FAQ 5.32 How can I tell whether there's a character waiting on a filehandle?

Discussion in 'Perl Misc' started by PerlFAQ Server, Apr 6, 2011.

  1. This is an excerpt from the latest version perlfaq5.pod, which
    comes with the standard Perl distribution. These postings aim to
    reduce the number of repeated questions as well as allow the community
    to review and update the answers. The latest version of the complete
    perlfaq is at http://faq.perl.org .


    5.32: How can I tell whether there's a character waiting on a filehandle?

    The very first thing you should do is look into getting the
    Term::ReadKey extension from CPAN. As we mentioned earlier, it now even
    has limited support for non-portable (read: not open systems, closed,
    proprietary, not POSIX, not Unix, etc.) systems.

    You should also check out the Frequently Asked Questions list in
    comp.unix.* for things like this: the answer is essentially the same.
    It's very system dependent. Here's one solution that works on BSD

    sub key_ready {
    my($rin, $nfd);
    vec($rin, fileno(STDIN), 1) = 1;
    return $nfd = select($rin,undef,undef,0);

    If you want to find out how many characters are waiting, there's also
    the FIONREAD ioctl call to be looked at. The *h2ph* tool that comes with
    Perl tries to convert C include files to Perl code, which can be
    "require"d. FIONREAD ends up defined as a function in the *sys/ioctl.ph*

    require 'sys/ioctl.ph';

    $size = pack("L", 0);
    ioctl(FH, FIONREAD(), $size) or die "Couldn't call ioctl: $!\n";
    $size = unpack("L", $size);

    If *h2ph* wasn't installed or doesn't work for you, you can *grep* the
    include files by hand:

    % grep FIONREAD /usr/include/*/*
    /usr/include/asm/ioctls.h:#define FIONREAD 0x541B

    Or write a small C program using the editor of champions:

    % cat > fionread.c
    #include <sys/ioctl.h>
    main() {
    printf("%#08x\n", FIONREAD);
    % cc -o fionread fionread.c
    % ./fionread

    And then hard code it, leaving porting as an exercise to your successor.

    $FIONREAD = 0x4004667f; # XXX: opsys dependent

    $size = pack("L", 0);
    ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
    $size = unpack("L", $size);

    FIONREAD requires a filehandle connected to a stream, meaning that
    sockets, pipes, and tty devices work, but *not* files.


    The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
    are not necessarily experts in every domain where Perl might show up,
    so please include as much information as possible and relevant in any
    corrections. The perlfaq-workers also don't have access to every
    operating system or platform, so please include relevant details for
    corrections to examples that do not work on particular platforms.
    Working code is greatly appreciated.

    If you'd like to help maintain the perlfaq, see the details in
    PerlFAQ Server, Apr 6, 2011
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