Engineering a list container. Part 1.

J

jacob navia

Lists
-----
Single linked lists use a single pointer to the next element. The data
for the element comes right behind that pointer to avoid the overhead
that yet another pointer would represent.

So, we use this structure:

typedef struct _list_element {
struct _list_element *Next;
char Data[MINIMUM_ARRAY_INDEX]; // See below
} list_element;

The list header uses this structure to store the elements. As you can
see, there is no space wasted in a pointer to the element stored. The
element stored is placed just behind the Next pointer. The downside of
this decision is that we can’t recycle this object to store other
different objects of different size.

The constant MINIMUM ARRAY INDEX is defined as 1 if we are compiling in
C90 mode or as nothing if we are compiling in C99 mode. In C99 mode we
have a flexible structure, that consists of a fixed and a variable part.
The fixed part is the pointer to the next element. The variable part is
the object we are storing in the list.

Alignment
---------
Some machines require that data be stored at particular addresses,
always a multiple of two. For instance SPARC machines require that
doubles be aligned at a multiple of 8. The structure for our list
element above would provoke a crash when used to store doubles 4.
In those machines the list element structure is defined as follows:

typedef struct _ListElement {
struct _ListElement *Next;
#ifdef SPARC32
void *alignment;
#endif
char Data[MINIMUM_ARRAY_INDEX];
} ListElement;

This assumes that sizeof(void *) is 4.
In machines that handle unaligned data gracefully without crashing
alignment re- quirements aren’t useless, since in most cases they
provoke a performance loss.

The list header
---------------

struct _List {
ListInterface *VTable;
size_t count;
unsigned Flags;
unsigned timestamp;
size_t ElementSize;
list_element *Last;
list_element *First;
CompareFunction Compare;
ErrorFunction RaiseError;
ContainerHeap *Heap;
ContainerAllocator *Allocator;
};
In the public containers.h header file we refer always to an abstract
structure List. We define it here. This schema allows other
implementation to use the same header with maybe radically different
implementations of their data structure.

The individual fields are as follows:

1.
1.1: Vtable. Pointer to a table of methods for this container
1.2: count. Number of elements in the list
1.3: Flags. Bit fields for flags about this list.
1.4: ElementSize. Number of bytes the data occupies.

2. timestamp. This field is incremented at each modification of the
list, and allows the iterators to detect if the container changes during
an iteration: they store the value of this field at the start of the
iteration, and before each iteration they compare it with its current
value. If there are any changes, they return NULL .

3. Last. Stores a pointer to the last element of the list. This allows
the addition of an element at the end of the list to be fast, avoiding a
complete rescan of the list. This field is an optimization, all
algorithms of a single linked list would work without this field.

4. First. The start of the linked list.

5. Compare. A comparison function for the type of elements stored in the
list.

6. RaiseError. A function that will be called when an error occurs. This
field is necessary only if you want to keep the flexibility of having a
different error function for each list that the client software builds.
An alternative implementation would store a pointer to an error function
in the interface.

7. Allocator. A set of functions that allocates memory for this list. In
an implementation that needs less flexibility and is more interested in
saving space it could be replaced by the default allocator.

The sample implementation has certainly a quite voluminous header
because of a design decision to keep things very flexible. Other
implementations could trim most of the fields, and an absolute minimal
implementation would trim Last, Compare, RaiseError, Heap,
and Allocator. If the implementation assumes that only one iterator per
container is allowed, the timestamp field could be replace by a single
bit (’changed’) in the Flags field.

The list interface
------------------
The list interface is a table of functions that holds all the methods
of the container. For the list container we have:

typedef struct tagListInterface {
int (*Add)(List *L,const void *newval);
int (*AddRange)(List *L, size_t n,const void *data);
void *(*Advance)(ListElement **pListElement);
int (*Append)(List *l1,List *l2);
int (*Apply)(List *L,int(Applyfn)(void *,void *),void *arg);
void *(*Back)(const List *l);
int (*Clear)(List *L);
int (*Contains)(const List *L,const void *element);
List *(*Copy)(const List *L);
int (*CopyElement)(const List *list,size_t idx,void *OutBuffer);
List *(*Create)(size_t element_size);
List *(*CreateWithAllocator)(size_t elementsize,
const ContainerAllocator *mm);
void *(*ElementData)(ListElement *le);
int (*Equal)(const List *l1,const List *l2);
int (*Erase)(List *L,const void *);
int (*EraseAll)(List *l,const void *);
int (*EraseAt)(List *L,size_t idx);
int (*EraseRange)(List *L,size_t start,size_t end);
int (*Finalize)(List *L);
ListElement *(*FirstElement)(List *l);
void *(*Front)(const List *l);
const ContainerAllocator *(*GetAllocator)(const List *list);
void *(*GetElement)(const List *L,size_t idx);
size_t (*GetElementSize)(const List *l);
unsigned (*GetFlags)(const List *L);
ContainerHeap *(*GetHeap)(const List *l);
List *(*GetRange)(const List *l,size_t start,size_t end);
int (*IndexOf)(const List *L,const void *SearchedElement,
void *ExtraArgs,size_t *result);
List *(*Init)(List *aList,size_t element_size);
int (*InitIterator)(List *L,void *buf);
List *(*InitWithAllocator)(List *aList,size_t element_size,
const ContainerAllocator *mm);
List *(*InitializeWith)(size_t elementSize,size_t n,
const void *data);
int (*InsertAt)(List *L,size_t idx,const void *newval);
int (*InsertIn)(List *l, size_t idx,List *newData);
ListElement *(*LastElement)(List *l);
List *(*Load)(FILE *stream, ReadFunction loadFn,void *arg);
Iterator *(*NewIterator)(List *L);
ListElement *(*NextElement)(ListElement *le);
int (*PopFront)(List *L,void *result);
int (*PushFront)(List *L,const void *str);
int (*RemoveRange)(List *l,size_t start, size_t end);
int (*ReplaceAt)(List *L,size_t idx,const void *newval);
int (*Reverse)(List *l);
int (*RotateLeft)(List *l, size_t n);
int (*RotateRight)(List *l,size_t n);
int (*Save)(const List *L,FILE *stream, SaveFunction saveFn,
void *arg);
int (*Select)(List *src,const Mask *m);
List *(*SelectCopy)(const List *src,const Mask *m);
CompareFunction (*SetCompareFunction)(List *l,CompareFunction fn);
DestructorFunction (*SetDestructor)(List *v,DestructorFunction fn);
int (*SetElementData)(List *l, ListElement *le,void *data);
ErrorFunction (*SetErrorFunction)(List *L,ErrorFunction);
unsigned (*SetFlags)(List *L,unsigned flags);
size_t (*Size)(const List *L);
size_t (*Sizeof)(const List *l);
size_t (*SizeofIterator)(const List *);
ListElement *(*Skip)(ListElement *l,size_t n);
int (*Sort)(List *l);
List *(*SplitAfter)(List *l, ListElement *pt);
int (*UseHeap)(List *L, const ContainerAllocator *m);
int (*deleteIterator)(Iterator *);
} ListInterface;

Note that ther is a single Vtable for all lists of the same type.
All list just a pointer to a common method table.

In a future post I will explain this functions in detail and propose
code for them.

Feel free to comment on this.

jacob
 
M

Malcolm McLean

typedef struct _list_element {

struct _list_element *Next;

char Data[MINIMUM_ARRAY_INDEX]; // See below

} list_element;

C handles this perfectly well as

typedef struct node
{
struct node *next;
int fielda;
char fieldb[100];
}

etc, but it's by no means easy to make it generic. As you say, if fielda is
a double, some systems will insert 4 bytes of padding between it and the next
pointer. No problem, unless you want a generic system.
The sample implementation has certainly a quite voluminous header
because of a design decision to keep things very flexible. Other
implementations could trim most of the fields, and an absolute minimal
implementation would trim Last, Compare, RaiseError, Heap,
and Allocator.
Feel free to comment on this.
OK, so keeping with our non-generic implementation, probably we'll be calling
malloc() to create new nodes, we'll have head pointer somewhere, we'll
frequently be iterating over the list with

for(ptr = head; ptr != 0; ptr = ptr->next)

How can we beat this for the generic? We can use a fast fixed block allocator.
We can implement bug-free insert / deletes at any position, whilst with the
simple non-generic implementation we can only insert at the head, and we've
got to remember not to let anyone hang on to the head pointer because it
becomes invalidated by an insert. We can automatically destroy the list.

What happens if the list itself changes during an iteration? Is it important
for the programmer to be able to toggle relatively easily between a list and
an array? Are out of memory conditions essentially impossible, or do we need
an elaborate system to handle them? With a flexible generic solution, you've
got to provide answers to all of those, and it has to be the more complex
answer. That's one snag. The generic can't be totally simple to use, because
there aren't simple ways of addressing these issues.

The other problem is the perennial one, dependency. You don't want to create
a dependency on another file, much less a library/compiler, just because you're
using a linked list to store your objects.
 
J

jacob navia

Le 08/12/2013 17:42, Malcolm McLean a écrit :
typedef struct _list_element {

struct _list_element *Next;

char Data[MINIMUM_ARRAY_INDEX]; // See below

} list_element;

C handles this perfectly well as

typedef struct node
{
struct node *next;
int fielda;
char fieldb[100];
}

etc, but it's by no means easy to make it generic. As you say, if fielda is
a double, some systems will insert 4 bytes of padding between it and the next
pointer. No problem, unless you want a generic system.

You have no problems with the generic system either. Just do

typedef struct myData {
int fielda;
char fieldb[100];
} Data;

You pass sizeof(Data) to the generic creation functions. No overhead at all.

OK, so keeping with our non-generic implementation, probably we'll be calling
malloc() to create new nodes, we'll have head pointer somewhere, we'll
frequently be iterating over the list with

for(ptr = head; ptr != 0; ptr = ptr->next)

How can we beat this for the generic? We can use a fast fixed block allocator.

Yes, a fast fixed block allocator is provided by the ccl. No need to use
a slow malloc function. But of course the generic implementation
needs to do the same loop and will have the same speed as the
non-generic.
We can implement bug-free insert / deletes at any position, whilst with the
simple non-generic implementation we can only insert at the head, and we've
got to remember not to let anyone hang on to the head pointer because it
becomes invalidated by an insert.

No. The ccl provides with "FirstElement" and "NextElement" functions
that return the whole (including the "next" pointer), for you to hack
away with the same liberty as you would using your personal list
implementation.


We can automatically destroy the list.

Sorry I do not understand that. You mean your personal non-generic
implementation uses garbage collection? Or what?
What happens if the list itself changes during an iteration?

If you use the provided iterators, the iteration stops.

Is it important
for the programmer to be able to toggle relatively easily between a list and
an array?

Maybe, who knows. In any case it is very easy to do with the ccl.

Are out of memory conditions essentially impossible, or do we need
an elaborate system to handle them? With a flexible generic solution, you've
got to provide answers to all of those, and it has to be the more complex
answer. That's one snag.

Yes, in your own list implementation you just crash if there is no more
memory left because you assumed it will always be available. The ccl
doesn't do that but this provokes NO EXTRA COMPLEXITY in your code since
this only be an issue if an allocation fails!

This is no "snag" but a feature that allows you to program AS IF there
was always memory available but in case you do not crash but a user
supplied recovery function is called!
The generic can't be totally simple to use, because
there aren't simple ways of addressing these issues.

The error handling design is quite complex but at the same time very
easy to use. By default there is an error function that is called when
an error occurs. The ccl supplies a default function that prints the
error in stderr and exits the program. You can override that with your
own function of course.

The other problem is the perennial one, dependency. You don't want to create
a dependency on another file, much less a library/compiler, just because you're
using a linked list to store your objects.

Yes, you program (and debug) the linked list package (that is always an
extra file excuse me) at each application that you program.

The idea of the ccl is that you use a debugged container library instead
of reinventng the wheel at each time.

In the case of a linked list it is possible to understand your view
point, but for other containers like a flexible (resizable) array or
a tree, a hashtable, etc the situation is different. You do not want
to program those, and then, instead of using a resizable array you
use a list even if it is slower!

I started with the linked list because is the simplest!

Thanks for your input Malcom.

jacob
 
I

Ian Collins

Malcolm said:
The other problem is the perennial one, dependency. You don't want to create
a dependency on another file, much less a library/compiler, just because you're
using a linked list to store your objects.

Where do you draw the line?

Somewhere between "You don't want to create a dependency just to output
a character" and "You don't want to create a dependency just to use a list"?
 
J

jacob navia

Le 08/12/2013 19:57, Ian Collins a écrit :
Where do you draw the line?

Somewhere between "You don't want to create a dependency just to output
a character" and "You don't want to create a dependency just to use a
list"?

In the case of a simple container like a single linked list it is easy
to just hack away a nth implementation of a list but for a hashtable
or a tree, or even an extensible array the effort is quite considerable
to get it right...


Then, you get stuck with using a list instead of a hashtable or
a flexible array!

I started with the linked list because is the simplest to explain.

jacob
 
M

Malcolm McLean

Malcolm McLean wrote:




Where do you draw the line?

Somewhere between "You don't want to create a dependency just to output
a character" and "You don't want to create a dependency just to use a list"?
Programming has to be done by reasonably intelligent human beings. There's
no simple or algorithmic answer.

Is it important to be able to take a single file, and associated header, and
drop it into another program? Does it matter if you have three or four
generic linked lists kicking about in your program source (because Malc used
Jacob's library whilst Ian used a version based on C++ stl and Fred rolled
his own)?

There aren't easy answers. Sometimes it's easy, if we know that the code will
be used only in one program, it's not going to be of any interest to anyone
else developing similar programs, and we've got a small, controlled group
working on that one program, then once we've taken the decision to go the
Jacob container library route, there's no further issue, we should use it
for every linked list.

With outputting a character, that's not a "bit shuffling operation", so
inherently it has some dependency. So I try to move my IO to separate modules,
so the bulk of the program can be written as portable, independent, standalone
functions.
 
J

Johann Klammer

jacob said:
Lists
-----
Single linked lists use a single pointer to the next element. The data
for the element comes right behind that pointer to avoid the overhead
that yet another pointer would represent.

So, we use this structure:

typedef struct _list_element {
struct _list_element *Next;
char Data[MINIMUM_ARRAY_INDEX]; // See below
} list_element;

The list header uses this structure to store the elements. As you can
see, there is no space wasted in a pointer to the element stored. The
element stored is placed just behind the Next pointer. The downside of
this decision is that we can’t recycle this object to store other
different objects of different size.

The constant MINIMUM ARRAY INDEX is defined as 1 if we are compiling in
C90 mode or as nothing if we are compiling in C99 mode. In C99 mode we
have a flexible structure, that consists of a fixed and a variable part.
The fixed part is the pointer to the next element. The variable part is
the object we are storing in the list.

Did you look at the linked lists inside linux(include/linux/list.h...
types.h... struct list_head)? AFAIK they use a header structure, that
gets included in structures that want to use it. Then there's usually
some pointer arithmetics(usually things involving offsetof) to get from
the header pointer to the containing struct and vice-versa. It has the
advantage to be able have some struct in multiple lists (perhaps a rare
use-case.. but sometimes..), by using several header vars..
Any particular reason not to do it that way? I always thought, it was
rather clever...
 
J

jacob navia

Le 08/12/2013 22:02, Johann Klammer a écrit :
Did you look at the linked lists inside linux(include/linux/list.h...
types.h... struct list_head)? AFAIK they use a header structure, that
gets included in structures that want to use it. Then there's usually
some pointer arithmetics(usually things involving offsetof) to get from
the header pointer to the containing struct and vice-versa. It has the
advantage to be able have some struct in multiple lists (perhaps a rare
use-case.. but sometimes..), by using several header vars..
Any particular reason not to do it that way? I always thought, it was
rather clever...

Well, the idea of having a header in each element is too heavy
for the containers since it is of the order of:

sizeof(elementHeader) * N

In my case this is just

sizeof(void *) * N // the "next item" pointer

The library doesn't know anything about any offsets into your data.
It treats the data as a container (a box for instance) into which
you store stuff and retrieve it. What is stored is invisible for the
container. You do not expect a box to handle the objects you store into
it any differently from each other!

You can have generic lists specialized for any conceivable type
if you use the generic list container, to be explained later.

Yes, in C. It uses the C preprocessor. Of course I have always read
that it can't be done. I believed that also, until... I wrote it.

It is actually very simple.

http://code.google.com/p/ccl/

All the code can be downloaded from there and hacked at will.

Enjoy!

jacob
 
B

Ben Bacarisse

jacob navia said:
Le 08/12/2013 22:02, Johann Klammer a écrit :

Well, the idea of having a header in each element is too heavy
for the containers since it is of the order of:

sizeof(elementHeader) * N

In my case this is just

sizeof(void *) * N // the "next item" pointer

The Linux lists are doubly linked, so two pointers are needed. The
overhead, though, is just that: two pointers plus any required padding.
Using the same technique for singly linked lists would presumably have
the same overhead as yours: one pointer plus any required padding.

<snip>
 
J

jacob navia

Le 09/12/2013 12:17, BartC a écrit :
I've looked at the PDF, and one word I wouldn't use to describe it is
simple! (Comprehensive, perhaps.)

Its 370 dense pages are 100 more than K&R 2 which describes the entire
language.

If you look at any book about the STL it is much thicker. Obviously
writing detailed documentation for each function is not a strength
of the package in your opinion.

In those 370 "dense" pages there is a lot of repetitions to facilitate
reading, a lot of diagrams, a rationale, and comparisons with other
languages like Java, C# and C++.

And if K&R 2 is 270 pages, the language standard is 683 pages.
 
M

Malcolm McLean

Le 09/12/2013 12:17, BartC a �crit :


If you look at any book about the STL it is much thicker. Obviously
writing detailed documentation for each function is not a strength
of the package in your opinion.
It's obviously not acceptable for someone to need to read 370 pages before
they can use the package.
You need two layers of documentation. In layer one, an overview of the scope
of the package, what it can do, and it's also convenient though sometimes a
bit embarrassing to point out what it cannot do. (Nothing wastes time so
much as looking for the "serialise" method when there isn't one). You also
need to tell the user how to achieve common tasks, eg set up a list, iterate
through it, add and delete items, get its length, destroy it, usually with
source code. He should be able to glance at the docs, and within a minute or
so have a functioning linked list.
In layer two, you need the 370 pages of detail. For instance what's meant to
happen if you add or delete elements from a list whilst iterating through it?
There's no good, obvious, or simple answer to that. In fact you increment an
edit counter field. So is the field reset to zero if we take a copy of the list? It's unlikely that many people will want to know the answer to that
question, but for someone it's going to be vital. So that needs to be buried
away somewhere.
 
A

Andrew Cooper

The Linux lists are doubly linked, so two pointers are needed. The
overhead, though, is just that: two pointers plus any required padding.
Using the same technique for singly linked lists would presumably have
the same overhead as yours: one pointer plus any required padding.

<snip>

A linux list_head is two pointers (plus whatever padding the compiler
deems is necessary - 0 for the common architectures)

In addition, you have an ability to turn an arbitrary structure into
linked list by embedding a list_head. You can put an arbitrary number
of list_heads into a structure, and have the structure in an arbitrary
number of independent lists.

The standard iteration tools will issue hardware preload instructions
for the subsequent entries as you walk the list, meaning fewer cache misses.

Judging from the presented code, the entire Linux implementation is
substantially smaller, and involves no function pointers.


The proposal presented here is desperately trying to be C++, and is
completely over engineered for the problem it is trying to solve; It is
not like lists are hard.

It seems silly to be concerned about an extra pointer in the structure,
with a vtable that large for each type of list.

~Andrew
 
B

BartC

The proposal presented here is desperately trying to be C++, and is
completely over engineered for the problem it is trying to solve; It is
not like lists are hard.

The idea of using a common set of functions to work with linked lists is
good - if you do just want a simple linked list.

However, I use linked lists a lot, and they are rarely simple! For example,
one struct I had I think was linked seven different ways to other structs
(mostly sideways, some up and down to form a tree).

The point is, my data structures, as many as I needed, were superimposed on
the struct, rather than the struct being shoe-horned into a (single) data
structure, which is less flexible.

I'd imagine it would also be awkward to point into the middle of a list, or
somehow share the same data with several list headers. And if I wanted to
insert a pointer to a list into a struct, it's no longer a pointer to the
first element, but a pointer to a formal header, which then points to the
data. It's just extra stuff to be aware of and which can complicate matters
rather than simplify them.

But, maybe it's a mistake to try and adapt existing code to use these
containers...
It seems silly to be concerned about an extra pointer in the structure,
with a vtable that large for each type of list.

There is just one instance of the vtable (for all singly-linked lists). And
one list-header for each entire list. But any extra fields per-element can
have a much bigger impact.
 
J

jacob navia

Le 10/12/2013 00:22, Andrew Cooper a écrit :
A linux list_head is two pointers (plus whatever padding the compiler
deems is necessary - 0 for the common architectures)

The same in the ccl.

In addition, you have an ability to turn an arbitrary structure into
linked list by embedding a list_head. You can put an arbitrary number
of list_heads into a structure, and have the structure in an arbitrary
number of independent lists.

You can store any data type into an arbitrary number of lists with the
ccl too.

The standard iteration tools will issue hardware preload instructions
for the subsequent entries as you walk the list, meaning fewer cache misses.

That is an implementation detail easily done in an architecture where
that is relevant. The code presented in the ccl is generic for *ANY*
architecture.
Judging from the presented code, the entire Linux implementation is
substantially smaller, and involves no function pointers.

The linux implementation is not the ccl, it is a kernel list
implementation that has only lists.


If you write code with the ccl, you can easily change your container
from a list to a flexible array just by changing a few lines. You
can do it if you measure that lists are taking too much CPU time.

I do not see how you would do that in a container that is isolated from
the others!
The proposal presented here is desperately trying to be C++, and is
completely over engineered for the problem it is trying to solve; It is
not like lists are hard.

No, it is not C++ even if I use some of the concepts of object oriented
programming. And mocking about it by saying "desperatly" doesn't really
convey any argument. By the way I am not "against" C++. I consider the
language as a whole a huge mistake, but there are good ideas within it.

It seems silly to be concerned about an extra pointer in the structure,
with a vtable that large for each type of list.

The vtable is shared by ALL lists. The elements do not have any pointer
to the vtable, only the list head.

And yes, lists are easy, that is why I presented them FIRST.

Next are the flexible arrays.
 
M

Malcolm McLean

However, I use linked lists a lot, and they are rarely simple! For example,
one struct I had I think was linked seven different ways to other structs
(mostly sideways, some up and down to form a tree).
That's often a problem with a library that "sits over you" rather than "sits
under you". By "sits over you", I mean provides structures that you plug
the subroutines and data items into, rather than calling with your data
items. It works fine, until you need two libraries that "sit over you".

Then they start interfering. Both want to control your main loop, both
want to deallocate your structures when they die, both want to insert a
special field at the head of the structure.
 
I

Ian Collins

Andrew said:
Judging from the presented code, the entire Linux implementation is
substantially smaller, and involves no function pointers.

But it is just a list.
The proposal presented here is desperately trying to be C++, and is
completely over engineered for the problem it is trying to solve; It is
not like lists are hard.

It isn't trying to be C++, it is striving to provide a subset of the C++
standard library in C. Making a small list is trivial, making a
collection of interchangeable containers isn't.

Lists are widely used in C because they are so simple. In C++, vector
(flexible array) is more common. It is more complex to implement, but
in most use cases it offers better performance. Having a standardised
collection of containers saves an awful lot of wheel reinventing. Being
able to quickly swap between them when requirements or the environment
changes is a big bonus.
 
J

jacob navia

Le 11/12/2013 20:09, (e-mail address removed) a écrit :
Anything particularly wrong with CoreFoundation?

http://en.wikipedia.org/wiki/Core_Foundation

Yes.

1) The Apple license. The CCL is in the public domain, it has
in fact no restrictions of any kind, and no, it is free, not GNU
so the FSF has nothing to say either.

2)
<quote>
Core Foundation is a library with a set of programming interfaces
conceptually derived from the Objective-C-based Foundation framework but
implemented in the C language. To do this, Core Foundation implements a
limited object model in C. Core Foundation defines opaque types that
encapsulate data and functions, hereafter referred to as “objects.”
<end quote>

This makes very strong assumptions about the objects being stored.
Basically the core foundation is just objective C "compiled" to C.

The CCL should be able to run in very limited memory setups, and be
portable to any machine. As far as I know the Core Foundation has been
only ported to windows, not to linux.

That said, many specifications of the ccl (for instance the observer
interface) have been influenced by the work of Apple's engineers.
 
J

jacob navia

Le 19/12/2013 07:36, Gareth Owen a écrit :
The STL container section of the C++ standard is around 100 pages.
Granted, its terse standardese, and almost entirely free of diagrams,
comparisons, examples etc.

Yes, I will write a shorter documentation, in 100 pages or so.
But the real saving is that the error handling (i.e. the interaction
between destructors and exceptions) is part of the language, not part of
the library.

It can be "savings" if you want the bloated code that this produces.
Granted, it is the compiler that writes that, but it does take
space.
 
J

jacob navia

Le 19/12/2013 18:14, Gareth Owen a écrit :
jacob navia said:
It can be "savings" if you want the bloated code that this produces.

[citation needed]

Exception handling has a long history in C++.

The first solutions were just a glorified longjmp that the compiler
inlined at each entry of a protected block.

This solutions is now obsolete but still used in some systems like
gcc under windows. Since each protected block needs to establish a
context (the setjmp call) you pay even if there is no exception.

The solution that evolved later is a table approach.

You build tables relating code points to stack descriptions, so that
the run time can call a bye code machine that interprets the tables
to unwind the stack searching for an exception. Lcc-win generates
this tables to be compatible with C++, and I can tell you those
tables are HUGE since each function entry/exit needs a lot of
descriptions for each instruction being emitted.

There isn't a big literature on this subject since only Microsoft
has really documented this stuff in a meaningful way. Gcc refers
always to the DWARF specifications where a documentation exists
but gcc doesn't follow their own specs for unknown reasons. So,
you have just to fgure out what gcc generates and then do the same.
 

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