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fsync(2) [suse man page]

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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fsync(3C)						   Standard C Library Functions 						 fsync(3C)

fsync - synchronize changes to a file SYNOPSIS
#include <unistd.h> int fsync(int fildes); DESCRIPTION
The fsync() function moves all modified data and attributes of the file descriptor fildes to a storage device. When fsync() returns, all in-memory modified copies of buffers associated with fildes have been written to the physical medium. The fsync() function is different from sync(), which schedules disk I/O for all files but returns before the I/O completes. The fsync() function forces all outstanding data operations to synchronized file integrity completion (see fcntl.h(3HEAD) definition of O_SYNC.) The fsync() function forces all currently queued I/O operations associated with the file indicated by the file descriptor fildes to the synchronized I/O completion state. All I/O operations are completed as defined for synchronized I/O file integrity completion. RETURN VALUES
Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error. If the fsync() function fails, outstanding I/O operations are not guaranteed to have been completed. ERRORS
The fsync() function will fail if: EBADF The fildes argument is not a valid file descriptor. EINTR A signal was caught during execution of the fsync() function. EIO An I/O error occurred while reading from or writing to the file system. ENOSPC There was no free space remaining on the device containing the file. ETIMEDOUT Remote connection timed out. This occurs when the file is on an NFS file system mounted with the soft option. See mount_nfs(1M). In the event that any of the queued I/O operations fail, fsync() returns the error conditions defined for read(2) and write(2). USAGE
The fsync() function should be used by applications that require that a file be in a known state. For example, an application that contains a simple transaction facility might use fsync() to ensure that all changes to a file or files caused by a given transaction were recorded on a storage medium. The manner in which the data reach the physical medium depends on both implementation and hardware. The fsync() function returns when notified by the device driver that the write has taken place. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Committed | +-----------------------------+-----------------------------+ |MT-Level |Async-Signal-Safe | +-----------------------------+-----------------------------+ |Standard |See standards(5). | +-----------------------------+-----------------------------+ SEE ALSO
mount_nfs(1M), read(2), sync(2), write(2), fcntl.h(3HEAD), fdatasync(3C), attributes(5), standards(5) SunOS 5.11 5 Feb 2008 fsync(3C)

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