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
I have a C-based DLL that I wrote years ago for a project and it exports a set of functions that define an API. Now I need to re-write this DLL's internals but keep the API exactly the same.
The user of the DLL used static linking and they do not want to or are unable to recompile their executable.
I've noticed that the RVAs of the exported functions are different. My understanding is that means the executable won't be able to find the functions unless it is re-linked with the updated lib file.
Is there a way in VS2017 to force an exported function to use a specific RVA? I checked the Microsoft LINK DEF file format and I didn't see an option in there.
Even if it is possible, is fixing the RVAs enough to ensure the old executable will be able to use the updated DLL or are there additional complications that make this a non-starter?
Thanks.
When you statically link an EXE module against a DLL, you do indeed link against the the DLL's import library (a .LIB) created alongside the DLL when the DLL was built. This is not the same thing as linking against a static library which is confusing because those are also .LIB files.
The first thing you should do is figure out if your EXE module has an import entry for said DLL using a tool like Dependency Walker, Dumpbin, pelook or your favorite PE analyzer tool. If there is no DLL import entry, you have have probably linked the EXE against a static library as described by #HAL9000 's answer. Short of reverse-engineering the EXE, your best bet would be to rebuild the module as suggested if possible.
Otherwise, if you do find an import for said DLL, then yes you can swap out a newly-built DLL provided you have the same export (function) names and/or ordinal values as the original. DLLs find function by export names and/or ordinal values, not RVAs which in this case are only an internal detail. Whether the DLL is implicitly loaded (from being statically-linked) during process (EXE) initialization (before the EXE's entry point is called) or explicitly loaded (via code using LoadLibrary, etc.) the whole point of being a DLL is that it is a module is designed to be dynamically replaced and Windows was designed around this concept. The internal RVAs both within the EXE (referencing the DLL) and the DLL itself do not need to match an old DLL's values; this bookkeeping is automatically handled by the Windows loader during a process also known as runtime linking.
In the event the EXE is linked against said DLL and ALSO specifies hard-coded addresses (RVAs) for the DLL's exported functions (a process known as static binding), Windows will still verify the addresses still internally reflect the correct values in the DLL that is actually loaded which may be a different, updated DLL. This is done via a timestamp check in the import section for the DLL. If there is a mismatch, the Windows loader tosses-out all of the static RVAs and updates them with the current values incurring a slight performance penalty, but the program will still load. FWIW the bind.exe tool to do this static binding no longer ships with the Visual C++ toolset as the performance gain in modern versions of Windows is minimal. This optimization used to be be common practice to speed up load times, especially in OS-supplied system DLLs, but shouldn't affect what you are trying to do one way or the other.
If the user has statically linked in your library, then it is not a DLL, and making a drop in replacement without relinking is not possible. At least not without some ugly hacks. The old library functions have been copied into the executable, so there are no way around editing the executable. If you can't recompile or relink, then it is probably easier to rewrite the executable from scratch.
Mucking around with adresses of functions in your new DLL, if possible, can't have any effect if the executable doesn't have any code to load a DLL at all.
I have a basic question that I cannot find the answer to even after searching the web many times.
Is it possible to compile a Fortran program by IFORT that uses (as dependencies) object files (i.e. .lib) that were compiled by Microsoft Visual Studio C?
Yes, it is possible, and is even rather easy. You have several options for how to do this:
Add the MSVC library project to your Fortran Visual Studio solution and then use Project > Dependencies to add the C project as a dependent of your Fortran project *does not work if the C project creates a DLL)
Add the .lib from the C project to the Fortran project as if it were a source file
Name the C .lib in the Linker > Input > Additional Dependencies project property.
I generally recommend the first choice, as it means you don't have to fuss with different settings for Debug and Release projects. You do need to make sure that the C library is built to specify the same run-time library type (Debug vs. Nondebug, DLL vs. Static) as the Fortran project. This is in the Code Generation property page for C.
There is a worked example "Fortran Calls C" in the Intel Parallel Studio XE for Windows Sample Bundle.
You'll also need to understand how to call a C procedure from Fortran and ensure that the C arguments have compatible Fortran types. It works best if you make use of the Fortran standard's "C interoperability" features.
The background is following: there is 3'rd party provider that provides us with a libveryfancylib.so, in 32b. Softaware that uses the library has quite a load of other linux library dependencies (like QT) also, but they are open source, so no problem for statical linking. The target platform is 64b and running Debian 7.
We can ship the program with binary + dynamical libraries, no problem, but i would rather see single static binary with no dependencies.
So my question is: why i cannot link the dynamical library into static binary? I mean what bit of information is there missing, or is it just feature that is rarely needed -> not implemented.
We can ship the program with binary + dynamical libraries, no problem, but i would rather see single static binary with no dependencies.
What is the problem you are trying to solve?
You can follow the model most commercial applications on Linux do: put your executable, shared libraries and other resources in one directory (possibly with subdirectories). When linking your executable against those shared libraries pass -Wl,-rpath,'$ORIGIN' (in make use -Wl,-rpath,'$$ORIGIN') to the linker, so that when starting your application the runtime linker looks for required shared libraries in the same directory where executable is.
Then archive that directory and give it to your users.
There are programs for MS Windows that can do so, eg DLL to Lib and DLL to Static Lib.
In the open source world, there isn't really much of an incentive to develop such a tool as you can always recompile from source (but of course it's possible that someone somewhere did it anyway).
It's because dynamic libraries and static libraries are two different things. A static library is just an archive of object files (much like a zip archive). A dynamic library is more like an executable program.
So you can't really link anything into a static library, you can only add more object files.
I am compiling a C code using the intel compiler. I integrated icc with visual studio 2010. I want to generate an optimized executable which will run on a windows machine. It is actually a virtual machine in the cloud. I don't have a chance to install any redistributable library to the target machine. I want to statically link all the required libraries. How can I do this?
I suppose you meant icl since you're mentioning VS2010/Windows (icc would be Linux/Mac version): just selecting 'Multi-threaded (/MT)' under Project settings->Configuration properties->C/C++->Code Generation should work. It'll cause both MSVC and Intel runtime to be statically linked into app.
But then it also depends which other libraries are you using, it might not work for all. In that case you can check the dependencies with depends.exe (http://www.dependencywalker.com/) and copy them side-by-side with your .exe to target machine.
Try adding -i-static -static-libcxa to the final linkage.
This should force static linking for intel libraries only.
(You can also try -static as littleadv suggested in the comment, but this will produce a huge static executable with no shared libraries at all)
One more note: A simple workaround would be to copy the executable with the required shared libraries (those that do not exist at the host) to the same directory. Then set LD_LIBRARY_PATH=. before running your dynamically linked executable. This will force searching for libraries in the current directory as well as system directories.
EDIT: I just noticed you said "windows machine". The above is relevant to UNIX machines so probably not useful to you. I'll leave it here in case someone needs the information.