| // Copyright 2025 The Fuchsia Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #include <cstddef> |
| |
| #include "log.h" |
| |
| namespace LIBC_NAMESPACE_DECL { |
| namespace { |
| |
| // Code using gLog may need to run before all normal constructors have run, and |
| // keep working after all normal destructors have run. The Log type is |
| // entirely constinit-compatible. But it needs a destructor, so even a |
| // constinit global induces a static constructor to register the destructor. |
| // Or else it might induce a direct `.fini_array` entry as a static |
| // registration of the destructor. In either case, it's hard or impossible to |
| // ensure that this destructor will run only after all others. It's important |
| // not to tear this down until there is no code (including any plain compiled |
| // code that's instrumented to possibly call into a sanitizer runtime) that |
| // might ever use gLog. So instead of a simple gLog global object here, a test |
| // on the Log type ensures that Log's default constructor is not only constinit |
| // but truly just makes all the bytes zero. This means it's sufficient to just |
| // pretend that the object was formally constructed, without ever actually |
| // doing it whether via a global defined with type Log or via placement new. |
| // |
| // So here gLogStorage is defined just as an array of bytes; but with the |
| // alignment and size of Log, internal linkage, and a known linkage name. |
| |
| using LogStorage = std::byte[sizeof(Log)]; |
| [[maybe_unused]] alignas(Log) constinit LogStorage gLogStorage |
| LIBC_ASM_LINKAGE_DECLARE(gLogStorage); |
| |
| } // namespace |
| |
| // Now the real LIBC_NAMESPACE::gLog symbol can be defined as just an alias to |
| // that storage with a different type. Outside this file, nothing appears |
| // different from `Log gLog;` here except that no destructor will ever run. |
| extern constinit Log gLog [[gnu::alias(LIBC_ASM_LINKAGE_STRING(gLogStorage))]]; |
| |
| } // namespace LIBC_NAMESPACE_DECL |
