What is the difference between a process and a thread?

Question

IT Interview Practice · Accepted Answer

A **process** is a running program with its own isolated memory (address space); a **thread** is a unit of execution *inside* a process. One process can hold many threads, and those threads **share** the process's memory — but each process is isolated from every other.

## What each one owns

```text
PROCESS A                          PROCESS B
┌─────────────────────────────┐    ┌───────────────┐
│ address space (code/heap)   │    │ address space │
│ file descriptors, PID       │    │  (isolated)   │
│ ┌────────┐ ┌────────┐       │    │  ┌────────┐   │
│ │thread 1│ │thread 2│  ...  │    │  │thread 1│   │
│ │ stack  │ │ stack  │       │    │  │ stack  │   │
│ │ regs/PC│ │ regs/PC│       │    │  └────────┘   │
│ └────────┘ └────────┘       │    └───────────────┘
│  ▲ shared heap, globals, fds│
└─────────────────────────────┘
```

- **Per process:** address space, open file descriptors, PID — fully isolated by the kernel.
- **Per thread:** its own **stack**, registers, and program counter (PC).
- **Shared among threads in a process:** the heap, global variables, and open files.

## The practical trade-off

- **Threads** are cheap to create and communicate instantly (shared memory) — but a bug in one thread (bad pointer, crash) takes down the whole process, and shared data needs locks.
- **Processes** are heavier and must use IPC to talk, but isolation means one crashing process won't corrupt another — the model browsers use to sandbox tabs.

Context-switching **between threads** of the same process is cheaper than between processes, because the memory map (and its TLB entries) doesn't have to change.

## Why it matters

"Process vs thread" is one of the most common OS interview questions because the answer drives real design choices: threads for shared-memory parallelism within a service, separate processes for fault isolation and security sandboxing. Knowing exactly what is shared (heap, fds) versus private (stack, registers) is also the foundation for understanding race conditions and why synchronization is needed.

What each one owns

The practical trade-off

Why it matters

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