Can't read a specific file under /sys filesystem

Question

While searching for ways to obtain various WiFi information, I found /sys/class/net/wlan7/speed file. Problem is, even though all read permissions are granted, I can't read it.

$ ls -l /sys/class/net/wlan7/speed                                                                                       
-r--r--r-- 1 root root 4096 3月  24 15:08 /sys/class/net/wlan7/speed

$ sudo cat /sys/class/net/wlan7/speed                                                                                    
[sudo] password for xieerqi: 
cat: /sys/class/net/wlan7/speed: Invalid argument

I can stat the file, I can list with ls, but every time I try to read it ( with hexdump, with cat, with Python in binary reading mode, etc. ) it shows invalid argument error.

strace shows that error apparently occurs at this point:

read(3, 0x7f6ad6b44000, 131072)         = -1 EINVAL (Invalid argument)
write(2, "cat: ", 5cat: )                    = 5
write(2, "/sys/class/net/wlan7/speed", 26/sys/class/net/wlan7/speed) = 26
open("/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 4

( link to full trace paste: http://paste.ubuntu.com/24252504/ )

By contrast , the corresponding file for the ethernet interface works:

$ cat /sys/class/net/eth3/speed                              
10

So basically my question is this: how do I read this file ?

OS: Ubuntu 16.04 LTS, kernel 4.4.0-67-generic , AR9565 WiFi adapter

Thomas · Accepted Answer · 2017-03-26 10:15:59Z

8

The file /sys/class/net/<iface>/speed is only valid for devices that support ethtool get_settings method, which mostly applies to Ethernet adapters.

To get the speed of a WiFi device, you can use iwconfig or iwlist.

iwconfig wlan7
iwlist wlan7 rate

From the Kernel documentation:

What:       /sys/class/net/<iface>/speed
...  
Description:
        Indicates the interface latest or current speed value. Value is
        an integer representing the link speed in Mbits/sec.

        Note: this attribute is only valid for interfaces that implement
        the ethtool get_settings method (mostly Ethernet ).

answered Mar 26, 2017 at 10:15

Thomas

6,6428 gold badges31 silver badges34 bronze badges

2

One small request: can you provide the link to the kernel documentation from which the quote comes ? I would like to read more there

Sergiy Kolodyazhnyy
– Sergiy Kolodyazhnyy

2017-03-26 10:59:10 +00:00
Commented Mar 26, 2017 at 10:59

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