[SOLVED] reading from hwmon temperature sensor 103

igok99 · 08-15-2019, 07:41 AM

hi,
i'm trying to write a driver which reads from "/sys/class/hwmon/hwmon0/temp1_input" on user space.
The first read is successful, but then from the second read I get only zero values.
Reading from the FD works only when closing and reopening the FD again.

is there a way/example how to read from hwmon without reopening & closing each time the FD?

thanks.

pan64 · 08-15-2019, 09:16 AM

is there a way to post your code?

BW-userx · 08-15-2019, 09:33 AM

Code:

while true
do
 cat  /sys/class/hwmon/hwmon0/temp1_input
sleep 1
done

??

gkrellm has a means to get and keep track of temps, open source, maybe take a peak at how they do it?

igok99 · 08-17-2019, 02:03 PM

this is my code:
fdHwmon0 = open("/sys/class/hwmon/hwmon0/temp1_input", O_RDONLY);
read(fdHwmon0, buf, 6);
close(fdHwmon0);

i'm trying to avoid opening and closing on every read.

BW-userx · 08-17-2019, 02:12 PM

again, I suggest looking at other apps that tap into the temps of systems and see how they are doing it so you can get a better understanding of what needs to be done, and how.
see post #3

do a little research

igok99 · 08-17-2019, 02:17 PM

ok thanks!

pan64 · 08-18-2019, 02:43 AM

Quote:

Originally Posted by igok99

i'm trying to avoid opening and closing on every read.

Why? I think this is not a real file, but something generated during open (dynamically). So there is no other way. But probably I missed something.

igok99 · 08-18-2019, 06:06 AM

Quote:

Originally Posted by pan64

Why? I think this is not a real file, but something generated during open (dynamically). So there is no other way. But probably I missed something.

So there is no other way- no other way but to open and close the fd on every read? why?

allend · 08-18-2019, 07:56 AM

From 'man 5 sysfs'

Quote:

The sysfs filesystem is a pseudo-filesystem which provides an interface to kernel data structures. (More precisely, the files and directories in sysfs provide a view of the kobject structures defined internally within the kernel.) The files under sysfs provide information about devices, kernel modules, filesystems, and other kernel components.

and from the kernel source file /Documentation/filesystems/sysfs.txt mentioned in the SEE ALSO section of the man page

Quote:

Attributes can be exported for kobjects in the form of regular files in the filesystem. Sysfs forwards file I/O operations to methods defined for the attributes, providing a means to read and write kernel attributes.

In line with the *nix philosophy that everything is a file, the information is returned on reading the file. The file needs to be read again to get updated data.

jpollard · 08-18-2019, 08:06 AM

Quite likely to read again you would have to seek to the beginning of the file.

The lm_sensors does open and close the file, but it appears to only read it once and then exits.

pan64 · 08-18-2019, 10:44 AM

yes, you can try to seek, but if you want to understand it you need to read the documentation starting for example here: https://www.mjmwired.net/kernel/Docu.../sysfs-api.txt (or the source code)

rtmistler · 08-18-2019, 11:17 AM

Using pan64's reference, I'd instead not try to read a /sys file and instead try to register a callback or obtain the actual structure from the driver, which contains the relevant information you need.

allend · 08-18-2019, 07:18 PM

From the kernel source file /Documentation/filesystems/sysfs.txt with emphasis added by me.

Quote:

sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the
method. Sysfs will call the method exactly once for each read or
write. This forces the following behavior on the method
implementations:

- On read(2), the show() method should fill the entire buffer.
Recall that an attribute should only be exporting one value, or an
array of similar values, so this shouldn't be that expensive.

This allows userspace to do partial reads and forward seeks
arbitrarily over the entire file at will. If userspace seeks back to
zero or does a pread(2) with an offset of '0' the show() method will
be called again, rearmed, to fill the buffer.

igok99 · 08-26-2019, 03:38 PM

pread did the job..

thanks for help!