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

Tanaka Akira
2005-08-05T05:48:25Z

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