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But code outside the standard library can’t do that, can it? fopen returns a pointer to a FILE, and you can’t know how a struct FILE should be copied.

You can’t just memcpy the bits and then mix calls to fread using pointers to the old and the new FILE struct, for example. I think the standard library need not even support calls using a pointer to a FILE struct it didn’t create.

You can’t just memcpy the bits...

You certainly shouldn't, but sadly this is something which people do.

Are there any POSIX or ISO guarantees on "FILE"? I think it's safe to assume that it isn't an incomplete type, but all functions that use it operate on pointers anyway. Storing a copy of a "FILE" object might result in each copy pointing to the same underlying file handle but having different internal state.

It happened to microsoft a while ago, they changed something in FILE and something else broke, so they went for opaque FILE.