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fsync(2) [netbsd man page]

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

NAME
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)						      BSD System Calls Manual							  FSYNC(2)

NAME
fsync -- synchronize a file's in-core state with that on disk SYNOPSIS
#include <unistd.h> int fsync(int fildes); DESCRIPTION
fsync() causes all modified data and attributes of fildes to be moved to a permanent storage device. This normally results in all in-core modified copies of buffers for the associated file to be written to a disk. Note that while fsync() will flush all data from the host to the drive (i.e. the "permanent storage device"), the drive itself may not physi- cally write the data to the platters for quite some time and it may be written in an out-of-order sequence. Specifically, if the drive loses power or the OS crashes, the application may find that only some or none of their data was written. The disk drive may also re-order the data so that later writes may be present, while earlier writes are not. This is not a theoretical edge case. This scenario is easily reproduced with real world workloads and drive power failures. For applications that require tighter guarantees about the integrity of their data, Mac OS X provides the F_FULLFSYNC fcntl. The F_FULLFSYNC fcntl asks the drive to flush all buffered data to permanent storage. Applications, such as databases, that require a strict ordering of writes should use F_FULLFSYNC to ensure that their data is written in the order they expect. Please see fcntl(2) for more detail. RETURN VALUES
The fsync() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ERRORS
The fsync() system call will fail if: [EBADF] fildes is not a valid descriptor. [EINTR] Its execution is interrupted by a signal. [EINVAL] fildes refers to a file type (e.g., a socket) that does not support this operation. [EIO] An I/O error occurred while reading from or writing to the file system. If a queued I/O operation fails, fsync() may fail with any of the errors defined for read(2) or write(2). SEE ALSO
fcntl(2), read(2), sync(2), write(2), sync(8), update(8) HISTORY
The fsync() function call appeared in 4.2BSD. 4.2 Berkeley Distribution June 4, 1993 4.2 Berkeley Distribution

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