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    From
    SubjectRe: [PATCH] nextfd(2)
    Date
    "H. Peter Anvin" <hpa@zytor.com> writes:

    > On 04/06/2012 02:54 AM, Alexey Dobriyan wrote:
    >>
    >> Without proc knowledge about fdtable is gathered linearly and still unreliable.
    >> With nextfd(2), even procful environments could lose several failure branches.
    >> And they can keep old dumb fd++ or smart /proc/self/fd loops for a change.
    >>
    >
    > Incidentally, if we were to create a system call for this -- which I so
    > far see no reason for -- I would make it return a select-style bitmask
    > of file descriptors in use, not a "next fd" which would require a system
    > call per iteration.

    It's already possible to do something a little like that with the
    existing "poll" system call:

    #include <stdio.h>
    #include <sys/poll.h>

    int
    main(void)
    {
    enum { N_FDS = 1024 };
    struct pollfd fds[N_FDS];
    int error;
    int i;

    for (i = 0; i < N_FDS; i++)
    {
    fds[i].fd = i;
    fds[i].events = 0;
    }

    error = poll(fds, N_FDS, 0);
    if (error < 0)
    perror("poll");
    else
    {
    printf("valid fds:");
    for (i = 0; i < N_FDS; i++)
    if (!(fds[i].revents & POLLNVAL))
    printf(" %d", fds[i].fd);
    printf("\n");
    }

    return 0;
    }

    blp@blp:~/tmp(0)$ gcc tmp.c -Wall
    blp@blp:~/tmp(0)$ ./a.out
    valid fds: 0 1 2
    blp@blp:~/tmp(0)$ ./a.out 5>/dev/null
    valid fds: 0 1 2 5
    blp@blp:~/tmp(0)$ ./a.out 5>/dev/null 3</dev/stdin
    valid fds: 0 1 2 3 5
    blp@blp:~/tmp(0)$

    --
    Ben Pfaff
    http://benpfaff.org


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