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Ganga
Guest
Posts: n/a

 01-31-2006
Please, will anyone explain me difference between 'register' and
'volatile' variables

Nick Keighley
Guest
Posts: n/a

 01-31-2006
Ganga wrote:

> Please, will anyone explain me difference between 'register' and
> 'volatile' variables

is this homework? Why don't you lookup the answer in your textbook?

--
Nick Keighley

Umesh
Guest
Posts: n/a

 01-31-2006

Ganga wrote:
> Please, will anyone explain me difference between 'register' and

Register keyword for a variable will force the compiler backend to give
register.
so by this compiler can acess the variable without store
instruction(i.e it can use mov instruction).
> 'volatile' variables

Volatile Keyword for a variable will prevent variable from
optimization.
i.e
main()
{
int i=2,j=3,a,b;
a=i+j;
b=i+j;

return a+b;
}

For above code the compiler try to optimize the code.
i.e
main()
{
int i=2,j=3,a,b
t=i+j; //where t is the temproray variable which is interal generated
by compiler.
a=t;//a & b are automatic variable.
b=t;
// by this we can save one addition.
return a+b;
}

main()
{
volatile int i=2,j=3,a,b;
a=i+j;
b=i+j;

return a+b;
}

but in the case volatile.it prevents variables(i,j) from optimization.
so above code remain same.

Umesh
Guest
Posts: n/a

 01-31-2006

Ganga wrote:
> Please, will anyone explain me difference between 'register' and

Register keyword for a variable will force the compiler backend to give
register.
so by this compiler can acess the variable without store
instruction(i.e it can use mov instruction).
> 'volatile' variables

Volatile Keyword for a variable will prevent variable from
optimization.
i.e
main()
{
int i=2,j=3,a,b;
a=i+j;
b=i+j;

return a+b;
}

For above code the compiler try to optimize the code.
i.e
main()
{
int i=2,j=3,a,b
t=i+j; //where t is the temproray variable which is interal generated
by compiler.
a=t;//a & b are automatic variable.
b=t;
// by this we can save one addition.
return a+b;
}

main()
{
volatile int i=2,j=3,a,b;
a=i+j;
b=i+j;

return a+b;
}

but in the case volatile.it prevents variables(i,j) from optimization.
so above code remain same.

Guest
Posts: n/a

 01-31-2006
Umesh wrote:
> Ganga wrote:
> > Please, will anyone explain me difference between 'register' and

> Register keyword for a variable will force the compiler backend to give
> register.
> so by this compiler can acess the variable without store
> instruction(i.e it can use mov instruction).

Not entirely correct. The `register` keyword will /suggest/ to the
compiler that a variable be stored in a register. Most modern compilers
are better at optimising than programmers, and will happily ignore the
`register` keyword.

> > 'volatile' variables

> Volatile Keyword for a variable will prevent variable from
> optimization.

Again, not entirely correct. The `volatile` keyword tells the compiler
that the variable may be changed by something outside the scope of the
current program (other programs in the system, hardware device, cosmic
rays...). Because of this, the compiler cannot be certain of the value
of the `volatile` variable, even if your code did not change it, and
will hence avoid optimisations that rely on that value not changing
between two points in the code.

For example, in the snippet:

int main(void)
{
volatile int i = 5;
int j = 6;

while (3 < i)
{
++j;
}

return 0;
}

The compiler cannot assume that `i` will be 5 when it is tested for the
`while` loop, and hence cannot replace it with the equivalent of `while
(1)`, but has to generate the code for the test. If `i` was not
`volatile`, the compiler could generate the equivavalent of:

int main(void)
{
int j = 6;

endless_loop:
++j;
goto endless_loop;
}

(Note that it could get rid of `i` altogether.)

<snipped not so good an example>

Cheers

Mike Wahler
Guest
Posts: n/a

 01-31-2006

"Ganga" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Please, will anyone explain me difference between 'register' and
> 'volatile' variables

The difference depends upon their values.

register int r = 42;
volatile int v = 25;

printf("Difference between %d and %d is %d\n", r, v, r - v);

the 'register' and 'volatile' keywords, the answer can
be found in any good C book.

-Mike

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