From: ION_G_M (ION_G_M_at_[hidden])
Date: 2004-11-23 06:03:20
Thank you Pavel for your comments, I would like to comment on some
>1. The HTML documentation does't allow to change font in browser.
>2. End of document can be marked so it doesn't
> look like cut in half during transmission.
>3. docs 3.2:
Sorry for errors and bad html. It was generated from MS Word so I will
try to rewrite it using plan html. I'll change that shortly.
>4. Example in 3.2: the "alignement" parameter in
> segment.create() isn't found in code.
I can't find the error you mention, I've just downloaded the zip file
and 3.2 does not have any example. In 4.2 segment.create is missing
>5. Example in 4.2:
> ("MyType instance", /*name of the object*/
> 10 /*number of elements*/,
> false /*if previously present, return error*/,
> 0 /*ctor first argument*/,
> 0 /*ctor second argument*/);
a.It returns false. Exceptions are used only to indicate memory errors
throwing bad_alloc. You are right there is info missing here. I will
add more documentation in examples.
b.If you find this approach more useful, I have no problem. I really
don't like the boolean parameter, but I wanted to have a "find or
create" functionality. If you like a find_or_named_new<>() additional
function to indicate that approach I find it more clear than with a
c.The syntax you propose is better, no doubt. I don't have experience
with it so to implement this I suppose named_new<> should return an
proxy object with overloaded operator()() functions. Is that right? If
you want to help me I'm open. If boosters prefer throwing exceptions
instead or returning false no problem here. A problem I see is that my
interface allows creating an array like new. Do you consider this
necessary? You prefer a different function? Maybe the proxy object
should have an operator that can be used to specify array allocation?
> std::pair< MyType *, std::size_t> res =
> segment.find_named_new< MyType > ("MyType instance");
>Why do I need the "size"? Doesn't a type have
>always the same size regardless?
Size contains the number of elements in case you allocate an array. with
("MyType instance", /*name of the object*/
10 /*number of elements*/, ...
you allocate an array of 10 elements. so it will return 10.
>6. Could you use namespace shmem_detail or so
> instead of "detal" to avoid possible clashes?
No problem. I've seen detail namespace in some projects so I thought it
was not a problem. detail namespace is inside boost::shmem namespace so
it wouldn't be necessary. You find it necessary even if detail
namespace is really boost::shmem::detail?
>a) file name should be shmem_exceptions.hpp or so
>b) does it make sense to have common base
> for both exceptions there?
My exception handling experience is null, so I based all in thread
exception examples, where lock_error inherits from thread_exception. No
problem changing this is you find it necessary.
>8. offset_ptr.hpp: full_offset_ptr class
>a) using char as de-facto bool class has usually
> no practical advantage and may be actually slowe.
No problem. You are right, with all platforms using 4 byte alignment
bool and char use the same space and it can be slower I suppose due to
>b) The flag could be eliminated completely.
> If m_offset == 0 it is NULL pointer and
> no data allocated in shmem will starts
> on the beginning. This would also eliminate
> need for min_offset_ptr.
The offset indicates the distance between the pointee and the this
pointer of offset_ptr, so m_offset == 0 indicates a pointer pointing to
itself and this is quite common in STL containers when empty, since
next pointer in the end node points to end node, resulting in a
m_offset = 0. Obviously this is different from a null pointer. If I
change the meaning of m_offset to offset from the beginning of the
segment I need the base address stored somewhere (and the base address
is different in each process), so that I can convert from offset_ptr<A>
to A* using get() or the constructor.
Using my approach I can convert between A* and offset_ptr<A> without
any additional data. Obviously, your approach is more performance
friendly since m_offset does not need to be changed with each
assignment and constructor. If boost people find the additional member
as a waste comparing to A* <-> offset_ptr<A> conversions I would change
it but I find my approach quite useful, since I don't want to deal with
base addresses stored somewhere for each process when building a
>c) swap() could be added and other operators.
Ok. I will add it. Which other operator you miss?
>9. Maybe the protection of mutext from shared ptr
> lib could be worked around
I don't understand your point. Could you give me an example please?
>10. The example in 4.3 uses very dirty C-like
> approach with casts. Cannot it be rewritten
> in C++ way with overloaded new?
If you refer to (list_node*)segment.allocate(...) line, segment in this
case is a low-level void* returning function. If you prefer an
allocate<Type> approach, I don't have any problem, but remember that
this segment object is of type shmem_alloc which allocs raw memory,
something like when writing your own void* operator new(size_t size)
for a class. If you have I workaround in mind, please let me know.
>11. Some source files use Unix line ends,
> some DOS line ends. Just bit strange.
It's because I've tested and changed things in both windows and linux.
I'll try to convert all to a common line feed.
>12. The simple algorithm to find fitting memory
> block may not be adequate for high-performance
> apps (who are most likely to use shared
You are right. The default algorithm is space-friendly, which I thought
it was more important than performance for fixed size segments. You can
write your own algorithm and use it since shmem_alloc is a typedef of
basic_shmem_alloc<default_algo>. If you prefer another algo like
segregated lists, I can try to write it, so that the user can choose
the allocation algorithm. I've written the pooled allocator due to
default algorithm slowness.
>13. Can be be possible to identify named objects
> with something else than C string? Say
> wchar_t* or numeral or other templated type?
Well, I've chosen a c string as an universal name, but other types
could be used. Do you think that the key type should be templatized? I
think that an integer key can speed up a lot searches but I have to
think about which classes should be templatized. When storing other
type of strings (for example std::string, I would need to build an
allocator for strings in shared memory but also a std::string since
it's probable that current STL won't work with Shemem STL allocator).
The key meaning would be different since right now, I copy the string
to shared memory but with configurable key type things are more
>14. What I would like to see:
>a) avoiding shmem specific containers/mutexes/etc
> as much as possible.
I think you can't avoid mutexes if you want to guarantee atomic memory
allocation, since I have no skills to write a lock-free memory
allocator. Regarding to containers, it was no my intention to write
them, but I needed some of them to store name-buffer mappings and a
node container to test the pooled allocator in several systems. For
now, I have only succeed using shmem STL allocators in a modified
Dinkum STL. STLport and libstc++ use suppose allocator::pointer to be a
raw pointer, so I can't use STL containers. I've chosen internal
containers to be public because I find them very useful, but if this is
not accepted they can be used only for internal uses and removed from
Regarding mutexes and etc... can you be more explicit? I propose
mutexes to be general. If boost want to implement process-shared
mutexes in other way, say in another library, I would use them, but for
the moment, this is all that I have.
>b) ability to "equalize" shared memory features
I would need some help in this because my operating system knowledge is
very limited. Mimic-ing UNIX way in windows can be very difficult, I
think, unless you use a mapped file. I would need some serious help
>c) support for inheritance in shmem using object
> factories, e.g. like one in Classloader
I'll check it. Thanks for the url.
>d) support for "transactions": I would like to
My knowledge in transaction world is null so I can't help you with
that. I suppose that a shared memory condition variable should be very
interesting to notify events to other processes, but I'm afraid this is
a work for more skilled programmers than me (people from
boost::threads, perhaps?). As far as I know in windows is difficult to
implement a shared memory condition variable, pthreads-win32 does no
support it and I don't know how cygwin solves this.
Thank you again for all your comments. I would like to make some
changes you've suggested, so I will wait your response regarding open
issues and comments from other boosters before doing any change.
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk