3.13. sync, fsync, and fdatasync Functions

Traditional implementations of the UNIX System have a buffer cache or page cache in the kernel through which most disk I/O passes. When we write data to a file, the data is normally copied by the kernel into one of its buffers and queued for writing to disk at some later time. This is called delayed write. (Chapter 3 of Bach [1986] discusses this buffer cache in detail.)

The kernel eventually writes all the delayed-write blocks to disk, normally when it needs to reuse the buffer for some other disk block. To ensure consistency of the file system on disk with the contents of the buffer cache, the sync, fsync, and fdatasync functions are provided.

#include <unistd.h> int fsync(int filedes); int fdatasync(int filedes);
Returns: 0 if OK, 1 on error
void sync(void);

The sync function simply queues all the modified block buffers for writing and returns; it does not wait for the disk writes to take place.

The function sync is normally called periodically (usually every 30 seconds) from a system daemon, often called update. This guarantees regular flushing of the kernel's block buffers. The command sync(1) also calls the sync function.

The function fsync refers only to a single file, specified by the file descriptor filedes, and waits for the disk writes to complete before returning. The intended use of fsync is for an application, such as a database, that needs to be sure that the modified blocks have been written to the disk.

The fdatasync function is similar to fsync, but it affects only the data portions of a file. With fsync, the file's attributes are also updated synchronously.

All four of the platforms described in this book support sync and fsync. However, FreeBSD 5.2.1 and Mac OS X 10.3 do not support fdatasync.