NAME

sync — synchronize disk block in-core status with that on disk

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <unistd.h>

void
sync(void);

DESCRIPTION

The sync() function forces a write of dirty (modified) buffers in the block buffer cache out to disk. The kernel keeps this information in core to reduce the number of disk I/O transfers required by the system. As information in the cache is lost after a system crash, kernel thread ioflush ensures that dirty buffers are synced to disk eventually. By default, a dirty buffer is synced after 30 seconds, but some file systems exploit ioflush features to sync directory data and metadata faster (after 15 and 10 seconds, respectively).

The function fsync(2) may be used to synchronize individual file descriptor attributes.

CAUTIONS

Many modern disks contain write-back caches. In theory sync() flushes these. In practice there are many possible ways for this mechanism to go astray. It is prudent (where possible) to allow a few seconds after syncing for everything to settle before e.g. turning off the power.

It may also be desirable to use dkctl(8) or scsictl(8) to disable the write-back cache entirely.

SEE ALSO

fsync(2), dkctl(8), scsictl(8), sync(8)

HISTORY

A sync() function call appeared in Version 6 AT&T UNIX.

Historically, sync() would schedule buffers for writing but not actually wait for the writes to finish. It was necessary to issue a second or sometimes a third call to ensure that all buffers had in fact been written out. In NetBSD, sync() does not return until all buffers have been written.