This document demonstrates a practical troubleshooting workflow using strace to diagnose an application failure. It walks through information gathering system call analysis, root cause identification, and implementation of both immediate and long-term remediation strategies.
This document presents a real-world troubleshooting scenario where an application fails to launch after a version upgrade. It demonstrates the systematic use of the strace diagnostic tool to trace system calls, identify a missing directory as the root cause, and implement appropriate remediation measures at both immediate and long-term levels.
A user reports that a certain application fails to open. Following established troubleshooting methodology, the first step involves gathering more information about the conditions that caused the failure. Key questions include what error message the user receives and whether the failure can be reproduced.
Through this inquiry, it is discovered that a new version of the software was recently released. After upgrading to this new version, the problem becomes reproducible on other systems. When attempting to run the program, no error message appears. The application simply exits immediately without any indication of what went wrong.
Even without an explicit error message, numerous tools can help understand what is happening with the system and applications. These tools extend knowledge of particular problems, provide different perspectives on program actions, and reveal necessary diagnostic information.
The strace tool allows deep inspection of what a program is doing by tracing system calls made by the program and reporting the result of each call. System calls are the requests that programs running on a computer make to the operating system kernel. Different system calls serve various purposes, and depending on the debugging context, some may be more relevant than others.
Running strace on the failing application generates substantial output showing all system calls the program made. While comprehensive, this volume of information requires effective management for practical analysis.
Two primary approaches exist for handling the extensive output from strace:
| Method | Command/Flag | Description |
|---|---|---|
| Piping to pager | strace app | less | Pipes output to less command for scrolling through text |
| Output to file | strace -o filename app | Uses -o flag to store output in a file for later analysis |
The -o flag approach offers the advantage of preserving the output for future reference, making it the preferred option for thorough investigation.
After storing the strace output to a file, it can be examined using any preferred text viewer. Opening the file with less and navigating to the end using Shift+G, then scrolling upward reveals the final operations before program termination.
Close to the end of the log file, a significant finding emerges: the application attempts to open a directory called .config/purple-box, which does not exist.
The suspicious log entry warrants detailed examination:
| Component | Value | Explanation |
|---|---|---|
| System call | openat | One of the calls used to open files or directories |
| Parameters | Path and flags | Includes the path being opened and operational flags |
| Flag | O_DIRECTORY | Indicates the program expects to open this path as a directory |
| Return code | -1 | Negative one indicates the operation failed |
The program attempts to open this directory, but since it does not exist, the operation fails. This failure occurs shortly before the program terminates, making it a strong candidate for the root cause.
To verify this hypothesis, creating the missing directory and attempting to start the program again provides confirmation. After creating the directory, the program launches successfully, confirming that the missing directory was indeed the root cause of the failure.
The troubleshooting process followed a systematic approach:
| Phase | Action Taken | Outcome |
|---|---|---|
| Information Gathering | Received user report about new version causing failure | Identified version change as trigger |
| Reproduction | Reproduced problem on local system | Confirmed consistent behavior |
| Investigation | Used strace to trace system calls | Generated detailed execution log |
| Analysis | Examined system call output | Found missing directory error |
| Hypothesis Testing | Created the missing directory | Program worked correctly |
| Root Cause | Identified missing directory | Confirmed as source of failure |
With the root cause identified, remediation occurs at multiple levels to address both immediate needs and long-term prevention.
The short-term solution involves instructing the user to create the missing directory manually. This approach allows affected users to resume work quickly without waiting for software updates.
1# Create the missing directory
2mkdir -p ~/.config/purple-box
The long-term solution requires contacting the software developers to inform them that the program fails to start when this directory is missing. Providing this feedback enables developers to fix the issue in subsequent versions, potentially by implementing automatic directory creation or providing clearer error messages.
Proper documentation ensures future efficiency. The finding should be recorded: this version of the software will not start if the .config/purple-box directory does not exist. This documentation helps others encountering the same issue to quickly identify the solution without repeating the entire diagnostic process.
Important
While this example demonstrated relatively straightforward troubleshooting using strace, not all problems resolve this easily. Complex issues may require multiple tools, various perspectives, and creative problem-solving approaches.
For deeper understanding of system calls encountered during debugging, manual pages provide comprehensive documentation for each system call. The man command provides access to this information:
1# View manual page for a specific system call
2man 2 openat
The section number 2 specifically references system calls, distinguishing them from other manual page categories.
This troubleshooting example illustrated the practical application of systematic problem-solving methodology. By gathering information about the failure conditions, using the strace diagnostic tool to examine system calls, identifying the missing directory as the root cause, and implementing both immediate and long-term remediation strategies, the issue was successfully resolved. The process highlighted the importance of using appropriate diagnostic tools, managing large volumes of output effectively, and documenting findings for future reference. While this case demonstrated relatively straightforward analysis, it established foundational techniques applicable to more complex debugging scenarios throughout the course.