int fsync(int fildes);
The fsync() function shall request that all data for the open file descriptor named by fildes is to be transferred to the storage device associated with the file described by fildes in an implementation-defined manner. The fsync() function shall not return until the system has completed that action or until an error is detected.
If _POSIX_SYNCHRONIZED_IO is defined, the fsync() function shall force all currently queued I/O operations associated with the file indicated by file descriptor fildes to the synchronized I/O completion state. All I/O operations shall be completed as defined for synchronized I/O file integrity completion.
Upon successful completion, fsync() shall return 0. Otherwise, -1 shall be returned and errno set to indicate the error. If the fsync() function fails, outstanding I/O operations are not guaranteed to have been completed.
The fsync() function shall fail if:
In the event that any of the queued I/O operations fail, fsync() shall return the error conditions defined for read() and write().
The following sections are informative.
The fsync() function should be used by programs which require modifications to a file to be completed before continuing; for example, a program which contains a simple transaction facility might use it to ensure that all modifications to a file or files caused by a transaction are recorded.
The fsync() function is intended to force a physical write of data from the buffer cache, and to assure that after a system crash or other failure that all data up to the time of the fsync() call is recorded on the disk. Since the concepts of "buffer cache", "system crash", "physical write", and "non-volatile storage" are not defined here, the wording has to be more abstract.
If _POSIX_SYNCHRONIZED_IO is not defined, the wording relies heavily on the conformance document to tell the user what can be expected from the system. It is explicitly intended that a null implementation is permitted. This could be valid in the case where the system cannot assure non-volatile storage under any circumstances or when the system is highly fault-tolerant and the functionality is not required. In the middle ground between these extremes, fsync() might or might not actually cause data to be written where it is safe from a power failure. The conformance document should identify at least that one configuration exists (and how to obtain that configuration) where this can be assured for at least some files that the user can select to use for critical data. It is not intended that an exhaustive list is required, but rather sufficient information is provided so that if critical data needs to be saved, the user can determine how the system is to be configured to allow the data to be written to non-volatile storage.
It is reasonable to assert that the key aspects of fsync() are unreasonable to test in a test suite. That does not make the function any less valuable, just more difficult to test. A formal conformance test should probably force a system crash (power shutdown) during the test for this condition, but it needs to be done in such a way that automated testing does not require this to be done except when a formal record of the results is being made. It would also not be unreasonable to omit testing for fsync(), allowing it to be treated as a quality-of-implementation issue.
sync() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>