#864 _ftelli64 returns wrong values with MinGW-w64 32-bit

[Johannes Wilde]

A very good day to You,
I stumbled upon a little problem when trying to read binary data from a file with a program compiled using the MinGW-w64 32-bit compiler (the one currently distributed with Qt - build-info is attached as a separate file) running under a Windows 10 64-bit system.
An MWI is attached as well - it first creates a file with meaningless data and then tries to read part of it back. The part to read is 2 times 4 bytes, i.e. 8 bytes total. Querying _ftelli64() afterwards however returns only "7" for me if the file stream is buffered (the default).
The MinGW-w64 64-bit compiler returns "8" (as does a current MSVC). For an unbuffered stream MinGW-w64 32-bit also returns "8".
Am I doing something wrong? Did I overlook something in the documentation which should prevent me from using _ftelli64() in this fashion? Or might this be a problem of Microsoft Windows 10 in relation with the MinGW-w64 32-bit compiler?
Feedback would be very much appreciated,
J. Wilde

[Johannes Wilde]

I should probably add that ftello64 returns "8" for all tested compilers.

[David Bryant]

Hi. I am seeing this exact same bug. It occurs only with the 32-bit compiler (the 64-bit compiler is fine) and the executables fail exactly the same way whether they are run on Wine or an XP VM (which I assume means they'll fail anywhere they will run). I grabbed the MS demo of ftell() and modified it to demonstrate with just a few lines:

$ i686-w64-mingw32-gcc --version
i686-w64-mingw32-gcc (GCC) 9.3-win32 20200320
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat crt_ftell.c
// crt_ftell.c
// This program opens a file named CRT_FTELL.C
// for reading and tries to read 100 characters. It
// then uses ftell to determine the position of the
// file pointer and displays this position.

#include <stdio.h>

FILE *stream;

int main( void )
{
   char list[100];
   if( (stream = fopen( "crt_ftell.c", "rb" )) != NULL )
   {
      // Move the pointer by reading data:
      fread( list, sizeof( char ), 100, stream );
      // Get position after read:
      printf( "Position after trying to read 100 bytes: %ld\n", ftell (stream));
      printf( "Position after trying to read 100 bytes: %I64d\n", _ftelli64 (stream));
      printf( "Position after trying to read 100 bytes: %I64d\n", ftello64 (stream));
      fclose( stream );
   }
}
$ i686-w64-mingw32-gcc -Wall crt_ftell.c -o crt_ftell.exe
$ wine ./crt_ftell.exe 
Position after trying to read 100 bytes: 100
Position after trying to read 100 bytes: 78
Position after trying to read 100 bytes: 100
$ 

Cheers!