fsync man page on CentOS

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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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