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.)

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