[libc] [llvm] [libc] Move libc_errno.h to libc/src/__support and make LIBC_ERRNO_MODE_SYSTEM to be header-only. (PR #143187)

[Alexey Samsonov via llvm-commits]

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@@ -0,0 +1,108 @@
+//===-- Implementation header for libc_errno --------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_LIBC_ERRNO_H
+#define LLVM_LIBC_SRC___SUPPORT_LIBC_ERRNO_H
+
+// This header is to be consumed by internal implementations, in which all of
+// them should refer to `libc_errno` instead of using `errno` directly from
+// <errno.h> header.
+
+// Unit and hermetic tests should:
+// - #include "src/__support/libc_errno.h"
+// - NOT #include <errno.h>
+// - Only use `libc_errno` in the code
+// - Depend on libc.src.errno.errno
+
+// Integration tests should:
+// - NOT #include "src/__support/libc_errno.h"
+// - #include <errno.h>
+// - Use regular `errno` in the code
+// - Still depend on libc.src.errno.errno
+
+// libc uses a fallback default value, either system or thread local.
+#define LIBC_ERRNO_MODE_DEFAULT 0
+// libc never stores a value; `errno` macro uses get link-time failure.
+#define LIBC_ERRNO_MODE_UNDEFINED 1
+// libc maintains per-thread state (requires C++ `thread_local` support).
+#define LIBC_ERRNO_MODE_THREAD_LOCAL 2
+// libc maintains shared state used by all threads, contrary to standard C
+// semantics unless always single-threaded; nothing prevents data races.
+#define LIBC_ERRNO_MODE_SHARED 3
+// libc doesn't maintain any internal state, instead the embedder must define
+// `int *__llvm_libc_errno(void);` C function.
+#define LIBC_ERRNO_MODE_EXTERNAL 4
+// libc uses system `<errno.h>` `errno` macro directly in the overlay mode; in
+// fullbuild mode, effectively the same as `LIBC_ERRNO_MODE_EXTERNAL`.
+// In this mode, the public C++ symbol `LIBC_NAMESPACE::libc_errno ` is still
+// exported and get redirected to the system `errno` inside its implementation.
+
+// TODO: Investigate deprecating LIBC_ERRNO_MODE_SYSTEM in favor of
+//       LIBC_ERRNO_MODE_SYSTEM_INLINE.
+//       https://github.com/llvm/llvm-project/issues/143454
+#define LIBC_ERRNO_MODE_SYSTEM 5
+// In this mode, the libc_errno is simply a macro resolved to `errno` from the
+// system header <errno.h>.  There is no need to link against the
+// `libc.src.errno.errno` object.
+#define LIBC_ERRNO_MODE_SYSTEM_INLINE 6
+
+#if !defined(LIBC_ERRNO_MODE) || LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_DEFAULT
+#undef LIBC_ERRNO_MODE
+#if defined(LIBC_FULL_BUILD) || !defined(LIBC_COPT_PUBLIC_PACKAGING)
+#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_THREAD_LOCAL
+#else
+#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_SYSTEM
+#endif
+#endif // LIBC_ERRNO_MODE
+
+#if LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_DEFAULT &&                              \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_UNDEFINED &&                            \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_THREAD_LOCAL &&                         \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SHARED &&                               \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_EXTERNAL &&                             \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SYSTEM &&                               \
+    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SYSTEM_INLINE
+#error LIBC_ERRNO_MODE must be one of the following values: \
+LIBC_ERRNO_MODE_DEFAULT, \
+LIBC_ERRNO_MODE_UNDEFINED, \
+LIBC_ERRNO_MODE_THREAD_LOCAL, \
+LIBC_ERRNO_MODE_SHARED, \
+LIBC_ERRNO_MODE_EXTERNAL, \
+LIBC_ERRNO_MODE_SYSTEM, \
+LIBC_ERRNO_MODE_SYSTEM_INLINE.
+#endif
+
+#if LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_SYSTEM_INLINE
+
+#include <errno.h>
+
+#define libc_errno errno
+
+#else // !LIBC_ERRNO_MODE_SYSTEM
----------------
vonosmas wrote:

!LIBC_ERRNO_MODE_SYSTEM_INLINE -- here and below

https://github.com/llvm/llvm-project/pull/143187