How To Find the Stinky Parts of Your Code: Code Smell 248 – Unreliable Copy

22 Apr 2024

Picture this: You copy a file and don't verify it

TL;DR: Don't rely on external solutions without good handlers


  • Silent Modifications

  • Least Surprise Principle violation

  • Fail Fast Principle Violation


  1. Ensure you meet your function's postconditions

  2. Use mature languages


The copy() function is used to copy files from one location to another.

However, when used on some systems, it can fail silently or make unexpected conversions.

For example, Windows interprets paths ending with a backslash () as directories.

If the intended destination file has the same name as a directory in the path, copy() will silently create an empty file with the intended filename within that directory.

This can be confusing and lead to data loss.

Sample Code



  $sourceFile = 'C:\temp\source.txt';
  $destination = 'C:\temp\destination.txt';
  $copyWasSuccessful = copy($sourceFile, $destination); // true
  $destinationFileExists = file_exists($destination); // true
  $sourceFile = 'C:\temp\source.txt';
  $destination = 'C:\temp\destination :txt';
  // The filename is simplified 
  // and might come from a programmatic construction

  $copyWasSuccessful = copy($sourceFile, $destination); 
  // true - this is a mistake

  $destinationFileExists = file_exists($destination); 
  // false since it was not created

  $destinationChangedFileExists = file_exists('C:\temp\destination '); 
  // true but unexpected



  $sourceFile = 'C:\temp\source.txt';
  $destination = 'C:\temp\destination :txt';
  // The filename is simplified
  // and might come from a programmatic construction

  $copyWasSuccessful = copy($sourceFile, $destination);  
  if (!$copyWasSuccessful || !$file_exists($destination)) {
    // Don't trust the function result. Handle the postcondition error


  • [x]Semi-Automatic

You can check all copy() handlers and wrap them


  • Fail Fast


  • [x]Beginner

AI Generation

Gemini is the only generator that avoided the problem dealing with ":" on file names

AI Detection

With this prompt:

What happens with this code on windows and what is the value of copyWasSuccessful

ChatGPT found the mistake and (wrongly) predicted the operation would fail.

Gemini, on the other hand, found the typo but also couldn't accurately predict the behavior of the operation.

Claude also noticed the mistake but refused to display the execution result


Always check important function's post-conditions even if you think you will have performance penalties. You can never be too safe.

Relation Code Smells

Code Smell 15 - Missed Preconditions

Disclaimer: Code Smells are my opinion.

Blaming programmers has been the prevailing approach for a half century of software development: It has not solved the problem yet, so it is time to look in different directions.

- Boris Beizer from Software Engineering Great works.

This article is part of the CodeSmell Series: How to Find the Stinky Parts of your Code

Photo Credit: Luke Jernejcic on Unsplash