Chapter 1.2 Lock-in

ACompressed recap

Threads share code, globals, heap, files; private stack, registers, PC. User-level threads: fast, all block together; kernel-level: independent, parallel. Models many-to-one, one-to-one, many-to-many. IPC: shared memory (fast, needs sync), message passing (kernel copies, blocking or not), pipes (one-way; named for unrelated processes); producer–consumer waits on full/empty.

BMerged rule card

1.2.1 Threads: what they share, user-level vs kernel-level threads

• Thread = path of execution inside a process; shares code, data, heap, open files; private stack, registers, PC, thread ID.
• Threads are cheaper to create and switch than processes.
• User-level: fast, kernel unaware, one block blocks all, no parallelism. Kernel-level: kernel-scheduled, independent blocking, parallel on multiple cores.
• Models: many-to-one, one-to-one, many-to-many.

1.2.2 Inter-process communication: shared memory, message passing and pipes

• Shared memory: map a common region; fast; programmer synchronizes.
• Message passing: send/receive via the kernel; direct or indirect (mailbox); blocking or non-blocking; works across machines; copies data.
• Ordinary pipe: one-way, parent–child; named pipe (FIFO) for unrelated processes; two-way needs two pipes.
• Producer waits when the buffer is full; consumer waits when empty.

CInterleaved GATE-style questions

Q1 · MCQ · 1 mark(s) — Threads

A process has 4 threads. How many stacks does it have?

1. (A) 1
2. (B) 2
3. (C) 8
4. (D) 4

Q2 · MSQ · 2 mark(s) — Kernel threads

Which are advantages of kernel-level threads over user-level threads?

1. (A) The kernel can schedule each thread separately.
2. (B) Thread switches never involve the kernel.
3. (C) A blocking call blocks only that thread.
4. (D) Threads can run in parallel on multiple CPUs.

Q3 · MCQ · 1 mark(s) — Sharing

Which of these is NOT shared between threads of the same process?

1. (A) Register values
2. (B) Heap memory
3. (C) Code
4. (D) Global variables

Q4 · MCQ · 1 mark(s) — Model

In the many-to-one model, the main drawback is:

1. (A) threads cannot share memory
2. (B) too many kernel threads
3. (C) a blocking system call blocks all threads
4. (D) threads cannot be created

Q5 · MCQ · 1 mark(s) — IPC

Which IPC mechanism requires the processes themselves to handle synchronization?

1. (A) Blocking pipes
2. (B) Mailboxes
3. (C) Message passing
4. (D) Shared memory

Q6 · NAT · 1 mark(s) — Pipes

Minimum number of ordinary pipes for full two-way communication between 2 processes?

Numerical answer: ________

Q7 · MCQ · 1 mark(s) — Named pipes

Two unrelated processes (not parent and child) want to use a pipe. They should use a:

1. (A) shared stack
2. (B) ordinary pipe
3. (C) named pipe (FIFO)
4. (D) thread

Q8 · MCQ · 1 mark(s) — Blocking

A consumer reads from an empty ordinary pipe. It:

1. (A) blocks until data is written
2. (B) terminates the producer
3. (C) reads 0 bytes and crashes
4. (D) reads old data