about memory model

[kumar]

hi
i want to know where & how the C variables gets stored
i mean like volatile , pointer and string variables gets stored ,
whether it is on stack or some other places
if is there any clear document , plz suggest the link

[Ian Collins]

Re: about memory model

kumar wrote:

hi
i want to know where & how the C variables gets stored
i mean like volatile , pointer and string variables gets stored ,
whether it is on stack or some other places
if is there any clear document , plz suggest the link

The word is "please".

The best place to find the answer is in your compiler or platform
documentation. The details of where variables are stored are
implementation specific.

--
Ian Collins.

[Szabolcs Borsanyi]

Re: about memory model

On Mon, May 26, 2008 at 12:29:21AM -0700, kumar wrote:

hi
i want to know where & how the C variables gets stored
i mean like volatile , pointer and string variables gets stored ,
whether it is on stack or some other places

The nice thing with C is that you do not have to think about this
when you program in standard C. Whenever you think about these details
you loose the portability of your program, since the storage of the
variables is left for the discretion of the implementors.

The concept of stack is not part of the language, but variables with
automatic storage (local variables without the static keyword) are
stored by most systems on some sort of stack. The global variables
and those declared with the static keyword are stored in a designated
section of memory, and they are initialised before main() is called.
The memory for dynamical variables are asked from the operating system,
whenever you call malloc(). There is an other storage class specifier,
register, which suggests that the variable should be stored in a cpu register,
but compilers are free to ignore that keyword (but all have to document
the effect of the register keyword.)
The qualifiers (e.g. const, volatile) do not affect the storage, but they
do have an impact on the access to those variables.

It is difficult to tell more without knowing your system and your intentions.
It is not polite to ask if you really need the information you have asked for,
but it is difficult to resist.

Szabolcs

[Flash Gordon]

Re: about memory model

Szabolcs Borsanyi wrote, On 26/05/08 10:18:

On Mon, May 26, 2008 at 12:29:21AM -0700, kumar wrote:

>hi
>i want to know where & how the C variables gets stored
>i mean like volatile , pointer and string variables gets stored ,
>whether it is on stack or some other places

>
The nice thing with C is that you do not have to think about this
when you program in standard C. Whenever you think about these details
you loose the portability of your program, since the storage of the
variables is left for the discretion of the implementors.
>
The concept of stack is not part of the language, but variables with
automatic storage (local variables without the static keyword) are
stored by most systems on some sort of stack. The global variables

On a lot of systems at least some of them are stored in registers and
never written to if the compiler can avoid it.

<snip>

The qualifiers (e.g. const, volatile) do not affect the storage, but they
do have an impact on the access to those variables.

Incorrect. On a lot of systems const will cause "variables" to be stored
in some form of read-only memory, either actual ROM or a page that the
OS will mark as read only when it loads the program.

It is difficult to tell more without knowing your system and your intentions.
It is not polite to ask if you really need the information you have asked for,
but it is difficult to resist.

Any questions about the specifics of how/where an implementation stores
variables belong on a group dedicated to that implementation rather than
here.
--
Flash Gordon

[Malcolm McLean]

Re: about memory model


"kumar" <raman.emb@gmai l.comwrote in message news:

hi
i want to know where & how the C variables gets stored
i mean like volatile , pointer and string variables gets stored ,
whether it is on stack or some other places
if is there any clear document , plz suggest the link
>

In C you have a stack and a heap. When you call a function, local variables
are pushed on the stack. The return address might also be pushed on the
stack, or there might be a special stack for it.
When you call malloc() you take a chunk for memory from the heap. This
doesn't get reused automatically, and persists until you explicitly call
free().

pointers are just ordinary variables. There's no special storage space for
them.

Global variables go into a special area of memory created at program
startup. They persist for the entire life of the program. Local variables
with "static" are really global variables in disguise. They also persist the
entire life of the program, and are stored in the same place as the globals.

However be aware that optimisers can produce any code whatsoever, as long as
it has the same effect as the code you would "naturally" expect from a
translation of C into assembly. So variables might be kept in registers, or
optimised away entirely, or funny things might be done to make cache usage
more efficient.

volatile variables can be modified outside the C program. So all these
optimisations have to be turned off. A volatile variable will always be kept
in the same place in memory so the outside routine - usually an interrupt -
can find it to modify it.

--
Free games and programming goodies.

http://www.personal.leeds.ac.uk/~bgy1mm

[Flash Gordon]

Re: about memory model

Malcolm McLean wrote, On 26/05/08 12:30:

<snip>

Ignoring the lack of requirement for a stack or heap the following is
just plain WRONG

volatile variables can be modified outside the C program. So all these
optimisations have to be turned off. A volatile variable will always be
kept in the same place in memory so the outside routine - usually an
interrupt - can find it to modify it.

If it is a local non-static volatile variable (i.e. an automatic
volatile object) then there is absolutely NO requirement that it is kept
in the same place in memory, and in general it will be created where
ever happens to be convenient when the scope is entered. If the scope is
a function that is called recursively it is almost impossible for the
variable to be always created at the same location!
--
Flash Gordon

[kumar]

Re: about memory model

On May 26, 4:30 pm, "Malcolm McLean" <regniz...@btin ternet.comwrote :

"kumar" <raman....@gmai l.comwrote in message news:

hi
i want to know where & how the C variables gets stored
i mean like volatile , pointer and string variables gets stored ,
whether it is on stack or some other places
if is there any clear document , plz suggest the link

>
In C you have a stack and a heap. When you call a function, local variables
are pushed on the stack. The return address might also be pushed on the
stack, or there might be a special stack for it.
When you call malloc() you take a chunk for memory from the heap. This
doesn't get reused automatically, and persists until you explicitly call
free().
>
pointers are just ordinary variables. There's no special storage space for
them.
>
Global variables go into a special area of memory created at program
startup. They persist for the entire life of the program. Local variables
with "static" are really global variables in disguise. They also persist the
entire life of the program, and are stored in the same place as the globals.
>
However be aware that optimisers can produce any code whatsoever, as long as
it has the same effect as the code you would "naturally" expect from a
translation of C into assembly. So variables might be kept in registers, or
optimised away entirely, or funny things might be done to make cache usage
more efficient.
>
volatile variables can be modified outside the C program. So all these
optimisations have to be turned off. A volatile variable will always be kept
in the same place in memory so the outside routine - usually an interrupt -
can find it to modify it.
>
--
Free games and programming goodies.http://www.personal.leeds.ac.uk/~bgy1m

i am practicing the system programming , that's why i am concerned
about variables storage
and now i got about it

[Barry Schwarz]

Re: about memory model

On Mon, 26 May 2008 07:28:06 -0700 (PDT), kumar <raman.emb@gmai l.com>
wrote:

>i am practicing the system programming , that's why i am concerned
>about variables storage
>and now i got about it

But there is no requirement for Compiler 1 to use the same approach to
storing variables as Compiler 2. The same is true for different
versions of Compiler 1. The answer to your original question remains
implementation specific.


Remove del for email

[Keith Thompson]

Re: about memory model

Flash Gordon <spam@flash-gordon.me.ukwri tes:

Szabolcs Borsanyi wrote, On 26/05/08 10:18:

[...]

>The qualifiers (e.g. const, volatile) do not affect the storage, but they
>do have an impact on the access to those variables.

>
Incorrect. On a lot of systems const will cause "variables" to be
stored in some form of read-only memory, either actual ROM or a page
that the OS will mark as read only when it loads the program.

[...]

That can happen only if the initial value can be determined at
compilation time.

For example, this is a valid declaration (if it appears within a
function, and assuming the required headers have been #included):

const time_t now = time(NULL);

For that matter, if an object's initial value can be determined at
compilation time and the compiler can determine that it's never
modified, the compiler is free to store it in ROM even if it's not
declared const (though in that case it *should* have been declared
const).

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Keith Thompson]

Re: about memory model

"Malcolm McLean" <regniztar@btin ternet.comwrite s:
[...]

In C you have a stack and a heap.

[...]

Wrong, and I'm sure you know better.

In most C *implementation s* you have a stack and a heap. The C
language itself (i.e., the standard) doesn't refer to either. It
states how certain objects are required to behave; the structures
known as a "stack" and as a "heap" are usually, but by no means
always, the most convenient way to meet those requirements.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Ian Collins]

Re: about memory model

Keith Thompson wrote:

>
For that matter, if an object's initial value can be determined at
compilation time and the compiler can determine that it's never
modified, the compiler is free to store it in ROM even if it's not
declared const (though in that case it *should* have been declared
const).
>

String literals being one example.

--
Ian Collins.

[Szabolcs Borsanyi]

Re: about memory model

On Mon, May 26, 2008 at 11:23:02AM +0100, Flash Gordon wrote:

Szabolcs Borsanyi wrote, On 26/05/08 10:18:

>>
>The concept of stack is not part of the language, but variables with
>automatic storage (local variables without the static keyword) are
>stored by most systems on some sort of stack. The global variables

>
On a lot of systems at least some of them are stored in registers and
never written to if the compiler can avoid it.
>

Thanks, you are right, indeed. Optimising compilers use registers extensively,
unless the address of the variable is asked for. (Or could they use registers
even then?...)

<snip>
>

>The qualifiers (e.g. const, volatile) do not affect the storage, but they
>do have an impact on the access to those variables.

>
Incorrect. On a lot of systems const will cause "variables" to be stored
in some form of read-only memory, either actual ROM or a page that the
OS will mark as read only when it loads the program.

Thank you for correcting me.
I wonder if the volatile qualifier could have an impact on the way of storage...

Szabolcs

[Jack Klein]

Re: about memory model

On Mon, 26 May 2008 12:30:24 +0100, "Malcolm McLean"
<regniztar@btin ternet.comwrote in comp.lang.c:

>
"kumar" <raman.emb@gmai l.comwrote in message news:

hi
i want to know where & how the C variables gets stored
i mean like volatile , pointer and string variables gets stored ,
whether it is on stack or some other places
if is there any clear document , plz suggest the link

In C you have a stack and a heap. When you call a function, local variables
are pushed on the stack. The return address might also be pushed on the
stack, or there might be a special stack for it.

There are, of course, several platforms where this is just plain
completely incorrect.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++

http://www.club.cc.cmu.edu/~ajo/docs/FAQ-acllc.html

[Malcolm McLean]

Re: about memory model


"Jack Klein" <jackklein@spam cop.netwrote in message

There are, of course, several platforms where this is just plain
completely incorrect.
>

The OP wants an explanation of how C's memory is laid out, not a formal
definition of the memory model.
--
Free games and programming goodies.

http://www.personal.leeds.ac.uk/~bgy1mm

[Keith Thompson]

Re: about memory model

"Malcolm McLean" <regniztar@btin ternet.comwrite s:

"Jack Klein" <jackklein@spam cop.netwrote in message

>There are, of course, several platforms where this is just plain
>completely incorrect.
>>

The OP wants an explanation of how C's memory is laid out, not a
formal definition of the memory model.

And you gave him an explanation of how *some* C implementations do it.

It's important to understand the distinction between the language and
an implementation of the language. Please don't blur that
distinction.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"