I am writing an C++ application that could be compiled under Linux (gcc 4.3) and Windows (MS VS08 Express).
My application uses third-party libraries,
On Linux , they are compiled as shared libraries, while
on Windows, there are two versions "Debug" and "Release".
I know that debug version provides extra support for debugging ( just like using -ggdb option in linux gcc, right? )
But I found that if my application is in debug version , the libraries must also be in debug version, otherwise the application will crash.
Why is there such a limit? It seems that there are no such limits in the linux world
Thank you very much!
The Debug configuration of your
program is compiled with full symbolic
debug information and no optimization.
Optimization complicates debugging,
because the relationship between
source code and generated instructions
is more complex.
The Release configuration of your
program contains no symbolic debug
information and is fully optimized.
Debug information can be generated in
PDB Files (C++) depending on the
compiler options used. Creating PDB
files can be very useful if you later
need to debug your release version.
Debug vs Release
It is also possible to debug your release build with the compiler flags.
Debugging Release Builds
Heap Layout - Guard Bytes to prevent overwriting
Compilation - Removing Assert/Debug Info
Pointer Support - Buffers around pointers to prevent seg faults
Optimization - Inline Functions
Common Problems When Creating Release Builds
To elaborate on Martin Tornwall. The various libraries linked when in Debug or Release
LIBCPMT.LIB, Multithreaded, static link, /MT, _MT
LIBCPMTD.LIB, Multithreaded, static link, /MTd, _DEBUG, _MT
CRT Libraries
Most likely, the Release and Debug versions are linked against different versions of the C++ Runtime library. Usually, a Debug build links against the "Multithreaded Debug DLL" runtime, whereas a Release build will generally link against "Multithreaded DLL". Loading DLLs whose runtime libraries are mismatched with that of the application will often lead to mysterious crashes.
You could try verifying that all your DLLs build against the same runtime library as your application, regardless of which configuration (Debug or Release) is active. Whether or not this is desirable is another question entirely.
The ability to choose which runtime library to link with enables the application developer to select the best feature set given her application's requirements. For example, a single-threaded application might incur performance penalties as a result of unnecessary thread synchronization if it is linked with a runtime library that is designed with thread safety in mind. Likewise, the consequences of linking a multi-threaded application against a single-threaded runtime could potentially be disastrous.
While I never intentionally link libraries that were built with different compiler settings, there isn't much point in doing so, I only know of the STL classes in the Dinkumware implementation (the one MSFT uses) to cause this problem.
They support a feature called 'iterator debugging' which is turned on by default in the Debug configuration. This adds members to the classes to aid the diagnostic code. Making them larger. This goes bad when you create the object in a chunk of code that was compiled with one setting and pass it to code that was compiled with the opposite setting. You can turn this off by setting the _HAS_ITERATOR_DEBUGGING macro to 0. Which is rather a major loss, the feature is excellent to diagnose mistakes in the way you use STL classes.
Passing objects or pointers between different libraries is always a problem if you don't carefully control the compile settings. Mixing and matching the CRT version and flavor gets you in trouble when you do so. This normally generates a warning from the linker, not sure what you did to not see it. There won't be one if the code lives in a DLL.
Related
I was working on a MS Visual Studio project and noticed that the Debug build was Linking to a Release library (DLL), when a Debug library with the same name was also available. I switched the path to Link the Debug library, and it seems to still work. Is there any way to know if my change was correct?
Additional Information:
Using the debug DLL triggers a small memory leak that wasn't triggered with the release DLL. Or possibly that is debug related cache data. That leak made me question if it was including the lib headers without _DEBUG somehow. Thus this question.
It is the preferred way to link Debug builds of your program with Debug version of external dynamic libraries as it guarantees that uniform Visual C++ runtime libraries will be used. Mixing libraries built in Debug and Release mode will result in both Debug and Release VC++ runtimes being linked and annoying linker warnings about multiple symbol definitions in conflicting Debug/Runtime libraries.
Sometimes it may be inevitable as i.e. only Release version of some external library may be available. Hence in VC++ there are linker settings allowing to ignore some specific libraries. So you may start checking in the Linker-Input settings of the Debug build if such ignores are already defined for existing conflicts. With some luck you might be able to remove them now.
If using the Debug version of some library breaks the whole program it might be an insight how to improve the whole system, it is that what a Debug build is for anyway.
Assuming that it is the right library (as in the same code compiled to a different configuration), you are right to link to the debug dll on your debug configuration (and should link to the release dll on your release configuration).
The differences tend to be related to the optimisation level (Debug is usually compiled without any optimisation at all) and any symbols that might be included to make it easier to develop with and maybe step into.
Brief introduction:
I am refactoring win32 application, I use VS 2008.
The application consists of both of my own dll and 3-rd party dll`s.
Issue:
When I run the application in the debug mode and execute some action the error is raised: the application programm has triggered a breakpoint, heap is corrupted.
Actions undertaken:
I have searched the internet and found that this error may be because of different build flags (multi-threaded debug /MD and multi-threaded debug dll /MDd) were used for dlls within the project(which results that they use different c runtime libraries, and for each library own list for work with memory is maintained this therfore can lead to the heap corruption).
I have checked my dlls - they all have the same flag: debug multithreaded dll. So I think that one of the 3-rd party DLL maybe was built with multi-threaded debug flag.
Questions:
Is it possible to find out with what flag 3-rd party library was
built, if so how can I do this.
How can I sort you my issue of
different build flags?
Is my guess about that error is due to
different build flags is correct?
Is it possible to find out with what flag 3-rd party library was built, if so how can I do this
Yes. C or C++ DLLs built with Visual Studio versions 2005 and 2008 require a manifest that states what version of the C runtime DLL they need. You can see it with VS, File + Open + File, select the DLL and open the node labeled "RT_MANIFEST". Resource ID 2 is the manifest that states the type and version of the CRT. It should match yours. Export it to make it easier to read. If it is missing then it either wasn't built with /MD or used a completely different version of VS, which in itself is bad news.
How can I sort you my issue of different build flags?
You can't. You'll need to contact the 3rd party and request a build that's compatible with yours.
Is my guess about that error is due to different build flags is correct?
It is possible but not terribly likely. Having a mismatch does not automatically cause a crash, a programmer can certainly design the DLL interface so that's never an issue. You can typically tell from the function signature and documentation. The problem case is where the DLL allocates an object and you are supposed to release it. It will be obvious when the function returns a pointer. Or a standard C++ class library object like std::string. Less obvious is when it throws an exception. Such a problem is also highly repeatable, it will bomb consistently, not occasionally.
The biggest mistake you are making is asking this question here. You should be talking to a programmer employed by that 3rd party that has worked on this DLL. He'll know the exact answer to your questions and can very easily solve your problem. If you cannot get this kind of support then you should not be using these DLLs, they'll make your life miserable for a long time to come.
I work for a camera company and we provide an SDK for our customers. Historically we only provided a release build of our SDK that was built against the non-debug CRT. As part of our SDK package we provide a number of examples on how to use the SDK. The examples have Debug project configurations which use the debug CRT. In some cases we run into strange behaviour due to the fact that these examples and the library that they link against use different CRT's.
My questions is what is the appropriate way to deal with this sort of situation? Should we be distributing a debug version of our library that uses the debug CRT? As long as we don't provide a pdb or at most a stripped pdb, then all proprietary information should still remain hidden. Is it correct to assume that in doing this there should be no other negative effects other then a larger, not optimised binary?
Is it common practice to distribute a debug binary linked against the debug CRT or should we just continue distributing only the release build?
Yes, you'll need to distribute Debug and Release builds of your library. Built with respectively /MDd and /MD so the CRT can be shared. Different versions too, built against, say, the VS2005, VS2008 and VS2010 versions of the CRT.
This is of course painful. To narrow it down to a single library, you'll need to carefully craft your public interface so it doesn't expose any C++ objects or any pointers that need to be released by the client code. Exceptions are also taboo. A common solution is to use COM. Especially an Automation compatible interface is useable by most any language runtime in common use on Windows.
You might consider an Optimized Debug build, where it is set to use the debug versions of the libraries but has all the optimization flags set as they would be in a Release build. This will prevent subtle differences in execution from affecting the user experience.
I've downloaded a 3rd party library, and built the .lib file in 'release' mode.
After adding the lib to my project, if i run my project in release mode, it's fine. But if i run my project in debug mode, i get an error:
_iterator_debug_level value '0' doesn't match value '2;
I could rebuild the library in debug mode, but I don't think I'll need to be debugging the library itself? And I've downloaded prebuilt 3rd party libraries before which only come with a release build (i assume?) that link fine whether my project is in debug or release. I'm wondering how that is done.
If you want to distribute a release library that others can use in either release or debug mode, you need to do two things:
Build a DLL, so that you get your own copy of the C runtime library
Not share CRT resources, such as the heap, across the library boundary. The biggest thing for C code is that dynamically allocated memory has to be deallocated on the same side of the boundary. For C++ code, you can use the std namespace inside your DLL, but not pass those objects across the boundary.
That's what the pre-built third-party libraries have most likely done. You can do the same thing with your library only if the external interface doesn't share CRT objects. Or you can build separate release and debug versions as static libraries.
Looks like your debug binary and the library you downloaded use incompatible iterator debug modes. Iterator debugging is usually controlled by macros. Depending on macro values the sizes of interators and many other objects can change. You are lucky that your program emitted useful error message instead of simply crushing.
Check the library documentation and make sure that your project uses the same iterator debug mode. You may also try recompiling the library in release mode. If that doesn't help, you would have to recompile the library in debug mode, even if you don't intend to debug the library itself.
In Visual Studio, there's the compile flags /MD and /MT which let you choose which kind of C runtime library you want.
I understand the difference in implementation, but I'm still not sure which one to use. What are the pros/cons?
One advantage to /MD that I've heard, is that this allows someone to update the runtime, (like maybe patch a security problem) and my app will benefit from this update. Although to me, this almost seems like a non-feature: I don't want people changing my runtime without allowing me to test against the new version!
Some things I am curious about:
How would this affect build times? (presumably /MT is a little slower?)
What are the other implications?
Which one do most people use?
By dynamically linking with /MD,
you are exposed to system updates (for good or ill),
your executable can be smaller (since it doesn't have the library embedded in it), and
I believe that at very least the code segment of a DLL is shared amongst all processes that are actively using it (reducing the total amount of RAM consumed).
I've also found that in practice, when working with statically-linked 3rd-party binary-only libraries that have been built with different runtime options, /MT in the main application tends to cause conflicts much more often than /MD (because you'll run into trouble if the C runtime is statically-linked multiple times, especially if they are different versions).
If you are using DLLs then you should go for the dynamically linked CRT (/MD).
If you use the dynamic CRT for your .exe and all .dlls then they will all share a single implementation of the CRT - which means they will all share a single CRT heap and memory allocated in one .exe/.dll can be freed in another.
If you use the static CRT for your .exe and all .dlls then they'll all get a seperate copy of the CRT - which means they'll all use their own CRT heap so memory must be freed in the same module in which it was allocated. You'll also suffer from code bloat (multiple copies of the CRT) and excess runtime overhead (each heap allocates memory from the OS to keep track of its state, and the overhead can be noticeable).
I believe the default for projects built through Visual Studio is /MD.
If you use /MT, your executable won't depend on a DLL being present on the target system. If you're wrapping this in an installer, it probably won't be an issue and you can go either way.
I use /MT myself, so that I can ignore the whole DLL mess.
P.S. As Mr. Fooz points out, it's vital to be consistent. If you're linking with other libraries, you need to use the same option they do. If you're using a third party DLL, it's almost certain that you'll need to use the DLL version of the runtime library.
I prefer to link statically with /MT.
Even though you do get a smaller executable with /MD, you still have to ship a bunch of DLLs to make sure the user gets the right version for running your program. And in the end your installer is going to be BIGGER than when linking with /MT.
What's even worse, if you choose to put your runtime libraries in the windows directory, sooner or later the user is going to install a new application with different libraries and, with any bad luck, break your application.
The problem you will run into with /MD is that the target version of the CRT may not be on your users machine (especially if you're using the latest version of Visual Studio and the user has an older operating system).
In that case you have to figure out how to get the right version onto their machine.
from http://msdn.microsoft.com/en-us/library/2kzt1wy3(VS.71).aspx:
/MT Defines _MT so that multithread-specific versions of the run-time routines are selected from the standard header (.h) files. This option also causes the compiler to place the library name LIBCMT.lib into the .obj file so that the linker will use LIBCMT.lib to resolve external symbols. Either /MT or /MD (or their debug equivalents /MTd or /MDd) is required to create multithreaded programs.
/MD Defines _MT and _DLL so that both multithread- and DLL-specific versions of the run-time routines are selected from the standard .h files. This option also causes the compiler to place the library name MSVCRT.lib into the .obj file.
Applications compiled with this option are statically linked to MSVCRT.lib. This library provides a layer of code that allows the linker to resolve external references. The actual working code is contained in MSVCR71.DLL, which must be available at run time to applications linked with MSVCRT.lib.
When /MD is used with _STATIC_CPPLIB defined (/D_STATIC_CPPLIB) it will cause the application to link with the static multithread Standard C++ Library (libcpmt.lib) instead of the dynamic version (msvcprt.lib) while still dynamically linking to the main CRT via msvcrt.lib.
So if I am interpreting it correctly then /MT links statically and /MD links dynamically.
If you are building executable that uses other dlls or libs than /MD option is preferred because that way all the components will be sharing same library. Of course this option should match for all the modules involved i.e dll/lib/exe.
If your executable doesn't uses any lib or dll than its anyone's call. The difference is not too much now because the sharing aspect is not into play.
So maybe you can start the application with /MT since there is no compelling reason otherwise but when its time to add a lib or dll, you can change it to /MD with that of the lib/dll which is easy.