- A dynamic link library (DLL) is defined as a file type containing code, data, and resources that can be shared among multiple programs to accomplish specific tasks.
- DLLs offer common functionality to programs that can be loaded into memory and executed on demand.
- This article explains the fundamentals of DLL, its types, and key advantages.
Table of Contents
What Is a Dynamic Link Library (DLL)?
A dynamic link library (DLL) is a file type that contains code, data, and resources that can be shared among multiple programs to accomplish specific tasks. DLLs offer common functionality to programs that can be loaded into memory and executed on demand.
The key advantage of using a DLL is that it can reduce the size of executable files and improve the performance of programs. Moreover, whenever a program needs to use a particular function or resource, it can load the target DLL into memory and execute the code. Since several programs use one DLL, it leads to a reduction in the overall memory consumed by the system.
For example, different programs might call upon the User32.dll file to incorporate core functionality related to user interface elements, such as windows, buttons, and menus, into their framework. The User32.dll is typically used by many programs, including web browsers, text editors, and other applications, to access UI components.
Commonly used DLLs
Just like the DLL mentioned above, several others provide a wide range of functionality to applications and operating systems. Let’s look at some of the most commonly used DLLs.
- Kernel32.dll: This is a Windows operating system DLL that provides features such as memory management, process management, and input/output operations. Several system-level programs, including device drivers and system utilities, use this type.
- OpenGL32.dll: This DLL supports the OpenGL graphics library used by many games and graphics-intensive applications. It provides functions to render 2D and 3D graphics.
- Advapi32.dll: This is a Windows DLL that provides advanced features such as cryptography, authentication, and registry access. Many security applications such as firewalls, antivirus programs, and encryption software use it.
- MSVCR100.dll: This is a Microsoft Visual C++ runtime DLL that offers standard C++ functions and libraries for programs built using the Microsoft Visual C++ IDE.
- DirectX.dll: This Microsoft’s DLL is known to provide multimedia support.
As seen in the examples listed above, DLLs are not limited to any one programming language. Instead, they are flexible and can be created in various programming languages, including C, C++, or any assembly language. Using DLLs, multiple programs can avail a wide range of functionalities such as networking capabilities or database access, which they lack.
DLLs can be updated independently of the programs that use them. This implies that if a bug is identified in a DLL or new functionality is needed, the DLL can be updated without making any changes to the programs that use it. This simplifies the overall process of maintaining and updating software.
On the flip side, DLLs also have some drawbacks. For instance, if a DLL is updated or replaced, it can break the programs that depend on its previous version. This can give rise to compatibility issues that would require significant effort to resolve. Additionally, as DLLs are loaded into memory at runtime, they are open to vectors such as malware and other security threats.
Although DLLs act as a powerful tool for software development, helping reduce complexity, improve performance, and simplify maintenance, you need to consider the potential risks and benefits of it.
How does a DLL work?
Applications that need to use a specific function or resource can do so through a DLL. This is how a typical workflow of a DLLs is.
- Create a DLL: A DLL is created using programming languages such as C or C++. The developer writes the DLL code and compiles it into a DLL file.
- Load the DLL: When an application wants to use a function provided by a DLL, it first checks whether the DLL is already loaded into memory. If not, the program loads the DLL by calling a function such as LoadLibrary or LoadLibraryEx. Then, the OS loads the DLL into memory and maps its code and data sections to the program’s memory space.
- Link to the DLL: Upon loading the DLL, the applications link to it by calling a function defined in the DLL. The application uses a dynamic linking mechanism to connect to the DLL at runtime. Dynamic linking implies that the entire code and data of the DLL are not included in the executable file. Instead, only a small section of code is included that can be resolved at runtime.
- Use the DLL: Upon linking the application to the desired DLL, the application is free to use the functions and resources provided by that DLL. The application calls the functions within the DLL, and subsequently, the DLL executes the code and returns the result to the application. The application can also use pointers or other methods to access data defined in the DLL.
- Unload the DLL: Once the application has finished using the DLL, it can then unload and free the DLL by calling the FreeLibrary function. This is an essential step since it frees up the resources and memory of the used DLL.
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Types of DLL
DLLs are primarily divided into two main types: static and dynamic libraries. Both these types have their unique features, benefits, and drawbacks. Let’s look closer at the various DLL types.
1. Static library (statically-linked library)
Static libraries refer to a set of routines, external functions, and variables compiled and linked into an executable file at compile time. This implies that the code within the static library is part of the executable file itself, making the file larger. As and when the program runs, this code, along with the executable file, is loaded into memory. This approach is suitable for small libraries used frequently since it can improve system performance by cutting down the unnecessary overhead of loading and unloading libraries at runtime.
Static libraries have the following benefits:
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- Since the library code is directly included in the executable file, the libraries are easy to use and fast.
- Static libraries do not require additional runtime files since the entire code is part of the executable file.
Despite the benefits, static libraries also have these drawbacks:
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- These libraries tend to increase the overall size of the executable file, which can be detrimental to large applications.
- If the library code is updated or changed, it demands recompilation, which is a time-intensive process.
2. Dynamic library
Dynamic libraries refer to shared libraries compiled separately from the main executable file and are only loaded during execution. When the program starts, dynamic library code is not loaded into memory; instead, a reference to the library is created. Whenever the program needs to use code from the library, the library is loaded into memory, and the code is subsequently executed. This approach is suitable for large libraries that are not used very often.
A few advantages of dynamic libraries include:
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- Dynamic libraries are more flexible than their static counterparts since they can be updated or changed without recompiling the entire application.
- These libraries reduce the duplication effort as they can be shared between multiple applications.
- Dynamic libraries require less memory since they are loaded and unloaded at runtime.
Dynamic libraries also have some disadvantages:
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- With dynamic libraries, applications should be able to load and unload them at runtime. As a result, the complexity of applications inevitably increases. Moreover, it also slows down the library due to the additional overhead of loading and unloading.
- These libraries sometimes must rely on other runtime files, such as DLLs, for the OS to be used by programs or applications.
Dynamic libraries are further divided into the following sub-types:
- Delay-loaded DLLs: Delay-loaded DLLs are loaded into memory only when needed. This improves the startup time of an application because the amount of memory required at startup is significantly reduced.
- Dynamic-link libraries for COM (Component Object Model): These DLLs are used for COM components that refer to reusable software components used by multiple applications. COM components are compatible with multiple programming languages and can be loaded and unloaded at runtime.
- Windows API DLLs: These DLLs are part of the Windows OS and offer accessibility to system functions and resources. They are loaded into memory when a program uses the Windows API to access a system function or resource.
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Advantages of DLL
DLLs are a popular choice for software development. Listed below are the reasons why developers need DLLs while designing software applications.
DLL Advantages
1. Code reuse
DLLs offer the code reuse facility. When multiple programs intend to employ the same functionality or resource, a DLL can be created that provides code for that very functionality. As a result, developers can write the code once and then allow multiple applications to use it. This reduces the duplication of effort, with developers simply focusing on writing new code rather than re-writing the same code multiple times. This allows significant time savings, increased productivity, and reduced development costs.
2. Improved application performance
DLLs can improve the performance of applications by keeping a check on the overall size of executable files. Whenever a DLL is used in an application, its code and data are loaded into memory only when needed. As a result, the amount of memory required to run the application is considerably reduced. This improves the application’s performance by reducing the time it takes to load the application into memory.
3. Modular architecture
Modular architecture of a dynamic link library refers to the design approach where the DLL is divided into smaller, independent modules that can be loaded and executed separately.
Each module within the DLL is responsible for specific functionality, and they communicate with each other through a defined interface. This approach allows developers to create complex systems by combining different modules rather than writing a monolithic application. Moreover, modularity provides flexibility in adding or removing features and efficiently using system resources.
4. Easy maintenance and updates
DLLs enable easy maintenance and updates. If a bug is found or a new function is added, developers can update the DLL and distribute it to all applications that use it. This reduces the time and effort required to maintain software. Moreover, by breaking an application into smaller, more manageable pieces, developers can make it easier to maintain and update it. This also makes it easier to debug an application since problems can be isolated to specific modules.
5. Versioning feature
DLLs provide version control functionality. Developers can version DLLs independently of the main application, and, as such, they can update and test new features without affecting the stability of the existing application. For example, if a new version of a DLL containing new features is released, an application can use the new version without having to modify its code.
6. Plug-in support
DLLs can be used to create plug-ins or add-ons for software applications. Plug-ins are small software components that can be loaded at runtime to add new functionality to an existing application without requiring the application to be recompiled.
Think of a video editing application that allows users to apply various effects to their videos. The application could be designed to use DLLs to provide plug-in support for new effects. Each DLL could contain a set of functions that implement a particular effect, along with an interface that allows the application to communicate with the DLL.
When the application is launched, it can load all the DLLs located in a particular directory. Each DLL can register itself with the application so that the application knows which effects are available. Users can then choose the effects they want to use, and the application can dynamically load the appropriate DLLs and apply the effects to the video.
Thus, plug-in support allows multiple developers to work on different application components simultaneously without interfering with each other’s work. It also allows users to customize the application to their specific needs by choosing the plug-ins they want to use.
7. Reduced memory usage
DLLs are memory efficient. By using dynamic linking, DLLs can be shared across multiple applications, reducing the overall memory footprint of the system. This can be particularly important for resource-constrained systems such as embedded or mobile devices.
8. Platform independence
DLLs can be platform-independent. By writing DLLs in a platform-independent language such as C or C++, developers can create code that can be used on multiple operating systems, saving time and effort in porting code to different platforms.
9. Security
DLLs support various security features. By providing a mechanism to isolate code from the main application, DLLs can help prevent the spread of viruses or malware. Additionally, DLLs can be digitally signed, which can help verify their authenticity and integrity.
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Takeaway
DLLs provide a powerful mechanism for code reuse, modularity, and memory efficiency while offering platform independence, version control, and security. They allow developers to create efficient, maintainable, and secure applications that enhance productivity, lower costs, and improve user experiences.
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