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Why ISO/ANSI didn't standardize C++ at the binary level? There are many portability issues with C++, which is only because of lack of it's standardization at the binary level.

Don Box writes, (quoting from his book Essential COM, chapter COM As A Better C++)

C++ and Portability


Once the decision is made to distribute a C++ class as a DLL, one is faced with one of the fundamental weaknesses of C++, that is, lack of standardization at the binary level. Although the ISO/ANSI C++ Draft Working Paper attempts to codify which programs will compile and what the semantic effects of running them will be, it makes no attempt to standardize the binary runtime model of C++. The first time this problem will become evident is when a client tries to link against the FastString DLL's import library from a C++ developement environment other than the one used to build the FastString DLL.

Are there more benefits Or loss of this lack of binary standardization?

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Is this better asked on programmers.stackexchange.com, seeing as how it's more of a subjective question? –  Stephen Furlani Dec 22 '10 at 15:59
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Related question of mine actually: stackoverflow.com/questions/2083060/… –  AraK Dec 22 '10 at 16:00
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Don Box is a zealot. Ignore him. –  John Dibling Dec 22 '10 at 16:16
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Well, C isn't standardized by ANSI/ISO in the binary level either; OTOH C has a de facto standard ABI rather than a de jure one. C++ doesn't have such a standardized ABI because different manufacturers had different goals with their implementations. For example, exceptions in VC++ piggyback on top of Windows SEH. POSIX has no SEH and therefore taking that model wouldn't have made sense (So G++ and MinGW don't use that model). –  Billy ONeal Dec 22 '10 at 16:18
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I see this as a feature not a weakness. If you bind an implementation to a specific ABI then we will never have innovation and new hardware will be bound to the design of the language (and since there is 15 years between each new version that's a long time in the hardware industry) and by stifling innovate new ideas to make the code execute more efficiently will not be made. The price is that all code in an executable must be built by the same compiler/version (a problem but not a major one). –  Martin York Dec 22 '10 at 19:29

9 Answers 9

up vote 13 down vote accepted

Languages with binary-compatible compiled form are a relatively new phase[*], for example the JVM and .NET runtimes. C and C++ compilers usually emit native code.

The advantage is that there is no need for a JIT, or a bytecode interpreter, or a VM, or any other such thing. For example, you can't write the bootstrap code that runs at machine startup as nice, portable Java bytecode, unless perhaps the machine can natively execute Java bytecode, or you have some kind of converter from Java to a non-binary-compatible native executable code (in theory: not sure this can be recommended in practice for bootstrap code). You could write it in C++, more or less, albeit not portable C++ even at the source level, since it will do a lot of messing with magic hardware addresses.

The disadvantage is that of course native code only runs at all on the architecture it was compiled for, and the executables can only be loaded by a loader that understands their executable format, and only link with and call into other executables for the same architecture and ABI.

Even if you get that far, linking two executables together will only actually work correctly as long as: (a) you don't violate the One Definition Rule, which is easy to do if they were compiled with different compilers/options/whatever, such that they were using different definitions of the same class (either in a header, or because they each statically linked against different implementations); and (b) all relevant implementation details such as structure layout are identical according to the compiler options in force when each was compiled.

For the C++ standard to define all of this would remove a lot of the freedoms currently available to implementers. Implementers are using those freedoms, especially when writing very low-level code in C++ (and C, which has the same issue).

If you want to write something that looks a bit like C++, for a binary-portable target, there's C++/CLI, which targets .NET, and Mono so that you can (hopefully) run .NET elsewhere than Windows. I think it's possible to persuade MS's compiler to produce pure CIL assemblies that will run on Mono.

There are also potentially things that can be done with for example LLVM to create a binary-portable C or C++ environment. I don't know that any widespread example has emerged, though.

But these all rely on fixing a lot of things that the C++ makes implementation-dependent (such as the sizes of types). Then the environment that understands the portable binaries, must be available on the system where the code is to run. By allowing non-portable binaries, C and C++ can go places where portable binaries can't, and that's why the standard doesn't say anything at all about binaries.

Then on any given platform, implementations usually still don't provide binary compatibility between different sets of options, although the standard isn't stopping them. If Don Box doesn't like that Microsoft's compilers can produce incompatible binaries from the same source, according to compiler options, then it's the compiler team he needs to complain about. The C++ language does not forbid a compiler or an OS from pinning down all the necessary details, so once you limit yourself to Windows it's not a fundamental problem with C++. Microsoft has chosen not to do so.

The differences often manifest as one more thing that you can get wrong and crash your program, but there may be considerable gains to be made in efficiency between, for example, incompatible debug vs release versions of a dll.

[*] I'm not sure when the idea was first invented, probably 1642 or something, but their current popularity is relatively new, compared to the time when C++ committed to the design decisions which prevent it defining binary-portability.

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@Steve : thanks for the detail explanation. :-) –  Nawaz Dec 24 '10 at 9:37

Cross-platform and cross-compiler compatibility were not the primary goals behind C and C++. They were born in an era, and intended for purposes for which platform-specific and compiler-specific minimizations of time and space were crucial.

From Stroustrup's "The Design and Evolution of C++":

"The explicit aim was to match C in terms of run-time, code compactness and data compactness. ... The ideal - which was achieved - was that C with Classes could be used for whatever C could be used for."

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+1 -- exactly. How would one build a standard ABI that worked on both ARM and Intel boxes? Wouldn't make sense! –  Billy ONeal Dec 22 '10 at 16:20
    
unfortunately, it failed in this. You can do everything C does... except dynamically load a C++ module at runtime. you have to 'revert' to using C functions in the exposed interface. –  gbjbaanb Mar 27 '12 at 19:13

The problem described in the quotation is caused by the quite deliberate avoidance of standardisation of symbol-name mangling schemes (I think "standardisation at the binary level" is a misleading phrase in this respect although the issue is related to a compiler's Application Binary Interface (ABI).

C++ encodes a function or data object's signature and type information, and its class/namespace membership into the symbol-name, and different compilers are allowed to use different schemes. Consequently a symbol in a static library, DLL, or object file will not link with code compiled using a different compiler (or possibly even a different version of the same compiler).

The issue is described and explained probably better than I can here, with examples of schemes used by different compilers.

The reasons for the deliberate lack of standardisation are also explained here.

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It's not a bug, it's a feature! This gives implementors freedom to optimize their implementation at the binary level. The little-endian i386 and its offspring are not the only CPUs that have or do exist.

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The aim of ISO/ANSI was to standardize the C++ language, issue that seems to be complex enough to require years to have an update of the language standards and compiler support.

The binary compatibility is much more complex, given that the binaries need to run on different CPUs' architectures and different OS environments.

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True, but the problem described in the quotation is in fact nothing to do with "binary level compatibility" (despite the author's use of the term) in any sense other than such things are defined in something called an "Application Binary Interface". He is in fact describing the issue of incompatible name mangling schemes. –  Clifford Dec 22 '10 at 16:53
    
@Clifford : name mangling scheme is just a subset of the binary level compatibility. the latter is more like an umbrella term! –  Nawaz Dec 24 '10 at 6:31
    
I doubt there's a problem with trying to run a Linux binary on a windows machine. Things would be a lot better if there was an ABI per-platform, as at least then a script language could dynamically load and run a binary on the same platform, or apps could use components built with a different compiler. You can't use a C dll on linux today, and no-one complains, but that C dll can still be loaded by a python app which is where the benefit accrues. –  gbjbaanb Mar 27 '12 at 19:04

As Andy said cross platform compatibility wasn't a big goal, whereas broad platform and hardware implementation was a goal, with the net result that you can write conforming implementations for a very wide selection of systems. Binary standardization would have made this practically unachievable.

C compatibility was also important and would have significantly complicated this.

There has subsequently been some efforts to standardise the ABI for a subset of implementations.

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Darn, I forgot C compatibility. Good point, +1! –  Andy Thomas-Cramer Dec 22 '10 at 16:14

I think the lack of a standard for C++ is a problem in today's world of de-coupled, modular programming. However, we have to define what we want from such a standard.

No-one in their right mind wants to define the implementation or platform for a binary. So you can't take a x86 Windows dll and start using it on a x86_64 Linux platform. That would be a bit much.

However, what people do want is the same thing we have with C modules - a standardised interface at the binary level (ie once compiled). Currently, if you want to load a dll in a modular app, you export C functions and bind to them at runtime. You cannot do that with a C++ module. It would be great if you could, which would also mean that dlls written with one compiler could be loaded by a different one. Sure, you still wouldn't be able to load a dll built for an incompatible platform, but that's not a problem that needs fixing.

So if the standards body defined what the interface a module exposed, then we'd have a lot more flexibility in loading C++ modules, we wouldn't have to expose C++ code as C code, and we'd probably get a lot more use of C++ in script languages.

We also wouldn't have to suffer things like COM that attempt to provide a solution to this problem.

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+1. Yeah, I agree. The other answers here basically handwave away the problem by saying that binary standardization would prohibit architecture-specific optimizations. But that's not the point. Nobody is arguing for some cross-platform binary executable format. The problem is that there's no standard interface to load C++ modules dynamically. –  Charles Salvia May 3 '12 at 15:06

I don't know why it doesn't standardize on a binary level. But I know what I do about it. On Windows I declare function extern "C" BOOL WINAPI. (Of course replace BOOL with whatever type the function is.) And they are exported cleanly.

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But if you declare it extern "C", it will use the C ABI, which a de facto standard on common PC hardware even though it's not imposed by any sort of committee. –  Billy ONeal Dec 22 '10 at 16:21

Use unzip foo.zip && make foo.exe && foo.exe if you want portability of your source.

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