Check for POSIX features with the build system

1 files changed, 54 insertions, 31 deletions

diff --git a/src/serd_config.h b/src/serd_config.h
index df73522b..e54b8688 100644
--- a/src/serd_config.h
+++ b/src/serd_config.h
@@ -1,15 +1,35 @@
-// Copyright 2021-2022 David Robillard <d@drobilla.net>
+// Copyright 2021-2023 David Robillard <d@drobilla.net>
 // SPDX-License-Identifier: ISC
 
 /*
-  Configuration header that defines reasonable defaults at compile time.
-
-  This allows compile-time configuration from the command line (typically via
-  the build system) while still allowing the source to be built without any
-  configuration.  The build system can define SERD_NO_DEFAULT_CONFIG to disable
-  defaults, in which case it must define things like HAVE_FEATURE to enable
-  features.  The design here ensures that compiler warnings or
-  include-what-you-use will catch any mistakes.
+  Configuration header that defines reasonable defaults at compile-time.
+
+  This allows configuration from the command-line (usually by the build system)
+  while still allowing the code to compile "as-is" with reasonable default
+  features on supported platforms.
+
+  This system is designed so that, ideally, no command-line or build-system
+  configuration is needed, but automatic feature detection can be disabled or
+  overridden for maximum control.  It should never be necessary to edit the
+  source code to achieve a given configuration.
+
+  Usage:
+
+  - By default, features are enabled if they can be detected or assumed to be
+    available from the build environment, unless `SERD_NO_DEFAULT_CONFIG` is
+    defined, which disables everything by default.
+
+  - If a symbol like `HAVE_SOMETHING` is defined to non-zero, then the
+    "something" feature is assumed to be available.
+
+  Code rules:
+
+  - To check for a feature, this header must be included, and the symbol
+    `USE_SOMETHING` used as a boolean in an `#if` expression.
+
+  - None of the other configuration symbols described here may be used
+    directly.  In particular, this header should be the only place in the
+    source code that touches `HAVE` symbols.
 */
 
 #ifndef SERD_CONFIG_H
@@ -21,62 +41,65 @@
 #if !defined(SERD_NO_DEFAULT_CONFIG)
 
 // We need unistd.h to check _POSIX_VERSION
-#  ifndef SERD_NO_POSIX
-#    ifdef __has_include
-#      if __has_include(<unistd.h>)
-#        include <unistd.h>
-#      endif
-#    elif defined(__unix__)
+#  ifdef __has_include
+#    if __has_include(<unistd.h>)
 #      include <unistd.h>
 #    endif
+#  elif defined(__unix__)
+#    include <unistd.h>
+#  endif
+
+// Define SERD_POSIX_VERSION unconditionally for convenience
+#  if defined(_POSIX_VERSION)
+#    define SERD_POSIX_VERSION _POSIX_VERSION
+#  else
+#    define SERD_POSIX_VERSION 0
 #  endif
 
 // POSIX.1-2001: fileno()
 #  ifndef HAVE_FILENO
-#    if defined(_POSIX_VERSION) && _POSIX_VERSION >= 200112L
-#      define HAVE_FILENO
+#    if SERD_POSIX_VERSION >= 200112L || defined(_WIN32)
+#      define HAVE_FILENO 1
 #    endif
 #  endif
 
 // POSIX.1-2001: posix_fadvise()
 #  ifndef HAVE_POSIX_FADVISE
-#    ifndef __APPLE__
-#      if defined(_POSIX_VERSION) && _POSIX_VERSION >= 200112L
-#        define HAVE_POSIX_FADVISE
-#      endif
+#    if SERD__POSIX_VERSION >= 200112L && !defined(__APPLE__)
+#      define HAVE_POSIX_FADVISE 1
 #    endif
 #  endif
 
 // POSIX.1-2001: posix_memalign()
 #  ifndef HAVE_POSIX_MEMALIGN
-#    if defined(_POSIX_VERSION) && _POSIX_VERSION >= 200112L
-#      define HAVE_POSIX_MEMALIGN
+#    if SERD_POSIX_VERSION >= 200112L
+#      define HAVE_POSIX_MEMALIGN 1
 #    endif
 #  endif
 
 #endif // !defined(SERD_NO_DEFAULT_CONFIG)
 
 /*
-  Make corresponding USE_FEATURE defines based on the HAVE_FEATURE defines from
-  above or the command line.  The code checks for these using #if (not #ifdef),
-  so there will be an undefined warning if it checks for an unknown feature,
-  and this header is always required by any code that checks for features, even
-  if the build system defines them all.
+  Unconditionally define symbols like USE_SOMETHING based on HAVE_SOMETHING, if
+  it's defined.  The code checks these using #if (not #ifdef), so there will be
+  a warning if it checks for an unknown feature or doesn't include this header.
+  This header should be the only file in the source code that touches symbols
+  like HAVE_SOMETHING.
 */
 
-#ifdef HAVE_FILENO
+#if defined(HAVE_FILENO) && HAVE_FILENO
 #  define USE_FILENO 1
 #else
 #  define USE_FILENO 0
 #endif
 
-#ifdef HAVE_POSIX_FADVISE
+#if defined(HAVE_POSIX_FADVISE) && HAVE_POSIX_FADVISE
 #  define USE_POSIX_FADVISE 1
 #else
 #  define USE_POSIX_FADVISE 0
 #endif
 
-#ifdef HAVE_POSIX_MEMALIGN
+#if defined(HAVE_POSIX_MEMALIGN) && HAVE_POSIX_MEMALIGN
 #  define USE_POSIX_MEMALIGN 1
 #else
 #  define USE_POSIX_MEMALIGN 0