Check for fdatasync, and fall back to fsync if missing#55
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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.
Rhys-T
commented
Jun 21, 2026
| * 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) |
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Another option here, if you trust macOS's undocumented fdatasync function (which is what's currently being used if you build with Apple's tools), would be:
#if !HAVE_DECL_FDATASYNC
#if defined(__APPLE__) && defined(HAVE_FDATASYNC)
int fdatasync(int);
#else
#define fdatasync(arg) fsync(arg)
#endif
#endif
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Not all systems define
fdatasync, andfsyncdoes a strict superset of whatfdatasyncdoes.macOS has an
fdatasyncfunction 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 forfcronto fail to build on macOS.