first commits inodes to buffers, and then buffers to disk.
This function is always successful.
SVr4, 4.3BSD, POSIX.1-2001.
Since glibc 2.2.2 the Linux prototype is as listed above,
following the various standards.
In libc4, libc5, and glibc up to 2.2.1
it was "int sync(void)", and
always returned 0.
According to the standard specification (e.g., POSIX.1-2001),
schedules the writes, but may return before the actual
writing is done.
However, since version 1.3.20 Linux does actually wait.
(This still does not guarantee data integrity: modern disks have