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To my surprise, the st_size field *is* undefined for
devices, according to
http://www.opengroup.org/onlinepubs/009695399/basedefs/sys/stat.h.html
My Linux consistently gives 0 for device nodes, but I guess
Unix might not.

jafo, loweis: read() from /dev/zero is special cased in
linux.  In 2.4.20-8, function read_zero_pagealined, a
comment says "for private mappings, just map in zero pages",
which will be very fast and not actually write any memory or
blocks of swap.