[libcxx-commits] [libcxx] [AIX][libc++] fopen() does not support the 'x' (exclusive/noreplace) mode suffix. (PR #206498)

[Nikolas Klauser via libcxx-commits]

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@@ -664,6 +674,9 @@ const char* basic_filebuf<_CharT, _Traits>::__make_mdstring(ios_base::openmode _
   case ios_base::in | ios_base::app | ios_base::binary:
     return "a+b" _LIBCPP_FOPEN_CLOEXEC_MODE;
 #    if _LIBCPP_STD_VER >= 23
+// AIX fopen() does not support the 'x' (noreplace) mode suffix; these cases fall through to
+// default (return nullptr), which is conformant per C23 7.21.5.3p5 (fail if atomic check-and-create is unsupported).
+#      if !defined(_AIX)
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philnik777 wrote:

Why do we need to handle this in libc++? If `fopen` was conforming it would fail and this code wouldn't need to be modified in order to be conforming. We would simply fail gracefully. What am I missing?

https://github.com/llvm/llvm-project/pull/206498