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# Addressing crashes from Swift runtime errors

Identify the signs of a Swift runtime error, and address the crashes runtime errors
cause.

## Overview

Swift uses memory safety techniques to catch programming mistakes early. Optionals
require you to think about how best to handle a `nil` value. Type safety prevents
casting an object to a type that doesn’t match the object’s actual type.

If you use the `!` operator to force unwrap an optional value that’s `nil`, or if
you force a type downcast that fails with the `as!` operator, the Swift runtime catches
these errors and intentionally crashes the app. If you can reproduce the runtime
error, Xcode logs information about the issue to the console. On ARM processors,
the exception info in the crash report looks like:

```other
Exception Type:  EXC_BREAKPOINT (SIGTRAP)
...
Termination Signal: Trace/BPT trap: 5
Termination Reason: Namespace SIGNAL, Code 0x5
```

On Intel processors (including apps for macOS, Mac Catalyst, and the simulators for
iOS, iPadOS, tvOS, and watchOS), the exception info in the crash report looks like:

```other
Exception Type:        EXC_BAD_INSTRUCTION (SIGILL)
...
Exception Note:        EXC_CORPSE_NOTIFY

Termination Signal:    Illegal instruction: 4
Termination Reason:    Namespace SIGNAL, Code 0x4
```

### Identify the location of the error

The crash report shows the thread that encountered the runtime error, with a frame
in the backtrace identifying the specific line of code.

```other
Thread 0 Crashed:
0   MyCoolApp                         0x0000000100a71a88 @objc ViewController.viewDidLoad() (in MyCoolApp) (ViewController.swift:18)
1   MyCoolApp                         0x0000000100a71a40 @objc ViewController.viewDidLoad() (in MyCoolApp) (ViewController.swift:18)
2   UIKitCore                         0x00000001c569e920 -[UIViewController _sendViewDidLoadWithAppearanceProxyObjectTaggingEnabled] + 100
3   UIKitCore                         0x00000001c56a3430 -[UIViewController loadViewIfRequired] + 936
4   UIKitCore                         0x00000001c56a3838 -[UIViewController view] + 28
```

In this example, thread 0 encountered the error. Frame 0 of this thread shows that
the runtime error occurs on line 18 of `ViewController.swift`, in the `viewDidLoad`
method:

```other
0   MyCoolApp                         0x0000000100a71a88 @objc ViewController.viewDidLoad() (in MyCoolApp) (ViewController.swift:18)
```

### Change your code

Look at the other frames in the backtrace to identify the exact function calls that
produced the error, and determine whether you used a force unwrap or a forced downcast.
A force unwrap uses the `!` operator. For example:

```swift
let image = UIImage(named: "aMissingIcon")!
print("Image size: \(image.size)")
```

Instead of the force unwrap, gracefully handle the `nil` value where it first appears
in your code by using optional binding:

```swift
if let image = UIImage(named: "aMissingIcon") {
    print("Image size: \(image.size)")
}
```

See the Swift documentation for more information on [Optionals](https://developer.apple.com/library/archive/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html#//apple_ref/doc/uid/TP40014097-CH5-ID330).

For type casts, a forced downcast uses the `as!` operator. This example crashes if
`library` contains something other than a `Song` type:

```swift
for item in library {
    let song = item as! Song
    print("Song: \(song.name), by \(song.artist)")
}
```

Instead of the forced downcast, gracefully handle the scenarios where the type of
the object doesn’t match the expected type by using a conditional downcast:

```swift
for item in library {
    if let song = item as? Song {
         print("Song: \(song.name), by \(song.artist)")
    }
}
```

See the Swift documentation for more information on [Type Casting](https://developer.apple.com/library/archive/documentation/Swift/Conceptual/Swift_Programming_Language/TypeCasting.html#//apple_ref/doc/uid/TP40014097-CH22).

## See Also

[Analyzing a crash report](/documentation/Xcode/analyzing-a-crash-report)

Identify clues in a crash report that help you diagnose problems.


---

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