Windows SOCKET is a handler to *kernel* object and as such
constitutes an *offset* in per-process handle table. As this
table can accommodate not more than 2^24 entries, it's always
*safe* to cast/truncate SOCKET to 32-bit value and back. Even on
64-bit Windows.
for reference,
http://msdn2.microsoft.com/en-us/library/ms724485(VS.85).aspx.
This says that there can be no more than 2^24 entries per process,
but it doesn't say that the value of the handle cannot exceed 2^24.
The fact that handle is *index* in a table [which is described in
Windows Internals by Russinovich and Solomon] means that values are
densely packed, and not 2^24 values wildly spread all around. One would
normally expect that they fit in 24 bit, but Windows allow for 2 *least*
significant bits to be used for application specific purposes (can't
find reference), so that value of handle to kernel object fits in 26 bits.
Windows could, for example, use the high bits to indicate what type
of handle it is. (Or might in a future version of Windows, causing
applications that truncate handles to suddenly find they're not as
portable as expected.)
Well, it doesn't and I find it hard to believe it will [kernel knows
better and shall not rely on it in either case, there is no reason to
expose it to user-land]. Secondly, 64-bit Windows allows for 64-bit
application to be executed in none "largeaddressaware" mode. This is to
allow for legacy code which interchangeably uses pointer and 32-bit
integer to be recompiled unmodified for 64-bit ISA and be executed
without risking data corruption. So even if it would use some kind of
decoration, it would be limited to 32-bit anyway [to enable legacy code
to generate revenue, as *they* would put it].
For example, I don't see any reason an LSP couldn't use the high bits
of a SOCKET handle to store some information which it strips off
before it passes the SOCKET value to the underlying kernel code.
Winsock explicitly supports user-level code between the Winsock
functions and the kernel.
LSP is not involved in all socket I/O, most notably not if application
uses [Read|Write]File. LSP is expected to be programmed with this in
mind and therefore can't rely on such decoration [nor substitute SOCKET
with a pointer to user-land object].
Does Microsoft actually specify somewhere that the integer value of a
handle cannot exceed 2^24?
Not in one sentence. A.
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