If i have global variable in A.dll, that depends on global variable in B.dll , is it guaranteed that B.dll will be loaded before A.dll? I made two sample dll projects in Visual Studio, and link A.dll with B.dll , and it seems, that B.dll is loaded first.So is this behavior guaranteed ?
This behavior is guaranteed by the OS, because otherwise it would be impossible to write proper dll-loading code. In particular, if A.dll imports B.dll, then when the dynamic linker attempts to load A.dll, it will see that dependency and load B.dll into the process first.
Related
As the title says, how to tell if an exe will load a DLL statically or dynamically by looking at the PE file header?
In other words, how to tell if the DLL is part of the executable, or will be called by the loader?
Thanks
Let me first clarify some terminology to avoid confusion.
Anything executed within a DLL is by definition dynamic. But, a DLL may be statically bound or dynamically bound to an executable.
With static binding, the EXE links against a DLL's import library (actually a .LIB file that is built alongside the DLL). Corresponding DLL function prototypes in header files will usually be declared with __declspec(dllimport). This results in the EXE being filled with stubs for each DLL symbol that are filled in by the Windows loader at runtime. This is facilitated by the final EXE having an import section structure in its PE headers listing all the DLLs to be resolved by the Windows loader at runtime and their symbolic names (e.g. functions). Windows then does all the dirty work to find and load these DLLs and referenced symbolic addresses before the EXE starts execution of the primary thread at its entry point. If Windows fails to find any DLL(s) or referenced symbolic addresses, the EXE won't start.
With dynamic binding, the EXE explicitly invokes code to load DLL(s) and resolve symbolic addresses. This is done using the two KERNEL32 API functions: LoadLibrary() and GetProcAddress(). If an EXE does this, there will be no associated import section describing this DLL and its symbols, and the Windows loader will happily load the EXE knowing nothing said DLL(s). It is then application defined as to how to handle success or failure of LoadLibrary() and /or GetProcAddress().
It is worth noting at this point, that libraries like the C-Runtime may be provided in DLL form (dynamic library) or static form (static library). If you link to one of these libraries statically, there will be no DLL import section in the built EXE's PE header and no function stubs to resolve at runtime for that library. Instead of stubs, these symbols (functions and/or data variables) become part of the EXE. Static library functions and/or data are copied into the EXE and are assigned relative addresses explicitly by the linker; no different than if those symbols were implemented directly by the EXE. Additionally, there will be no LoadLibrary() or GetProcAddress() resolution either implicitly (by the Windows loader) or explicitly in code for these functions as they will be directly present and self-contained within the final EXE. As a side-note, debugging symbols may be used in this case to try and differentiate between EXE implemented functions and library implemented functions (should you care) but this is highly dependent on the settings used to build both the EXE and the static library.
With terminology cleared up, let me attempt to answer your question! :)
Let me also add I'm not going to go into the specifics of bound and unbound import symbols for a module's import section because this distinction has nothing to do with the original question and have more to do with speeding up the work done by the Windows loader. If you are interested in those details however, you can read up on Microsoft's PE COFF Specification.
To see if an EXE is statically bound to a DLL, you can either parse the PE headers yourself to locate the DLL imports section or use one of dozens of tools to do this for you, such as Dependency Walker. If you load your EXE in Dependency Walker for example, you will see a list of all statically bound DLLs in the top-left pane underneath the EXE itself. If any of these DLLs are not found at runtime, the program will fail to load. In the right pane, top table, you will see symbols (e.g. functions) that are referenced in the EXE for the selected DLL. All of these symbols must additionally be found for the EXE to load. The lower table simply shows all of the symbols exported by the DLL, referenced or not.
If the EXE uses dynamic binding (also called it manual binding) for a given DLL, there will be no import section for that DLL and thus you won't see it referenced in tools like Dependency Walker. BUT, you can click on KERNEL32.DLL in Dependency Walker (all EXEs will have this dependency, though there are exceptions to this rule I won't get in to here) and locate references to LoadLibrary() and GetProcAddress(). Unfortunately most EXEs reference these functions in boilerplate code such as the C-Runtime so this won't tell you too much.
If you want to dig deeper into trying to figure out which DLLs are manually loaded by an application, the first thing to try is to and locate that DLL name string by searching the EXE for the DLL name. Note that the DLL name string need not end in ".DLL" as LoadLibrary() automatically assumes this extension if not provided. The standard tool for searching for strings within a binary module is Sysinternals Strings. This works great for modules that make no attempt to hide what they are doing.
With that said, obfuscated code (found in unpackers, viruses and the like) may obfuscate or encrypt DLL names as well as the functions referenced. Code may even choose to resolve LoadLibrary() and GetProcAddress() at runtime to further hinder efforts to figure out what they are doing. Your best bet in these situations is to use a tool like Sysinternals Process Monitor or a debugger with verbose logging enabled to watch the DLLs being loaded as the program runs. You can also use a disassembler (such as IDA) to try and piece together what the code is doing. To find out what DLL symbols are being used, you might start the EXE in a debugger and wait for the initial break at the entry-point. Then add a breakpoint on the first instruction in KERNEL32.GetProcAddress. Run the program. When that breakpoint is hit, the stack arguments will contain the symbol trying to be resolved.
As you can see, if an application resolves DLL symbols manually (dynamic binding), the process of figuring out what DLLs are being referenced is not as straightforward.
One of the points with component oriented software is that metadata has replaced linking.
This means that in the absence of strong naming and versioning of assemblies changing and recompiling an assembly B that is referenced by an assembly A should not require the recompilation of A.
But this is what happens is Visual Studio!
The output window contains the text "B.dll" is newer than output file "A.dll" so it stems from the target definition and moreover the assembly A.dll is actually changed.
I would like to know the reason for this and what is changed in A.dll. Looking at the old and new versions of A.dll in IL Spy they look identical.
When does Windows Operating System load a DLL into memory?
Does the operation occur when the application starts or when the application first calls one of the procedures in the DLL?
Could a DLL be unloaded once it has been loaded?
When does Windows Operating System
load a DLL into memory?
If you've linked your EXE to a DLL implicitly through a .lib file, like you normally do for most windows apis such as user32.dll and kernel32.dll, then the defautl behavior is for the DLL to get loaded when the process starts and before your WinMain/main function is called. See below for delay loading...
If one DLL depends on another, it will load its dependencies first if they are not already loaded.
If you are explicitly loading code through a DLL (LoadLibrary, CoCreateInstance, etc...), then it will get loaded upon making these calls
Does the operation occur when the
application starts or when the
application first calls one of the
procedures in the DLL?
You can have it both ways. By default, DLL is loaded at app startup. If you used the /DELAYLOAD linker flag, the DLL may be able to defer being loaded until its actually needed. This is "best effort" - if there are weird export dependencies with global variables, it may not work.
Could a DLL be unloaded once it has been loaded?
Short answer is "no" for implicit DLL dependencies that you've linked. FreeLibrary and CoFreeUnusedLibrary can be used for LoadLibrary/CoCreateInstance calls.
I'm going to assume we are talking .net. It is garanteed to happen before you need the code. But you can use late binding to do it at some other time. See this pagelink text
In the windows API, you can explicitly control the loading and unloading of a .dll.
See LoadLibrary and FreeLibrary as a starting point.
Depending on the language/tools you are using many of the details of loading libraries will be taken care of for you, but usually you can still get explicit control if you really want it.
I want to know the exact difference between the dll and exe file.
I don't know why everybody is answering this question in context of .NET. The question was a general one and didn't mention .NET anywhere.
Well, the major differences are:
EXE
An exe always runs in its own address space i.e., It is a separate process.
The purpose of an EXE is to launch a separate application of its own.
DLL
A dll always needs a host exe to run. i.e., it can never run in its own address space.
The purpose of a DLL is to have a collection of methods/classes which can be re-used from some other application.
DLL is Microsoft's implementation of a shared library.
The file format of DLL and exe is essentially the same. Windows recognizes the difference between DLL and EXE through PE Header in the file. For details of PE Header, You can have a look at this Article on MSDN
EXE:
It's a executable file
When loading an executable, no export is called, but only the module entry point.
When a system launches new executable, a new process is created
The entry thread is called in context of main thread of that process.
DLL:
It's a Dynamic Link Library
There are multiple exported symbols.
The system loads a DLL into the context of an existing process.
For More Details: http://www.c-sharpcorner.com/Interviews/Answer/Answers.aspxQuestionId=1431&MajorCategoryId=1&MinorCategoryId=1
http://wiki.answers.com/Q/What_is_the_difference_between_an_EXE_and_a_DLL
Reference: http://www.dotnetspider.com/forum/34260-What-difference-between-dll-exe.aspx
The difference is that an EXE has an entry point, a "main" method that will run on execution.
The code within a DLL needs to be called from another application.
There are a few more differences regarding the structure you could mention.
Both DLL and EXE share the same file structure - Portable Executable, or PE. To differentiate between the two, one can look in the Characteristics member of IMAGE_FILE_HEADER inside IMAGE_NT_HEADERS. For a DLL, it has the IMAGE_FILE_DLL (0x2000) flag turned on. For a EXE it's the IMAGE_FILE_EXECUTABLE_IMAGE (0x2) flag.
PE files consist of some headers and a number of sections. There's usually a section for code, a section for data, a section listing imported functions and a section for resources. Some sections may contain more than one thing. The header also describes a list of data directories that are located in the sections. Those data directories are what enables Windows to find what it needs in the PE. But one type of data directory that an EXE will never have (unless you're building a frankenstein EXE) is the export directory. This is where DLL files have a list of functions they export and can be used by other EXE or DLL files. On the other side, each DLL and EXE has an import directory where it lists the functions and DLL files it requires to run.
Also in the PE headers (IMAGE_OPTIONAL_HEADER) is the ImageBase member. It specifies the virtual address at which the PE assumes it will be loaded. If it is loaded at another address, some pointers could point to the wrong memory. As EXE files are amongst the first to be loaded into their new address space, the Windows loader can assure a constant load address and that's usually 0x00400000. That luxury doesn't exist for a DLL. Two DLL files loaded into the same process can request the same address. This is why a DLL has another data directory called Base Relocation Directory that usually resides in its own section - .reloc. This directory contains a list of places in the DLL that need to be rebased/patched so they'll point to the right memory. Most EXE files don't have this directory, but some old compilers do generate them.
You can read more on this topic # MSDN.
This answer was a little more detailed than I thought but read it through.
DLL:
In most cases, a DLL file is a library. There are a couple of types of libraries, dynamic and static - read about the difference. DLL stands for dynamic link library which tells us that it's a part of the program but not the whole thing. It's made of reusable software components (library) which you could use for more than a single program. Bear in mind that it's always possible to use the library source code in many applications using copy-paste, but the idea of a DLL/Static Library is that you could update the code of a library and at the same time update all the applications using it - without compiling.
For example:
Imagine you're creating a Windows GUI component like a Button. In most cases you'd want to re-use the code you've written because it's a complex but a common component - You want many applications to use it but you don't want to give them the source code You can't copy-paste the code for the button in every program, so you decide you want to create a DL-Library (DLL).
This "button" library is required by EXEcutables to run, and without it they will not run because they don't know how to create the button, only how to talk to it.
Likewise, a DLL cannot be executed - run, because it's only a part of the program but doesn't have the information required to create a "process".
EXE:
An executable is the program. It knows how to create a process and how to talk to the DLL. It needs the DLL to create a button, and without it the application doesn't run - ERROR.
hope this helps....
Both DLL and EXE are Portable Executable(PE) Formats
A Dynamic-link library (DLL) is a library and therefore can not be executed directly. If you try to run it you will get an error about a missing entry point. It needs an entry point (main function) to get executed, that entry point can be any application or exe. DLL binding occurs at run-time. That is why its called "Dynamic Link" library.
An Executable (EXE) is a program that can be executed. It has its own entry point. A flag inside the PE header indicates which type of file it is (irrelevant of file extension). The PE header has a field where the entry point for the program resides. In DLLs it isn't used (or at least not as an entry point).
There are many software available to check header information. The only difference causing both to work differently is the bit in header as shown in below diagram.
EXE file has only single main entry means it is isolated application, when a system launches exe, a new process is created while DLLs have many entry points so when application use it no new process started, DLL can be reused and versioned. DLL reduces storage space as different programs can use the same dll.
Dll v/s Exe
1)DLL file is a dynamic link library which can be used in exe files and
other dll files.
EXE file is a executable file which runs in a separate
process which is managed by OS.
2)DLLs are not directly executable . They are separate files containing functions that can be called by programs and other DLLs to perform computations and functions.
An EXE is a program that can be executed . Ex :Windows program
3)Reusability
DLL: They can be reused for some other application. As long as the coder knows the names and parameters of the functions and procedures in the DLL file .
EXE: Only for specific purpose .
4)A DLL would share the same process and memory space of the calling application while an
EXE creates its separate process and memory space.
5)Uses
DLL: You want many applications to use it but you don't want to give them the source code You can't copy-paste the code for the button in every program, so you decide you want to create a DL-Library (DLL).
EXE: When we work with project templates like Windows Forms Applications, Console Applications, WPF Applications and Windows Services they generate an exe assembly when compiled.
6)Similarities :
Both DLL and EXE are binary files have a complex nested structure defined by the Portable Executable format, and they are not intended to be editable by users.
Two things: the extension and the header flag stored in the file.
Both files are PE files. Both contain the exact same layout. A DLL is a library and therefore can not be executed. If you try to run it you'll get an error about a missing entry point. An EXE is a program that can be executed. It has an entry point. A flag inside the PE header indicates which file type it is (irrelevant of file extension). The PE header has a field where the entry point for the program resides. In DLLs it isn't used (or at least not as an entry point).
One minor difference is that in most cases DLLs have an export section where symbols are exported. EXEs should never have an export section since they aren't libraries but nothing prevents that from happening. The Win32 loader doesn't care either way.
Other than that they are identical. So, in summary, EXEs are executable programs while DLLs are libraries loaded into a process and contain some sort of useful functionality like security, database access or something.
The .exe is the program. The .dll is a library that a .exe (or another .dll) may call into.
What sakthivignesh says can be true in that one .exe can use another as if it were a library, and this is done (for example) with some COM components. In this case, the "slave" .exe is a separate program (strictly speaking, a separate process - perhaps running on a separate machine), but one that accepts and handles requests from other programs/components/whatever.
However, if you just pick a random .exe and .dll from a folder in your Program Files, odds are that COM isn't relevant - they are just a program and its dynamically-linked libraries.
Using Win32 APIs, a program can load and use a DLL using the LoadLibrary and GetProcAddress API functions, IIRC. There were similar functions in Win16.
COM is in many ways an evolution of the DLL idea, originally concieved as the basis for OLE2, whereas .NET is the descendant of COM. DLLs have been around since Windows 1, IIRC. They were originally a way of sharing binary code (particularly system APIs) between multiple running programs in order to minimise memory use.
An EXE is visible to the system as a regular Win32 executable. Its entry
point refers to a small loader which initializes the .NET runtime and tells
it to load and execute the assembly contained in the EXE.
A DLL is visible to the system as a Win32 DLL but most likely without any
entry points. The .NET runtime stores information about the contained
assembly in its own header.
dll is a collection of reusable
functions where as an .exe is an
executable which may call these
functions
An exe is an executible program whereas A DLL is a file that can be loaded and executed by programs dynamically.
● .exe and dll are the compiled version of c# code which are also called as
assemblies.
● .exe is a stand alone executable file, which means it can executed directly.
● .dll is a reusable component which cannot be executed directly and it requires
other programs to execute it.
For those looking a concise answer,
If an assembly is compiled as a class library and provides types for other assemblies to use, then it has the ifle extension .dll (dynamic link library), and it cannot be executed standalone.
Likewise, if an assembly is compiled as an application, then it has the file extension .exe (executable) and can be executed standalone. Before .NET Core 3.0, console apps were compiled to .dll fles and had to be executed by the dotnet run command or a host executable. - Source
Difference in DLL and EXE:
1) DLL is an In-Process Component which means running in the same memory space as the client process. EXE is an Out-Process Component which means it runs in its own separate memory space.
2) The DLL contains functions and procedures that other programs can use (promotes reuability) while EXE cannot be shared with other programs.
3) DLL cannot be directly executed as they're designed to be loaded and run by other programs. EXE is a program that is executed directly.
The major exact difference between DLL and EXE that DLL hasn't got an entry point and EXE does. If you are familiar with c++ you can see that build EXE has main() entry function and DLL doesn't :)
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?
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.
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.
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.
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".
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.
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
DLL is a File Extension & Known As “dynamic link library” file format used for holding multiple codes and procedures for Windows programs. Software & 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/
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.
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