There is no portable way. Under some versions of Unix you can call statfs. Under
MS-DOS, use interrupt 0x21 subfunction 0x36, or perhaps a routine such as diskfree.
Another possibility is to use popen to invoke and read the output of a ``disk free''
command (df on Unix).
(Note that the amount of free space apparently available on a disk may not match
the size of the largest file you can store, for all sorts of reasons.)