FSYNC(2)

NAME
fsync - synchronize a file's in-core state with that on disk

SYNOPSIS
#include <unistd.h>
int
fsync(int fd);

DESCRIPTION
Fsync() causes all modified data and attributes of fd to be moved to a permanent storage device. This normally results in all in-core modified copies of buffers for the associated file to be written to a disk.

Note that while fsync() will flush all data from the host to the drive (i.e. the "permanent storage device"), the drive itself may not physically write the data to the platters for quite some time and it may be written in an out-of-order sequence.

Specifically, if the drive loses power or the OS crashes, the application may find that only some or none of their data was written. The disk drive may also re-order the data so that later writes may be present while earlier writes are not.

This is not a theoretical edge case. This scenario is easily reproduced with real world workloads and drive power failures.

For applications that require tighter guarantess about the integrity of their data, MacOS X provides the F_FULLFSYNC fcntl. The F_FULLFSYNC fcntl asks the drive to flush all buffered data to permanent storage. Applications such as databases that require a strict ordering of writes should use F_FULLFSYNC to ensure their data is written in the order they expect. Please see fcntl(2) for more detail.

RETURN VALUES
A 0 value is returned on success. A -1 value indicates an error.

ERRORS
The fsync() fails if:

[EBADF]
    fd is not a valid descriptor.

[EINVAL]
    fd refers to a socket, not to a file.

[EIO]
    An I/O error occurred while reading from or writing to the file system.

SEE ALSO
sync(2), sync(8), update(8), fcntl(2)

HISTORY
The fsync() function call appeared in 4.2BSD.