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Add Preemption Using SIGALRM to Switch User-Space Threads

hard3/10

Implement Mutex Using Compare-And-Swap (CAS)Implement a Semaphore Using Futex

Language

Compiler

2.0s Limit 256MB

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Implementation

Implement preemptive scheduling for green threads using SIGALRM.

Requirements:

Implement SIGALRM signal handler
Set up interval timer with setitimer(ITIMER_REAL)
Handler calls thread_yield() to force context switch
Use separate signal stack with sigaltstack()
Ensure handler is async-signal-safe
Restart timer after context switch
Implement signal-safe thread yield
Prevent reentrant handler calls (block SIGALRM during yield)
Test with CPU-intensive thread (prevents starvation)
Show preemption working with time quanta

Test Scenarios:

CPU-intensive thread gets preempted
Multiple threads get fair CPU time
Timer fires at regular intervals
No thread monopolizes CPU
Signal safety maintained

Why This Matters

This is how early user-level threading libraries like GNU Pth added preemption on top of cooperative scheduling, and the async-signal-safety constraint you'll hit is a real, frequently-violated rule: calling malloc() or printf() inside a signal handler is a documented source of deadlocks and heap corruption in production C code, which is exactly why POSIX maintains an explicit async-signal-safe function list.

Hints

0 / 3 revealed

Validation Suite

main.c

Workspace 1.0

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