November 14, 2003 FSYNC 2

### NAME

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

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```int` fsync(```int fd`)
```int` fsync_range(```int fd``, int how``, off_t start``, off_t length`)

### 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.

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

fsync_range(``) causes all modified data starting at ```start` for length ```length` of ```fd` to be written to permanent storage. Note that fsync_range(``) requires that the file ```fd` must be open for writing.

fsync_range(``) may flush the file data in one of two manners:

```FDATASYNC`
  Synchronize the file data and sufficient meta-data to retrieve the data for the specified range.

```FFILESYNC`
  Synchronize all modified file data and meta-data for the specified range.

By default, fsync_range(``) does not flush disk caches, assuming that storage media are able to ensure completed writes are transfered to media. The ```FDISKSYNC` flag may be included in the ```how` parameter to trigger flushing of all disk caches for the file.

If the ```length` parameter is zero, fsync_range(``) will synchronize all of the file data.

### RETURN VALUES

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

### ERRORS

fsync(``) or fsync_range(``) fail 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.

Additionally, fsync_range(``) fails if:

[```EBADF`]
  ```fd` is not open for writing.

[```EINVAL`]
  ```start` + ```length` is less than ```start`.

### NOTES

For optimal efficiency, the fsync_range(``) call requires that the file system containing the file referenced by ```fd` support partial synchronization of file data. For file systems which do not support partial synchronization, the entire file will be synchronized and the call will be the equivalent of calling fsync(``).

### SEE ALSO

sync(2), sync(8)

### HISTORY

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

The fsync_range(``) function call first appeared in NetBSD2.0 and is modeled after the function available in AIX.