpfile()). So we
    // detect and diagnose this.
    //
#ifdef __FreeBSD__
    {
      struct stat s;
      if (stat (f, &s) == 0 && S_ISDIR (s.st_mode))
        throw_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.
    //
    bool pass_perm (of & _O_CREAT);

    if (pass_perm && file_exists (path (f)))
    {
      // 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_ios_failure (EEXIST);

      of &= ~_O_CREAT;
      pass_perm = false;
    }

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

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

#endif

    if (fd == -1)
      throw_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_ios_failure (e);
      }
    }

    return auto_fd (fd);
  }

#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 nfd (::dup (fd));
      if (nfd.get () == -1)
        throw_ios_failure (errno);

      return nfd;
    };

    int f (fcntl (fd, F_GETFD));
    if (f == -1)
      throw_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 nfd (dup ());

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

    return nfd;
  }

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

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

    if (fd == -1)
      throw_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_ios_failure (errno);

    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");

      int new_flags (
        m == fdstream_mode::non_blocking
        ? flags | O_NONBLOCK
        : flags & ~O_NONBLOCK);

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

    return fdstream_mode::binary |
      ((flags & O_NONBLOCK) == O_NONBLOCK
       ? fdstream_mode::non_blocking
       : fdstream_mode::blocking);
  }

  fdstream_mode
  stdin_fdmode (fdstream_mode m)
  {
    return fdmode (STDIN_FILENO, m);
  }

  fdstream_mode
  stdout_fdmode (fdstream_mode m)
  {
    return fdmode (STDOUT_FILENO, m);
  }

  fdstream_mode
  stderr_fdmode (fdstream_mode m)
  {
    return fdmode (STDERR_FILENO, m);
  }

  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_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_ios_failure (errno);
    }

    return r;
  }

#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.
    //
    // We can not ammend file descriptors directly (nor obtain the flag value),
    // so need to resolve them to Windows HANDLE first. Such handles are closed
    // either when CloseHandle() is called for them or when _close() is called
    // for the associated file descriptors. Make sure that either the original
    // file descriptor or the resulting HANDLE is closed but not both of them.
    //
    auto handle = [] (int fd) -> HANDLE
    {
      HANDLE h (reinterpret_cast<HANDLE> (_get_osfhandle (fd)));
      if (h == INVALID_HANDLE_VALUE)
        throw_ios_failure (errno); // EBADF (POSIX value).

      return h;
    };

    auto dup = [fd] () -> auto_fd
    {
      auto_fd nfd (_dup (fd));
      if (nfd.get () == -1)
        throw_ios_failure (errno);

      return nfd;
    };

    DWORD f;
    if (!GetHandleInformation (handle (fd), &f))
      throw_ios_system_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 nfd (dup ());
    if (!SetHandleInformation (handle (nfd.get ()), HANDLE_FLAG_INHERIT, 0))
      throw_ios_system_failure (GetLastError ());

    return nfd;
  }

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

  auto_fd
  fdnull (bool temp) noexcept
  {
    // 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_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_ios_failure (errno);

      return auto_fd (fd);
    }
    catch (const bad_alloc&)
    {
      throw_ios_failure (ENOMEM);
    }
    catch (const system_error& e)
    {
      // Make sure that the error denotes errno portable code.
      //
      assert (e.code ().category () == generic_category ());

      throw_ios_failure (e.code ().value ());
    }
  }

  fdstream_mode
  fdmode (int fd, fdstream_mode m)
  {
    m &= fdstream_mode::text | fdstream_mode::binary;

    // Should be exactly one translation flag specified.
    //
    // It would have been natural not to change translation mode if none of
    // text or binary flags are passed. Unfortunatelly there is no (easy) way
    // to obtain the current mode for the file descriptor without setting a
    // new one. This is why not specifying one of the modes is an error.
    //
    if (m != fdstream_mode::binary && m != fdstream_mode::text)
      throw invalid_argument ("invalid translation mode");

    // 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_ios_failure (errno); // EBADF or EINVAL (POSIX values).

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

  fdstream_mode
  stdin_fdmode (fdstream_mode m)
  {
    int fd (_fileno (stdin));
    if (fd == -1)
      throw_ios_failure (errno);

    return fdmode (fd, m);
  }

  fdstream_mode
  stdout_fdmode (fdstream_mode m)
  {
    int fd (_fileno (stdout));
    if (fd == -1)
      throw_ios_failure (errno);

    return fdmode (fd, m);
  }

  fdstream_mode
  stderr_fdmode (fdstream_mode m)
  {
    int fd (_fileno (stderr));
    if (fd == -1)
      throw_ios_failure (errno);

    return fdmode (fd, m);
  }

  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_ios_failure (errno);

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