On Sun, 21 Sep 2025, Pali Rohár wrote:
C99 allows to pass NULL path into freopen() function which just changes the
mode of FILE*. Both msvcrt and UCRT returns error when NULL path is passed.
The primary use case of this C99 feature is to change stdin / stdout mode
from text to binary and so allow processing of binary data on stdin or
stdout.
Implement support for NULL path by renaming existing freopen symbol to
_freopen and then providing mingw-w64 wrapper function freopen() which will
either call renamed _freopen() or just changes the mode on existing FILE*
stream.
Because FILE struct has different offsets to members between msvcrt and
UCRT builds, it is needed to have separate compilation unit for msvcrt and
UCRT builds.
---
Overall, I'm a little sceptical to this patch.
First off - this patchset adds a lot of things to public headers that
aren't supposed to be in public headers.
If we feel that we do want to make such assumptions about UCRT internals,
then we should keep those private defines in mingw-w64-crt internal
headers, not expose those defines to everybody else. But I'm not sure if
this usecase warrants hardcoding assumptions about UCRT internals.
I'm also a little sceptical about the use case overall.
Yes, C99 says you should be able to do this. But currently, on Unix,
nobody needs to care about binary vs non-binary stdio modes. On Windows,
no existing code works with freopen this way, as neither mingw nor MSVC
support this. So there is no preexisting code that would need this. All
code that targets Windows that wants to do this already does it
differently already. So by implementing this, we gain a little standards
compliance, but we also divide the ecosystem (any code written with
mingw-w64 assuming this behaviour won't work on official SDKs).
Therefore, plus the fact that it requires poking into UCRT internals,
makes me less inclined to accept this change.
If it would be a C standard function which is commonly used in a portable
fashion on other platforms already, I would be more inclined to support it
in mingw, even though MSVC might not support it.
Also in this case, it could be good to file a bug report to MS about this
deviation from the C99 standard.
// Martin
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