On Wednesday 19 November 2025 15:35:30 Martin Storsjö wrote:
> On Sat, 15 Nov 2025, Pali Rohár wrote:
>
> > Currently the fsetpos emulation supports only 32-bit offsets.
> >
> > When the new position does not fit into fsetpos32 argument then call
> > fd-based _lseeki64() function. To ensure that FILE* stream does not have
> > in-use buffers, call fflush() and fsetpos32() functions which should drop
> > remaining write and read buffers.
> >
> > With this change mingw-w64 should fully support for all CRT builds the
> > fsetpos function with 64-bit offset. It also proves _fseeki64 and fseeko64
> > emulations which call fsetpos.
> > ---
> > mingw-w64-crt/stdio/fsetpos.c | 28 ++++++++++++++++++++--------
> > 1 file changed, 20 insertions(+), 8 deletions(-)
> >
> > diff --git a/mingw-w64-crt/stdio/fsetpos.c b/mingw-w64-crt/stdio/fsetpos.c
> > index 4bb9e5cb7e47..4d56648d12d9 100644
> > --- a/mingw-w64-crt/stdio/fsetpos.c
> > +++ b/mingw-w64-crt/stdio/fsetpos.c
> > @@ -7,21 +7,33 @@
> > #include <stdio.h>
> > #include <errno.h>
> > #include <limits.h>
> > +#include <io.h>
> >
> > -/* Define 64-bit fsetpos() function via 32-bit fsetpos32() function */
> > +/* Define 64-bit fsetpos() function via 32-bit fsetpos32() and 64-bit
> > _lseeki64() functions */
> > _CRTIMP int __cdecl fsetpos32(FILE *__restrict__ _File, long *__restrict__
> > _Pos);
> > int __cdecl fsetpos(FILE *__restrict__ _File, const fpos_t *__restrict__
> > _Pos)
> > {
> > - long pos32;
> > -
> > - if (*_Pos < LONG_MIN || *_Pos > LONG_MAX)
> > + if (*_Pos < LONG_MIN)
> > {
> > - errno = EOVERFLOW;
> > + errno = EINVAL;
> > return -1;
> > }
> > -
> > - pos32 = *_Pos;
> > - return fsetpos32(_File, &pos32);
> > + else if (*_Pos > LONG_MAX)
> > + {
> > + /* fflush() and fsetpos32() clear read and write buffers, so the
> > followup
> > + * _lseeki64() call correctly change offset of the FILE* stream too. */
> > + if (fflush(_File) != 0)
> > + return -1;
> > + if (fsetpos32(_File, &(long){LONG_MAX}) != 0)
> > + return -1;
> > + if (_lseeki64(fileno(_File), *_Pos, SEEK_SET) == -1)
> > + return -1;
>
> So there's no FILE* level caching of the file offset, which this bypasses?
> E.g. a ftell() after this won't give the wrong result?
>
> If there's no such issue, then this looks fine.
>
> // Martin
I did some tests and seems that the successful fsetpo32 call flushes
those caches. So when I called lseek64 between fsetpos32 and
fgetpos32/ftell32, I got from the fgetpos32/ftell32 correct result of
the lseek64 change.
That is why there is fsetpos32(_File, &(long){LONG_MAX}) call, it
flushes those FILE* caches and makes the fd-_lseeki64 be in sync with FILE*.
But we do not have full emulation of fgetpos64/ftell64. So I checked
that only for offsets which fits into 32-bit number.
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