gfortran localizing a bug that does not occur in the debugger

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Localizing a bug that doesn't occur in the debugger can be challenging, but there are several techniques you can use to track down the issue in your Fortran code. Here are some steps you can take:

  1. Reproduce the Issue: First, make sure you can reliably reproduce the bug outside the debugger. This might involve running your program multiple times with the same input data or conditions.

  2. Check for Timing Issues: Some bugs might be related to timing or race conditions that might not occur consistently under the debugger. Try adding explicit delays or waiting periods in your code to see if the bug becomes more reproducible.

  3. Logging: Insert print statements or log messages at critical points in your code to help track the flow of execution and the values of variables. Write the log output to a file so you can review it even after the program exits.

  4. Diagnostics Output: Add more diagnostics output specifically related to the part of the code where you suspect the bug might be occurring. This can help you narrow down the location of the issue.

  5. Code Review: Have someone else review your code. Sometimes, a fresh pair of eyes can spot something you might have missed.

  6. Simplify the Code: Try to create a simplified version of your program that still reproduces the bug. By removing unrelated parts, you might make it easier to pinpoint the source of the problem.

  7. Binary Search: If the bug depends on a particular section of your code, consider using a binary search approach to identify which part of that section is causing the problem. Temporarily comment out half of the code and see if the bug still occurs. If it does, the problem is likely in the remaining half. Continue narrowing down the search until you've isolated the problematic code.

  8. Static Analysis Tools: Consider using static analysis tools that can identify potential issues in your code without running it. Tools like gfortran's -Wall option can help catch some common coding mistakes.

  9. Memory Tools: Use memory analysis tools or sanitizers to identify memory-related issues that might not be caught by the debugger.

  10. Unit Testing: If possible, create unit tests for different parts of your code. Unit tests can help isolate specific issues and make it easier to identify the problem.

  11. Stress Testing: Increase the load on your program by using larger input data or more iterations. This might trigger the bug more consistently.

  12. Platform Differences: If your program works fine on one platform (like the debugger) but not on another (running standalone), consider checking for platform-specific differences. It could be related to environment variables, file paths, system resources, etc.

  13. External Dependencies: Make sure that any external dependencies (libraries, data files, etc.) are accessible and in the expected state when running your program outside the debugger.

  14. Ask for Help: If you're still struggling to localize the bug, consider reaching out to forums, mailing lists, or communities related to Fortran programming. Sometimes, others who are experienced with the language can provide valuable insights.

Remember that debugging can be a process of trial and error, and patience is key. By combining these techniques, you should be able to track down the source of the bug and address it.