Cfrtjryk

Kernel space device drivers usually implement directories and file that show through /sys or /proc. Can the long running user space programs do this as well?

I have a daemon or long running program that needs to be able to be queried for some data and have some data set by external programs while it runs.

I could do a full blown sockets interface, but that's a lot of overhead for the program and the external requestors.

As the linux kernel developers found, using the "everything is a file" model was useful for tweaking kernel setting. I'd like to do the same.

Some may think the /sys directory is the sacred space of the kernel, but I don't see an important line between what is what is the "system" and some other services/servers/applications.

Using FUSE...
I've decided to use FUSE, the 'File system in USErspace' package libfuse3.so.
(After writing a wrapper for it...) I can define an array of structs, one per access variable/file:

struct fileObj files = {

  {"mode", mode, getFunc, putFunc},

  {"numbProcs", numbProcs, getFunc, putFunc},

  {"svrHostPort", hostPort, getFunc, putFunc},

  {"somethingWO", jakeBuf, NULL, putFunc},  // Write only file (why?)

  {"timestamp", NULL, getTimestampFunc, NULL},  // Returns timestamp, R/O

  {0}

};

The mountpoint for the FUSE filesystem is '/ssm/fuse'... The 'ls -l' shows that each entry in the 'files' array shows up as a file, some R/O, some R/W, one W/O. The 'getTimestampFunc in the 'get' function position shows that a special function can be associated with a file to perform calculate repsonses.

ribo@box:~/c$ ls -l /ssm/fuse

total 0

-rw-r--r-- 1 ribo ribo 10 Dec 28 17:17 mode

-rw-r--r-- 1 ribo ribo  1 Dec 28 17:17 numbProcs

--w------- 1 ribo ribo  3 Dec 28 17:17 somethingWO

-rw-r--r-- 1 ribo ribo  5 Dec 28 17:17 svrHostPort

-r--r--r-- 1 ribo ribo 32 Dec 28 17:17 timestamp

ribo@box:~/c$ cat /ssm/fuse/timestamp

18/12/28 17:17:27ribo@box:~/c$cat  /ssm/fuse/mode

hyperSpeedribo@box:~/c$ echo slow >/ssm/fuse/mode

ribo@box:~/c$ cat  /ssm/fuse/mode

slow

The 'echo >' shows passing a value into the program. So its easy for me to peek and poke various parameters of the program as it runs.

I don’t think there’s any way to add /sys or /proc entries outside the kernel. For /sys it wouldn’t make much sense anyway — it’s a direct representation of kobject data structures.

You can however provide similar interfaces from userspace, for example using FIFOs; see mkfifo for details. You can see an implementation of this in sysvinit with its initctl FIFO.

Surely they can. You can mount anything (actual disk filesystems, fuse filesystems, overlayfs, tmpfs, bind mounts, etc) below /sys or /proc, extending and/or replacing the interfaces provided by the kernel.

Whether that's a good idea, it's a completely different matter.

Example:

# unshare -m

# touch /tmp/foo

# mount -B /tmp/foo /proc/1/status

# echo FOR GREAT JUSTICE > /proc/1/status

# cat /proc/1/status