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The ‘pseudo’ file system

,

In the magic Linux world 🐧 there is one specific entity with the label ‘pseudo file system’. It sits tight and altough is a little bit jealous that it’s not part of the /etc/fstab it’s the kernel best sharing friend and exposes all of its secrets.

After this introduction it’s time to give a down to earth explanation. In Linux you have a directory with the path and name /proc, which for the most part exposes kernel information in user land. Many internal commands that you use like top are actually getting their data from there.

Also for each process that is currently in place you have a lot of information shared. That gives you valuable internal if you are debugging or simply want to understand more.

If you wake up one day really curious about the number of open files a process is currently handling this is the best place to go.


Let’s visit this land with our best below buddy.

import threading
import time
import os
    
print("PID: {0}".format(os.getpid()))
        
def write(filename, n):
    with open(filename, "w") as f:
        f.write("vip msg")
        f.flush()
        time.sleep(100)
    
threads = []
    
for i in range(0, 10):
    filename = "file-{0}".format(str(i))
    t = threading.Thread(target=write, args=(filename, i,))
    threads.append(t)
for t in threads:
    t.start()
for t in threads:
    t.join()


Long story short we are opening 10 files at once, write very important message to each one of them and wait for some time each worker. 

python3 write.py
PID: 16899

Now let’s go to our visit place and see how it’s doing on one of its addresses 16899.

cd /proc/16899/
/proc/16899$ ls
arch_status         exe                mem            personality   status
attr                fd                 mountinfo      projid_map    syscall
autogroup           fdinfo             mounts         root          task
auxv                gid_map            mountstats     sched         timens_offsets
cgroup              io                 net            schedstat     timers
clear_refs          ksm_merging_pages  ns             sessionid     timerslack_ns
cmdline             ksm_stat           numa_maps      setgroups     uid_map
comm                latency            oom_adj        smaps         wchan
coredump_filter     limits             oom_score      smaps_rollup
cpu_resctrl_groups  loginuid           oom_score_adj  stack
cwd                 map_files          pagemap        stat
environ             maps               patch_state    statm

That’s a lot of things to handle. Let’s summarize a few of those that I consider most important, it all really depends on what kind of process you are looking at.

environ - the environment variables initially passed to the process. There are also environment variables which could be later on passed and you would not find them here.

fd – all the file descriptors that the process has opened.

fdinfo – for each opened file descriptors above there is a few fields here, including the ino field which reflects the inode number of the target file.

limits – here you would find how much maximum number of open file descriptors your process might have, as well as the max stack size.

Let’s open the fd and see what is the current state there.

ls -l
total 0
lrwx------ 1 gosho gosho 64 Mar 14 19:27 0 -> /dev/pts/3
lrwx------ 1 gosho gosho 64 Mar 14 19:27 1 -> /dev/pts/3
l-wx------ 1 gosho gosho 64 Mar 14 19:27 10 -> /home/gosho/Blog/02.Python/file-7
l-wx------ 1 gosho gosho 64 Mar 14 19:27 11 -> /home/gosho/Blog/02.Python/file-8
l-wx------ 1 gosho gosho 64 Mar 14 19:27 12 -> /home/gosho/Blog/02.Python/file-9
lrwx------ 1 gosho gosho 64 Mar 14 19:27 2 -> /dev/pts/3
l-wx------ 1 gosho gosho 64 Mar 14 19:27 3 -> /home/gosho/Blog/02.Python/file-0
l-wx------ 1 gosho gosho 64 Mar 14 19:27 4 -> /home/gosho/Blog/02.Python/file-1
l-wx------ 1 gosho gosho 64 Mar 14 19:27 5 -> /home/gosho/Blog/02.Python/file-2
l-wx------ 1 gosho gosho 64 Mar 14 19:27 6 -> /home/gosho/Blog/02.Python/file-3
l-wx------ 1 gosho gosho 64 Mar 14 19:27 7 -> /home/gosho/Blog/02.Python/file-4
l-wx------ 1 gosho gosho 64 Mar 14 19:27 8 -> /home/gosho/Blog/02.Python/file-5
l-wx------ 1 gosho gosho 64 Mar 14 19:27 9 -> /home/gosho/Blog/02.Python/file-6

You could see a set of symbolic links towards those files. We also have started counting from 3, since 0, 1 and 2 are reserved for each process to present the standard input, output and error.

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