race condition

**Race Condition** — a concurrency bug where the program's behavior depends on the unpredictable timing of thread execution, leading to inconsistent or incorrect results. **How It Happens** Two threads access the same shared variable, and at least one writes: ``` Thread A: read counter (=5) Thread B: read counter (=5) Thread A: write counter = 5+1 = 6 Thread B: write counter = 5+1 = 6 ← Should be 7! ``` **Types** - **Data Race**: Unsynchronized concurrent access to shared memory (undefined behavior in C/C++) - **Race Condition**: Logic error due to timing, even with proper synchronization (e.g., TOCTOU — time-of-check-to-time-of-use) **Prevention** - **Mutex/Lock**: Only one thread enters critical section at a time - **Atomic Operations**: Hardware-guaranteed read-modify-write (e.g., `atomic_fetch_add`) - **Immutable Data**: If nobody writes, no race possible - **Thread-Local Storage**: Each thread has its own copy - **Message Passing**: Communicate by sending messages, not sharing memory **Detection Tools** - ThreadSanitizer (TSan) — dynamic detection at runtime - Helgrind (Valgrind-based) - Intel Inspector **Race conditions** are among the most difficult bugs to find — they may only manifest under specific timing conditions that are hard to reproduce.