[libcxx-commits] [PATCH] D59727: [libc++] Fix return value of snprintf_l() on Windows when buffer is too small

Tom Anderson via Phabricator via libcxx-commits libcxx-commits at lists.llvm.org
Fri Mar 22 16:17:56 PDT 2019

thomasanderson created this revision.
thomasanderson added a reviewer: EricWF.
Herald added a subscriber: ldionne.

When the output buffer is too small to contain the output, `vsnprintf()`
fills the buffer and returns the number of characters that __would have__
been written if the buffer was sufficiently large.

`_vnsprintf_s()` on the other hand fills the buffer and returns -1 when this
happens.  We want the former behavior, but we also want to be able to
pass in a locale to prevent having to call `setlocale()`.

`__stdio_common_vsprintf()` is the only function general enough to get
the behavior we want.

  rCXX libc++



Index: src/support/win32/locale_win32.cpp
--- src/support/win32/locale_win32.cpp
+++ src/support/win32/locale_win32.cpp
@@ -93,7 +93,9 @@
     va_list ap;
     va_start( ap, format );
 #if defined(_LIBCPP_MSVCRT)
-    int result = _vsnprintf_l( ret, n, format, loc, ap );
+    int result = __stdio_common_vsprintf(
+        ret, n, format, loc, ap);
     int result = vsnprintf( ret, n, format, ap );

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