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This article describes what is a dynamic link library (DLL) and various problems that may occur when using DLL. Then, this article describes some advanced issues that should be considered when developing your own DLL. What is d...
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This article describes what is a dynamic link library (DLL) and various problems that may occur when using DLL.Then, this article describes some advanced issues that should be considered when developing your own DLL. This article describes the Dynamic Link Method, DLL dependency, DLL entry point, export DLL functions, and DLL troubleshooting tools.
At the end of this articleProgramSet as a comparison.
Back to TopIntroduction
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For Microsoft Windows operating systems listed in the "apply to" section, a large number of features of the operating system are provided by the dynamic link library (DLL)
Use DLL
FacilitatingCodeModular, code reuse, effective use of memory, and reduced disk space. Therefore, the operating system and program can load and run faster, and occupy less disk space in the computer.
When the program uses
DLL, a problem called dependency may cause the program to fail to run. When the program uses DLL
A dependency is created. If other programs rewrite and corrupt the dependency, the original program may not be able to run successfully.
When Microsoft. NET is introduced
After the framework, most dependency problems have been eliminated by using the assembly.
What is DLL? DLL is a library that contains code and data that can be used by multiple programs at the same time. For example, in a Windows operating system, comdlg32 DLL runs Common Dialog Box-related operations...
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What is DLL?
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DLL is a library that contains code and data that can be used by multiple programs at the same time. For example, in windows, comdlg32 DLL
Execute common functions related to the dialog box. Therefore, each program can use the DLL
To enable the "open" dialog box. This helps promote code reuse and effective memory usage.
Use
DLL, the program can be modularized, composed of relatively independent components. For example, an accounting program can be sold by module. Each module can be loaded into the main program at runtime (if the corresponding module is installed ). Because modules are independent from each other, the program loading speed is faster, and the module is loaded only when the corresponding functions are requested.
In addition, it is easier to apply updates to each module without affecting other parts of the program. For example, you may have a salary calculation program, and the tax rate is changed every year. When these changes are isolated
DLL, you do not need to regenerate or install the entire program to update the application.
The following table describes some files implemented as DLL in Windows:
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The following table describes some advantages of using DLL:
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When a program or DLL uses a DLL in another DLL
When the function is used, the dependency is created. Therefore, the program is no longer independent, and if the dependency is damaged, the program may encounter problems. For example, if one of the following operations occurs, the program may not run:
These operations are usually called DLL
Conflict. If backward compatibility is not enforced, the program may fail to run.
The following table describes how to minimize dependency issues in Microsoft
Changes introduced in Windows 2000 and later versions:
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You can use multiple tools to solve the DLL problem. The following are some of the tools.
Dependency Walker
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The dependency Walker tool can recursively scan all the dependent DLL used by the program. When you are in the dependency Walker
When the program is opened, dependency Walker performs the following checks:
By using dependency Walker, you can record all the DLL used by the program. This may help avoid and correct the DLL that may occur in the future.
Problem. When you install Microsoft Visual Studio 6.0, dependency Walker is located in the following directory:
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The DLL universal problem solver (dups) tool is used to review, compare, record, and display DLL information. The following table describes the composition
Dups tool utility:
Related
For more information about the dups tool, clickArticleTo view the articles in the Microsoft Knowledge Base:
Example: Use dups.exe to solve DLL compatibility problems
DLL help Database
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DLL helps you find a specific version of DLL installed by Microsoft software products. Related DLL
For more information about the help database, visit the following Microsoft Website:
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This section describes the issues and requirements you should consider when developing your own DLL.
DLL type
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When you load a DLL in an application, you can use two link methods to call the exported DLL function. These two link methods are dynamic links during loading and dynamic links during runtime.
Dynamic Link during loading
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In the dynamic link during loading, the application calls the exported DLL just like a local function.
The function is called explicitly. To use the dynamic link during loading, provide the header file (. h) and imported file (. Lib) when compiling and linking the application ). When you do this, the linker will provide the system with DLL Loading
The required information, and parse the location of the exported DLL function during loading.
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In the dynamic link at runtime, the application callsLoadlibrary
Function orLoadLibraryEx
Function to load the DLL at runtime. After successfully loading the DLL, you can useGetprocaddress
Function to obtain the exported DLL
The address of the function. When using dynamic links during running, you do not need to use import/export files.
The following list describes the conditions for when to use the dynamic link at loading and when to use the dynamic link at runtime:
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When creating a DLL, you can choose to specify the entry point function. When a process or thread attaches itself to a DLL or
When splitting, the entry point function is called. You can use the entry point function according to the DLL
To initialize or destroy the data structure. In addition, if the application is multi-threaded, you can use the Thread Local Storage (TLS) in the entry point function)
To allocate dedicated memory for each thread. The following code is an example of a DLL entry point function.
Bool apientry dllmain (
Handle hmodule, // handle to DLL module
DWORD ul_reason_for_call, // reason for calling function
Lpvoid lpreserved) // Reserved
{
Switch (ul_reason_for_call)
{
Case dll_process_attached:
// A process is loading the DLL.
Break;
Case dll_thread_attached:
// A process is creating a new thread.
Break;
Case dll_thread_detach:
// A thread Exits normally.
Break;
Case dll_process_detach:
// A process unloads the DLL.
Break;
}
Return true;
}
When the entry point function returnsFalse
If you are using a dynamic link during loading, the application will not start. If you are using a dynamic link during runtime, only a few DLL
Not loaded.
The entry point function should only execute simple initialization tasks and should not call any other DLL to load or terminate functions. For example, the entry point function should not be called directly or indirectly.Loadlibrary
Function orLoadLibraryEx
Function. In addition, it should not be called at Process Termination.Freelibrary
Function.
Note:
: In multi-threaded applications, ensure that the access to global DLL data is synchronized (thread security) to avoid possible data corruption. To do this, use TLS
Provide unique data for each thread.
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To export a DLL function, you can add a function keyword to the exported DLL function, or create a module definition file (. Def) to list the exported DLL.
Function.
To use the function keyword, you must use the following keywords to declare the functions to be exported:
_ Declspec (dllexport)
To use the exported DLL function in an application, you must use the following keywords to declare the functions to be imported:
_ Declspec (dllimport)
Generally, you 'd better useDefine
Statement andIfdef
Statement header file to separate the Export Statement and Import Statement.
You can also use the module definition file to declare the exported DLL.
Function. When you use a module definition file, you do not need to add function keywords to the exported DLL function. In the module definition file, you can declareLibrary
Statement andExports
Statement. The following code is an example of a definition file.
Example DLL and Application// Sampledll. Def
//
Library "sampledll"
Exports
Helloworld
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In Microsoft Visual C ++ 6.0, you can select the "Win32 dynamic link library" project type or the "MFC Application Wizard"
(DLL) to create a DLL.
The following code is an example of a DLL created using the "Win32 dynamic link library" project type in Visual C ++.
// Sampledll. cpp
//
# Include "stdafx. H"
# Define exporting_dll
# Include "sampledll. H"
Bool apientry dllmain (handle hmodule,
DWORD ul_reason_for_call,
Lpvoid lpreserved
)
{
Return true;
}
Void helloworld ()
{
MessageBox (null, text ("Hello World"), text ("in a DLL"), mb_ OK );
}
// File: sampledll. h
//
# Ifndef indll_h
# Define indll_h
# Ifdef exporting_dll
Extern _ declspec (dllexport) void helloworld ();
# Else
Extern _ declspec (dllimport) void helloworld ();
# Endif
# Endif
The following code is an example of a "Win32 application" project. This example calls the export DLL function in sampledll.
// Sampleapp. cpp
//
# Include "stdafx. H"
# Include "sampledll. H"
Int apientry winmain (hinstance,
Hinstance hprevinstance,
Lpstr lpcmdline,
Int ncmdshow)
{
Helloworld ();
Return 0;
}
Note:
: In the dynamic link during loading, you must link sampledll. Lib created when the sampledll project is generated.
Import to warehouse.
In the dynamic link during runtime, you should use code similar to the following code to call sampledll. DLL to export the DLL function.
...
Typedef void (* dllproc) (lptstr );
...
Hinstance hinstdll;
Dllproc helloworld;
Bool ffreedll;
Hinstdll = loadlibrary ("sampledll. dll ");
If (hinstdll! = NULL)
{
Helloworld = (dllproc) getprocaddress (hinstdll, "helloworld ");
If (helloworld! = NULL)
(Helloworld );
Ffreedll = freelibrary (hinstdll );
}
...
When you compile and link the sampledll application, the Windows operating system searches for sampledll in the following order:
Note:
:Getsystemdirectory
The function returns the path of the Windows system folder.
Note:
:Getwindowsdirectory
The function returns the path of the Windows folder.
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After Microsoft. NET and. NET Framework are introduced, most
The associated problems have been eliminated by using the assembly. An assembly is a logical functional unit that runs under the control of the. NET Common Language Runtime Library (CLR. The Assembly is actually used as a. dll file or
The. exe file exists. However, internally, the Assembly and Microsoft Win32 DLL
Big differences.
The Assembly file contains the Assembly List, type metadata, and Microsoft intermediate language (msil)
Code and other resources. The Assembly List contains Assembly metadata to provide all the information required to make the Assembly a self-describing assembly. The Assembly list contains the following information:
The msil code contained in the program cannot be directly executed.
. By default, when you create an assembly, the Assembly is proprietary to the application. To create a shared assembly, you must assign a strong name to the Assembly and publish the assembly in the Global Assembly Cache.
The following table describes some functions of the Assembly and
The functions of Win32 DLL are compared:
For more information about DLL and. NET Framework Assembly, visit the following Microsoft Website: DLL help database http://support.mic...
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For more information about DLL and. NET Framework Assembly, visit the following Microsoft Website:DLL help Database
DLL conflict (DLL conflicts)
Http://msdn2.microsoft.com/en-us/library/ms811694.aspx
Http://msdn2.microsoft.com/en-us/library/ms811694.aspx)
Implementing parallel component sharing in applications (implementing side-by-side component sharing in applications)
Http://msdn2.microsoft.com/en-us/library/ms811700.aspx
Http://msdn2.microsoft.com/en-us/library/ms811700.aspx)
How to generate and maintain independent applications and parallel assembly for Windows XP (how to build and service isolated applications and side-by-side assemblies for Windows XP)
Http://msdn2.microsoft.com/en-us/library/ms997620.aspx
Http://msdn2.microsoft.com/en-us/library/ms997620.aspx)
Use. NET Framework to simplify deployment and resolve DLL conflicts (Simplifying deployment and solving DLL conflicts with the. NET Framework)
Http://msdn2.microsoft.com/en-us/netframework/aa497268.aspx
Http://msdn2.microsoft.com/en-us/netframework/aa497268.aspx)
. NET Framework developer guide: Assembly
Http: // msdn2.microsoft.com/en-us/library/hk5f 40ct (vs.71). aspx
(Http://msdn2.microsoft.com/en-us/library/hk5f 40ct (vs.71). aspx)
Dynamic Link during running
Http://msdn2.microsoft.com/en-us/library/ms685090.aspx
Http://msdn2.microsoft.com/en-us/library/ms685090.aspx)
Thread Local Storage
http://msdn2.microsoft.com/en-us/library/ms686749.aspx
(http://msdn2.microsoft.com/en-us/library/ms686749.aspx)
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