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fsync(2) [mojave man page]

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

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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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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