[R-pkg-devel] ucrt is not 64-bit clean

[Tomas Kalibera]

On 9/22/25 18:46, Vladimir Dergachev wrote:
>
> Just wanted to share something I just run into and fixed - when an R 
> package is compiled for 64-bit Windows using "ucrt" C library, 
> functions like ftell() and fseek() use 32-bit arguments for offsets, 
> rather than 64-bit.
>
> What's worse the type "off_t" is 32-bit as well, which defeats the 
> purpose of introducing off_t in the first place.
>
> The solution for me was to replace ftell()/fseek() with MS-specific 
> _lseeki64(), _ftelli64() and _fseeki64(), and redefine off_t to be 64-bit
> "long long int".
>
> So typical MS - there is a C-library to compile against but without 
> changes the code only works up to 2GB.
>
> It might be a good idea to have an R-shim to isolate ucrt() and 
> provide 64-bit functions for R packages.

I think this is something mingw-w64 should take care off, and having a 
quick look at the sources it seems it probably does.

I would start from the POSIX specification and then check how much of it 
is supported by mingw-w64, perhaps using _FILE_OFFSET_BITS and lseek(), 
ftello(), fseeko() would work.

Best
Tomas


>
> What do you think?
>
> best
>
> Vladimir Dergachev
>
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