Windows application links dynamically CRT.After the application executes part of its code it loads shared library.The shared library is linked statically with CRT.
My question is :
If the application made some changes to the "CRT state" (for example reopens stdout in some file),
will these changes be valid to in the dll code?
Thank you
No, a statically linked CRT is completely independent from a dynamically loaded CRT. The file table is different, the heaps are different, everything is different.
Related
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'm still a noob in C, so I have a question about linking.
We have two programs "A" and "B", which links to the dynamic linked library "C".
Now we start program "A" and "B".
What happened now to "C". Will it be loaded once for both programs, or two times for every program?
And what is, when program B is a Python program, which make use of the foreign function interface?
It all depends on the operating system, but for e.g. Linux or Windows the shared library will only be loaded once, but it will be mapped twice. Each process using the shared library will have the library mapped, but those mappings all lead to the same single loaded library.
The mapping is done on a per-process basis, it doesn't really matter what the process does or is (if it's a program you made, a Python interpreter, or something completely different).
Searching google for Dynamic linking C gave me the following result (Shared libraries are dynamically loaded)
Shared libraries are loaded into memory by programs when they start. When a shared library is loaded properly, all programs that start later automatically use the already loaded shared library.
In the case of Windows, only the DLL (dynamic link library) code is shared between processes. Each process has it's own virtual memory address space, including the data used by a DLL. This means that a DLL normally can't have a static buffer that is shared between processes. The process or DLL code can set up shared memory, and that shared memory can be shared between processes.
MSDN articles:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms681914(v=vs.85).aspx
http://msdn.microsoft.com/en-us/library/windows/desktop/ms682594(v=vs.85).aspx
http://msdn.microsoft.com/en-s/library/windows/desktop/ms686958(v=vs.85).aspx
In case of a shared library between two applications, does each application use it's own copy of the library during run time ? If they use the same instance of the library, what happens to the global variables inside the library ?
It depends on the operating system. On most Unix-like systems, shared libraries use position-independent code, so the memory used by the code segment(which holds instructions and read-only variables) can be shared between processes, but each process still has its own data segment(which holds other variables).
For Unix-like operating systems, when you first execute your applications, the page tables of the two processes which map the library address space will point to the same frames in memory where the library is loaded.
However, the page tables which map the data section of the library are handled with Copy on Write mechanism. As soon as you try to write a global variable, the OS will create a process specific copy of the page containing the variable and will remap the page table of the process accordingly.
Each program creates a new instance of the library in its own memory space. They are not shared and the 2 programs will not see each other's data.
Take a look at how dynamic libraries are loaded: http://eli.thegreenplace.net/2011/08/25/load-time-relocation-of-shared-libraries
The same is true for statically linked libraries except instead of being loaded at runtime, they are linked at compile time.
Assume we have a very small embedded system consisting only of the linux kernel and a single statically linked binary run as init. We want the binary to be able to dynamically load external plugins in runtime.
Is it possible on linux? Dlopen only works with shared libraries and dynamic linking cause static binaries don't export any symbols to the outside world, so is there any other way to do it?
You could run the "plugins" as child processes, and communicate over IPC (shared memory, pipes, or so forth).
They would exist in their own process space, so you couldn't directly call functions in them (besides, if they're also statically linked, you won't have any function entry points other than main that you could reach), but you could (e.g.) send a command over a named pipe, or pass data in a shared memory structure.
Note that, the moment you load the second binary, you have lost one of the main benefits of static linking (because now you have two copies of your libc loaded), so you might want to consider just biting the bullet and using dynamic linking. You'll burn a few 100K's in adding the dynamic linking support, but the GNU libc is about 2M, so if you're loading one plug-in, you've gained maybe 1.8M in savings already; and for each additional plug-in you load, you're saving some 2M.
Dlopen only works with shared libraries and dynamic linking cause static binaries don't export any symbols to the outside world
You can dlopen a shared library from a statically linked binary when using glibc. If you need your plugin to reference symbols from the main executable, you would have to pass in pointers to them into the plugin, similar to this.
is there any other way to do it?
You could also write your own module loader. The Linux kernel does this, and so does Xorg.
I have a dll which I'd like to use in a c program,
Do you think is efficient to have a dll (lots of common functions) and then create a program that will eventually use them, or have all the source code?
To include the dll, What syntax must be followed?
Do you think is efficient to have a dll (lots of common functions) and then create a program that will eventually use them,or have all the source code.
For memory and disk space, it is more efficient to use a shared library (a DLL is the Windows implementation of shared libraries), assuming that at least two programs use this component. If only one program will ever use this component, then there is no memory or disk space savings to be had.
Shared libraries can be slightly slower than statically linking the code; however, this is likely to be incredibly minor, and shared libraries carry a number of benefits that make it more than worthwhile (such as the ability to load and handle symbols dynamically, which allows for plugin-like architectures). That said, there are also some disadvantages (if you are not careful about where your DLLs live, how they are versioned, and who can update them, then you can get into DLL hell).
To include the dll, What syntax must be followed?
This depends. There are two ways that shared libraries can be used. In the first way, you tell the linker to reference the shared library, and the shared library will automatically be loaded on program startup, and you would basically reference the code like normal (include the various headers and just use the name of the symbol when you want to reference it). The second way is to dynamically load the shared library (on Windows this is done via LoadLibrary while it is done on UNIX with dlopen). This second way makes it possible to change the behavior of the program based on the presence or absence of symbols in the shared library and to inspect the available set of symbols. For the second way, you would use GetProcAddress (Windows) or dlsym (UNIX) to obtain a pointer to a function defined in the library, and you would pass around function pointers to reference the functions that were loaded.
You can put your functions into either a static library ( a .lib) which is merged into your application at compile time and is basically the same as putting the .c files in the project.
Or you can use a dll where the functions are included at run time. the advantage of a dll is that two programs which use the same functions can use the same dll (saving disk space) and you can upgrade the dll without changing the program - neither of these probably matters for you.
The dll is automatically loaded when your program runs there is nothing special you need to do to include it ( you can load a dll specifically in your code - there are sometimes special reasons to do this)
Edit - if you need to create a stub lib for an existing dll see http://support.microsoft.com/kb/131313