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    SubjectRe: [patch 7/8] fdmap v2 - implement sys_socket2
    Kyle Moffett writes:

    > 1) Linear FD allocation makes it IMPOSSIBLE for libraries to
    > reliably use persistent FDs behind an application's back. For

    That's not completely true; for example, openlog() opens a file
    descriptor for the library's own use, as does sethostent(). I agree
    that it creates difficulties if the library implementor wants to use a
    file descriptor in a set of functions that didn't previously use one,
    but with a bit of assistance from the kernel, that can be solved
    without breaking the ABI.

    > for (i = 0; i < NR_OPEN; i++)
    > if (!fd_is_special_to_us(i))
    > close(i);
    >
    > Note that this is conceptually buggy, but occurs in several major C
    > programming books, most of the major shells, and a lot of other
    > software to boot.

    Buggy in what way? In the use of the NR_OPEN constant?

    > 3) In order to allocate new FDs, the kernel has to scan over a
    > (potentially very large) bitmap. A process with 100,000 fds (not
    > terribly uncommon) would have 12.5kbyte of FD bitmap and would trash
    > the cache every time it tried to allocate an FD.

    For specialized programs like that we can offer alternative fd
    allocation strategies if necessary (although I expect that with
    100,000 fds other things will limit performance more than
    allocation).

    None of those things is an excuse for breaking the ABI, however.
    As I said to Davide, I was really protesting about the attitude that
    we can just break the ABI however and whenever we like and force
    programs to adapt.

    Paul.
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