[Newbie] What is sizeof() ?

[Vincent Pierroux]

Hi,

I search the header file of sizeof() function but i don't find.
Where can find that ?
Sizeof() is a function ? macro ? "os function" ?
I don't understand what is exactly sizeof.

Vincent Pierroux

 

[Eric]

Hi,

I search the header file of sizeof() function but i don't find.
Where can find that ?
Sizeof() is a function ? macro ? "os function" ?
I don't understand what is exactly sizeof.

It's an operator.

You might find it useful to read the questions in the C FAQ reguarding
the sizeof operator at:

http://www.eskimo.com/~scs/C-faq/questions.html

Even though sizeof is an operator, I will still write my code as sizeof(
.... ) because I find it easier to read that way.

 

[Jeremy Yallop]

Sizeof() is a function ? macro ? "os function" ?

It's an operator.

I don't understand what is exactly sizeof.

The sizeof operator yields the size in bytes of its operand, which may
be an expression or a parenthesized type name.

Jeremy.

 

[Dave Vandervies]

Hi,

I search the header file of sizeof() function but i don't find.
Where can find that ?
Sizeof() is a function ? macro ? "os function" ?
I don't understand what is exactly sizeof.

sizeof is an operator. It's built into the language, not a macro or
function, so you won't find it in your header files. Your textbook
should mention it somewhere, with more details.

Given an expression, it returns a size_t representing the size (in bytes)
of a variable with the type of that expression. Given a parenthesized
type, it returns the size of a variable with that type.


dave

 

[Vincent Pierroux]

Thanks for your answer.

So, if it is a operator i don't have any argument push on the stack like a
function.

If i make onething like that

for(foo=0;foo<1000;i++)
foo+= sizeof myStruct ;

it take the same processing time this

var= sizeof myStruct
for(foo=0;foo<1000;i++)
foo+= var;

(it is a stupid code but it is for the sample )

Vincent Pierroux

 

[Richard Heathfield]

Si tu hoc leger scis, tu nimis argutior es.

If you can read this, you are a nimis argutior, right?

 

[Mark A. Odell]

Thanks for your answer.

So, if it is a operator i don't have any argument push on the stack like
a function.

Yeah, I guess.

If i make onething like that

for(foo=0;foo<1000;i++)
foo+= sizeof myStruct ;

it take the same processing time this

var= sizeof myStruct
for(foo=0;foo<1000;i++)
foo+= var;

That would depend upon your compiler but they'd be pretty similar. Here's
a another usage of sizeof:

....

int idx;
float arr[1000];

....

for (idx = 0; idx < sizeof fload / sizeof *float; ++idx)
{
/* work */
}

Sometimes I make a macro of this:

#define NUM_OF(x) (sizeof (x) / sizeof *(x))

No function call overhead whatsoever.

 

[Dave Vandervies]

If you can read this, you are a nimis argutior, right?

uhmm... something like that, I think, depending on what you mean by "a
nimis argutior", especially if you mean "an excessively educated person".

I got that one from a coworker's tshirt, and she translated it for me as
"If you can read this, you are far too educated".


dave

--
Dave Vandervies (e-mail address removed)

Wrong.

May I be excused for showing incomplete/incorrect knowledge of C++ knowledge
in a C newsgroup? --Michael Rubenstein and Ian Woods in comp.lang.c

 

[Joona I Palaste]

uhmm... something like that, I think, depending on what you mean by "a
nimis argutior", especially if you mean "an excessively educated person".

I got that one from a coworker's tshirt, and she translated it for me as
"If you can read this, you are far too educated".

I prefer the name of a level in "Pogo Joe":
IF U CN RD THS, U R WRD!

--
/-- Joona Palaste ([email protected]) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"You can pick your friends, you can pick your nose, but you can't pick your
relatives."
- MAD Magazine

 

[Irrwahn Grausewitz]

If you can read this, you are a nimis argutior, right?

Drop the 'a' and you are indeed nimis arguitor. ;-)

 

[Irrwahn Grausewitz]

That would depend upon your compiler but they'd be pretty similar. Here's
a another usage of sizeof:
...
int idx;
float arr[1000];
...
for (idx = 0; idx < sizeof fload / sizeof *float; ++idx)

^^^^^ ^^^^^^
Err, ... I mean, what???
ITYM:
for (idx = 0; idx < sizeof arr / sizeof *arr; ++idx)

{
/* work */
}

<snip>

 

[Mark A. Odell]

That would depend upon your compiler but they'd be pretty similar.
Here's a another usage of sizeof:
...
int idx;
float arr[1000];
...
for (idx = 0; idx < sizeof fload / sizeof *float; ++idx)

^^^^^ ^^^^^^
Err, ... I mean, what???

^^^ ^^^^
Sorry, I meant: ||| ||||
arr *arr

 

[Irrwahn Grausewitz]

^^^ ^^^^
Sorry, I meant: ||| ||||
arr *arr

So you already had your fload? Arr. ;-)

 

[Peter Pichler]

So, if it is a operator i don't have any argument push on the stack like a
function.

If i make onething like that

for(foo=0;foo<1000;i++)
foo+= sizeof myStruct ;

it take the same processing time this

var= sizeof myStruct
for(foo=0;foo<1000;i++)
foo+= var;

Most likely the first one will take shorter: sizeof myStruct is a constant,
so the compiler might just replace the whole loop with

foo += 1000 * sizeof myStruct;

On the other hand, so it could in the second case, should it be clever
enough.

 

[Christian Bau]

Most likely the first one will take shorter: sizeof myStruct is a constant,
so the compiler might just replace the whole loop with

foo += 1000 * sizeof myStruct;

On the other hand, so it could in the second case, should it be clever
enough.

You mean stupid enough? Take a very good look at the for loop. It is
most likely _not_ executed 1000 times.

 

[Peter Pichler]

You mean stupid enough? Take a very good look at the for loop. It is
most likely _not_ executed 1000 times.

Doh! You are right, of course. I fell for "do what I want, not what I say"

 

[Keith Thompson]

I prefer the name of a level in "Pogo Joe":
IF U CN RD THS, U R WRD!

LL YR VWL R BLNG T S

 

[Martin Ambuhl]

Hi,

I search the header file of sizeof() function

It is not a function, but an operator.

but i don't find.
Where can find that ?
Sizeof() is a function ? macro ? "os function" ?

No, it is not a function, macro, or os function. 'sizeof' is an operator,
as are such things as '/', '+', '&', and '||'.

 

[Irrwahn Grausewitz]

LL YR VWL R BLNG T S

Ok, here you go:

#include <stdio.h>
int main(void){puts("AEIOUaeiou");return 0;}

 

[Vittal]

Hi,

I search the header file of sizeof() function but i don't find.
Where can find that ?
Sizeof() is a function ? macro ? "os function" ?
I don't understand what is exactly sizeof.

Vincent Pierroux

Niether its a function nor its a macro. Its an operator, which returns
the size of a perticular data type.