fsync(2)



FSYNC(2)                   Linux Programmer's Manual                  FSYNC(2)

NAME
       fsync,  fdatasync  -  synchronize  a  file's in-core state with storage
       device

SYNOPSIS
       #include <unistd.h>

       int fsync(int fd);

       int fdatasync(int fd);

DESCRIPTION
       fsync() transfers ("flushes") all modified in-core data of (i.e., modi-
       fied  buffer cache pages for) the file referred to by the file descrip-
       tor fd to the disk device (or other  permanent  storage  device)  where
       that  file  resides.  The call blocks until the device reports that the
       transfer has completed.  It also flushes  metadata information  associ-
       ated with the file (see stat(2)).

       Calling  fsync()  does  not  necessarily  ensure  that the entry in the
       directory containing the file has  also  reached  disk.   For  that  an
       explicit fsync() on a file descriptor for the directory is also needed.

       fdatasync() is similar to fsync(), but does not flush modified metadata
       unless  that  metadata  is  needed  in order to allow a subsequent data
       retrieval to be correctly handled.  For example, changes to st_atime or
       st_mtime  (respectively, time of last access and time of last modifica-
       tion; see stat(2)) do not not require flushing  because  they  are  not
       necessary  for  a subsequent data read to be handled correctly.  On the
       other hand, a change to the file size (st_size, as made by  say  ftrun-
       cate(2)), would require a metadata flush.

       The  aim  of  fdatasync(2)  is to reduce disk activity for applications
       that do not require all metadata to be synchronised with the disk.

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.

       EIO    An error occurred during synchronization.

       EROFS, EINVAL
              fd  is  bound  to a special file which does not support synchro-
              nization.

NOTES
       If the underlying hard disk has write caching enabled,  then  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 implicitly 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.1-2001

SEE ALSO
       bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8),  mount(8),
       sync(8), update(8)

Linux 1.3.85                      2006-04-28                          FSYNC(2)

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