Debugging in Delphi is a blend of built-in IDE features and external tools that allow you to peer into the inner workings of your application. Whether you are hunting for a memory leak or trying to trace a complex logic flow, these three methods are your primary lines of defense.

I am only mentioning three methods in this article and I appreciate that there are many other forms of debugging, especially when discussing more modern applications. So, for instance, using Postman, to test your REST API.

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OutputDebugString and External Viewers

OutputDebugString is a Windows API function that sends a string to the system debugger. It is one of the most non-intrusive ways to monitor an application’s behavior in real-time without pausing execution.

How to use it in Delphi

You simply call the procedure from the Winapi.Windows unit:

uses Winapi.Windows;

procedure TForm1.LogValue(const AMsg: string);
begin
  OutputDebugString(PChar('MyAppLog: ' + AMsg));
end;

Viewing the Output

Since this output isn’t visible in your standard application UI, you need a “listener” or “viewer”:

• DebugView (Sysinternals): The classic, lightweight industry standard for capturing kernel and Win32 debug output.

• DebugView++: A more modern, powerful alternative that supports filtering, highlighting, and even tailing log files.

Note: You aren’t limited to third-party tools. You can actually create your own custom listener client for OutputDebugString directly in Delphi using a shared memory window (named “DBWIN_BUFFER”). I will be releasing Delphi code to build your own listener soon.

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Loggers: Custom and Sophisticated

Sometimes you need a permanent record of what happened during a session, especially when debugging issues that only occur on a client’s machine where a debugger isn’t installed.

Simple File Logging

Writing a simplistic logger in Delphi is straightforward—it usually involves a TStringList or a TFileStream that appends text to a *.log file on disk.

• Why use a logger? It provides a chronological history that survives application crashes.

• What’s coming: Creating a robust, thread-safe logger is an art in itself. I will be producing code for a custom Delphi logger shortly that handles file locking and timestamps efficiently.

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Using Breakpoints

Breakpoints are the “bread and butter” of the Delphi IDE. They allow you to pause your program at a specific line of code to inspect the current state of the CPU, memory, and variables.

Key Features of Breakpoints:

• Standard Breakpoints: Just click the gutter to the left of your code. Execution stops, and you can hover over variables to see their current values.

• Conditional Breakpoints: You can right-click a breakpoint to set a condition ( e.g., i > 100 ). The program will only pause if that condition is met.

• Logging Breakpoints: You can configure a breakpoint to log a message or play a sound instead of actually pausing the program—very useful for loops where pausing would be too tedious.

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Resources and Links

• Sysinternals DebugView: Download from Microsoft

• DebugView++: GitHub Releases (CobaltFusion)