What is the difference between a process and a thread?

Question

IT Interview Practice · Accepted Answer

**A process is an isolated program with its own memory; a thread is a unit of execution that shares memory with other threads in the same process.** One process can contain many threads.

## How it works

- **Process**: own address space, file descriptors, and heap. Isolated by the OS — a crash or memory bug in one process cannot corrupt another. Context switching and communication (IPC) are relatively expensive.
- **Thread**: lives inside a process, shares its heap and globals, but has its own stack and registers. Cheap to create and switch, but sharing memory means you need synchronization.

```text
Process A                     Process B
├─ heap (shared by threads)   ├─ heap
├─ Thread 1 (own stack)       └─ Thread 1
└─ Thread 2 (own stack)
   ↑ threads share heap → must lock shared data
```

## Example

```python
import threading, multiprocessing
# Threads share 'counter' — needs a lock
counter = 0
threading.Thread(target=lambda: work(counter)).start()

# Processes get separate copies — isolated, pass data via IPC/queue
multiprocessing.Process(target=work, args=(data,)).start()
```

The scheduler assigns CPU time to threads; the OS groups threads under processes for resource ownership and isolation.

## Why it matters

The trade-off drives design choices. Threads are lighter and share data directly but risk race conditions; processes are safer and scale across machines but pay a communication cost. Knowing this explains, for example, why Python uses multiprocessing to sidestep the GIL for CPU work.

How it works

Example

Why it matters

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