Relation between Thread ID and Process ID

Question

I am having some confusion between Process Id and Thread Id. I have gone through several web-post including stack overflow here, Which says

starting a new process gives you a new PID and a new TGID, while starting a new thread gives you a new PID while maintaining the same TGID.

So when I run a program why all the threads created from the program don't have different PID?

I know in programming we usually say that the main is a thread and execution starts from main , So if I create multiple thread from main, all the threads will have the same PID which is equal to the main's PID.

So what I wanted to ask is as below:

1) When we run a program it will run as a process or a thread?

2) Is there any difference between main thread creating threads and Process creating threads?

3) Is there any difference between thread and process in linux? Since I read somewhere that linux doesn't differentiate between Thread and Process.

Answer 1

This confusion comes from the Linux concept of tasks.

In Linux there is little difference between a task and a thread though.

Every process is a self contained VM running at least one task.

Each task is an independent execution unit within a process scope.

The main task of a process gives it's task id (TID) to the process as it's process id (PID).

Every new thread that you spawn within a process creates a new task within it. In order to identify then individually in the kernel they get assigned their own individual task id (TID).

All tasks within a process share the same task group id (TGID).

Answer 2

Simplifying a bit:

1. The PID is the process ID, TID is the thread ID. The thing is that for the first thread created by fork(), PID=TID. If you create more threads within the process, with a clone() command, then PID and TID will be different, PID will always be smaller than TID.

2. No, there is no difference, except maybe that if main is killed, all other threads are also killed.

3. Yes, the thread is what actually gets scheduled. Technically, the process is only a memory mapping of the different segments of code (text, bss, stack, heap and the OS).