rundll32.exe: The Windows DLL Runner

rundll32.exe is a crucial, legitimate system process in Microsoft Windows. Its primary function is to load and execute 32-bit Dynamic Link Libraries (DLLs). Think of it as a "launcher" for functionality contained within DLL files. It's not a standalone program with a user interface; instead, it acts as a bridge between the user (or another program) and the code residing in a DLL.

Origin and Purpose

rundll32.exe has been a core component of Windows since the introduction of 32-bit DLLs. It was designed to provide a standardized way to invoke functions within DLLs, simplifying the process for developers and the operating system itself. The name itself reflects its function: "Run DLL as a 32-bit application."

Before rundll32.exe, executing DLL functions directly was more complex and less standardized. This utility streamlined the process. The need for rundll32.exe arises because DLLs, unlike executable (.exe) files, cannot be run directly. They contain libraries of code that other programs use, rather than being self-contained applications.

How it Works

rundll32.exe works by taking two main arguments on the command line:

1. The DLL file to load: This specifies the path to the DLL file containing the function to be executed. This can be a full path (e.g., C:\Windows\System32\shell32.dll) or, if the DLL is in a standard system directory, just the DLL name (e.g., shell32.dll).

2. The function to execute: This specifies the name of the function within the DLL that should be called. This is often referred to as the "entry point." The function must be specifically designed to be called by rundll32.exe (it must have a specific function signature, detailed below).

The general syntax is: