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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