How can I get the Linux PID for a thread? - linux

Question

I'm interested in obtaining the PID of a thread created inside a Rust program. As stated in the documentation, thread::id() does not work for this purpose. I found Get the current thread id and process id as integers? that seemed like the answer, but my experiments show it doesn't work.
This is the code:
extern crate rand;
extern crate libc;
use std::thread::{self, Builder};
use std::process::{self, Command};
use rand::thread_rng;
use rand::RngCore;
use std::time::Duration;
use std::os::unix::thread::JoinHandleExt;
use libc::pthread_t;
fn main() {
let main_pid = process::id();
println!("This PID {}", main_pid);
let b = Builder::new().name(String::from("LongRunningThread")).spawn(move || {
let mut rng = thread_rng();
let spawned_pid = process::id();
println!("Spawned PID {}", spawned_pid);
loop {
let u = rng.next_u64() % 1000;
println!("Processing request {}", u);
thread::sleep(Duration::from_millis(u));
}
}).expect("Could not spawn worker thread");
let p_threadid : pthread_t = b.as_pthread_t();
println!("Spawned p_threadid {}", p_threadid);
let thread_id = b.thread().id();
println!("Spawned thread_id {:?}", thread_id);
thread::sleep(Duration::from_millis(60_000));
}
The output from running the program inside a Linux machine is the following:
This PID 8597
Spawned p_threadid 139858455706368. <-- Clearly wrong
Spawned thread_id ThreadId(1) <-- Clearly wrong
Spawned PID 8597
Processing request 289
Processing request 476
Processing request 361
Processing request 567
The following is an excerpt from the output of htop in my system:
6164 1026 root 20 0 98M 7512 6512 S 0.0 0.0 0:00.03 │ ├─ sshd: dash [priv]
6195 6164 dash 20 0 98M 4176 3176 S 0.0 0.0 0:00.20 │ │ └─ sshd: dash#pts/11
6196 6195 dash 20 0 22964 5648 3408 S 0.0 0.0 0:00.09 │ │ └─ -bash
8597 6196 dash 20 0 2544 4 0 S 0.0 0.0 0:00.00 │ │ └─ ./process_priorities
8598 6196 dash 20 0 2544 4 0 S 0.0 0.0 0:00.00 │ │ └─ LongRunningThre
The PID I want from the spawned thread is 8598, but I can't figure out how to obtain it in a Rust program. Any ideas?

Answer 1

I found the answer using an existing crate called Palaver. It includes a gettid() that works across platforms. The only caveat is that the default configuration of the crate uses nightly features, so if you are on stable, make sure to disable them like this palaver = { version = "*", default-features = false }
Now, when I run the code that uses gettid(), this is the output:
This PID 9009
Priority changed for thread
Spawned PID 9010
Processing request 803
Processing request 279
Processing request 624
And the output from my htop:
6164 1026 root 20 0 98M 7512 6512 S 0.0 0.0 0:00.03 │ ├─ sshd: dash [priv]
6195 6164 dash 20 0 98M 4176 3176 S 0.0 0.0 0:00.21 │ │ └─ sshd: dash#pts/11
6196 6195 dash 20 0 22964 5648 3408 S 0.0 0.0 0:00.10 │ │ └─ -bash
9009 6196 dash 20 0 2544 4 0 S 0.0 0.0 0:00.00 │ │ └─ ./process_priorities
9010 6196 dash 20 0 2544 4 0 S 0.0 0.0 0:00.00 │ │ └─ LongRunningThre