Modifying Variable Values During LLDB Debugging in Xcode

28 February 2025 Stephan Petzl Leave a comment Xcode

Debugging is a critical aspect of software development, and Xcode’s LLDB debugger offers powerful capabilities for developers. One frequent requirement during debugging is the ability to alter variable values on the fly. Here’s a comprehensive guide on how to achieve this using LLDB in Xcode.

Using the LLDB Expression Command

The expr command in LLDB allows developers to evaluate and change variable values within the current program context. This command is versatile, supporting C, Objective-C, and C++ expressions.

To modify a variable, use the syntax:

expr variableName = newValue

For example, to change a string variable:

expr myString = @"Foo"

This command evaluates the expression and updates the variable in the current scope, allowing you to test different code paths without recompiling.

Practical Examples

• Updating a Structure Member: expr my_struct->a = my_array[3]
• Binary Format Expression: expr -f bin -- (index * 8) + 5
• Modifying Character Arrays: expr char c[] = "foo"; c[0]

Adjusting Variables for Testing

For testing specific code flows, you might want to set a variable to nil or another test value. After setting a breakpoint, execute:

expression dict = nil

This command sets the variable dict to nil, allowing you to observe how your application behaves under these conditions.

Automating Variable Changes

To automate variable changes at a breakpoint, follow these steps:

1. Create a breakpoint where you want to modify the variable.
2. Right-click and select “Edit Breakpoint”.
3. Choose “Debugger Command” in the action section.
4. Enter the command: e yourStringName = "Your new value"
5. Check “Automatically continue after evaluating actions” to streamline the debugging process.

Enhancing Debugging with Repeato

While LLDB provides robust debugging capabilities, automating tests can further streamline your development workflow. This is where Repeato comes into play. Repeato is a no-code test automation tool that leverages computer vision and AI to create, run, and maintain automated tests for iOS, Android, and web apps.

With Repeato, recording tests is remarkably fast, and the ability to run command line scripts or JavaScript code allows for automating complex tasks. It supports data-driven and keyword-driven testing, making it a practical alternative to other automation tools. Unlike closed-source tools, Repeato saves all test and workspace data in text and JSON format, facilitating seamless version control and collaboration within teams.

Incorporating Repeato into your testing strategy can significantly enhance your debugging and testing efficiency, ensuring your applications are robust and ready for deployment.

Check it out!

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