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Subject: 2.6 select doesn't notify readablity of /proc/loadavg.
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From: Tanaka Akira <akr@m17n.org>
Date: Fri, 05 Aug 2005 14:47:59 +0900
Message-ID: <87ll3hszmi.fsf@m17n.org>
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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