How to Use the Command 'rr' (with Examples)

The rr tool is a powerful debugging utility that allows developers to record and replay program executions. Its primary function is to capture the exact sequence of a program’s operations, enabling developers to trace back any bugs or issues by replaying this recorded execution. This functionality is invaluable for diagnosing and fixing tricky bugs that may not always be reproducible. The tool is particularly useful for complex applications where traditional debugging methods might fall short.

The rr tool consists of two primary operations: recording an application’s execution and replaying that execution. Each use case is detailed below with examples.

Use case 1: Record an Application

Code:

rr record path/to/binary --arg1 --arg2

Motivation:

The recording functionality of rr is crucial when you need to capture the exact state and sequence of events during a program’s execution. This is especially useful in scenarios where a bug occurs intermittently, making it hard to reproduce consistently. By recording an application’s execution, you ensure that you can scrutinize each step of the execution later on. This can be highly effective for debugging, as it allows you to explore an application’s behavior at any given point in its runtime.

Explanation:

• rr: This is the invocation of the rr command, indicating that we wish to use this debugging tool.
• record: This subcommand instructs rr to start recording the application’s execution.
• path/to/binary: This should be replaced with the actual path to the executable binary you wish to record. It’s crucial to specify the correct path so that rr can properly attach to and record the execution.
• --arg1 --arg2: These are example command-line arguments that your application might require during execution. Replace them with any arguments your specific program requires. Supplying these arguments ensures the application runs in the desired configuration/state so the recording captures the necessary behavior.

Example Output:

rr: Starting recording...
rr: Recording working directory was /home/user/project
rr: Output will be stored in /home/user/.local/share/rr/project-NNN
rr: Running `path/to/binary --arg1 --arg2`
...
rr: Recording complete.

Use case 2: Replay Latest Recorded Execution

Code:

rr replay

Motivation:

Once you have recorded a program’s execution, you may need to examine the recorded trace to understand an issue better. The replay functionality of rr allows you to step through each instruction of your application exactly as it executed during the recording session. This can greatly facilitate the identification of complex bugs, race conditions, and subtle logic errors by allowing the developer to inspect the exact flow of execution and program state at any given point.

Explanation:

• rr: This command activates the rr utility, setting the stage for either recording or replaying an execution.
• replay: This subcommand operates on the latest recorded session, playing back the execution so that a developer can investigate how the program ran. This includes all syscalls, signals, and even what data was read or written.

Example Output:

rr: Replaying execution from /home/user/.local/share/rr/project-NNN
(rr) [Replaying pid 12345. Press 'Ctrl+C' to interrupt, 'c' to continue]
...
(rr) Continuing...
(rr) Execution reached breakpoint 1
(rr) Inspect variables...

Conclusion:

The rr command provides indispensable functionality for developers dealing with debugging challenges. By allowing for the accurate recording and later replaying of a program’s execution, rr stands out as a vital tool in diagnosing bugs that are difficult to reproduce. Understanding how to effectively leverage rr can significantly improve a developer’s ability to identify and fix elusive bugs within complex software.