The mincore function requests a vector describing which pages of a file
are in core and can be read without disk access. The kernel will supply data
for length bytes following the start address. On return, the
kernel will have filled vec with bytes, of which the least significant
bit indicates if a page is core resident. (The other bits are undefined,
reserved for possible later use.) Of course this is only a snapshot - pages
that are not locked in core can come and go any moment, and the contents of
vec may be stale already when this call returns.
For mincore to return successfully, start must lie on a page
boundary. It is the caller's responsibility to round up to the nearest page.
The length parameter need not be a multiple of the page size. The
vector vec must be large enough to contain (length+PAGE_SIZE-1) /
PAGE_SIZE bytes. One may obtain the page size from getpagesize(2).