Help understanding lsof and threads

Question

When i use -p $PID on lsof i get only the immediate thread of the process. why and how to work with threads and lsof effectively?

sudo lsof -p 5344 > lsof-p5344.log
sudo lsof | grep  '\<5344\>' > lsofgrep5344.log

$ wc -l lsof*5344.log
  1,120,208 lsofgrep5344.log
      2,166 lsof-p5344.log

Those are lines for the exact process. This is not about children process. Here I am using awk to filter by exactly the process PID numbers (Second column of lsof output) being on field two of the output.

$ cat lsof-p5344.log  | awk '{print $2}' | grep '^5344$' | wc -l
2,165

$ cat lsofgrep5344.log  | awk '{print $2}' | grep '^5344$' | wc -l
1,120,207

now if i filter by the first column "COMMAND", then i see different threads of the java application, while the lsof -p one will only show the main thread (e.g. "COMMAND=java_daemon"), the lsof | grep will have many (e.g. "COMMAND=pool123, JMX, Java2D, Quartz, etc", with "PID" all equal to 5344)

Answer 1

On my system, lsof -k displays thread numbers. But I guess the ressources (files, memory maps...) are allocated for the whole process (A file opened by a thread can be used by an other), then digging to the thread doesn't seem pertinent.

On my Linux system, I create a thread which terminates immediately after opening a file. lsof continues to see the file while the thread which had opened it is not there anymore.