From feedback it is now clear that when opening a font file using TTF_OpenFont
the file on disk does not remain open. Therefore once the font has been opened it is in memory for use as normal and can then be released with TTF_CloseFont
when no longer required.
The initial question was a result of confusion around the file remaining open (or not as it turned out) after the call to TTF_OpenFont
Credit to @Armin for this