This document defines the coverage for the fsync() function:

Assertion	Tested?
1		NO
2		NO
3		NO ** Synchronized I/O
4		YES
5		YES
6		NO
7		YES
8		NO: ** Not sure how to generate an I/O error
9		NO: ** Synchronized I/O

NOTE:

Assertion 1 is hard to test. Here cited the specification:

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.