NAME

fsync, fdatasync — synchronize a file's in-core state with that on disk

SYNOPSIS

#include <unistd.h>

int
fsync(int fd);

int
fdatasync(int fd);

DESCRIPTION

The fsync() function 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.

The fdatasync() function is similar to fsync() except that it only guarantees modified data (and metadata necessary to read that data) is committed to storage. Other file modifications may be left unsynchronized.

fsync() and fdatasync() should be used by programs that require a file to be in a known state, for example, in building a simple transaction facility.

If fsync() or fdatasync() fail with EIO, the state of the on-disk data may have been only partially written. To guard against potential inconsistency, future calls will continue failing until all references to the file are closed.

RETURN VALUES

The fsync() and fdatasync() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

ERRORS

The fsync() and fdatasync() functions fail if:

[EBADF]

fd is not a valid descriptor.

[EINVAL]

fd does not refer to a file which can be synchronized.

[EIO]

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

SEE ALSO

sync(2), sync(8)

STANDARDS

The fsync() and fdatasync() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

HISTORY

The fsync() system call first appeared in 4.1cBSD, and the fdatasync() function has been available since OpenBSD 5.4.

BUGS

The fdatasync() function is currently a wrapper around fsync(), so it synchronizes more state than necessary.