From 7382127f0c08b3e6b269ae4ee33eafe70f0993af Mon Sep 17 00:00:00 2001
From: Rhys-T <108157737+Rhys-T@users.noreply.github.com>
Date: Sun, 21 Jun 2026 16:13:40 -0400
Subject: [PATCH] Check for fdatasync, and fall back to fsync if missing

Not all systems define fdatasync, and fsync does a strict superset of
what fdatasync does.

macOS has an fdatasync function in its standard library, but doesn't
declare or document it. Apple Clang hides the warning/error when you try
to call it without a declaration, but other compilers (including
upstream Clang) don't. This is currently causing the Nix package for
fcron to fail to build on macOS.
---
 config.h.in  | 6 ++++++
 configure.in | 2 ++
 global.h     | 8 ++++++++
 3 files changed, 16 insertions(+)

diff --git a/config.h.in b/config.h.in
index 2ae5644..60b2d22 100644
--- a/config.h.in
+++ b/config.h.in
@@ -240,6 +240,12 @@
 /* Define if you have the flock function.  */
 #undef HAVE_FLOCK
 
+/* Define if you have the fdatasync function.  */
+#undef HAVE_FDATASYNC
+
+/* Define if the headers actually declare the fdatasync function.  */
+#undef HAVE_DECL_FDATASYNC
+
 /* Define if you have the lockf function.  */
 #undef HAVE_LOCKF
 
diff --git a/configure.in b/configure.in
index 8e57f96..0b611e3 100644
--- a/configure.in
+++ b/configure.in
@@ -106,6 +106,8 @@ fi
 AC_CHECK_FUNCS(getcwd gettimeofday mktime putenv strerror setenv unsetenv gethostname)
 AC_CHECK_FUNCS(getopt_long)
 AC_CHECK_FUNCS(mkstemp)
+AC_CHECK_FUNCS(fdatasync)
+AC_CHECK_DECLS(fdatasync)
 AC_CHECK_FUNCS(flock lockf)
 AC_CHECK_FUNCS(setlinebuf)
 AC_CHECK_FUNCS(sigaction)
diff --git a/global.h b/global.h
index 12094ed..2d502f7 100644
--- a/global.h
+++ b/global.h
@@ -168,6 +168,14 @@
 #endif
 
 /* macros */
+/* macOS has an fdatasync to link against, but doesn't declare or document it.
+ * Apple Clang suppresses the warning/error when calling it, which causes
+ * HAVE_FDATASYNC to be defined. But other compilers (including upstream Clang)
+ * don't. So we have to check both to see if we can safely use it. */
+#if !defined(HAVE_FDATASYNC) || !HAVE_DECL_FDATASYNC
+#define fdatasync(arg) fsync(arg)
+#endif
+
 #ifndef HAVE_SETEUID
 #define seteuid(arg) setresuid(-1,(arg),-1)
 #endif