FSYNC

Section: Linux Programmer's Manual (2)
Updated: 2001-04-18
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NAME

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

SYNOPSIS

#include <unistd.h>

int fsync(int fd);

int fdatasync(int fd);

DESCRIPTION

fsync copies all in-core parts of a file to disk, and waits until the device reports that all parts are on stable storage. It also updates metadata stat information. It does not necessarily ensure that the entry in the directory containing the file has also reached disk. For that an explicit fsync on the file descriptor of the directory is also needed.

fdatasync does the same as fsync but only flushes user data, not the meta data like the mtime or atime.

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS

EBADF: fd is not a valid file descriptor open for writing.
EROFS, EINVAL: fd is bound to a special file which does not support synchronization.
EIO: An error occurred during synchronization.

NOTES

In case the hard disk has write cache enabled, the data may not really be on permanent storage when fsync/fdatasync return.

When an ext2 file system is mounted with the sync option, directory entries are also implicitely synced by fsync.

On kernels before 2.4, fsync on big files can be inefficient. An alternative might be to use the O_SYNC flag to open(2).

CONFORMING TO

POSIX.1b (formerly POSIX.4)

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