LWG Poll 8: P0943R6 Support C atomics in C++

source/atomics.tex

@@ -20,7 +20,8 @@
 \ref{atomics.types.generic} & Class template \tcode{atomic}   &  \\
 \ref{atomics.nonmembers}  & Non-member functions &  \\
 \ref{atomics.flag}  & Flag type and operations   &  \\
-\ref{atomics.fences}  & Fences   &  \\
+\ref{atomics.fences}  & Fences   &  \\ \rowsep
+\ref{stdatomic.h.syn}  & C compatibility   & \tcode{<stdatomic.h>} \\
 \end{libsumtab}
 
 \rSec1[atomics.syn]{Header \tcode{<atomic>} synopsis}
@@ -3702,3 +3703,140 @@
 that \tcode{atomic_thread_fence} would have inserted are not emitted.
 \end{note}
 \end{itemdescr}
+
+\rSec1[stdatomic.h.syn]{C compatibility}
+
+The header \libheaderdef{stdatomic.h} provides the following definitions:
+
+\begin{codeblock}
+template<class T>
+  using @\exposid{std-atomic}@ = std::atomic<T>;        // exposition only
+
+#define _Atomic(T) @\exposid{std-atomic}@<T>
+
+#define ATOMIC_BOOL_LOCK_FREE @\seebelow@
+#define ATOMIC_CHAR_LOCK_FREE @\seebelow@
+#define ATOMIC_CHAR16_T_LOCK_FREE @\seebelow@
+#define ATOMIC_CHAR32_T_LOCK_FREE @\seebelow@
+#define ATOMIC_WCHAR_T_LOCK_FREE @\seebelow@
+#define ATOMIC_SHORT_LOCK_FREE @\seebelow@
+#define ATOMIC_INT_LOCK_FREE @\seebelow@
+#define ATOMIC_LONG_LOCK_FREE @\seebelow@
+#define ATOMIC_LLONG_LOCK_FREE @\seebelow@
+#define ATOMIC_POINTER_LOCK_FREE @\seebelow@
+
+using std::@\libglobal{memory_order}@                // \seebelow
+using std::@\libglobal{memory_order_relaxed}@        // \seebelow
+using std::@\libglobal{memory_order_consume}@        // \seebelow
+using std::@\libglobal{memory_order_acquire}@        // \seebelow
+using std::@\libglobal{memory_order_release}@        // \seebelow
+using std::@\libglobal{memory_order_acq_rel}@        // \seebelow
+using std::@\libglobal{memory_order_seq_cst}@        // \seebelow
+
+using std::@\libglobal{atomic_flag}@                 // \seebelow
+
+using std::@\libglobal{atomic_bool}@                 // \seebelow
+using std::@\libglobal{atomic_char}@                 // \seebelow
+using std::@\libglobal{atomic_schar}@                // \seebelow
+using std::@\libglobal{atomic_uchar}@                // \seebelow
+using std::@\libglobal{atomic_short}@                // \seebelow
+using std::@\libglobal{atomic_ushort}@               // \seebelow
+using std::@\libglobal{atomic_int}@                  // \seebelow
+using std::@\libglobal{atomic_uint}@                 // \seebelow
+using std::@\libglobal{atomic_long}@                 // \seebelow
+using std::@\libglobal{atomic_ulong}@                // \seebelow
+using std::@\libglobal{atomic_llong}@                // \seebelow
+using std::@\libglobal{atomic_ullong}@               // \seebelow
+using std::@\libglobal{atomic_char8_t}@              // \seebelow
+using std::@\libglobal{atomic_char16_t}@             // \seebelow
+using std::@\libglobal{atomic_char32_t}@             // \seebelow
+using std::@\libglobal{atomic_wchar_t}@              // \seebelow
+using std::@\libglobal{atomic_int8_t}@               // \seebelow
+using std::@\libglobal{atomic_uint8_t}@              // \seebelow
+using std::@\libglobal{atomic_int16_t}@              // \seebelow
+using std::@\libglobal{atomic_uint16_t}@             // \seebelow
+using std::@\libglobal{atomic_int32_t}@              // \seebelow
+using std::@\libglobal{atomic_uint32_t}@             // \seebelow
+using std::@\libglobal{atomic_int64_t}@              // \seebelow
+using std::@\libglobal{atomic_uint64_t}@             // \seebelow
+using std::@\libglobal{atomic_int_least8_t}@         // \seebelow
+using std::@\libglobal{atomic_uint_least8_t}@        // \seebelow
+using std::@\libglobal{atomic_int_least16_t}@        // \seebelow
+using std::@\libglobal{atomic_uint_least16_t}@       // \seebelow
+using std::@\libglobal{atomic_int_least32_t}@        // \seebelow
+using std::@\libglobal{atomic_uint_least32_t}@       // \seebelow
+using std::@\libglobal{atomic_int_least64_t}@        // \seebelow
+using std::@\libglobal{atomic_uint_least64_t}@       // \seebelow
+using std::@\libglobal{atomic_int_fast8_t}@          // \seebelow
+using std::@\libglobal{atomic_uint_fast8_t}@         // \seebelow
+using std::@\libglobal{atomic_int_fast16_t}@         // \seebelow
+using std::@\libglobal{atomic_uint_fast16_t}@        // \seebelow
+using std::@\libglobal{atomic_int_fast32_t}@         // \seebelow
+using std::@\libglobal{atomic_uint_fast32_t}@        // \seebelow
+using std::@\libglobal{atomic_int_fast64_t}@         // \seebelow
+using std::@\libglobal{atomic_uint_fast64_t}@        // \seebelow
+using std::@\libglobal{atomic_intptr_t}@             // \seebelow
+using std::@\libglobal{atomic_uintptr_t}@            // \seebelow
+using std::@\libglobal{atomic_size_t}@               // \seebelow
+using std::@\libglobal{atomic_ptrdiff_t}@            // \seebelow
+using std::@\libglobal{atomic_intmax_t}@             // \seebelow
+using std::@\libglobal{atomic_uintmax_t}@            // \seebelow
+
+using std::@\libglobal{atomic_is_lock_free}@                         // \seebelow
+using std::@\libglobal{atomic_load}@                                 // \seebelow
+using std::@\libglobal{atomic_load_explicit}@                        // \seebelow
+using std::@\libglobal{atomic_store}@                                // \seebelow
+using std::@\libglobal{atomic_store_explicit}@                       // \seebelow
+using std::@\libglobal{atomic_exchange}@                             // \seebelow
+using std::@\libglobal{atomic_exchange_explicit}@                    // \seebelow
+using std::@\libglobal{atomic_compare_exchange_strong}@              // \seebelow
+using std::@\libglobal{atomic_compare_exchange_strong_explicit}@     // \seebelow
+using std::@\libglobal{atomic_compare_exchange_weak}@                // \seebelow
+using std::@\libglobal{atomic_compare_exchange_weak_explicit}@       // \seebelow
+using std::@\libglobal{atomic_fetch_add}@                            // \seebelow
+using std::@\libglobal{atomic_fetch_add_explicit}@                   // \seebelow
+using std::@\libglobal{atomic_fetch_sub}@                            // \seebelow
+using std::@\libglobal{atomic_fetch_sub_explicit}@                   // \seebelow
+using std::@\libglobal{atomic_fetch_or}@                             // \seebelow
+using std::@\libglobal{atomic_fetch_or_explicit}@                    // \seebelow
+using std::@\libglobal{atomic_fetch_and}@                            // \seebelow
+using std::@\libglobal{atomic_fetch_and_explicit}@                   // \seebelow
+using std::@\libglobal{atomic_flag_test_and_set}@                    // \seebelow
+using std::@\libglobal{atomic_flag_test_and_set_explicit}@           // \seebelow
+using std::@\libglobal{atomic_flag_clear}@                           // \seebelow
+using std::@\libglobal{atomic_flag_clear_explicit}@                  // \seebelow
+
+using std::@\libglobal{atomic_thread_fence}@                         // \seebelow
+using std::@\libglobal{atomic_signal_fence}@                         // \seebelow
+\end{codeblock}
+
+\pnum
+Each \grammarterm{using-declaration} for some name $A$ in the synopsis above
+makes available the same entity as \tcode{std::$A$}
+declared in \libheaderrefx{atomic}{atomics.syn}.
+Each macro listed above other than \tcode{_Atomic(T)}
+is defined as in \libheader{atomic}.
+It is unspecified whether \libheader{stdatomic.h} makes available
+any declarations in namespace \tcode{std}.
+
+\pnum
+Each of the \grammarterm{using-declaration}s for
+\tcode{int$N$_t}, \tcode{uint$N$_t}, \tcode{intptr_t}, and \tcode{uintptr_t}
+listed above is defined if and only if the implementation defines
+the corresponding typedef name in \ref{atomics.syn}.
+
+\pnum
+Neither the \tcode{_Atomic} macro,
+nor any of the non-macro global namespace declarations,
+are provided by any \Cpp{} standard library header
+other than \libheader{stdatomic.h}.
+
+\pnum
+\recommended
+Implementations should ensure
+that C and \Cpp{} representations of atomic objects are compatible,
+so that the same object can be accessed as both an \tcode{_Atomic(T)}
+from C code and an \tcode{atomic<T>} from \Cpp{} code.
+The representations should be the same, and
+the mechanisms used to ensure atomicity and memory ordering
+should be compatible.

source/compatibility.tex

@@ -2674,9 +2674,7 @@
 
 \pnum
 There are no \Cpp{} headers for the C standard library's headers
-\libnoheader{stdatomic.h},
-\libnoheader{stdnoreturn.h},
-and \libnoheader{threads.h},
+\libnoheader{stdnoreturn.h} and \libnoheader{threads.h},
 nor are these headers from the C standard library headers themselves part of \Cpp{}.
 
 \pnum

source/future.tex

@@ -243,17 +243,18 @@
 \libheaderdef{signal.h} \\
 \libheader{stdalign.h} \\
 \libheaderdef{stdarg.h} \\
+\libheader{stdatomic.h} \\
 \libheader{stdbool.h} \\
 \libheaderdef{stddef.h} \\
-\libheaderdef{stdint.h} \\
 \columnbreak
+\libheaderdef{stdint.h} \\
 \libheaderdef{stdio.h} \\
 \libheaderdef{stdlib.h} \\
 \libheaderdef{string.h} \\
 \libheader{tgmath.h} \\
 \libheaderdef{time.h} \\
-\libheaderdef{uchar.h} \\
 \columnbreak
+\libheaderdef{uchar.h} \\
 \libheaderdef{wchar.h} \\
 \libheaderdef{wctype.h} \\
 \end{multicolfloattable}
@@ -365,6 +366,7 @@
 \libdeprheaderref{complex.h},
 \libdeprheaderref{iso646.h},
 \libdeprheaderref{stdalign.h},\newline
+\libheaderref{stdatomic.h},
 \libdeprheaderref{stdbool.h}, and
 \libdeprheaderref{tgmath.h},
 each of

source/lib-intro.tex

@@ -1013,7 +1013,6 @@
 \begin{footnote}
 It is intentional that there is no \Cpp{} header
 for any of these C headers:
-\libnoheader{stdatomic.h},
 \libnoheader{stdnoreturn.h},
 \libnoheader{threads.h}.
 \end{footnote}

source/support.tex

@@ -675,6 +675,7 @@
 #define @\defnlibxname{cpp_lib_ssize}@                             201902L // also in \libheader{iterator}
 #define @\defnlibxname{cpp_lib_stacktrace}@                        202011L // also in \libheader{stacktrace}
 #define @\defnlibxname{cpp_lib_starts_ends_with}@                  201711L // also in \libheader{string}, \libheader{string_view}
+#define @\defnlibxname{cpp_lib_stdatomic_h}@                       202011L // also in \libheader{stdatomic.h}
 #define @\defnlibxname{cpp_lib_string_contains}@                   202011L // also in \libheader{string}, \libheader{string_view}
 #define @\defnlibxname{cpp_lib_string_udls}@                       201304L // also in \libheader{string}
 #define @\defnlibxname{cpp_lib_string_view}@                       201803L // also in \libheader{string}, \libheader{string_view}