What exactly are DLL files and how do they work

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How exactly do DLL files work  There seems to be an awful lot of them  but I don t know what they are or how they work   So  what s the deal with them

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DLL is a File Extension  amp  Known As    dynamic link library    file format used for holding multiple codes and procedures for Windows programs  Software  amp  Games runs on the bases of DLL Files  DLL files was created so that multiple applications could use their information at the same time    IF you want to get more information about DLL Files or facing any error read the following post  https   www bouncegeek com fix-dll-errors-windows-586985

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What is a DLL   DLL files are binary files that can contain executable code and resources like images  etc  Unlike applications  these cannot be directly executed  but an application will load them as and when they are required  or all at once during startup    Are they important   Most applications will load the DLL files they require at startup  If any of these are not found the system will not be able to start the process at all   DLL files might require other DLL files  In the same way that an application requires a DLL file  a DLL file might be dependent on other DLL files itself  If one of these DLL files in the chain of dependency is not found  the application will not load  This is debugged easily using any dependency walker tools  like Dependency Walker   There are so many of them in the system folders  Most of the system functionality is exposed to a user program in the form of DLL files as they are a standard form of sharing code   resources  Each functionality is kept separately in different DLL files so that only the required DLL files will be loaded and thus reduce the memory constraints on the system   Installed applications also use DLL files  DLL files also becomes a form of separating functionalities physically as explained above  Good applications also try to not load the DLL files until they are absolutely required  which reduces the memory requirements  This too causes applications to ship with a lot of DLL files   DLL Hell  However  at times system upgrades often breaks other programs when there is a version mismatch between the shared DLL files and the program that requires them  System checkpoints and DLL cache  etc  have been the initiatives from M  to solve this problem  The  NET platform might not face this issue at all   How do we know what s inside a DLL file   You have to use an external tool like DUMPBIN or Dependency Walker which will not only show what publicly visible functions  known as exports  are contained inside the DLL files and also what other DLL files it requires and which exports from those DLL files this DLL file is dependent upon   How do we create   use them   Refer the programming documentation from your vendor  For C    refer to LoadLibrary in MSDN

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Let   s say you are making an executable that uses some functions found in a library   If the library you are using is static  the linker will copy the object code for these functions directly from the library and insert them into the executable   Now if this executable is run it has every thing it needs  so the executable loader just loads it into memory and runs it   If the library is dynamic the linker will not insert object code but rather it will insert a stub which basically says this function is located in this DLL at this location   Now if this executable is run  bits of the executable are missing  i e the stubs  so the loader goes through the executable fixing up the missing stubs  Only after all the stubs have been resolved will the executable be allowed to run   To see this in action delete or rename the DLL and watch how the loader will report a missing DLL error when you try to run the executable   Hence the name Dynamic Link Library  parts of the linking process is being done dynamically at run time by the executable loader   One a final note  if you don t link to the DLL then no stubs will be inserted by the linker  but Windows still provides the GetProcAddress API that allows you to load an execute the DLL function entry point long after the executable has started

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DLL files contain an Export Table which is a list of symbols which can be looked up by the calling program  The symbols are typically functions with the C calling convention    stcall   The export table also contains the address of the function   With this information  the calling program can then call the functions within the DLL even though it did not have access to the DLL at compile time   Introducing Dynamic Link Libraries has some more information

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DLLs  dynamic link libraries  and SLs  shared libraries  equivalent under UNIX  are just libraries of executable code which can be dynamically linked into an executable at load time   Static libraries are inserted into an executable at compile time and are fixed from that point   They increase the size of the executable and cannot be shared   Dynamic libraries have the following advantages   1  They are loaded at run time rather than compile time so they can be updated independently of the executable  all those fancy windows and dialog boxes you see in Windows come from DLLs so the look-and-feel of your application can change without you having to rewrite it    2  Because they re independent  the code can be shared across multiple executables - this saves memory since  if you re running 100 apps with a single DLL  there may only be one copy of the DLL in memory   Their main disadvantage is advantage  1 - having DLLs change independent your application may cause your application to stop working or start behaving in a bizarre manner   DLL versioning tend not to be managed very well under Windows and this leads to the quaintly-named  DLL Hell

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What is a DLL   Dynamic Link Libraries  DLL s are like EXEs but they are not directly executable   They are similar to  so files in Linux Unix   That is to say  DLLs are MS s implementation of shared libraries     DLLs are so much like an EXE that the file format itself is the same   Both EXE and DLLs are based on the Portable Executable  PE  file format   DLLs can also contain COM components and  NET libraries   What does a DLL contain   A DLL contains functions  classes  variables  UIs and resources  such as icons  images  files       that an EXE  or other DLL uses   Types of libraries   On virtually all operating systems  there are 2 types of libraries   Static libraries and dynamic libraries   In windows the file extensions are as follows  Static libraries   lib  and dynamic libraries   dll    The main difference is that static libraries are linked to the executable at compile time  whereas dynamic linked libraries are not linked until run-time   More on static and dynamic libraries   You don t normally see static libraries though on your computer  because a static library is embedded directly inside of a module  EXE or DLL    A dynamic library is a stand-alone file    A DLL can be changed at any time and is only loaded at runtime when an EXE explicitly loads the DLL   A static library cannot be changed once it is compiled within the EXE  A DLL can be updated individually without updating the EXE itself   Loading a DLL   A program loads a DLL at startup  via the Win32 API LoadLibrary  or when it is a dependency of another DLL   A program uses the GetProcAddress to load a function or LoadResource to load a resource   Further reading   Please check MSDN or Wikipedia for further reading    Also the sources of this answer

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According to Microsoft   DLL  Dynamic link libraries are files that contain data  code  or resources needed for the running of applications  These are files that are created by the windows ecosystem and can be shared between two or more applications    When a program or software runs on Windows  much of how the application works depends on the DLL files of the program   For instance  if a particular application had several modules  then how each module interacts with each other is determined by the Windows DLL files    If you want detailed explanation  check these useful resources  What are dll files   About Dll files

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DLLs  Dynamic Link Libraries  contain resources used by one or more applications or services   They can contain classes  icons  strings  objects  interfaces  and pretty much anything a developer would need to store except a UI

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http   support microsoft com kb 815065     A DLL is a library that contains code   and data that can be used by more than   one program at the same time  For   example  in Windows operating systems    the Comdlg32 DLL performs common   dialog box related functions    Therefore  each program can use the   functionality that is contained in   this DLL to implement an Open dialog   box  This helps promote code reuse and   efficient memory usage       By using a DLL  a program can be   modularized into separate components    For example  an accounting program may   be sold by module  Each module can be   loaded into the main program at run   time if that module is installed    Because the modules are separate  the   load time of the program is faster    and a module is only loaded when that   functionality is requested       Additionally  updates are easier to   apply to each module without affecting   other parts of the program  For   example  you may have a payroll   program  and the tax rates change each   year  When these changes are isolated   to a DLL  you can apply an update   without needing to build or install   the whole program again    http   en wikipedia org wiki Dynamic-link library

[windows] What exactly are DLL files, and how do they work?

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