Improve company productivity with a Business Account.Sign Up

x
?
Solved

Structure Alignment

Posted on 1998-08-20
2
Medium Priority
?
364 Views
Last Modified: 2010-04-15
What is Structure alignment? How does it affect performance?
Why can not two dlls having different structure alignments
share their structures?
0
Comment
Question by:krpadman
  • 2
2 Comments
 
LVL 3

Accepted Solution

by:
Norbert earned 300 total points
ID: 1252206
Answer is comming
0
 
LVL 3

Expert Comment

by:Norbert
ID: 1252207
To answer your questions I will use this struct as an example:
struct Something{
          char aChar;
          short int aShortInt;
          long aLong;
          double aDouble;
} Something;
sizeof (char)=1 Byte
sizeof (short int)=2 Byte
sizeof (long)=4 Byte
sizeof (double)=8 Byte
1 Byte = 8 Bits
Raw size of Something =15 Byte
>>What is Structure alignment?
Structure alignmet is how the compiler organizes the memory for a structure.
With No Alignment sizeof(something)=15
with 4 byte Alignment sizeof(Something)=20:
struct Something{
          char aChar;
          char Gap1[3];
          short int aShortInt;
          char Gap2[2]
          long aLong;
          double aDouble;
} Something;
tha gaps are invisible to your program. That means you can't write
struct Something Some;
Some.Gap1[1]='X';
>>How does it affect performance?
Each computer has a Preferred Adressing mode depending on its bus width
A computer with a bus width of 16 or 32 bit can access even adresses faster than odd adresses. Some computers even can only fetch 16 bit values so accessing 8 bit values on odd adresses is a 16 bit fetch on a the Odd Adress-1 and a swap of the bytes. To Speed up the program the compiler can fill in gaps
to move the elments to even 16 bit or 32 bit adresses:

>>Why can not two dlls having different structure alignments
share their structures?
That is because the position inside the memory is different.
Remember a Struct is only a logical thing to tell the compiler how to interpret the data on a given memory adress. The compiler adresses the a member of a struct by calculating baseAdress of struct + offset to member
so adressing the aLong member will calculate with no aligment as
Base+3 and with 4 byte alignment as Base+8

I hope i have answered your question fullfilled
0

Featured Post

Improve Your Query Performance Tuning

In this FREE six-day email course, you'll learn from Janis Griffin, Database Performance Evangelist. She'll teach 12 steps that you can use to optimize your queries as much as possible and see measurable results in your work. Get started today!

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

Preface I don't like visual development tools that are supposed to write a program for me. Even if it is Xcode and I can use Interface Builder. Yes, it is a perfect tool and has helped me a lot, mainly, in the beginning, when my programs were small…
Examines three attack vectors, specifically, the different types of malware used in malicious attacks, web application attacks, and finally, network based attacks.  Concludes by examining the means of securing and protecting critical systems and inf…
The goal of this video is to provide viewers with basic examples to understand how to use strings and some functions related to them in the C programming language.
The goal of this video is to provide viewers with basic examples to understand and use switch statements in the C programming language.

584 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question