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fsync_range(2) [netbsd man page]

FSYNC(2)						      BSD System Calls Manual							  FSYNC(2)

fsync, fsync_range -- synchronize a file's in-core state with that on disk LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> 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 modi- fied 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 synchro- nization 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 NetBSD 2.0 and is modeled after the function available in AIX. BSD
May 17, 2010 BSD

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

fsync, fdatasync - synchronize a file's in-core state with storage device SYNOPSIS
#include <unistd.h> int fsync(int fd); int fdatasync(int fd); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): fsync(): _BSD_SOURCE || _XOPEN_SOURCE || /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L fdatasync(): _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500 DESCRIPTION
fsync() transfers ("flushes") all modified in-core data of (i.e., modified buffer cache pages for) the file referred to by the file descriptor 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 associated 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 modi- fication; see stat(2)) do 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 ftruncate(2)), would require a metadata flush. The aim of fdatasync() is to reduce disk activity for applications that do not require all metadata to be synchronized with the disk. RETURN VALUE
On success, these system calls return zero. 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 synchronization. CONFORMING TO
On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRONIZED_IO is defined in <unistd.h> to a value greater than 0. (See also sysconf(3).) NOTES
Applications that access databases or log files often write a tiny data fragment (e.g., one line in a log file) and then call fsync() imme- diately in order to ensure that the written data is physically stored on the harddisk. Unfortunately, fsync() will always initiate two write operations: one for the newly written data and another one in order to update the modification time stored in the inode. If the mod- ification time is not a part of the transaction concept fdatasync() can be used to avoid unnecessary inode disk write operations. 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). In Linux 2.2 and earlier, fdatasync() is equivalent to fsync(), and so has no performance advantage. SEE ALSO
bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8), mount(8), sync(8), update(8) COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at Linux 2008-11-07 FSYNC(2)
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