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Manual Pages  — FSYNC


fdatasync, – synchronise changes to a file



Standard C Library (libc, -lc)


#include <unistd.h>

fdatasync(int fd);

fsync(int fd);


The fsync() system call causes all modified data and attributes of the file referenced by the file descriptor 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() system call causes all modified data of fd to be moved to a permanent storage device. Unlike fsync(), the system call does not guarantee that file attributes or metadata necessary to access the file are committed to the permanent storage.

The fsync() system call should be used by programs that require a file to be in a known state, for example, in building a simple transaction facility. If the file metadata has already been committed, using fdatasync() can be more efficient than fsync().

Both fdatasync() and fsync() calls are cancellation points.


The fsync function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.


The fsync() and fdatasync() calls fail if:
  The fd argument is not a valid descriptor.
  The fd argument refers to a socket, not to a file.
[EIO] An I/O error occurred while reading from or writing to the file system.


fsync(1), sync(2), syncer(4), sync(8)


The fsync() system call appeared in BSD 4.2 . The fdatasync() system call appeared in FreeBSD 11.1 .


The fdatasync() system call currently does not guarantee that enqueued aio(4) requests for the file referenced by fd are completed before the syscall returns.

FSYNC (2) February 6, 2018

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