sync(2) -- Linux man page
NAMEsync - commit buffer cache to disk
DESCRIPTIONsync first commits inodes to buffers, and then buffers to disk.
ERRORSThis function is always successful.
CONFORMING TOSVr4, SVID, X/OPEN, BSD 4.3
BUGSAccording to the standard specification (e.g., SVID), sync() schedules the writes, but may return before the actual writing is done. However, since version 1.3.20 Linux does actually wait. (This still does not guarantee data integrity: modern disks have large caches.)
NOTESSince glibc 2.2.2 the Linux prototype is as listed above, following the various standards. In libc4, libc5, and glibc up to 2.2.1 it was "int sync(void)", and sync always returned 0.
SEE ALSObdflush(2), fsync(2), fdatasync(2), update(8), sync(8)