About the only one is the underlying OS file descriptor; GHC's runtime implements its own native file handles, it does not wrap C stdio, and you cannot round-trip between a stdio (FILE *) and a GHC Handle.

Using the file descriptor should work on both POSIX and Windows, as long as it's an ordinary file (that is, not a device, fifo, socket, directory (netbsd/freebsd), mailbox, etc.). You will need to use fdToHandle / handleToFd on the Haskell side and fdopen()/fileno() on the C side; the Haskell ones will ensure the buffers are properly dealt with, the C ones you must make sure to do so yourself.
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