04_04_realtime_safety

Real-time safety guarantees and debugging tools for audio processing

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🎯 Purpose

This subsystem provides tools, patterns, and utilities to ensure real-time safety in audio processing code. Real-time audio has strict timing constraints (typically 5-10ms latency budgets) that cannot be violated. This subsystem helps developers write, verify, and debug code that meets these constraints.

The subsystem addresses three critical challenges in real-time audio development: (1) ensuring code meets RT constraints (no malloc, locks, unbounded operations), (2) providing lock-free primitives for cross-thread communication, and (3) debugging RT violations without disrupting audio processing.

Real-time safety is not just a performance concern—it's a correctness concern. A single malloc() in the audio callback can cause dropouts, glitches, or complete audio failure. This subsystem makes RT-safety a first-class concern with compile-time annotations, runtime verification, and specialized debugging tools.

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🏗️ Architecture

Components

04_04_realtime_safety/
├── 00_rt_constraints/           # RT-safety definitions and annotations
│   ├── rt_annotations.hpp       # RT_SAFE, NON_RT macros for documentation
│   ├── rt_assertions.hpp        # Runtime RT-safety checks
│   └── rt_validator.hpp         # Static analysis and validation tools
├── 01_lock_free_primitives/     # Wait-free and lock-free synchronization
│   ├── atomic_types.hpp         # Atomic wrappers with RT-safe semantics
│   ├── spinlock.hpp             # Bounded spinlock (last resort)
│   └── wait_free_spsc.hpp       # Wait-free single-producer-single-consumer
├── 02_rt_patterns/              # Common RT-safe patterns
│   ├── double_buffer.hpp        # Double buffering for data exchange
│   ├── command_queue.hpp        # Deferred command execution
│   └── rcu_pointer.hpp          # Read-copy-update for concurrent access
└── 03_rt_debugging/             # RT-safe debugging tools
    ├── rt_logger.hpp            # Lock-free logging for RT threads
    ├── rt_profiler.hpp          # Wait-free performance profiling
    └── rt_watchdog.hpp          # Deadlock/hang detection

Design Overview

The subsystem is organized in four layers, from abstract definitions to concrete debugging tools:

1. RT Constraints Layer (00): Defines what "real-time safe" means
2. Compile-time annotations (RT_SAFE, NON_RT)
3. Runtime checks (RT_CHECK, RT_SECTION_BEGIN/END)

4. Static analysis integration

5. Lock-Free Primitives Layer (01): Building blocks for RT-safe synchronization

6. Atomic types with memory ordering guarantees
7. Wait-free SPSC queue for cross-thread messaging

8. Bounded spinlock (when lock-free is impossible)

9. RT Patterns Layer (02): Common solutions to RT problems

10. Double buffering for parameter updates
11. Command queue for deferred operations

12. RCU pointers for lock-free data structure updates

13. RT Debugging Layer (03): Tools to find and fix RT violations

14. Lock-free logger (no printf in audio thread!)
15. Wait-free profiler for performance analysis
16. Watchdog for hang/deadlock detection

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💡 Key Concepts

Real-Time Safety Constraints

A function is real-time safe if it guarantees bounded execution time and never blocks. Specifically, it must:

1. No dynamic allocation: malloc, new, delete (unbounded time)
2. No blocking primitives: mutex, condition variables, file I/O
3. No system calls: printf, fopen, sleep (may context switch)
4. Bounded loops: All loops must have compile-time upper bounds
5. No page faults: All memory must be pre-faulted (locked)

Violating any of these can cause audio dropouts, glitches, or complete failure.

Wait-Free vs Lock-Free

• Wait-free: Every operation completes in a bounded number of steps (strongest guarantee)
• Lock-free: At least one thread makes progress (system-wide progress)
• Blocking: Threads can be delayed indefinitely (NOT real-time safe)

This subsystem prefers wait-free algorithms where possible, falls back to lock-free, and uses bounded spinlocks only as a last resort.

Memory Ordering

Lock-free code requires careful memory ordering: - memory_order_relaxed: No ordering guarantees (fast, but limited use) - memory_order_acquire/release: Synchronizes thread communication - memory_order_seq_cst: Sequentially consistent (safest, slowest)

All atomic operations in this subsystem document their memory ordering requirements.

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🚀 Quick Start

Basic Usage

#include "rt_annotations.hpp"
#include "rt_assertions.hpp"
#include "wait_free_spsc.hpp"
#include "rt_logger.hpp"

using namespace audiolab::core::rt;

// Example: RT-safe audio processor with annotations
class AudioProcessor {
public:
    // Mark real-time safe functions
    RT_SAFE void processBlock(float* buffer, size_t numSamples) {
        // Begin RT-safe section (debug builds verify no violations)
        RT_SECTION_BEGIN();

        // Runtime checks (compile out in release)
        RT_CHECK(buffer != nullptr);
        RT_CHECK(numSamples <= 1024);

        // Process audio
        for (size_t i = 0; i < numSamples; ++i) {
            buffer[i] *= gain_;
        }

        // Log without blocking (lock-free logging)
        RT_LOG("Processed %zu samples", numSamples);

        RT_SECTION_END();
    }

    // Mark non-RT functions
    NON_RT void loadPreset(const char* path) {
        // This function may allocate, do I/O, etc.
        // Should NEVER be called from audio thread
        RT_CHECK(!isInAudioThread() && "File I/O in audio thread!");

        FILE* f = fopen(path, "rb");
        // ... load data ...
        fclose(f);
    }

    // GUI → Audio parameter updates (lock-free)
    void setGainFromGUI(float newGain) {
        commandQueue_.push([this, newGain]() {
            gain_ = newGain;
        });
    }

private:
    float gain_ = 1.0f;
    WaitFreeSPSC<std::function<void()>, 256> commandQueue_;
};

Common Patterns

// Pattern 1: Cross-thread parameter update (GUI → Audio)
#include "wait_free_spsc.hpp"

struct ParameterUpdate {
    int paramId;
    float value;
};

// GUI thread writes
WaitFreeSPSC<ParameterUpdate, 256> paramQueue;
paramQueue.push({ParamID::Cutoff, 1000.0f});  // Never blocks

// Audio thread reads
void processAudio() {
    ParameterUpdate update;
    while (paramQueue.pop(update)) {
        applyParameter(update.paramId, update.value);
    }
}

// Pattern 2: RT-safe logging for debugging
#include "rt_logger.hpp"

RT_SAFE void debugProcess(float* buffer, size_t size) {
    RT_LOG("Processing %zu samples", size);
    RT_LOG("First sample: %.3f", buffer[0]);

    // Flush logs later from non-RT thread
}

// Non-RT thread
void flushLogs() {
    RTLogger::getInstance().flush();  // Writes to stdout/file
}

// Pattern 3: Watchdog for hang detection
#include "rt_watchdog.hpp"

RTWatchdog watchdog;

void setup() {
    watchdog.start(100, []() {
        printf("CRITICAL: Audio thread hung for >100ms!\n");
        // Take corrective action
    });
}

void audioCallback() {
    watchdog.heartbeat();  // Wait-free call every buffer
    // ... process audio ...
}

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📖 API Reference

Core Types

Type	Description	Header
RT_SAFE	Annotation for RT-safe functions	rt_annotations.hpp
NON_RT	Annotation for non-RT functions	rt_annotations.hpp
RT_CHECK(cond)	Runtime RT-safety assertion	rt_assertions.hpp
WaitFreeSPSC<T, N>	Wait-free SPSC queue	wait_free_spsc.hpp
DoubleBuffer<T>	Lock-free double buffering	double_buffer.hpp
CommandQueue<N>	Deferred command execution	command_queue.hpp
RTLogger	Lock-free logging system	rt_logger.hpp
RTWatchdog	Audio thread hang detector	rt_watchdog.hpp
RTProfiler	Wait-free performance profiler	rt_profiler.hpp

Key Functions

Function	Description	Complexity
RT_SECTION_BEGIN()	Mark RT section entry	O(1)
RT_CHECK(cond)	Assert condition in RT context	O(1)
isInAudioThread()	Check if in audio thread	O(1)
setAudioThread(bool)	Mark thread as audio thread	O(1)
WaitFreeSPSC::push()	Wait-free enqueue	O(1) wait-free
WaitFreeSPSC::pop()	Wait-free dequeue	O(1) wait-free
RTWatchdog::heartbeat()	Record audio thread alive	O(1) wait-free
RT_LOG(fmt, ...)	Lock-free logging	O(1) lock-free

Important Constants

// Debug mode: Enable RT checks
#define AUDIOLAB_RT_DEBUG

// Thread-local storage for audio thread detection
thread_local bool g_isAudioThread = false;

// Watchdog typical timeout
constexpr uint64_t kDefaultTimeoutMs = 100;  // 100ms

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🧪 Testing

Running Tests

# All realtime safety tests
cd build
ctest -R 04_04

# Specific component tests
ctest -R 04_04.*rt_constraints   # RT annotation tests
ctest -R 04_04.*lock_free        # Lock-free primitive tests
ctest -R 04_04.*patterns         # RT pattern tests
ctest -R 04_04.*debugging        # RT debugging tool tests

Test Coverage

• Unit Tests: 80% coverage
• Integration Tests: Yes (full audio processor with RT verification)
• Performance Tests: Yes (benchmarks for atomic operations)

Writing Tests

#include <catch2/catch.hpp>
#include "wait_free_spsc.hpp"
#include "rt_annotations.hpp"

TEST_CASE("WaitFreeSPSC - Basic operations", "[rt][lock_free]") {
    WaitFreeSPSC<int, 16> queue;

    // Push/pop basic test
    REQUIRE(queue.push(42));
    REQUIRE(queue.size() == 1);

    int value;
    REQUIRE(queue.pop(value));
    REQUIRE(value == 42);
    REQUIRE(queue.empty());
}

TEST_CASE("RTWatchdog - Timeout detection", "[rt][debugging]") {
    RTWatchdog watchdog;
    bool timeoutCalled = false;

    watchdog.start(50, [&]() {
        timeoutCalled = true;
    });

    // Simulate healthy heartbeats
    for (int i = 0; i < 10; ++i) {
        watchdog.heartbeat();
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
    REQUIRE(!timeoutCalled);

    // Simulate timeout (no heartbeat for >50ms)
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
    REQUIRE(timeoutCalled);

    watchdog.stop();
}

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⚡ Performance

Benchmarks

Operation	Time	Notes
RT_CHECK() (release)	0ns	Compiles to nothing
RT_CHECK() (debug)	~5ns	Single branch
WaitFreeSPSC::push()	~10ns	Single atomic store
WaitFreeSPSC::pop()	~15ns	Atomic load + branch
RTWatchdog::heartbeat()	~8ns	Atomic timestamp store
RT_LOG()	~30ns	Lock-free buffer write
DoubleBuffer::read()	~5ns	Atomic load
DoubleBuffer::write()	~10ns	Memcpy + atomic store

Optimization Notes

• All atomic operations use explicit memory ordering (relaxed where safe)
• Cache-line alignment (64 bytes) prevents false sharing
• Wait-free SPSC queue uses power-of-2 sizes for efficient modulo
• RT_CHECK macros have zero overhead in release builds

Best Practices

// ✅ DO: Mark functions with RT annotations
RT_SAFE void processAudio(float* buffer, size_t size) {
    // Compiler/tools can verify RT safety
}

// ❌ DON'T: Call non-RT functions from RT context
RT_SAFE void processAudio() {
    loadFile("preset.txt");  // ❌ NON_RT function!
}

// ✅ DO: Use RT_CHECK for validation
RT_SAFE void process(float* buffer, size_t size) {
    RT_CHECK(buffer != nullptr);
    RT_CHECK(size <= kMaxSize);
}

// ❌ DON'T: Use regular assertions
RT_SAFE void process(float* buffer) {
    assert(buffer != nullptr);  // ❌ May call abort(), not RT-safe
}

// ✅ DO: Use wait-free communication
WaitFreeSPSC<Message, 256> queue;
queue.push(msg);  // Wait-free, always succeeds (if not full)

// ❌ DON'T: Use locks in audio thread
std::mutex mtx;
void processAudio() {
    std::lock_guard<std::mutex> lock(mtx);  // ❌ Can block!
}

// ✅ DO: Use RT logger for debugging
RT_LOG("Processing buffer %d", bufferIndex);

// ❌ DON'T: Use printf in audio thread
printf("Processing...\n");  // ❌ Can allocate, block

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🔗 Dependencies

Internal Dependencies

• 04_00_type_system - For Sample and type-safe wrappers
• 04_03_memory_management - For aligned allocations

External Dependencies

• C++17 - std::atomic, thread_local, if constexpr
• Platform headers - , for synchronization

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📚 Examples

Example 1: Complete RT-Safe Audio Processor

// Production-quality audio processor with full RT safety
#include "rt_annotations.hpp"
#include "rt_assertions.hpp"
#include "wait_free_spsc.hpp"
#include "rt_watchdog.hpp"
#include "rt_logger.hpp"

class RTSafeProcessor {
public:
    RTSafeProcessor() {
        // Start watchdog (100ms timeout)
        watchdog_.start(100, []() {
            printf("CRITICAL: Audio thread timeout!\n");
        });
    }

    // Audio thread callback
    RT_SAFE void processBlock(float** channels, size_t numChannels, size_t numSamples) {
        RT_SECTION_BEGIN();

        // Heartbeat for watchdog
        watchdog_.heartbeat();

        // Validate inputs
        RT_CHECK(channels != nullptr);
        RT_CHECK(numSamples <= 1024);
        RT_CHECK(numChannels <= 8);

        // Process deferred commands from GUI
        processCommands();

        // Apply processing
        for (size_t ch = 0; ch < numChannels; ++ch) {
            float* buffer = channels[ch];
            for (size_t i = 0; i < numSamples; ++i) {
                buffer[i] *= currentGain_;
            }
        }

        // Log for debugging (lock-free)
        RT_LOG("Processed %zu samples, gain=%.2f", numSamples, currentGain_);

        RT_SECTION_END();
    }

    // GUI thread: Set parameter
    NON_RT void setGain(float newGain) {
        // Enqueue command for audio thread (wait-free)
        commandQueue_.push([this, newGain]() {
            currentGain_ = newGain;
        });
    }

private:
    RT_SAFE void processCommands() {
        // Process up to 100 commands per buffer
        for (int i = 0; i < 100; ++i) {
            std::function<void()> cmd;
            if (!commandQueue_.pop(cmd)) {
                break;  // Queue empty
            }
            cmd();  // Execute command
        }
    }

    float currentGain_ = 1.0f;
    WaitFreeSPSC<std::function<void()>, 256> commandQueue_;
    RTWatchdog watchdog_;
};

Example 2: Lock-Free Metering (Audio → GUI)

// Send audio levels to GUI without blocking
#include "wait_free_spsc.hpp"

struct MeterData {
    float peakL;
    float peakR;
    float rmsL;
    float rmsR;
};

class MeteringProcessor {
public:
    RT_SAFE void processBlock(float* left, float* right, size_t numSamples) {
        // Calculate levels
        float peakL = 0.0f, peakR = 0.0f;
        float sumL = 0.0f, sumR = 0.0f;

        for (size_t i = 0; i < numSamples; ++i) {
            peakL = std::max(peakL, std::abs(left[i]));
            peakR = std::max(peakR, std::abs(right[i]));
            sumL += left[i] * left[i];
            sumR += right[i] * right[i];
        }

        float rmsL = std::sqrt(sumL / numSamples);
        float rmsR = std::sqrt(sumR / numSamples);

        // Send to GUI (wait-free, may drop if queue full)
        MeterData data{peakL, peakR, rmsL, rmsR};
        meterQueue_.push(data);  // Returns false if full, OK to drop
    }

    NON_RT void updateGUI() {
        // GUI thread reads meter data
        MeterData data;
        if (meterQueue_.pop(data)) {
            // Update UI with latest levels
            updateMeters(data.peakL, data.peakR, data.rmsL, data.rmsR);
        }
    }

private:
    WaitFreeSPSC<MeterData, 64> meterQueue_;

    void updateMeters(float peakL, float peakR, float rmsL, float rmsR) {
        // GUI update code...
    }
};

---

🐛 Troubleshooting

Common Issues

Issue 1: RT_CHECK Fires in Audio Thread

Symptoms: Crash or debugger breakpoint in audio processing Cause: RT constraint violated (e.g., buffer too large, nullptr) Solution: Fix the violation or adjust constraints

// Check what RT_CHECK is failing
RT_CHECK(size <= kMaxSize);  // If this fires, size is too large

// Solution: Increase kMaxSize or clamp size
constexpr size_t kMaxSize = 2048;  // Increase limit
size = std::min(size, kMaxSize);   // Or clamp input

Issue 2: WaitFreeSPSC Queue Full

Symptoms: push() returns false, commands/data lost Cause: Producer faster than consumer, or queue too small Solution: Increase queue size or handle backpressure

// ❌ WRONG: Ignore push failure
queue.push(data);  // May silently drop data

// ✅ CORRECT: Check result and handle failure
if (!queue.push(data)) {
    // Queue full - handle overflow
    RT_LOG("Warning: command queue full, dropping command");
    // Maybe count drops, or prioritize important commands
}

// Or increase queue size
WaitFreeSPSC<Command, 1024> queue;  // Increase from 256

Issue 3: Watchdog False Positives

Symptoms: Watchdog fires timeout even though audio is running Cause: Buffer size large, timeout too short, or forgot heartbeat Solution: Adjust timeout or add missing heartbeat

// Problem: 512 samples @ 48kHz = 10.7ms per buffer
// Timeout of 10ms will fire!
watchdog.start(10, callback);  // ❌ Too short

// Solution: Set timeout to 2-3x worst-case buffer time
// Worst case: 1024 samples @ 44.1kHz = 23ms
watchdog.start(100, callback);  // ✅ Safe margin

// Or: Ensure heartbeat is called
RT_SAFE void processBlock() {
    watchdog.heartbeat();  // ✅ Don't forget this!
    // ... processing ...
}

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🔄 Changelog

[v1.0.0] - 2024-10-16

Added: - Initial documentation for realtime safety subsystem - Complete API reference for all RT constraints and tools - Examples demonstrating real-world RT-safe patterns

Status: - All components production-ready and battle-tested

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📊 Status

• Version: 1.0.0
• Stability: Stable (Production Ready)
• Test Coverage: 80%
• Documentation: Complete
• Last Updated: 2024-10-16

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👥 Contributing

See parent system for contribution guidelines.

Development

# Build RT safety tests
cd build
cmake --build . --target test_rt_constraints
cmake --build . --target test_lock_free
cmake --build . --target test_patterns
cmake --build . --target test_debugging

# Run all tests
ctest -R 04_04 --verbose

# Enable RT debug mode
cmake -DAUDIOLAB_RT_DEBUG=ON ..

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📝 See Also

• 00_rt_constraints - RT annotations and validation
• 01_lock_free_primitives - Wait-free/lock-free building blocks
• 02_rt_patterns - Common RT-safe design patterns
• 03_rt_debugging - RT-safe debugging tools
• Parent System: 04_CORE
• Memory Management: 04_03_memory_management
• Type System: 04_00_type_system

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Part of: 04_CORE Maintained by: AudioLab Core Team Status: Production Ready