STL and over 2GB memory

[W Randolph Franklin]

Hi,

Does STL work on a 32-bit machine when a process's total memory is
over 2GB? E.g., a Pentium allows the process memory to be as
large as 3GB.

Ditto Boost multi arrays.

Are there any other C++ problems that I ought to know about? I've
actually bought half a dozen C++, STL, and FAQ books and don't
happen to remember this ever being mentioned.

Thanks.

 

[Peter van Merkerk]

Does STL work on a 32-bit machine when a process's total memory is

over 2GB? E.g., a Pentium allows the process memory to be as
large as 3GB.

That question cannot be answered without knowing which implementation of
the standard library your are using. The ISO C++ standard says nothing
about the architecture or the organisation process memory of the target
platform.. Nor does the standard dictate how the standard library is to
be implemented. It is up to implementers of the standard library to
decide how to deal with the issues you mentioned. Maybe the
documentation that came with your standard library implementation can
provide you with an answer, or otherwise the developers of that library
may be able help you.

Ditto Boost multi arrays.

Are there any other C++ problems that I ought to know about? I've
actually bought half a dozen C++, STL, and FAQ books and don't
happen to remember this ever being mentioned.

If they are about _standard_ C++ they can't because it is beyond the
scope of the language. It is an implementation issue.

 

[Mike Wahler]

Hi,

Does STL work on a 32-bit machine when a process's total memory is
over 2GB?

If someone has created an implementation for that
machine, sure.

E.g., a Pentium allows the process memory to be as
large as 3GB.

That's irrelevant to a platform-specific language
like C++.

Ditto Boost multi arrays.

Ask about boost stuff on their mailing list.

Are there any other C++ problems that I ought to know about?

None of the above issues are about C++, thus they're
not 'C++ problems'. The problem is your misconception.

I've
actually bought half a dozen C++, STL, and FAQ books and don't
happen to remember this ever being mentioned.

You never saw "things like this" discussed in a C++
book because C++ is a platform-indepenedent language,
which does not depend upon or specify such platform
details.

-Mike

 

[Sarah Thompson]

You never saw "things like this" discussed in a C++
book because C++ is a platform-indepenedent language,
which does not depend upon or specify such platform
details.

This is an interesting point, worthy of expansion. C++ is indeed a
platform independent language, whose specification does a good job of
avoiding platform-specific language details. However, it is not correct
to state that C++ does not depend upon platform details. If one is
writing code with portability in mind, then striving for exactly this
kind of platform independence is a worthy goal. However, the semantics
of any C++ program are entirely dependent on the underlying architecture.

As a trivial example:

int main()
{
unsigned int x = 0xFFFFFFFF;
x++;

if(x == 0)
{
// 32 bit integers, probably.
return 0;
}
else
{
// Some other sized integers, probably.
return 1;
}
}

The above program is legal C++ (and for that matter legal C, but let's
stay on topic here). However, its runtime semantics critically depend
upon the details of its implementation. The language standard may have
nothing much to say about the word length of integers, but the above
program will behave completely differently depending on whether it is
compiled for a 32 bit or a 64 bit architecture.

C++, therefore, can not be regarded as truly platform independent, since
the semantics of quite simple code is entirely dependent on the details
of the implementation. With clever programming, it is often possible to
get around such limitations, but the language doesn't give you this for
free.

Arguably, if the C++ standard really did standardise data types, giving
them well-defined meanings (e.g. int8, unsigned16, ieeefloat, etc.) with
a fully specified semantics, this would make the language *more*
portable. The existing definition is sufficiently wooly that programming
for portability can often be extremely challenging.

Yes, this may be regarded as a little heretical, but I'm not trolling
here -- this is an important issue. I've written more than one C++
library with portability in mind (e.g. the signal/slot library on my web
site, as well as a new lexical analysis/parsing library that I'll
release sometime when I've finally gotten around to finishing its
documentation), and can't say I find C++ terribly helpful in this regard.

Sarah

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[Michiel Salters]

Hi,

Does STL work on a 32-bit machine when a process's total memory is
over 2GB? E.g., a Pentium allows the process memory to be as
large as 3GB.

STL the concept, yes. Your STL implementation, may be. However if
it doesn't you can fix things by provinding your own allocator
capable of addressing 3GB. The allocator is the only part that
deals with memory allocations.

Regards,

 

[Ron Natalie]

STL the concept, yes. Your STL implementation, may be. However if
it doesn't you can fix things by provinding your own allocator
capable of addressing 3GB. The allocator is the only part that
deals with memory allocations.

Well the other problem is making sure the size_type can hold numbers
that big. (By the way, pentiums have no problem with 4GB address
spaces, it's only windows that insists on the kernel and the user rings
sharing the 4Gigs that leads to the 2 or 3 gb limit).

 

[David Cattarin]

You never saw "things like this" discussed in a C++
book because C++ is a platform-indepenedent language,
which does not depend upon or specify such platform
details.

This is an interesting point, worthy of expansion. C++ is indeed a
platform independent language, whose specification does a good job of
avoiding platform-specific language details. However, it is not correct
to state that C++ does not depend upon platform details. If one is
writing code with portability in mind, then striving for exactly this
kind of platform independence is a worthy goal. However, the semantics
of any C++ program are entirely dependent on the underlying architecture. [snip]

C++, therefore, can not be regarded as truly platform independent,

[snip]

Actually, you mean a C++ program, the language is platform
independent.

[snip]

Arguably, if the C++ standard really did standardise data types, giving
them well-defined meanings (e.g. int8, unsigned16, ieeefloat, etc.) with
a fully specified semantics, this would make the language *more*
portable. The existing definition is sufficiently wooly that programming
for portability can often be extremely challenging.

Well, I think you are missing a key point. C++, and C, were defined to
be run on as many platforms and evironments as possible. That includes
embedded systems and systems where you could have bit sizes of 9, 12,
and even 40 bits. To avoid penalizing such systems, the standard
avoids mandating byte and type sizes; for example, whether a char
(byte) is 8 bits or not, and even whether it is signed or unsigned.
Also note that with such systems, mandating the sizes does not
necessarily simplify portability.

As a result, the programmer is responsible for determining how
portable a program is, which is as it should be. As for me, I can't
think of any program that I'd want to run on every conceivable
platform. But there are classes of platforms (32/64 bit UNIX and
WinNT) that I do want to program for. I make my design choices
accordingly.

Yes, this may be regarded as a little heretical, but I'm not trolling
here -- this is an important issue. I've written more than one C++
library with portability in mind (e.g. the signal/slot library on my web
site, as well as a new lexical analysis/parsing library that I'll
release sometime when I've finally gotten around to finishing its
documentation), and can't say I find C++ terribly helpful in this regard.

Well, I agree with much of what you say, but IMO, C++ can run on more
platforms and is a truly platform independent language because it does
avoid "hardware" requirements.

Dave

 

[Ron Natalie]

No, linux has the same problem. My uninformed feeling is that it
would be rather difficult for an operating system on a Pentium to
let the user have more than 3GB of VM while still having any
security.

I didn't say the problem was unique to WIN32. What does security
have to do with it? It's a performance/convenience thing to keep the
user space mapped while in kernel mode, but it is not required.

No one else has expressed a concern about something that had been
worrying me, which is negative addresses if the pointers were
signed ints. I could easily imagine that causing problems for
sloppy code.

Pointers aren't ints at all. I fail to understand what difference that makes.
You're not likely to have an issue with pointers. What you're likely to
have issue with is the sizes of things (which will be some sort of integer).
Addresses don't get negative (at least not on any machine I've ever used).
There were a couple with the address breaks for kernel vs. user hard coded
into them but the Pentium isn't one of these.

Nevertheless, if the authors have that knowledge, including some
references to a few specific widely used implementations, and
including info about the tools' limits, would increase the books'
usefulness, especially since that info is not to be obtained
anywhere else.

What usefulness? What limitations? All implementations are going to
have limitations, but it's not part of the language. You're going to have to
consult the individual implementation. Just gecause G++ on win32 supports
one thing doesn't mean VC++ on the same platform will have the same limitations
etc...

BTW, I had thought that the goal of boost was to become part of
standard C++, as STL basically is now, and that it has an overlap
of personnel. Could someone correct me on this?

Some of boost has merit for inclusion, some of it doesn't. As far as I know
there is no intention by the standards committee to take boost as a whole.

 

[Sarah Thompson]

Well, I think you are missing a key point. C++, and C, were defined to
be run on as many platforms and evironments as possible. That includes
embedded systems and systems where you could have bit sizes of 9, 12,
and even 40 bits. To avoid penalizing such systems, the standard
avoids mandating byte and type sizes; for example, whether a char
(byte) is 8 bits or not, and even whether it is signed or unsigned.
Also note that with such systems, mandating the sizes does not
necessarily simplify portability.

As a result, the programmer is responsible for determining how
portable a program is, which is as it should be. As for me, I can't
think of any program that I'd want to run on every conceivable
platform. But there are classes of platforms (32/64 bit UNIX and
WinNT) that I do want to program for. I make my design choices
accordingly.

There are really two choices here. Either standardise types in the
language, so that programs have a well-defined semantics regardless of
platform, or don't standardise types, and have no standardised semantics
either. C++ makes the wrong choice, in my opinion.

It *should not* be necessary to go to considerable lengths to guarantee
portability. Programs should be portable by default, unless they are
specifically written to interface with something nonstandard (low level
drivers, a GUI, etc.). This is not the case with C++.

Well, I agree with much of what you say, but IMO, C++ can run on more
platforms and is a truly platform independent language because it does
avoid "hardware" requirements.

C++ runs on lots of platforms mostly because lots of platforms have C++
compilers implemented for them, which itself is a consequence of the
language's popularity. This probably owes more to its C heritage, and to
C's own enormous market share in the late 80s and early 90s, than it
does to any specific features.

C++ doesn't avoid hardware requirements, because its semantics are
entirely hardware dependent, whether or not you choose to use
nonstandard libraries. In this sense, it is pushing the point a bit to
truthfully call the language genuinely platform independent, or for that
matter particularly portable. It's just that it happens to be supported
on lots of platforms -- this isn't the same thing. Moving code between
architectures involves quite a lot of pot luck -- however much effort
has been put into making the code portable, *something* typically breaks
if the code is non-trivial.

It is possible to write very portable C++. I know, I've written lots of
it myself. My argument is that such code is by no means easy to write,
nor is it the default for 'typical' code, because of C++'s incompletely
standardised semantics.

Frankly, being able to compile unmodified code for 9 bit bytes or 56 bit
machine words (say) is all very well, but this is something that is so
spectacularly rare that sacrificing genuine portability for this is not
really a supportable design choice.

Whoops, too late. We're stuck with it now!

--
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