[Bug libstdc++/64064] New: basic_filebuf seekoff return value is unusable for files opened in text mode on Windows

[Bug libstdc++/64064] New: basic_filebuf seekoff return value is unusable for files opened in text mode on Windows

[lukeallardyce at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64064

            Bug ID: 64064
           Summary: basic_filebuf seekoff return value is unusable for
                    files opened in text mode on Windows
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lukeallardyce at gmail dot com

Discussion here

https://gcc.gnu.org/ml/libstdc++/2014-11/msg00145.html

#include <fstream>
#include <iostream>

int main(int, char* argv[])
{
  using traits   = std::filebuf::traits_type;
  using int_type = std::filebuf::int_type;

  std::filebuf fb;
  fb.open(argv[1], std::ios::in);

  while (!traits::eq_int_type(fb.sbumpc(), traits::eof()))
    std::cout << fb.pubseekoff(0, std::ios::cur, std::ios::in) << ' ';

  std::cout << '\n';

  fb.close();
  fb.pubsetbuf(nullptr, 0);
  fb.open(argv[1], std::ios::in);

  while (!traits::eq_int_type(fb.sbumpc(), traits::eof()))
    std::cout << fb.pubseekoff(0, std::ios::cur, std::ios::in) << ' ';
}

With the following 3-line Windows-style text file:
hello
world

Produces the following:
4 5 6 7 8 9 10 11 12 13 14 15 16
1 2 3 4 5 7 8 9 10 11 12 14 16

On a buffered stream, the value returned by pubseekoff cannot be used to seek
back to that point due to the way Windows implements the POSIX read and lseek64
functions (i.e. read performs end of line conversion, lseek64 doesn't). The
value is off by one for each unconsumed end of line in the buffer.

It is still unclear whether this should be fixed, if possible, by libstdc++ or
mingw-w64.

[Bug libstdc++/64064] basic_filebuf seekoff return value is unusable for files opened in text mode on Windows

[lh_mouse at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64064

LIU Hao <lh_mouse at 126 dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lh_mouse at 126 dot com

--- Comment #1 from LIU Hao <lh_mouse at 126 dot com> ---
AFAICT the only issue here is that one can't pass the result of `pubseekoff()`
back to `pubseekpos()`. Both MSVCRT GCC and UCRT GCC are affected, but MSVC
isn't.


```
#include <fstream>
#include <iostream>

int main(int, char* argv[])
{
  using traits   = std::filebuf::traits_type;
  using int_type = std::filebuf::int_type;

  std::filebuf fb;
  fb.open(argv[1], std::ios::in);

  int_type c;
  while (!traits::eq_int_type(c = fb.sbumpc(), traits::eof()))
  {
    std::cout << static_cast<char>(c);
    auto pos = fb.pubseekoff(0, std::ios::cur, std::ios::in);
    fb.pubseekpos(pos, std::ios::in);
  }

  std::cout << '\n';
}
```

This outputs
```
hl
ol
```

[Bug libstdc++/64064] basic_filebuf seekoff return value is unusable for files opened in text mode on Windows

[lh_mouse at 126 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64064

--- Comment #2 from LIU Hao <lh_mouse at 126 dot com> ---
It looks that MS STL calls `_fseeki64(fp, ...)` [1], while libstdc++ calls
`lseek(_fileno(fp), ...)` [2].

The seek operation shall take the buffering of `FILE` struct into account,
hence I think the libstdc++ implementation is incorrect.


[1]
https://github.com/microsoft/STL/blob/442029c6fa37f1b6f9203357de09672d5704077c/stl/inc/__msvc_filebuf.hpp#L624
[2]
https://github.com/gcc-mirror/gcc/blob/9693459e030977d6e906ea7eb587ed09ee4fddbd/libstdc%2B%2B-v3/config/io/basic_file_stdio.cc#L425

[Bug libstdc++/64064] basic_filebuf seekoff return value is unusable for files opened in text mode on Windows

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64064

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/fseek-fseeki64?view=msvc-170#remarks

The only way to use _fseeki64 for text mode files is with __off=0 (and any
value of __way) or __way=beg and __off obtained from _ftelli64.

So I think for __way=cur or __way=end we would need to calculate a file offset
relative to the start of the file and then use that.