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
Jun 27 '08
33  1832 
Joachim Schmitz wrote:
Martin wrote:
>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?
If you simply examine limits.h you will probably find that UINT_MAX
is roughly twice the size of INT_MAX. To me, this indicates your
probability is roughly 2:1. Anything over 0 is bad.
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.
** Posted from http://www.teranews.com **
CBFalconer wrote:
Joachim Schmitz wrote:
>Martin wrote:
>>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?
If you simply examine limits.h you will probably find that UINT_MAX
is roughly twice the size of INT_MAX. To me, this indicates your
probability is roughly 2:1. Anything over 0 is bad.
So what? The basic daty types have sizes of 1 to 8 bytes. Sensibel structs
and arrays are most probaly well smaller than 32K, which is the minimun for
INT_MAX, so again: how big is the chance that a real life data type is
larger than that?
Bye, Jojo
Joachim Schmitz wrote:
CBFalconer wrote:
>Joachim Schmitz wrote:
>>Martin wrote:
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?
If you simply examine limits.h you will probably find that UINT_MAX
is roughly twice the size of INT_MAX. To me, this indicates your
probability is roughly 2:1. Anything over 0 is bad.
So what? The basic daty types have sizes of 1 to 8 bytes. Sensibel
structs and arrays are most probaly well smaller than 32K, which is
the minimun for INT_MAX, so again: how big is the chance that a real
life data type is larger than that?
On top of that: here it is about 2 or 4, which even on the DS9K would easily
fit into INT_MAX.
Bye, Jojo
Joachim Schmitz wrote:
On top of that: here it is about 2 or 4, which even on the DS9K would easily
fit into INT_MAX.
Not on the DS9K.
unsigned short can have INT_MAX padding bytes on the DS9K.
--
pete
pete wrote:
Joachim Schmitz wrote:
>On top of that: here it is about 2 or 4, which even on the DS9K
would easily fit into INT_MAX.
Not on the DS9K.
unsigned short can have INT_MAX padding bytes on the DS9K.
Chapter and verse please
Joachim Schmitz wrote:
pete wrote:
>Joachim Schmitz wrote:
>>On top of that: here it is about 2 or 4, which even on the DS9K
would easily fit into INT_MAX.
Not on the DS9K.
unsigned short can have INT_MAX padding bytes on the DS9K.
Chapter and verse please
N869
6.2.6.2 Integer types
[#1] For unsigned integer types other than unsigned char,
the bits of the object representation shall be divided into
two groups: value bits and padding bits (there need not be
any of the latter). If there are N value bits, each bit
shall represent a different power of 2 between 1 and 2N-1,
so that objects of that type shall be capable of
representing values from 0 to 2N-1 using a pure binary
representation; this shall be known as the value
representation. The values of any padding bits are
unspecified.
--
pete
Martin <m@b.cwrites:
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?
Yes. In this particular case, the expression ``sizeof s'' is the size
of an object of type unsigned short, so it's practically certain to be
no greater than INT_MAX. But using int rather than unsigned long was
just laziness in my part; IMHO it's better to get into the habit of
using unsigned long whether it's necessary or not, rather than wasting
time figuring out whether int is ok in this or that particular
instance.
In C99, you can use
printf("Size of s: %zu bytes\n", sizeof s);
to print a size_t value directly. I look forward eagerly to the day
when C99 is widely implemented and that code is reasonably portable.
--
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"
pete <pf*****@mindsp ring.comwrites:
Joachim Schmitz wrote:
>On top of that: here it is about 2 or 4, which even on the DS9K
would easily fit into INT_MAX.
Not on the DS9K.
unsigned short can have INT_MAX padding bytes on the DS9K.
Padding *bits*, not padding bytes.
But I think you have to pay for the commercial version of the DS9K C
compiler to get unsigned short with CHAR_BIT*INT_MA X padding bits; the
free version only has sizeof(unsigned short)==INT_MAX .
--
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"
Joachim Schmitz wrote:
>
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?
As I recall, "huge model" on real-mode x86 processors use 32-bit
pointers, yet 16-bit ints. As such, I would expect that size_t
could hold values larger than INT_MAX.
--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer .h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com>
Kenneth Brody <ke******@spamc op.netwrites:
Joachim Schmitz wrote:
>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?
As I recall, "huge model" on real-mode x86 processors use 32-bit
pointers, yet 16-bit ints. As such, I would expect that size_t
could hold values larger than INT_MAX.
Which is not particularly relevant in this case. The issue, as
Joachim wrote, is whether "a data type is really larger than INT_MAX".
It's not about whether the data type (unsigned short in this case) is
able to hold values larger than INT_MAX; it's about whether the size
of the data type is larger than INT_MAX bytes.
If I had an integer type bigger than 262136 bits, I don't think I'd
call it "short".
--
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" This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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