What is process and thread?

Question

Yes, I have read many materials related to operating system. And I am still reading. But it seems all of them are describing the process and thread in a "abstract" way, which makes a lot of high level elabration on their behavior and logic orgnization. I am wondering what are they physically? In my opinion, they are just some in-memory "data structures" which are maintained and used by the kernel codes to facilitate the execution of program. For example, operating system use some process data structure (PCB) to describe the aspects of the process assigned for a certain program, such as its priority, its address space and so on. Is this all right?

Answer 1

First thing you need to know to understand the difference between a process and a thread, is a fact, that processes do not run, threads do.

So, what is a thread? Closest I can get explaining it is an execution state, as in: a combination of CPU registers, stack, the lot. You can see a proof of that, by breaking in a debugger at any given moment. What do you see? A call stack, a set of registers. That's pretty much it. That's the thread.

Now, then, what is a process. Well, it's a like an abstract "container" entity for running threads. As far as OS is concerned in a first approximation, it's an entity OS allocates some VM to, assigns some system resources to (like file handles, network sockets), &c.

How do they work together? The OS creates a "process" by reserving some resources to it, and starting a "main" thread. That thread then can spawn more threads. Those are the threads in one process. They more or less can share those resources one way or another (say, locking might be needed for them not to spoil the fun for others &c). From there on, OS is normally responsible for maintaining those threads "inside" that VM (detecting and preventing attempts to access memory which doesn't "belong" to that process), providing some type of scheduling those threads, so that they can run "one-after-another-and-not-just-one-all-the-time".

Answer 2

Normally when you run an executable like notepad.exe, this creates a single process. These process could spawn other processes, but in most cases there is a single process for each executable that you run. Within the process, there can be many threads. Usually at first there is one thread, which usually starts at the programs "entry point" which is the main function usually. Instructions are executed one by one in order, like a person who only has one hand, a thread can only do one thing at a time before it moves on to the next.

That first thread can create additional threads. Each additional thread has it's own entry point, which is usually defined with a function. The process is like a container for all the threads that have been spawned within it.

That is a pretty simplistic explanation. I could go into more detail but probably would overlap with what you will find in your textbooks.

EDIT: You'll notice there are lot's of "usually"'s in my explanation, as there are occasionally rare programs that do things drastically different.