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sarathy
*nix forums beginner
Joined: 17 Jul 2006
Posts: 12
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 Posted: Thu Jul 20, 2006 1:58 pm Post subject:
Questions on bitfields and struct
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Hi,
1. How it that the results for the size of struct1 and struct2 (below)
are 4 and 3
# include <stdio.h>
struct struct1
{
const :16;
volatile :4;
};
struct struct2
{
int :1;
unsigned :1;
const :16;
volatile :4;
};
int main()
{
printf ("Size of struct 1 = %d\n",sizeof(struct struct1)); /*
Prints 4 */
printf ("Size of struct 2 = %d\n",sizeof(struct struct2)); /*
Prints 3 */
}
2. Also what is meant by "incomplete type" in C?
3. What does tag refer to in a struct/union declaration?
Regards,
Sarathy |
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Walter Roberson
*nix forums Guru
Joined: 19 Feb 2005
Posts: 1300
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| Posted: Thu Jul 20, 2006 3:01 pm Post subject:
Re: Questions on bitfields and struct
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In article <1153403911.312195.193670@p79g2000cwp.googlegroups.com>,
sarathy <sps.sarathy@gmail.com> wrote:
| Quote: | 1. How it that the results for the size of struct1 and struct2 (below)
are 4 and 3
struct struct1
{
const :16;
volatile :4;
};
struct struct2
{
int :1;
unsigned :1;
const :16;
volatile :4;
};
|
With SGI's IRIX cc compiler, both are 3. With gcc 3.3 on the same
machine, the first is 4 and the second is 3. Which is to say that the
alignment mechanisms are implementation dependant, and are not required
to be consistant.
It is, for example, entirely legal for the compiler to examine the
first structure, note that the first field is the same size as a short
int on that implementation, and decide that the structure shall be
built for fast access as a pair of shorts aligned on a short boundary,
for a total of 4 bytes.
The same compiler could look at the second structure and decide that it
is complicated enough that space is the important factor rather than
speed, and decide to pull bits out of 3 bytes.
But compilers are not required to allow bitfields to cross word
boundaries, so the compiler -could- have decided to put the first two
fields into one short, the third field into a second short, and the
fourth field into a third short, for a total of 6 bytes. Or it could
have decided that since the total fits within 32 bits that it would
pull bits out of a long, for a total of 4 bytes.
Decisions about when to move to the next word and how much padding
to use before that word are completely up to the compiler: the
closest that the C standard comes on this point is to say "if it fits".
| Quote: | 2. Also what is meant by "incomplete type" in C?
|
The C faq probably talks about that in better words than I could
come up with in a reasonable time.
| Quote: | 3. What does tag refer to in a struct/union declaration?
|
A structure tag is a programmer-given name for that variety of
structure. It is not a variable, but rather a reference to the type.
As a rough analogy: "Form 37/J" might be a name given to a particular
layout of income tax form, but "Form 37/J" is not a particular -copy-
of the form, it is the name of the -kind- of form.
--
I was very young in those days, but I was also rather dim.
-- Christopher Priest |
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sarathy
*nix forums beginner
Joined: 17 Jul 2006
Posts: 12
|
| Posted: Thu Jul 20, 2006 6:30 pm Post subject:
Re: Questions on bitfields and struct
|
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|
Walter Roberson wrote:
| Quote: | In article <1153403911.312195.193670@p79g2000cwp.googlegroups.com>,
sarathy <sps.sarathy@gmail.com> wrote:
1. How it that the results for the size of struct1 and struct2 (below)
are 4 and 3
struct struct1
{
const :16;
volatile :4;
};
struct struct2
{
int :1;
unsigned :1;
const :16;
volatile :4;
};
With SGI's IRIX cc compiler, both are 3. With gcc 3.3 on the same
machine, the first is 4 and the second is 3. Which is to say that the
alignment mechanisms are implementation dependant, and are not required
to be consistant.
It is, for example, entirely legal for the compiler to examine the
first structure, note that the first field is the same size as a short
int on that implementation, and decide that the structure shall be
built for fast access as a pair of shorts aligned on a short boundary,
for a total of 4 bytes.
The same compiler could look at the second structure and decide that it
is complicated enough that space is the important factor rather than
speed, and decide to pull bits out of 3 bytes.
But compilers are not required to allow bitfields to cross word
boundaries, so the compiler -could- have decided to put the first two
fields into one short, the third field into a second short, and the
fourth field into a third short, for a total of 6 bytes. Or it could
have decided that since the total fits within 32 bits that it would
pull bits out of a long, for a total of 4 bytes.
Decisions about when to move to the next word and how much padding
to use before that word are completely up to the compiler: the
closest that the C standard comes on this point is to say "if it fits".
2. Also what is meant by "incomplete type" in C?
The C faq probably talks about that in better words than I could
come up with in a reasonable time.
3. What does tag refer to in a struct/union declaration?
A structure tag is a programmer-given name for that variety of
structure. It is not a variable, but rather a reference to the type.
As a rough analogy: "Form 37/J" might be a name given to a particular
layout of income tax form, but "Form 37/J" is not a particular -copy-
of the form, it is the name of the -kind- of form.
--
I was very young in those days, but I was also rather dim.
-- Christopher Priest
|
Thanks a lot. I got the things clear now. The next fun starts...
Consider the code
struct struct1
{
short a1:16;
short b1:4;
};
struct struct2
{
int a2:16;
unsigned b2:1;
short c2 :16;
short d2 :4;
};
int main()
{
struct struct1 *str1=(struct struct1 *)malloc(sizeof(struct
struct1));
str1->a1=24;
str1->b1=4;
print_bits(str1,sizeof(str1));
printf ("Size of struct1 = %d\n",sizeof(struct struct1));
/* Prints 4 */
printf ("Size of str1 = %d\n",sizeof(str1)); /* Prints 4
*/
struct struct2 *str2=(struct struct2 *)malloc(sizeof(struct
struct2));
str2->a2=1;
str2->b2=1;
str2->c2=6;
str2->d2=12;
print_bits(str2,sizeof(struct struct2));
printf ("Size of struct2 = %d\n",sizeof(struct struct2));
/* Prints 8 */
printf ("Size of str2 = %d\n",sizeof(str2)); /* Prints 4
*/
}
Output:
----------
00000000 00000100 00000000 00011000
Size of struct1 = 4
Size of str1 = 4
00000000 00001100 00000000 00000110 00000000 00000001 00000000 00000001
Size of struct2 = 8
Size of str2 = 4
Is there any difference in sizeof (struct tag_name) and
sizeof(struct_instance_identifier)
In the third case, clearly from the results, 8 is correct due to
padding. But why is 4 being printed in the last printf. |
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Walter Roberson
*nix forums Guru
Joined: 19 Feb 2005
Posts: 1300
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| Posted: Thu Jul 20, 2006 9:52 pm Post subject:
Re: Questions on bitfields and struct
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In article <1153420215.987105.86630@p79g2000cwp.googlegroups.com>,
sarathy <sps.sarathy@gmail.com> wrote:
| Quote: |
Walter Roberson wrote:
|
[lots]
Please trim down quotations to just the portion needed for discussion.
| Quote: | Consider the code
struct struct2 *str2=(struct struct2 *)malloc(sizeof(struct
struct2));
printf ("Size of str2 = %d\n",sizeof(str2)); /* Prints 4
*/
Size of str2 = 4
|
| Quote: | Is there any difference in sizeof (struct tag_name) and
sizeof(struct_instance_identifier)
|
No.
| Quote: | In the third case, clearly from the results, 8 is correct due to
padding. But why is 4 being printed in the last printf.
|
Because str2 is not an instance of struct struct2.
str2 is a *pointer* to an instance of struct struct2.
sizeof(str2) is printing out the size of the pointer.
--
Okay, buzzwords only. Two syllables, tops. -- Laurie Anderson |
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