How to calculate the
offset of
fields in a
(list) struct/object
- Suppose
we have a struct
definition as
follows:
struct myStruct
{
... ^
... | offset
... |
... v
int x;
...
...
}
|
Then:
- The
offset of
the field
x
(from the base address of
the struct
variable) is:
number of bytes used for the variables prior to x
|
|
|
How to calculate the
offset of
fields in a
(list) struct/object
Consider the following
struct Node (object)
definition:
struct Node
{
int value; // Data field
struct Node * next; // Link field
};
|
How to calculate the
offset of
fields in a
(list) struct/object
Consider the following
struct Node (object)
definition:
struct Node
{
int value; // Data field offset = 0
struct Node * next; // Link field offset = 4
};
|
The
offsets of the
fields in
each
struct Node
variable/object are
as follows:
How to calculate the
offset of
fields in a
(list) struct/object
- Example 2
Consider the following
struct Node (object)
definition:
struct Node
{
struct Node *next; // Link field
int value1; // Data field 1
short value2; // Data field 2
short value3; // Data field 3
};
|
How to calculate the
offset of
fields in a
(list) struct/object
- Example 2
Consider the following
List (object)
definition:
struct Node
{
struct Node *next; // Link field offset = 0
int value1; // Data field 1 offset = 4
short value2; // Data field 2 offset = 8
short value3; // Data field 3 offset = 10
};
|
The
offsets of the
fields in
each
struct Node
variable/object are
as follows:
How to calculate the
offset of
fields in a
(list) struct/object
- Example 3
Given the following
struct Node (object)
definition:
struct Node
{
int value1; // Data field 1
short value2; // Data field 2
short value3; // Data field 3
struct Node *next; // Link field
};
|
The
offset
of
the
fields in
Node variables defined
by the
above
struct definition are:
Offset(value1) = 0
Offset(value2) = 4
Offset(value3) = 6 (4+2)
Offset(next) = 8 (4+2+2)
|
We discuss
how to
obtain the
base address of
a Node object in the
next slides.
Final comment: offset computation is
actually more complicated
- The complete process to
compute the
offset of a
field is
more complicated
due to:
- memory alignment
requirments...
|
- Alignment requirements:
- An
short type
field must be
placed at an
address that is
divisible by 2
- An
int type
field must be
placed at an
address that is
divisible by 4
- Etc, etc
|
- The memory alignment
requirement will
introduce
gaps
(= unused memory bytes)
within the
struct variable
(or object)
- The gaps will make the
offset computation more
complicated
|
- Simplifying
practice:
- In
CS255, we will
only use
struct definitions that
has
no gaps
between fields.
|
|
❮
❯