EN_CS_Process_Thread

Q4 & Q5: Program, Process, and Thread

Question: Explain the concepts of program, process, and thread. Describe the memory structure differences between processes and threads, IPC methods, and how to handle concurrency issues in multithreading.

Key Terms

Term	Description
Program	Static executable file stored on disk
Process	Running instance of a program (independent memory space)
Thread	Lightweight execution unit within a process (shared memory)
IPC	Inter-Process Communication — mechanisms for processes to communicate
Context Switching	CPU switching between running processes or threads
Race Condition	Non-deterministic result when threads concurrently access shared data
Deadlock	Two threads waiting on each other's lock indefinitely
Mutex	Mutual exclusion lock — only one thread enters the critical section at a time

Program → Process → Thread

Program (static file on disk)
    ↓ execute
Process (independent memory space, assigned PID)
    ↓ contains
Thread (shared memory within process, assigned TID)

In summary: A program is code; a process is what happens when the code runs.

Memory Structure

Processes (independent):               Threads (shared):
┌─────────────┐ ┌─────────────┐       ┌──────────────────────────────┐
│  Stack (A)  │ │  Stack (B)  │       │ Stack T1 │ Stack T2 │  ...   │ ← independent
├─────────────┤ ├─────────────┤       ├──────────────────────────────┤
│  Heap  (A)  │ │  Heap  (B)  │       │          Shared Heap          │ ← shared
├─────────────┤ ├─────────────┤       ├──────────────────────────────┤
│  Data  (A)  │ │  Data  (B)  │       │       Shared Data Segment     │ ← shared
├─────────────┤ ├─────────────┤       ├──────────────────────────────┤
│  Code  (A)  │ │  Code  (B)  │       │       Shared Code Segment     │ ← shared
└─────────────┘ └─────────────┘       └──────────────────────────────┘
  Process A       Process B                   Single Process

Process vs Thread Comparison

Aspect	Process	Thread
Memory	Independent (fully isolated)	Shared (only Stack is independent)
Creation Cost	High (fork)	Low (pthread_create)
Communication	Requires IPC	Direct via shared memory
Independence	Fully independent	Crash affects entire process
Context Switch	Slow	Fast
Keyword	Isolation	Concurrency

IPC (Inter-Process Communication)

Method	Characteristics	Speed	Use Case
Pipe	Unidirectional, parent-child	Fast	Simple data transfer
Message Queue	Async message-based	Medium	Async inter-process messaging
Shared Memory	Direct memory sharing	Very fast	Large data sharing
Socket	Network-capable	Slow	Distributed systems

Multithreading Concurrency Issues

Race Condition

Multiple threads access a shared variable simultaneously → result depends on execution order.

Solution: Mutex (Lock) Restrict critical section entry to one thread at a time.

Deadlock

Thread A holds Lock1 and waits for Lock2; Thread B holds Lock2 and waits for Lock1 → infinite wait.

Solution: Lock Ordering Enforce all threads to acquire locks in the same order.

Concurrency vs Parallelism

Concept	Description
Concurrency	Multiple tasks interleaved (possible on 1 CPU)
Parallelism	Multiple tasks truly simultaneous (requires multiple CPUs)

Reference

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