«

hi,
in Visula C++ 6.0 I have declared a struct like this:

typedef struct _WRITE_INPUT {

ULONG DeviceNumber;
ULONG RegisterNumber;
union {
USHORT ShortData;
UCHAR CharData;
};
} WRITE_INPUT;

I can't understand why, sizeof(WRITE_INPUT) returns 12. It should
return 10, shouldn't it?

sizeof(ULONG) = 4
sizeof(ULONG) = 4
sizeof(USHORT) = 2 (longest union field)

4 + 4 + 2 = 10

thanks
bye
luke

luke wrote:
> hi,
> in Visula C++ 6.0 I have declared a struct like this:
>
> typedef struct _WRITE_INPUT {
>
> ULONG DeviceNumber;
> ULONG RegisterNumber;
> union {
> USHORT ShortData;
> UCHAR CharData;
> };
> } WRITE_INPUT;
>
> I can't understand why, sizeof(WRITE_INPUT) returns 12. It should
> return 10, shouldn't it?
>
> sizeof(ULONG) = 4
> sizeof(ULONG) = 4
> sizeof(USHORT) = 2 (longest union field)
>
> 4 + 4 + 2 = 10
>
> thanks
> bye
> luke
>
c.l.c FAQ #2.13 would answer your question

thank you,
I thought it was a union problem
bye

"luke" <> writes:
> in Visula C++ 6.0 I have declared a struct like this:
>
> typedef struct _WRITE_INPUT {

Don't use identifiers starting with an underscore; they're reserved to
the implementation. (It's slightly more complex than that, but it's
safest just to avoid them.)

> ULONG DeviceNumber;
> ULONG RegisterNumber;
> union {
> USHORT ShortData;
> UCHAR CharData;
> };
> } WRITE_INPUT;
>
> I can't understand why, sizeof(WRITE_INPUT) returns 12. It should
> return 10, shouldn't it?
>
> sizeof(ULONG) = 4
> sizeof(ULONG) = 4
> sizeof(USHORT) = 2 (longest union field)
>
> 4 + 4 + 2 = 10

The compiler is free to add padding after any member of a structure.
In this case, it's probably adding 2 bytes of padding at the end to
make the size of the structure a multiple of 4, so the ULONG members
will be aligned properly if you have an array of structures.

Incidentally, the names ULONG, USHORT, and UCHAR aren't particularly
helpful. I presume they're typedefs (or macros?) for unsigned long,
unsigned short, and unsigned char, respectively. Why not just use the
names directly?

--
Keith Thompson (The_Other_Keith) kst- <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

luke wrote:
> hi,
> in Visula C++ 6.0 I have declared a struct like this:
>
> typedef struct _WRITE_INPUT {
>
> ULONG DeviceNumber;
> ULONG RegisterNumber;
> union {
> USHORT ShortData;
> UCHAR CharData;
> };
> } WRITE_INPUT;
>
> I can't understand why, sizeof(WRITE_INPUT) returns 12. It should
> return 10, shouldn't it?

Please check the FAQ before posting.
You might find <http://www.eskimo.com/~scs/C-faq/q2.13.html> enlightening.

BTW: Avoid nonstandard typenames in postings. If ULONG, USHORT, and
UCHAR are in fact unsigned long, unsigned short, and unsigned char, why
not use the real typenames?

In article <. com>,
"luke" <> wrote:

> hi,
> in Visula C++ 6.0 I have declared a struct like this:
>
> typedef struct _WRITE_INPUT {
>
> ULONG DeviceNumber;
> ULONG RegisterNumber;
> union {
> USHORT ShortData;
> UCHAR CharData;
> };
> } WRITE_INPUT;

What is ULONG? What is USHORT? What is UCHAR?

> I can't understand why, sizeof(WRITE_INPUT) returns 12. It should
> return 10, shouldn't it?
>
> sizeof(ULONG) = 4
> sizeof(ULONG) = 4
> sizeof(USHORT) = 2 (longest union field)
>
> 4 + 4 + 2 = 10

Compilers usually add padding (unused bytes) between members of a struct
or at the end of a struct to align the data in the struct and to allow
the processor to access them faster.

If one member of a struct has a size of four bytes, then quite often the
size of the struct will be increased to a multiple of four bytes, if
necessary.

On 30 Sep 2005 00:23:18 -0700, "luke" <> wrote in
comp.lang.c:

> hi,
> in Visula C++ 6.0 I have declared a struct like this:

Aside from the other replies you received, your code is not C at all,
and does not belong in this newsgroup in any way, shape, or form.

>
> typedef struct _WRITE_INPUT {
>
> ULONG DeviceNumber;
> ULONG RegisterNumber;
> union {
> USHORT ShortData;
> UCHAR CharData;
> };

There is no such thing as an anonymous union in C. A conforming C
compiler must issue a diagnostic for this.

Although it appears that Visual C++ 6.0 is non-conforming in this
respect.

> } WRITE_INPUT;

[snip]

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html

Keith Thompson wrote:

> "luke" <> writes:
>
>>in Visula C++ 6.0 I have declared a struct like this:
>>
>>typedef struct _WRITE_INPUT {
>
>
> Don't use identifiers starting with an underscore; they're reserved to
> the implementation. (It's slightly more complex than that, but it's
> safest just to avoid them.)
>
>
sorry to be nitty, but since some of us here do work at the OS/kernel
level and use C, I think using underscore when declaring objects are
fine (it would be a part of bug fix ) ?

- Ravi


>> ULONG DeviceNumber;
>> ULONG RegisterNumber;
>> union {
>> USHORT ShortData;
>> UCHAR CharData;
>> };
>>} WRITE_INPUT;
>>
>>I can't understand why, sizeof(WRITE_INPUT) returns 12. It should
>>return 10, shouldn't it?
>>
>>sizeof(ULONG) = 4
>>sizeof(ULONG) = 4
>>sizeof(USHORT) = 2 (longest union field)
>>
>>4 + 4 + 2 = 10
>
>
> The compiler is free to add padding after any member of a structure.
> In this case, it's probably adding 2 bytes of padding at the end to
> make the size of the structure a multiple of 4, so the ULONG members
> will be aligned properly if you have an array of structures.
>
> Incidentally, the names ULONG, USHORT, and UCHAR aren't particularly
> helpful. I presume they're typedefs (or macros?) for unsigned long,
> unsigned short, and unsigned char, respectively. Why not just use the
> names directly?
>

Ravi Uday a écrit :
>
> Keith Thompson wrote:
>
>> "luke" <> writes:
>>
>>> in Visula C++ 6.0 I have declared a struct like this:
>>>
>>> typedef struct _WRITE_INPUT {
>>
>> Don't use identifiers starting with an underscore; they're reserved to
>> the implementation. (It's slightly more complex than that, but it's
>> safest just to avoid them.)
>>
> sorry to be nitty, but since some of us here do work at the OS/kernel
> level and use C, I think using underscore when declaring objects are
> fine (it would be a part of bug fix ) ?

Implementation means compiler and compiler's libraries. Linux Kernel
Modules or the like are part of the system, not of the implmentation.

Emmanuel Delahaye wrote:

> Implementation means compiler and compiler's libraries. Linux Kernel
> Modules or the like are part of the system, not of the implmentation.

I think "implementation" means everything, including hardware.
If you change the system, then you change the implementation.

N869
3.10
[#1] implementation
a particular set of software, running in a particular
translation environment under particular control options,
that performs translation of programs for, and supports
execution of functions in, a particular execution
environment

--
pete