To ::close() or to ::fclose()?

To ::close() or to ::fclose()?

We see a strange issue sometimes when we call boost::filesystem::copy() to
copy a file from a normal local partition to one hosted on Lustre.
Normally if we do cp of files we notice that the destination file exists
correctly and is available immediately after cp returns, however with the
boost operation, when it returns, the file may be all there or not (at the
destination.)
Looking through the boost code, I see:
if ( ::close( infile) < 0 ) sz_read = -1;
if ( ::close( outfile) < 0 ) sz_read = -1;
I'm wondering if this is correct, will this correctly flush the file to
the destination, or should this call ::fclose() to explicitly flush and
then close the file? I don't see any explicit ::fflush() calls preceding
the close, so not sure if the file really is flushed to the destination
correctly...