Why use DLL files?

From http://hi.baidu.com/liuxiaoustb/item/beb0750b9719b83c4ac4a3ba

 

View DLL files

In the Windows world, when there are several active continents, they all have a common name-dynamic link library. Now, let's go into these magical activity continents and find out the secrets they have been hiding for a long time!

First glance: the cornerstone of Windows

Open a system directory and you will see a lot of dll extension files at a glance. These are what we often call "Dynamic Link Library ", DLL is the abbreviation of dynamic link library ("Dynamic Link Library. Since Microsoft launched its first version of Windows, the dynamic link library has been the basis of this operating system.

1. check what is in the DLL.

To solve what DLL is, let's first look at what dll has. Like the EXE file, the DLL file contains the binary code of the program and the resources required by the program (such as the comparison mark, dialog box, and string). Why do you need to put the code in the DLL file, instead of EXE? In fact, the DLL code appears in the form of API functions. In other words, the program code contained in the DLL is made into small modules, and the application presses the specific button in the required DLL, to call the Functions Represented by this button in the DLL. When using a program such as Notepad, if you want to save or open the file, a general file dialog box is displayed, selecting the file location. As you know, this is the interface that calls the General dialog box in the underlying DLL of the system.

2. Several important DLL files in the system

Windows has three very important underlying dll: kernel32.dll, user32.dll, and gdi32.dll. Kernel32.dll, as its name implies, is a kernel-related function. It mainly includes functions used to manage memory, processes, and threads. user32.dll contains functions used to execute User Interface tasks, for example, you can pass the user's mouse-click operation to the window so that the window can execute the scheduled event based on the user's click. The gdi32.dll name is abbreviated, the full name is graphical device interface, which contains functions used for drawing and displaying text. For example, to display a program window, you call the function to draw the window.

3. Why use DLL?

When talking about this problem just now, I only explained the principle of DLL encapsulating program code into functions. Why is it possible to encapsulate a function into a large number of DLL resources in the system?

① Extend the application

The dll can be dynamically loaded into the memory by the application. Therefore, the application can load the DLL into the memory only when needed, which makes the maintainability of the program very high. For example, if the video function of QQ needs to be upgraded, programmers responsible for compiling QQ do not have to rewrite all QQ code. They only need to rewrite DLL files related to the video function.

② Facilitates coders to cooperate

This has little to do with the end user and is only for your understanding. We all know that there are many programming tools, such as VB, Vc, and Delphi. If several people work together to compile a large program, some may use VB and others use VC, each person is responsible for different programming languages. Which compiler is used for compilation? This is better than a group of people from different countries writing an article together. If they use different languages, how can the written articles be written together? With the DLL, the VC programmer can write a DLL, and then the VB programmer calls it in the program, without worrying about how to compile them into a separate exe.

③ Memory saving

If multiple applications call the same dynamic link library, the DLL file will not be repeatedly loaded into the memory, instead, these applications share the same DLL with loaded memory. In an office, a water dispenser is rarely configured for every employee. Instead, a water dispenser is placed in a public location. All employees who need water can share the water dispenser, reduces costs and saves space.

④ Sharing program resources

Including the general file dialog box mentioned above, DLL files provide the possibility of sharing resources between applications. Resources can be program dialogs, strings, icons, or sound files.

⑤ Solve application localization problems

After downloading the Chinese package of a program, open the Chinese description. you can often see that the DLL file in the downloaded package overwrites the original DLL of the program. These programs are compiled separately from the Execution Code and application interface. Therefore, the Chinese users only need to simply compile and release the DLL related to the program interface.

Curious: Exploring the truth about DLL

Who knows how many functions are in the DLL, and who knows which DLL functions are called by the EXE? In fact, this problem is not difficult to solve. The dependency Walker tool used to analyze the EXE file (depends). Today, it is an adventure tool for everyone to explore the DLL truth.

1. Check the number of functions in the DLL.

Step 1: Download and decompress depends, run depends.exe, select "File> open" (File> open), select the DLL file to be analyzed in the file selection box, and open it. Select qqzip In the QQ directory. DLL.

Step 2: In the tree column on the left of the program, it lists the functions of other DLL used by the DLL (Other DLL ^ o ^ can be called in the original DLL ), the two column lists on the right show the function input and output tables respectively. The function output table is the total list of functions provided by the DLL to other EXE or DLL calls.

Step 3: In the function column of the function output table, it is the name of the output function (see figure 1). Two functions are found in qqzip. dll: unzip and zip. Therefore, it can be determined that the DLL is responsible for the compression and decompression tasks in the QQ program.

2. Review the DLL used by the EXE

The list of DLL files called by the dig command (see figure 2). If you expand these DLL branches, you will find other DLL files. This shows that these DLL files called by QQ are still possible (almost certainly) call another DLL. This is like buying a new DVD machine, where the core may be Sony, and a small capacitor in this movement may be owned by another company. This is the same principle.

Figure 2 DLL called by qq.exe

3. Use DLL to see the true face of exe

I just got the list of DLL used by qq.exe. In fact, this list can also analyze a lot of other information. Similar to mfc42.dll, you can use visual c ++ to compile qq.exe. wsock32.dll indicates that this program has the network communication function (nonsense! If QQ cannot communicate via the Internet, what's the use ......). The following is a simple table. When analyzing other EXE files, you can use the DLL to preliminarily judge its functions.

The EXE information that can be determined by the DLL file name

Mfc42.dll is written using vc5.0/6.0.
Vbrun *. dll "*" indicates the digital version, which is written in vb3.0/4.0.
Msvbvm50.dll is written in VB5.0 and comes with this DLL on Windows 98 (SE.
Msvbvm60.dll is written in VB6.0 and is provided on Windows ME/2000/XP and other systems.
Advapi32.dll may perform registry operations.
Wsock32.dll supports network communication.
Ws2_32.dll supports network communication.
Wininet. dll provides HTTP browsing and downloading functions. Typical examples are browsers and download tools.
Winmm. dll supports multimedia playback.
Ddraw. dll games and advanced image processing tools.
D3d *. dll 3D games, or animation processing tools.
4. dll is a big treasure

In addition to the DLL used by the program to call functions, there is another DLL used to save resources, such as qqres under the QQ directory. DLL, opened with depends and found that there is no output function, is it a chicken-rib DLL? However, use the resource hacker (: http://www.onlinedown.net/soft/12420.htm) to open this DLL, it is found that the original save so many QQ resources, including icons, music, pictures, strings, dialog box ...... (See figure 3)

Not complete ...............