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+// Copyright © Tavian Barnes <tavianator@tavianator.com>
+// SPDX-License-Identifier: 0BSD
+
+/**
+ * Praeludium.
+ *
+ * This header is automatically included in every translation unit, before any
+ * other headers, so it can set feature test macros[1][2].  This sets up our own
+ * mini-dialect of C, which includes
+ *
+ *   - Standard C17 and POSIX.1 2024 features
+ *   - Portable and platform-specific extensions
+ *   - Convenience macros like `bool`, `alignof`, etc.
+ *   - Common compiler extensions like __has_include()
+ *
+ * Further bfs-specific utilities are defined in "bfs.h".
+ *
+ * [1]: https://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html
+ * [2]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/V2_chap02.html
+ */
+
+#ifndef BFS_PRELUDE_H
+#define BFS_PRELUDE_H
+
+// Feature test macros
+
+/**
+ * Linux and BSD handle _POSIX_C_SOURCE differently: on Linux, it enables POSIX
+ * interfaces that are not visible by default.  On BSD, it also *disables* most
+ * extensions, giving a strict POSIX environment.  Since we want the extensions,
+ * we don't set _POSIX_C_SOURCE.
+ */
+// #define _POSIX_C_SOURCE 202405L
+
+/** openat() etc. */
+#define _ATFILE_SOURCE 1
+
+/** BSD-derived extensions. */
+#define _BSD_SOURCE 1
+
+/** glibc successor to _BSD_SOURCE. */
+#define _DEFAULT_SOURCE 1
+
+/** GNU extensions. */
+#define _GNU_SOURCE 1
+
+/** Use 64-bit off_t. */
+#define _FILE_OFFSET_BITS 64
+
+/** Use 64-bit time_t. */
+#define _TIME_BITS 64
+
+/** macOS extensions. */
+#if __APPLE__
+#  define _DARWIN_C_SOURCE 1
+#endif
+
+/** Solaris extensions. */
+#if __sun
+#  define __EXTENSIONS__ 1
+// https://illumos.org/man/3C/getpwnam#standard-conforming
+#  define _POSIX_PTHREAD_SEMANTICS 1
+#endif
+
+/** QNX extensions. */
+#if __QNX__
+#  define _QNX_SOURCE 1
+#endif
+
+// Get the convenience macros that became standard spellings in C23
+#if __STDC_VERSION__ < 202311L
+
+/** _Static_assert() => static_assert() */
+#include <assert.h>
+/** _Alignas(), _Alignof() => alignas(), alignof() */
+#include <stdalign.h>
+/** _Bool => bool, true, false */
+#include <stdbool.h>
+
+/**
+ * C23 deprecates `noreturn void` in favour of `[[noreturn]] void`, so we expose
+ * _noreturn instead with the other attributes in "bfs.h".
+ */
+// #include <stdnoreturn.h>
+
+/** Part of <threads.h>, but we don't use anything else from it. */
+#define thread_local _Thread_local
+
+#endif // !C23
+
+// Feature detection
+
+// https://clang.llvm.org/docs/LanguageExtensions.html#has-attribute
+#ifndef __has_attribute
+#  define __has_attribute(attr) false
+#endif
+
+// https://clang.llvm.org/docs/LanguageExtensions.html#has-builtin
+#ifndef __has_builtin
+#  define __has_builtin(builtin) false
+#endif
+
+// https://en.cppreference.com/w/c/language/attributes#Attribute_testing
+#ifndef __has_c_attribute
+#  define __has_c_attribute(attr) false
+#endif
+
+// https://clang.llvm.org/docs/LanguageExtensions.html#has-feature-and-has-extension
+#ifndef __has_feature
+#  define __has_feature(feat) false
+#endif
+
+// https://en.cppreference.com/w/c/preprocessor/include
+#ifndef __has_include
+#  define __has_include(header) false
+#endif
+
+// Sanitizer macros (GCC defines these but Clang does not)
+
+#if __has_feature(address_sanitizer) && !defined(__SANITIZE_ADDRESS__)
+#  define __SANITIZE_ADDRESS__ true
+#endif
+#if __has_feature(memory_sanitizer) && !defined(__SANITIZE_MEMORY__)
+#  define __SANITIZE_MEMORY__ true
+#endif
+#if __has_feature(thread_sanitizer) && !defined(__SANITIZE_THREAD__)
+#  define __SANITIZE_THREAD__ true
+#endif
+
+#endif // BFS_PRELUDE_H