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Finding_bugs_using_XCode_runtime_tools.md

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WWDC 2017

Table of Contents

Finding Bugs Using XCode Runtime Tools

Session video and resources: https://developer.apple.com/videos/play/wwdc2017/406/

Main Thread Checker

  • Detects misuses of common APIs
  • Available in the diagnostics tab of a scheme, under "Run"
    • Also can pause on issues for a better understanding
  • Supports AppKit, UIKit, WebKit
  • Swift and C languages
  • Does not require recompilations
  • Enabled by default on XCode 9

Designing asynchronous APIs

  • Good APIs should also provide queue as a parameter to specify where the code will be running

Address Sanitizer

  • Detects use-after-scope
  • Detects use-after-return
  • These both prevents access to the pointers created in previous scopes to prevent leaks and crashes
  • Better to use if there is a C & Obj-C mixed project with pointers

Thread Sanitizer

  • Detects multithreading issues
  • Find races even if they did not manifest
  • Only on 64 bit macOS and simulators

Data Races

  • Unsync access to shared mutable vars
class EventLog {
  private var lastEventSource: LogSource?

  func log(source: LogSource, message: String) {
    print(message)
    lastEventSource = source
  }
}

// Thread 1
eventlog.log()...

// Thread 2
eventlog.log()...

/// This is a data race, solve it by dispatch queues so func() runs asynchronously in a GCD Queue
  • Associate your data with serial dispatch queues

Races on Collections

  • Swift and Obj-C collection races are also detected

Key Points

  • Use GCD to synchronize access to the data

Undefined Behaviour Sanitizer

  • Runtime bug finder
  • Checks unsafe constructs in the C language family
  • Compatible with other runtime tools

Integer Overflow

  • Detects a possible integer overflow

Alignment Violation

  • Unaligned load or store
  • Causes crashes in Release builds

Nonnull Return Value Violation

  • Undefined Behaviour Sanitizer is available under build settings

Using Runtime Tools Effectively (Important)

  • Exercise more code
    • Use runtime tools for daily development
    • Use them before every release
    • Use CI with these tools turned on (WWDC 2015 -> CI and Code Coverage Session)
  • Combine different tools
  • Turning these tools on can cause execution and memory overhead up to 10x times