Structure Padding
Most processors require specific memory alignment on variables certain types. Normally the minimum alignment is the size of the basic type in question, fo instance this is common
char variables can be byte aligned and appear at any byte boundary
short (2 byte) variables must be 2 byte aligned, they can appear at any even byte boundary. This means that 0x10004567 is not a valid location for a short variable but 0x10004566 is.
long (4 byte) variables must be 4 byte aligned, they can only appear at byte boundarys that are a multiple of 4 bytes. This means that 0x10004566 is not a valid location for a long variable but 0x10004568 is.
Structure padding occurs because the members of the structure must appear at the correect byte boundary, to achieve this the compiler puts in padding bytes (or bits if bit fields are in use) so that the structure members appear in the correct location. Additionally the size of the structure must be such that in an array of the structures all the structures are correctly aligned in memory so there may be padding bytes at the end of the structure too
struct example {
char c1;
short s1;
char c2;
long l1;
char c3;
}
In this structure, assuming the alignment scheme I have previously stated then
c1 can appear at any byte boundary, however s1 must appear at a 2 byte boundary so there is a padding byte between c1 and s1.
c2 can then appear in the available memory location, however l1 must be at a 4 byte boundary so there are 3 padding bytes between c2 and l1
c3 then appear in the available memory location, however because the structure contains a long member the structure must be 4 byte aligned and must be a multiple of 4 bytes in size. Therefore there are 3 padding bytes at the end of the structure. it would appear in memory in this order
c1
padding byte
s1 byte 1
s1 byte 2
c2
padding byte
padding byte
padding byte
l1 byte 1
l1 byte 2
l1 byte 3
l1 byte 4
c3
padding byte
padding byte
padding byte
The structure would be 16 bytes long.
re-written like this
struct example {
long l1;
short s1;
char c1;
char c2;
char c3;
}
Then l1 appears at the correct byte alignment, s1 will be correctly aligned so no need for padding between l1 and s1. c1, c2, c3 can appear at any location. The structure must be a multiple of 4 bytes in size since it contains a long so 3 padding bytes appear after c3
It appears in memory in the order
l1 byte 1
l1 byte 2
l1 byte 3
l1 byte 4
s1 byte 1
s1 byte 2
c1
c2
c3
padding byte
padding byte
padding byte
and is only 12 bytes long.
I should point out that structure packing is platform and compiler (and in some cases compiler switch) dependent.
For info abt memory pools and memory leaks
Memory Pools are just a section of memory reserved for allocating temporarily to other parts of the application
A memory leak occurs when you allocate some memory from the heap(or a pool) and then delete all references to that memory without returning it to the pool it was allocated from.
|
|
Inappropriate
|
See Answer
To Email:
Subject:
Note to go along with the question: (Optional, no
more than 1,000 characters)
#include<stdio.h>
int main()
{
struct bit field{
int x:4;
char c:6;
};
return 0;
}
(or)
struct bit_field{
unisgned int x:3;
unsigned int y:6;
unsigned int z:8;
};
int main()
{
struct bit_field bit;
bit.x=3;
bit.y=15;
bit.z=12;
}
}
Create Date
:
Wednesday, November 07, 2007
Click here
to improve the Interview Question, Answer and other fields.
|
|
Inappropriate
|
See Answer
To Email:
Subject:
Note to go along with the question: (Optional, no
more than 1,000 characters)
A job that allows me to grow in the organization
A job where i can make a positive contribution to the growth of the firm
A job where i am comfortable with my coworkers
A job that pays a decent salary
A job that is so interesting that i am motivated to continue educating myself to be the best that i can be in the field
Create Date
:
Thursday, August 14, 2008
Click here
to improve the Interview Question, Answer and other fields.
|
|
Inappropriate
|
See Answer
To Email:
Subject:
Note to go along with the question: (Optional, no
more than 1,000 characters)
Start with the present and tell why you are well qualified for the position. Remember that the key to all successful interviewing is to match your qualifications to what the interviewer is looking for. In other words you must sell what the buyer is buying. This is the single most important strategy in job hunting.
So, before you answer this or any question it's imperative that you try to uncover your interviewer's greatest need, want, problem or goal.
To do so, make you take these two steps:
Do all the homework you can before the hr interview to uncover this person's wants and needs (not the generalized needs of the industry or company)
As early as you can in the interview, ask for a more complete description of what the position entails. You might say: "I have a number of accomplishments I'd like to tell you about, but I want to make the best use of our time together and talk directly to your needs. To help me do, that, could you tell me more about the most important priorities of this position? All I know is what I (heard from the recruiter, read in the classified ad, etc.)"
Then, ALWAYS follow-up with a second and possibly, third question, to draw out his needs even more. Surprisingly, it's usually this second or third question that unearths what the interviewer is most looking for.
You might ask simply, "And in addition to that?..." or, "Is there anything else you see as essential to success in this position?:
This process will not feel easy or natural at first, because it is easier simply to answer questions, but only if you uncover the employer's wants and needs will your answers make the most sense. Practice asking these key questions before giving your answers, the process will feel more natural and you will be light years ahead of the other job candidates you're competing with.
After uncovering what the employer is looking for, describe why the needs of this job bear striking parallels to tasks you've succeeded at before. Be sure to illustrate with specific examples of your responsibilities and especially your achievements, all of which are geared to present yourself as a perfect match for the needs he has just described.
Create Date
:
Monday, October 01, 2007
Click here
to improve the Interview Question, Answer and other fields.
|
|
Inappropriate
|
See Answer
To Email:
Subject:
Note to go along with the question: (Optional, no
more than 1,000 characters)
i can solve anytrouble by myself, i can work independently
Create Date
:
Monday, June 15, 2009
Click here
to improve the Interview Question, Answer and other fields.