功能描述：

　　同步内存中所有已修改的文件数据到储存设备。

　　用法：

　　#include

　　int fsync(int fd);

　　参数：

　　fd：文件描述词。

　　返回说明：

　　成功执行时，返回0。失败返回-1，errno被设为以下的某个值

　　EBADF： 文件描述词无效

　　EIO ： 读写的过程中发生错误

　　EROFS, EINVAL：文件所在的文件系统不支持同步

　　强制把系统缓存写入文件sync和fsync函数,, fflush和fsync的联系和区别2010-05-10 11:25传统的U N I X实现在内核中设有缓冲存储器，大多数磁盘I / O都通过缓存进行。当将数据写

　　到文件上时，通常该数据先由内核复制到缓存中，如果该缓存尚未写满，则并不将其排入输出

　　队列，而是等待其写满或者当内核需要重用该缓存以便存放其他磁盘块数据时，再将该缓存排

　　入输出队列，然后待其到达队首时，才进行实际的I / O操作。这种输出方式被称之为延迟写

　　(delayed write)(Bach 〔1 9 8 6〕第3章详细讨论了延迟写)。延迟写减少了磁盘读写次数，但是

　　第4章文件和目录8 7

　　下载

　　却降低了文件内容的更新速度，使得欲写到文件中的数据在一段时间内并没有写到磁盘上。当

　　系统发生故障时，这种延迟可能造成文件更新内容的丢失。为了保证磁盘上实际文件系统与缓

　　存中内容的一致性，U N I X系统提供了s y n c和f s y n c两个系统调用函数。

　　#include

　　void sync(void);

　　int fsync(intf i l e d e s) ;

　　返回：若成功则为0，若出错则为-1

　　s y n c只是将所有修改过的块的缓存排入写队列，然后就返回，它并不等待实际I / O操作结束。

　　系统精灵进程(通常称为u p d a t e )一般每隔3 0秒调用一次s y n c函数。这就保证了定期刷新内

　　核的块缓存。命令s y n c ( 1 )也调用s y n c函数。

　　函数f s y n c只引用单个文件(由文件描述符f i l e d e s指定)，它等待I / O结束，然后返回。f s y n c可

　　用于数据库这样的应用程序，它确保修改过的块立即写到磁盘上。比较一下f s y n c和O _ S Y N C标

　　志(见3 . 1 3节)。当调用f s y n c时，它更新文件的内容，而对于O _ S Y N C，则每次对文件调用w r i t e

　　函数时就更新文件的内容。

　　fflush和fsync的联系和区别

　　[zz ] http://blog.chinaunix.net/u2/73874/showart_1421917.html

　　1.提供者fflush是libc.a中提供的方法，fsync是系统提供的系统调用。2.原形fflush接受一个参数FILE *.fflush(FILE *);fsync接受的时一个Int型的文件描述符。fsync(int fd);3.功能fflush:是把C库中的缓冲调用write函数写到磁盘[其实是写到内核的缓冲区]。fsync：是把内核缓冲刷到磁盘上。

　　c库缓冲-----fflush---------〉内核缓冲--------fsync-----〉磁盘

　　再转一篇英文的

　　Write-back support

　　UBIFS supports write-back, which means that file changes do not go to the flash media straight away, but they are cached and go to the flash later, when it is absolutely necessary. This helps to greatly reduce the amount of I/O which results in better performance. Write-back caching is a standard technique which is used by most file systems like ext3 or XFS.

　　In contrast, JFFS2 does not have write-back support and all the JFFS2 file system changes go the flash synchronously. Well, this is not completely true and JFFS2 does have a small buffer of a NAND page size (if the underlying flash is NAND). This buffer contains last written data and is flushed once it is full. However, because the amount of cached data are very small, JFFS2 is very close to a synchronous file system.

　　Write-back support requires the application programmers to take extra care about synchronizing important files in time. Otherwise the files may corrupt or disappear in case of power-cuts, which happens very often in many embedded devices. Let's take a glimpse at Linux manual pages:

　　$ man 2 write

　　....

　　NOTES

　　A successful return from write() does not make any guarantee that data

　　has been committed to disk. In fact, on some buggy implementations, it

　　does not even guarantee that space has successfully been reserved for

　　the data. The only way to be sure is to call fsync(2) after you are

　　done writing all your data.

　　...

　　This is true for UBIFS (except of the "some buggy implementations" part, because UBIFS does reserves space for cached dirty data). This is also true for JFFS2, as well as for any other Linux file system.

　　However, some (perhaps not very good) user-space programmers do not take write-back into account. They do not read manual pages carefully. When such applications are used in embedded systems which run JFFS2 - they work fine, because JFFS2 is almost synchronous. Of course, the applications are buggy, but they appear to work well enough with JFFS2. But the bugs show up when UBIFS is used instead. Please, be careful and check/test your applications with respect to power cut tolerance if you switch from JFFS2 to UBIFS. The following is a list of useful hints and advices.

　　If you want to switch into synchronous mode, use the -o sync option when mounting UBIFS; however, the file system performance will drop - be careful; Also remember that UBIFS mounted in synchronous mode provides less guarantees than JFFS2 - refer this section for details.

　　Always keep in mind the above statement from the manual pages and run fsync() for all important files you change; of course, there is no need to synchronize "throw-away" temporary files; Just think how important is the file data and decide; and do not use fsync() unnecessarily, because this will hit the performance;

　　If you want to be more accurate, you may use fdatasync(), in which cases only data changes will be flushed, but not inode meta-data changes (e.g., "mtime" or permissions); this might be more optimal than using fsync() if the synchronization is done often, e.g., in a loop; otherwise just stick with fsync();

　　In shell, the sync command may be used, but it synchronizes whole file system which might be not very optimal; and there is a similar libc sync() function;

　　You may use the O_SYNC flag of the open() call; this will make sure all the data (but not meta-data) changes go to the media before the write() operation returns; but in general, it is better to use fsync(), because O_SYNC makes each write to be synchronous, while fsync() allows to accumulate many writes and synchronize them at once;

　　It is possible to make certain inodes to be synchronous by default by setting the "sync" inode flag; in a shell, the chattr +S command may be used; in C programs, use the FS_IOC_SETFLAGS ioctl command; Note, the mkfs.ubifs tool checks for the "sync" flag in the original FS tree, so the synchronous files in the original FS tree will be synchronous in the resulting UBIFS image.

　　Let us stress that the above items are true for any Linux file system, including JFFS2.

　　fsync() may be called for directories - it synchronizes the directory inode meta-data. The "sync" flag may also be set for directories to make the directory inode synchronous. But the flag is inherited, which means all new children of this directory will also have this flag. New files and sub-directories of this directory will also be synchronous, and their children, and so forth. This feature is very useful if one needs to create a whole sub-tree of synchronous files and directories, or to make all new children of some directory to be synchronous by default (e.g., /etc).

　　The fdatasync() call for directories is "no-op" in UBIFS and all UBIFS operations which change directory entries are synchronous. However, you should not assume this for portability (e.g., this is not true for ext2). Similarly, the "dirsync" inode flag has no effect in UBIFS.

　　The functions mentioned above work on file-descriptors,