Explain the difference between a process and a thread.

A process and a thread are both units of execution in a computer system, but they have some fundamental differences.

1. Definition:
- A process is an instance of a program that is being executed. It has its own memory space, resources, and state. It can be considered as an independent program running on the operating system.
- A thread, on the other hand, is a subset of a process. It is a lightweight unit of execution within a process. Multiple threads can exist within a single process, and they share the same memory space and resources of that process.

2. Memory and Resources:
- Each process has its own memory space, which means that processes do not share memory with other processes. They have separate address spaces, and communication between processes requires inter-process communication mechanisms like pipes or sockets.
- Threads, on the other hand, share the same memory space within a process. They can directly access and modify the memory of other threads within the same process. This shared memory allows for efficient communication and data sharing between threads.

3. Creation and Termination:
- Processes are created and terminated independently. Each process has its own process control block (PCB) that contains information about the process, such as its state, program counter, and register values. Processes can be created by the operating system or by other processes using system calls like fork().
- Threads, on the other hand, are created within a process. When a process is created, it starts with a single thread called the main thread. Additional threads can be created within the process using threading libraries or language-specific constructs. Threads are terminated when they complete their execution or when explicitly terminated by the program.

4. Scheduling and Context Switching:
- Processes are scheduled and executed independently by the operating system. The operating system allocates CPU time to each process based on scheduling algorithms. Context switching between processes involves saving and restoring the entire process state, which is a relatively expensive operation.
- Threads, on the other hand, are scheduled and executed within a process. The operating system schedules threads based on thread-specific scheduling algorithms. Context switching between threads within the same process is faster and less expensive compared to context switching between processes since threads share the same memory space.

5. Fault Isolation:
- Processes provide a higher level of fault isolation. If one process crashes or encounters an error, it does not affect other processes. Each process runs in its own protected memory space, ensuring that a failure in one process does not impact the stability of other processes.
- Threads, on the other hand, share the same memory space within a process. If one thread encounters an error or crashes, it can potentially affect the stability of other threads within the same process.

In summary, a process is an independent program with its own memory space and resources, while a thread is a lightweight unit of execution within a process that shares the same memory space and resources. Processes provide better fault isolation, while threads allow for efficient communication and data sharing within a process.