Four or Two Bytes?

[Hahnemann]

Does anybody know the answer to the following? An unsigned short is 2
bytes long. Why is the following file created as 4 bytes instead of
2?

file = fopen("data.bin ", "wb");
if (file != NULL)
{
unsigned short s = 65535;
printf("Size of unsigned short: %d bytes\n", sizeof(unsigned
short)); // 2 bytes - OK
fwrite(&s, sizeof(unsigned short), sizeof(s), file);
fclose(file); // File written as 4 bytes... hmm
}

- Hahnemann

[Chris McDonald]

Hahnemann <ha************ *@gmail.comwrit es:

>Does anybody know the answer to the following? An unsigned short is 2
bytes long. Why is the following file created as 4 bytes instead of
2?

>file = fopen("data.bin ", "wb");
if (file != NULL)
{
unsigned short s = 65535;
printf("Size of unsigned short: %d bytes\n", sizeof(unsigned
short)); // 2 bytes - OK
fwrite(&s, sizeof(unsigned short), sizeof(s), file);
fclose(file); // File written as 4 bytes... hmm
}

Because you're asking it to?
Check the 3rd parameter of fwrite().

--
Chris.

[Hahnemann]

On May 29, 9:39*pm, Chris McDonald <[e-mail] .edu.auwrote:

Hahnemann <hahnemann.or.. [e-mail] es:

Does anybody know the answer to the following? An unsigned short is 2
bytes long. *Why is the following file created as 4 bytes instead of
2?
file = fopen("data.bin ", "wb");
if (file != NULL)
{
* *unsigned short s = 65535;
* *printf("Size of unsigned short: %d bytes\n", sizeof(unsigned
short)); // 2 bytes - OK
* *fwrite(&s, sizeof(unsigned short), sizeof(s), file);
* *fclose(file); // File written as 4 bytes... hmm
}

Because you're asking it to?
Check the 3rd parameter of fwrite().

--
Chris.

But notice that:

printf("Size of s: %d bytes\n", sizeof(s)) ;

returns 2 bytes. How can I make the file 2 bytes using:

unsigned short x = 65535;

Is this even possible?

[Bill Leary]

"Hahnemann" <ha************ *@gmail.comwrot e in message
news:2f******** *************** ***********@d1g 2000hsg.googleg roups.com...

Does anybody know the answer to the following? An unsigned short is 2
bytes long. Why is the following file created as 4 bytes instead of
2?

file = fopen("data.bin ", "wb");
if (file != NULL)
{
unsigned short s = 65535;
printf("Size of unsigned short: %d bytes\n", sizeof(unsigned
short)); // 2 bytes - OK

fwrite(&s, sizeof(unsigned short), sizeof(s), file);

fwrite(
pointer to buffer ... ok
size of an element of that buffer ... 2
number of elements to write ... 2
handle for file write should occur to ... ok

So, 2 elements of 2 bytes each should write four bytes.

fclose(file); // File written as 4 bytes... hmm

So, fwrite worked correctly. More or less, since it wrote out 4 bytes from
a 2 byte object. It could just about as easily have crashed when it went
beyond the bounds of the object.

}

- Bill

[Peter Nilsson]

Hahnemann <hahnemann.or.. [e-mail] e:

Chris McDonald <[e-mail] .edu.auwrote:

Hahnemann <hahnemann.or.. [e-mail] es:

Does anybody know the answer to the following?
An unsigned short is 2 bytes long.

You mean _if_?

Why is the following file created as 4 bytes
instead of 2?

Because 2 lots of 2 bytes is 4 bytes.

* *unsigned short s = 65535;
* *fwrite(&s, sizeof(unsigned short), sizeof(s), file);

Because you're asking it to?
Check the 3rd parameter of fwrite().

But notice that:

printf("Size of s: %d bytes\n", sizeof(s)) ;

returns 2 bytes. *How can I make the file 2 bytes
using:

By only outputting 2 bytes once, instead of twice.

unsigned short x = 65535;

Is this even possible?

Hangliding is a topic best researched _before_ you
through yourself off the cliff. ;)

--
Peter

[Keith Thompson]

Hahnemann <ha************ *@gmail.comwrit es:

On May 29, 9:39*pm, Chris McDonald <[e-mail] .edu.auwrote:

>Hahnemann <hahnemann.or.. [e-mail] es:

>Does anybody know the answer to the following? An unsigned short is 2
bytes long. *Why is the following file created as 4 bytes instead of
2?
file = fopen("data.bin ", "wb");
if (file != NULL)
{
* *unsigned short s = 65535;
* *printf("Size of unsigned short: %d bytes\n", sizeof(unsigned
short)); // 2 bytes - OK
* *fwrite(&s, sizeof(unsigned short), sizeof(s), file);
* *fclose(file); // File written as 4 bytes... hmm
}

Because you're asking it to?
Check the 3rd parameter of fwrite().

But notice that:

printf("Size of s: %d bytes\n", sizeof(s)) ;

returns 2 bytes.

Yes, but ...

"%d" expects an argument of type int; you're giving it an argument of
type size_t. You should cast the value to int (one of the few cases
where a cast is appropriate):

printf("Size of s: %d bytes\n", (int)sizeof s);

And though unsigned short is 2 bytes on your implementation, that's
not guaranteed; it could be different on other implementations .

How can I make the file 2 bytes using:

unsigned short x = 65535;

Is this even possible?

Certainly.

fwrite() writes a specified number of elements. In your case, the
element(s) are of type unsigned short. The second argument is the
size of each element (you're passing ``sizeof(unsign ed short)'', which
is ok). The third argument is the number of elements (you're passing
``sizeof(s)''). How many elements do you want to write (hint: 1).

--
Keith Thompson (The_Other_Keit h) ks***@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"

[Martin]

On Fri, 30 May 2008 04:59:22 +0100, Keith Thompson <ks***@mib.orgw rote:

"%d" expects an argument of type int; you're giving it an argument of
type size_t. You should cast the value to int (one of the few cases
where a cast is appropriate):

printf("Size of s: %d bytes\n", (int)sizeof s);

Generally, isn't it better to use

printf("Size of s: %lu bytes\n", (unsigned long)sizeof s);

as recommended in the answer to FAQ question 7.15?

--
Martin

[Joachim Schmitz]

Martin wrote:

On Fri, 30 May 2008 04:59:22 +0100, Keith Thompson <ks***@mib.or g>
wrote:

>"%d" expects an argument of type int; you're giving it an argument of
type size_t. You should cast the value to int (one of the few cases
where a cast is appropriate):

printf("Size of s: %d bytes\n", (int)sizeof s);

Generally, isn't it better to use

printf("Size of s: %lu bytes\n", (unsigned long)sizeof s);

as recommended in the answer to FAQ question 7.15?

Guess Keith's point is that you should cast it to whatever the format
specified expects.

Bye, Jojo

[Martin]

On Fri, 30 May 2008 10:19:31 +0100, Joachim Schmitz
<no*********@sc hmitz-digital.dewrote :

Guess Keith's point is that you should cast it to whatever the format
specified expects.

I'll wait for Keith's response, but I would say that sometimes it may be
better to change the format specified. Casting an unsigned type to signed
type of equal or less width has the potential for data loss.

--
Martin

[Joachim Schmitz]

Martin wrote:

On Fri, 30 May 2008 10:19:31 +0100, Joachim Schmitz
<no*********@sc hmitz-digital.dewrote :

>Guess Keith's point is that you should cast it to whatever the format
specified expects.

I'll wait for Keith's response, but I would say that sometimes it may
be better to change the format specified. Casting an unsigned type to
signed type of equal or less width has the potential for data loss.

Yes it might have, but how likely is it that a data type is really larger
than INT_MAX?