2.6 select doesn't notify readablity of /proc/loadavg.

From: Tanaka Akira (akr_at_m17n.org)
Date: 08/05/05

  • Next message: Mukund JB.: "RE: 2.6 partition support driver methods" 
    To: linux-kernel@vger.kernel.org
Date:	Fri, 05 Aug 2005 14:47:59 +0900

    Hi. select in Linux 2.6.12.3 doesn't notify readability of
    /proc/loadavg and several other files under /proc.

    It is reproducible as follows. The sample program follow blocks
    because select doesn't notify readability.

    % uname -a
    Linux debiansarge 2.6.12.3 #1 Thu Aug 4 20:23:16 JST 2005 i686 GNU/Linux
    % cat /proc/version
    Linux version 2.6.12.3 (akr@debiansarge) (gcc version 3.3.5 (Debian 1:3.3.5-13)) #1 Thu Aug 4 20:23:16 JST 2005
    % cat t.c
    #include <stdio.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <sys/select.h>

    int main(int argc, char **argv)
    {
      int fd, ret;
      fd_set readfds;

      fd = open(argv[1], O_RDONLY);
      if (fd == -1) { perror("open"); exit(1); }

      FD_ZERO(&readfds);
      FD_SET(fd, &readfds);
      ret = select(fd+1, &readfds, NULL, NULL, NULL);
      printf("select returns %d\n", ret);
      return 0;
    }

    % gcc t.c
    % ./a.out /proc/loadavg

    This program blocks here.

    /proc/loadavg can be read by cat.

    % ./a.out /proc/loadavg
    ^C
    % cat /proc/loadavg
    0.00 0.00 0.00 2/36 2597

    Also, /proc/cmdline, /proc/devices, /proc/execdomains, /proc/fb,
    /proc/filesystems, /proc/locks, /proc/meminfo, /proc/uptime and
    /proc/version are blocks too.

    Is this behaviour intentional?

    The output of ver_linux is follows.

    % sh scripts/ver_linux
    If some fields are empty or look unusual you may have an old version.
    Compare to the current minimal requirements in Documentation/Changes.
     
    Linux debiansarge 2.6.12.3 #1 Thu Aug 4 20:23:16 JST 2005 i686 GNU/Linux
     
    Gnu C 3.3.5
    Gnu make 3.80
    binutils 2.15
    util-linux 2.12p
    mount 2.12p
    module-init-tools 3.2-pre1
    e2fsprogs 1.37
    reiserfsprogs line
    reiser4progs line
    nfs-utils 1.0.6
    Linux C Library 2.3.2
    Dynamic linker (ldd) 2.3.2
    Procps 3.2.1
    Net-tools 1.60
    Console-tools 0.2.3
    Sh-utils 5.2.1
    Modules Loaded md5 ipv6 evdev ehci_hcd usbcore shpchp pci_hotplug intel_agp piix pcnet32 mii crc32 agpgart ide_disk ide_cd cdrom ide_core rtc unix

    Note that select notify readability of /proc/loadavg on Linux 2.4.20.

    % uname -a
    Linux serein 2.4.20-686 #1 Mon Jan 13 22:22:30 EST 2003 i686 GNU/Linux
    % ./a.out /proc/loadavg
    select returns 1 

    -- 
Tanaka Akira
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  • Next message: Mukund JB.: "RE: 2.6 partition support driver methods"