What is the meaning of building a dll as export library ? I just googled it.I found its a dynamic link library.Can anyone please explain what actually dll is ? and why do we need to add these statement in the .dll file
extern "c" _declspec(dllexport)
I studied the static and shared libraries but Im not sure why do we go for dll files.I learnt .dll is used for the run time. But can you help me and give me more information.Thank you in advance
I may have been a bit harsh in my comments. I am not an authority on dlls, but I have a bit of working knowledge of them, so I will try to give a short explanation.
The difference between static and shared libraries should be easy to find in a web search, but basically the code in a static library gets included into the final executable, so after the linking stage, the actual library file is not needed anymore to run the program; on the other hand, code in a shared library doesn't get included in the main program - the two parts remain separate, so the shared library (called dll on windows) will be needed every time the program is run.
"Building a dll as export library" is a bit of a confusing term. I had not heard of it before, and during a short search could only find it on a cygwin page, which you might have read, considering your initial tags. A dll can export some or all of its functions and data. Exporting means that they are available for other programs and dlls to use. Which names get exported can be controlled in various ways. One of those is inserting _declspec(dllexport) in the declaration of the function. Another way is by using a definition file with an exports section.
When creating a dll, an import library can be created. This is a file that can then be used when building an executable that uses the dll, during the linking stage, to let it know which names are exported from the dll, so the program knows how to resolve references to those functions; in other words: how to import them. (This is not always necessary. Many linkers allow you to directly link against the dll itself, thereby removing the need for an import library.)
I realize it can be confusing, but try to find a tutorial and some small examples to see how it works, and play with it a bit.
Related
How can one programmatically determine which type libraries (GUID and version) a given native, VB6-generated DLL/OCX depends on?
For background: The VB6 IDE chokes when opening a project where one of the referenced type libraries can't load one of its dependencies, but it's not so helpful as to say which dependency can't be met--or even which reference has the dependency that can't be met. This is a common occurrence out my company, so I'm trying to supplement the VB6 IDE's poor troubleshooting information.
Relevant details/attempts:
I do have the VB source code. That tells me the GUIDs and versions as of a particular revision in the repo, but when analyzing a DLL/OCX/TLB file I don't know which version of the repo (if any--could be from a branch or might never have been committed to a branch) a given DLL/OCX corresponds to.
I've tried using tlbinf32.dll, but it doesn't appear to be able to list imports.
I don't know much about PE, but I popped open one of the DLLs in a PE viewer and it only shows MSVBVM60.dll in the imports section. This appears to be a special quirk of VB6-produced type libraries: they link only to MSVBVM60 but have some sort of delay-loading mechanism for the rest of the dependencies.
Even most of the existing tools I've tried don't give the information--e.g., depends.exe only finds MSVBVM60.dll.
However: OLEView, a utility that used to ship with Visual Studio, somehow produces an IDL file, which includes the importlib directives. Given that VB doesn't use IDL files, it's clearly generating the information somehow. So it's possible--I just have no idea how.
Really, if OLEView didn't do it I'd have given it up by now as impossible. Any thoughts on how to accomplish this?
It turns out that I was conflating basic DLL functionality and COM. (Not all DLLs are COM DLLs.)
For basic DLLs, the Portable Executable format includes a section describing its imports. The Optional Header's directory 1 is about the DLL's imports. Its structure is given by IMAGE_IMPORT_DESCRIPTOR. This is a starting point for learning about that.
COM DLLs don't seem to have an equivalent as such, but you can discover which other COM components its public interface needs: for each exposed interface, list out the types of their properties and their method arguments, and then use the Registry to look up where those types come from. tlbinf32.dll provides some of the basic functionality for listing members, etc. Here's and intro to that.
Does anyone know where I can find a Firebreath sample (either Mac OS X or Windows) that illustrates how to create a plugin that includes 1 or more other libraries (.DLLs or .SOs) that each rely on other sub-projects built as static libraries (LIBs)?
For example, let's say that the Firebreath plugin is called PluginA, and that PluginA calls methods from DLL_B and DLL_C. DLL_B and DLL_C are C++ projects. DLL_B calls methods from another project called LIB_D, and DLL_C calls methods from a project called DLL_E.
Therefore, the final package should contain the following files:
PluginA.dll
DLL_B.dll (which also incorporates LIB_D)
DLL_C.dll
DLL_E.dll
I am currently forced to dump all source files in the pluginA solution, but this is just a bottleneck (for example I cannot call libraries written in other languages, such as Objective-C on Mac OS X).
I tried following the samples on Firebreath, but couldn't get them to work, and I found no samples from other users that claimed they were able to get it to work. I tried using CMAKE, and also running the solutions directly from X-Code, but the end result was the same (received linking errors, after deployment DLL_C couldn't find DLL_E etc.)
Any help would be appreciated - thank you,
Mihnea
You're way overthinking this.
On windows:
DLLs don't depend on a static library because if they did it would have been compiled in when they were built.
DLLs that depend on another DLL generally just need that other DLL to be present in the same location or otherwise in the DLL search path.
Those two things taken into consideration, all you need to do is locate the .lib file that either is the static library or goes with the .dll and add a target_link_library call for each one. There is a page on firebreath.org that explains how to do this.
On linux it's about the same but using the normal rules for finding .so files.
I'm trying to create an D application which uses a (third party) COM .dll so I can scrape a text box of another application so I can sound an error when a certain string shows up.
However the third party doesn't provide .lib, .def or .h files that go with the dll (atleast with the free trial version). I can create the .lib file with the implib tool but I don't see any of the library's functions in the created .lib.
Their (visual c++) samples use the #import directive to link it in however that is of no use for me ...
On a side note how can I get the proper interfaces (in a .di with boilerplate that does the linking) of the dll automatically? I ask so the correctness of the linkage doesn't depend on my (likely to be incorrect) translation of the functions. They do have a webpage which gives all functions but the object model is a bit chaotic to say the least.
From what I know, COM libraries only expose a few functions, required to (un)register the library and to create objects.
You can however view the interfaces and functions in a COM .dll using the OLE/COM Object Viewer. It seems it might be able to output header files (.h). Afterwards, maybe you could use htod as a starting point to converting everything to D interfaces.
The DMD distribution seems to include a .COM sample (chello.d, dclient.d, dserver.d), and at first glance it doesn't look like it would require any LIBs explicitly.
Unfortunately, I've never actually used COM in D, so I can't advise any further. I hope this helps in some way.
While I have yet to actually do COM work myself, I am trying to revive Juno over on Github/he-the-great. Part of the project is tlbimpd which is what will output a D file from a DLL.
I've tested the examples and successfully run tlbimpd. Please do try things out for your use and submit any issues.
I am in the midst of evaluating the benefits of changing our program from 30+ statically linked libraries to 30+ dynamically linked libraries. We hope by changing to DLL, it will reduce the link time.
One immediate problem is the requirement to add __declspec in front of all the classes to create the lib file for other dlls to link. Is there a way to get around that? Is there a flag in the compiler to force a lib generation so to make all classes inside the DLL available for export? If not, is there any existing script/program that will do that? That will certainly make the switch from statically linked library to a dynamic one a lot easier. If not, what is the rationale behind __declspec? Why not an option to make all dll functions exportable?
Thank you.
Perhaps it's too late, but have you looked into using a DEF file?
There is one another way to solve your problem.
You just need to create one definition file(.def) and export all the methods or class you want to share.
U will also have to set :
Properties->Linker->Input->Module Definition File -> add name of your created .def file.
Now use run time dynamic linking:
In project where you want to call the exported methods use LoadLibrary to get handle of your Dll and call the required method using GetProcAddress.
I know very little about DLL's and LIB's other than that they contain vital code required for a program to run properly - libraries. But why do compilers generate them at all? Wouldn't it be easier to just include all the code in a single executable? And what's the difference between DLL's and LIB's?
There are static libraries (LIB) and dynamic libraries (DLL) - but note that .LIB files can be either static libraries (containing object files) or import libraries (containing symbols to allow the linker to link to a DLL).
Libraries are used because you may have code that you want to use in many programs. For example if you write a function that counts the number of characters in a string, that function will be useful in lots of programs. Once you get that function working correctly you don't want to have to recompile the code every time you use it, so you put the executable code for that function in a library, and the linker can extract and insert the compiled code into your program. Static libraries are sometimes called 'archives' for this reason.
Dynamic libraries take this one step further. It seems wasteful to have multiple copies of the library functions taking up space in each of the programs. Why can't they all share one copy of the function? This is what dynamic libraries are for. Rather than building the library code into your program when it is compiled, it can be run by mapping it into your program as it is loaded into memory. Multiple programs running at the same time that use the same functions can all share one copy, saving memory. In fact, you can load dynamic libraries only as needed, depending on the path through your code. No point in having the printer routines taking up memory if you aren't doing any printing. On the other hand, this means you have to have a copy of the dynamic library installed on every machine your program runs on. This creates its own set of problems.
As an example, almost every program written in 'C' will need functions from a library called the 'C runtime library, though few programs will need all of the functions. The C runtime comes in both static and dynamic versions, so you can determine which version your program uses depending on particular needs.
Another aspect is security (obfuscation). Once a piece of code is extracted from the main application and put in a "separated" Dynamic-Link Library, it is easier to attack, analyse (reverse-engineer) the code, since it has been isolated. When the same piece of code is kept in a LIB Library, it is part of the compiled (linked) target application, and this thus harder to isolate (differentiate) that piece of code from the rest of the target binaries.
One important reason for creating a DLL/LIB rather than just compiling the code into an executable is reuse and relocation. The average Java or .NET application (for example) will most likely use several 3rd party (or framework) libraries. It is much easier and faster to just compile against a pre-built library, rather than having to compile all of the 3rd party code into your application. Compiling your code into libraries also encourages good design practices, e.g. designing your classes to be used in different types of applications.
A DLL is a library of functions that are shared among other executable programs. Just look in your windows/system32 directory and you will find dozens of them. When your program creates a DLL it also normally creates a lib file so that the application *.exe program can resolve symbols that are declared in the DLL.
A .lib is a library of functions that are statically linked to a program -- they are NOT shared by other programs. Each program that links with a *.lib file has all the code in that file. If you have two programs A.exe and B.exe that link with C.lib then each A and B will both contain the code in C.lib.
How you create DLLs and libs depend on the compiler you use. Each compiler does it differently.
One other difference lies in the performance.
As the DLL is loaded at runtime by the .exe(s), the .exe(s) and the DLL work with shared memory concept and hence the performance is low relatively to static linking.
On the other hand, a .lib is code that is linked statically at compile time into every process that requests. Hence the .exe(s) will have single memory, thus increasing the performance of the process.