Side-by-side technology is a standard for executable files in Microsoft Windows XP and later versions that attempts to reduce DLL hell. Side-by-side technology is also known as WinSxS or SxS. Executables that include an SxS manifest are designated SxS assemblies.
DLL hell designates a group of problems that arise from the use of dynamic-link libraries in Microsoft Windows. Problems include version conflicts, missing DLLs, duplicate DLLs, and incorrect or missing registration. In SxS, Windows stores multiple versions of a DLL in the WinSXS subdirectory of the Windows directory, and loads them on demand. This reduces dependency problems for applications that include an SxS manifest.
Microsoft Visual C++ 2005 and later employ SxS with all C runtime libraries.
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Operation
An application that employs SxS must have a manifest. Manifests are typically a section embedded in the application's executable file but may also be an external file. When the operating system loads the application and detects the presence of a manifest, the operating system DLL loader is directed to the version of the DLL corresponding to that listed in the manifest. If there is no manifest, the DLL loader loads a default version of all DLL dependencies.
Manifest format
The manifest is internally represented as XML. The URN associated with SxS manifests is "urn:schemas-microsoft-com:asm.v1".
Several other recent Microsoft technologies such as ClickOnce employ the same manifest format.
Example manifest
The following is an example of a manifest for an application that depends on a C runtime DLL.
<?xml version='1.0' encoding='UTF-8' standalone='yes'?> <assembly xmlns='urn:schemas-microsoft-com:asm.v1' manifestVersion='1.0'> <trustInfo xmlns="urn:schemas-microsoft-com:asm.v3"> <security> <requestedPrivileges> <requestedExecutionLevel level='asInvoker' uiAccess='false' /> </requestedPrivileges> </security> </trustInfo> <dependency> <dependentAssembly> <assemblyIdentity type='win32' name='Microsoft.VC90.CRT' version='9.0.21022.8' processorArchitecture='x86' publicKeyToken='1fc8b3b9a1e18e3b' /> </dependentAssembly> </dependency> </assembly>
Advantages
- For applications that have been built with SxS, multiple applications may coexist that depend on different versions of the same DLL. This is in contrast to non-SxS DLL environments where an original DLL in a shared system folder may be overwritten by the subsequent installation of another program that depends on a different version of the same DLL.
- The XML formatting of the manifest is human-legible and thus makes it easier for developers to determine the dependencies of an application and their versions.
Disadvantages
- The benefits of SxS are not completely retroactive; that is, DLL hell may still occur for applications with no manifest or for applications that do not follow the SxS DLL installation strategy.
- Thus far, Windows XP and later versions are the only operating systems to implement SxS, so none of the advantages of SxS are portable to older versions of Windows. However, Wine for GNU/Linux does support SxS in its emulation of Windows XP.
See also
External links
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