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]

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.

 

[Szabolcs Borsanyi]

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]

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

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.

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 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!

 

[kumar]

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.

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

 

[Barry Schwarz]

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]

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]

In C you have a stack and a heap.

[...]

Wrong, and I'm sure you know better.

In most C *implementations* 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.

 

[Ian Collins]

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.

 

[Szabolcs Borsanyi]

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

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?...)

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

 

[Keith Thompson]

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.

 

[Flash Gordon]

Szabolcs Borsanyi wrote, On 26/05/08 20:34:

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?...)

If it is not declared volatile the compiler may be able to keep it in a
register for long stretches of code, an easy example being any stretch
of code where no pointers are used.

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

That's easy. If the variable is declared volatile const rather than just
const is may well *not* me placed in read-only memory

 

[Flash Gordon]

Keith Thompson wrote, On 26/05/08 19:21:

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.
True.

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).

True, but rather harder for the implementation to prove where there are
multiple compilation units. I know for definite of one compiler for
embedded systems which puts all const qualified variable with static
storage duration in a seperate memory section specifically so that you
can allocate it to ROM but it does not put non-const qualified variables
in that section even if they are never modified. I suspect (without
proof) that this is probably quite common.

 

[Chris Torek]

On a lot of systems at least some [local variables] 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?...)

They could, although exactly how depends on both the compiler and
the target architecture. Consider, e.g., the following C code
fragment:

for (i = 0; i < n; i++)
do_something(i);
maybe_alter(&i);
if (i != n)
do_something_else();
for (i = 0; i < n; i++)
do_third_thing(i);

Here, "i" is a likely candidate for "register-ization" to make
the two loops run faster. In between the two loops, however, we
pass &i to a function that might change it.

A reasonably clever compiler could easily rewrite this as:

for (i1 = 0; i1 < n; i1++)
do_something(i1);
i2 = i1;
maybe_alter(&i2);
if (i2 != n)
do_something_else();
for (i3 = 0; i3 < n; i3++)
do_third_thing(i3);
i2 = i3; /* if needed -- actually "i4" (if i is used below) */

It is now clear that i1 and i3 can be stored in a machine register
(the same machine register, in fact) even if i2 must live in RAM.

If the CPU has the ability to take the address of a register --
this is rare these days but was not uncommon once, and could perhaps
become popular again someday -- one need not even rewrite the
variable names to make this optimization possible.

I wonder if the volatile qualifier could have an impact on the
way of storage...

Yes, it often does. (In fact, cases where it fails to prevent
"registerization" tend to be compiler bugs. These bugs do happen
though.)