build: Move feature test macros to prelude.h

This replaces the explicit CPPFLAGS list in flags.mk with just
`-include src/prelude.h`, shortening our compiler command lines and
allowing them to be easily documented.

1 files changed, 56 insertions, 2 deletions

diff --git a/src/prelude.h b/src/prelude.h
index 5f7203e..a0cc2a1 100644
--- a/src/prelude.h
+++ b/src/prelude.h
@@ -2,12 +2,66 @@
 // SPDX-License-Identifier: 0BSD
 
 /**
- * Configuration and feature/platform detection.
+ * 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
+
 // Get the convenience macros that became standard spellings in C23
 #if __STDC_VERSION__ < 202311L
 
@@ -20,7 +74,7 @@
 
 /**
  * C23 deprecates `noreturn void` in favour of `[[noreturn]] void`, so we expose
- * _noreturn instead with the other attributes.
+ * _noreturn instead with the other attributes in "bfs.h".
  */
 // #include <stdnoreturn.h>