idn: avoid allocations and wcslen on Windows #19798
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Pull request overview
This PR optimizes the Windows IDN (Internationalized Domain Name) conversion functions by eliminating heap allocations and redundant string length calculations. The changes replace dynamically allocated intermediate buffers with fixed-size stack buffers and reuse length information returned by Windows APIs to avoid calling wcslen().
Key changes:
- Removed the helper function
idn_curlx_convert_UTF8_to_wchar()that performed heap allocation - Modified
idn_curlx_convert_wchar_to_UTF8()to accept an explicit character count parameter - Refactored
win32_idn_to_ascii()andwin32_ascii_to_idn()to use stack-allocated buffers and reuse length values
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vszakats
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Dec 15, 2025
https://learn.microsoft.com/windows/win32/api/stringapiset/nf-stringapiset-multibytetowidechar https://learn.microsoft.com/windows/win32/api/stringapiset/nf-stringapiset-widechartomultibyte https://learn.microsoft.com/windows/win32/api/winnls/nf-winnls-idntoascii https://learn.microsoft.com/windows/win32/api/winnls/nf-winnls-idntounicode WINE source code: https://gitlab.winehq.org/wine/wine/-/blob/wine-10.20/dlls/kernelbase/locale.c https://gitlab.winehq.org/wine/wine/-/blob/wine-10.20/dlls/ntdll/locale.c https://gitlab.winehq.org/wine/wine/-/blob/wine-10.20/dlls/ntdll/locale_private.h Ref: curl#19976 (comment) Follow-up to 6694a42 curl#19798
vszakats
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Dec 16, 2025
Add comments to clarify that a terminating null is always present in the buffers returned to the caller. The curl APIs `win32_idn_to_ascii()` or `win32_ascii_to_idn()` receive a null-terminated UTF-8 string as input. They first convert it to wide chars by first asking `MultiByteToWideChar()` to calculate the length, by passing -1. This API returns the length with the null char included (= `strlen() + 1`), does the conversion, with the output also null-terminated. `IdnTo*()` preserve this null character as documented. Then we pass this null-terminated, fixed-length buffer ito `WideCharToMultiByte()`, which keeps preserving the null, ending up in the buffer returned to the caller. Refs: https://learn.microsoft.com/windows/win32/api/stringapiset/nf-stringapiset-multibytetowidechar https://learn.microsoft.com/windows/win32/api/stringapiset/nf-stringapiset-widechartomultibyte https://learn.microsoft.com/windows/win32/api/winnls/nf-winnls-idntoascii https://learn.microsoft.com/windows/win32/api/winnls/nf-winnls-idntounicode WINE source code: https://gitlab.winehq.org/wine/wine/-/blob/wine-10.20/dlls/kernelbase/locale.c https://gitlab.winehq.org/wine/wine/-/blob/wine-10.20/dlls/ntdll/locale.c https://gitlab.winehq.org/wine/wine/-/blob/wine-10.20/dlls/ntdll/locale_private.h Ref: curl#19976 (comment) Follow-up to 6694a42 curl#19798 Closes curl#19980
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Eliminate a heap buffer in both
win32_idn_to_ascii()andwin32_ascii_to_idn(), by replacing it with stack buffer. The maximumsize is fixed in these cases, and small enough to fit there.
Also reuse length returned by the UTF-8 to wchar conversion, allowing
to drop
wcslen()call in both functions, and allowing to callthe wchar to UTF-8 conversion API
WideCharToMultiByte()with the knownlength, saving length calculations within that API too.
Ref: #19748 (comment)