codexion

C project · featured · pthreads, mutexes, scheduling

Multithreaded twist on the Dining Philosophers: coders compete for shared dongles, with FIFO/EDF scheduling.

The problem

Dining philosophers with coders: each coder is a thread that needs two shared USB dongles to compile, and burns out if it goes too long without compiling. A monitor must log a burnout within 10 ms.

What I built

• Dongles are taken atomically, both or none, under one mutex, so hold-and-wait (and so deadlock) can never happen.
• A hand-rolled binary heap orders waiting coders: by arrival for FIFO, or by deadline for EDF, so the coder closest to burning out goes first.
• Waiters use pthread_cond_timedwait with a fresh deadline each time round, so a dongle that comes off cooldown mid-wait is always noticed.
• A monitor thread that sleeps until the next possible burnout instead of polling, and a separate print mutex so log lines never interleave.

The hard part

Missed wake-ups. A released dongle stays unavailable for a cooldown, so a broadcast on release can arrive before the dongle is usable again, and a coder that simply waits for the next signal sleeps forever. The fix was a timed wait with a deadline recomputed every loop, plus re-checking the condition on every wake-up instead of trusting the signal.

What it looks like

$ ./codexion 5 800 200 100 100 3 10 edf
1478 3 has taken a dongle
1478 3 is compiling
1683 3 is debugging
1788 3 is refactoring

Built with

• C
• POSIX threads
• mutexes + condition variables
• binary heap
• Norminette

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