fsync / fdatasync - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Signature – int fsync(int fd); int fdatasync(int fd); – unistd.h.

Libc side – block until the kernel has flushed: fsync all data plus metadata, fdatasync data only (metadata may lag, manual, Synchronizing I/O operations). Returns 0 or -1+errno (EBADF, EINVAL in the manual’s list; EIO/EROFS at the syscall layer, see the sync family entry of Descriptors and File I/O). fsync is a cancellation point. glibc never calls these implicitly – the manual is explicit that write returning (and therefore fflush completing) does not imply the media got the data; programs that need durability call fsync(fileno(f)) after fflush(f). The whole-system sync and per-file syncfs variants are documented at the syscall layer.