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[libc] Add _Returns_twice to C++ code #153602

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Merged
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Aug 19, 2025
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[libc] Add _Returns_twice to C++ code #153602

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[libc] Add _Returns_twice to C++ code
saturn691 Aug 14, 2025
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Add suggestion from @petrhoesk
saturn691 Aug 18, 2025
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3 changes: 3 additions & 0 deletions libc/include/__llvm-libc-common.h
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Expand Up @@ -47,6 +47,9 @@
#define __NOEXCEPT throw()
#endif

#undef _Returns_twice
#define _Returns_twice [[gnu::returns_twice]]

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given that this is on both sides of the #ifdef __cplusplus branch is there a way we can unify it instead of having two copies? Also is there a reason you used the gnu attribute instead of the generic one?

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  1. returns_twice needs to be namespaced (https://en.cppreference.com/w/cpp/language/attributes.html)
❯ clang test.c                                                               
test.c:3:3: warning: unknown attribute 'returns_twice' ignored [-Wunknown-attributes]
    3 | [[returns_twice]] int f() {
      |   ^~~~~~~~~~~~~
1 warning generated.
  1. [[attribute]] syntax is only supported in C++11. It seems to compile for older versions and this has even been back-ported to C code in clang. I'll push an update to unify this.

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@petrhosek petrhosek Aug 17, 2025 •
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The [[attribute]] syntax was only introduced in C20 so we shouldn't be using it in C code since we want our headers are compatible with older C standards.

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@saturn691 saturn691 Aug 18, 2025 •
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Good point, I've settled with preferring to generate the new attribute syntax if the compiler supports it (C++11) onwards.

// This macro serves as a generic cast implementation for use in both C and C++,
// similar to `__BIONIC_CAST` in Android.
#undef __LLVM_LIBC_CAST
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