What is
Slack Byte or structure padding in Structure?
CPU/Computer stores structures using the concept of "word boundary".
What is word boundary?
Word boundary is the
size of word(bytes) that CPU can process(read/write)
at a time. Usually it is the Accumulator size of a processor(CPU). The size of word
boundary is machine dependent.
Slack
Byte(Padding byte)
In computer with 4 bytes word
boundary, the members of structure are stores left aligned on the wordboundary, as shown below. A character data takes one byte and an integer takes 4
bytes. 3 bytes(Padding bytes) between them is left unoccupied. This unoccupied
byte is known as the slack byte.
 |
| Structure Padding or Slack bytes |
Why Padding or slack bytes are required?
Slack byte used for speed optimization. It aligns bytes so that, that
can be read/write from structure faster. It is an extra byte, sometimes placed
between structure members to align the structure.
Some Platforms like x86,
CPU can access data aligned on absolutely any boundary, not
only on "word boundary". The mis-aligned access might be less
efficient than aligned access. If data is aligned properly, CPU can access the
memory(structure members) efficiently and faster.
Note: As far as
CPU is concerned, it can access any data on any boundary.
When we declare structure variables, each one of them may contain the
slack bytes and the values stored in such slack bytes are undefined(garbage).
Due to this, even though the members of two variable values are equal, their structures
do not necessarily to be equal. Therefore C does not permit compassion of
structures. However, we need to design our own function that can compare the
individual members of structure to decide whether the structures are equal or
not.
For understanding more, run the below program and check for memory
mapping of the structure variable.
#include "stdio.h"
void main()
{
struct st_demo
{
char ch;
int i;
};
struct st_demo s;
printf("
Size of int = %d \n\n Size of char = %d \n\n Size of struct = %d ",sizeof(s.i), sizeof(s.ch),sizeof(s));
getch();
}
Let assume the size of char
is 1 byte and int is 4 bytes(Variable sizes
depends on Platforms/Compilers).
According to this, the size of structure variable s
should be 5(1+4) bytes.
 |
| Output of the demo program |
But from the above programs Output, Size of struct is 8 bytes.
So
from where we got this extra 3 bytes?
From the memory map we can observe that there are three bytes between
variable is unoccupied. These bytes are called slack bytes.
Note: It is not
always necessary that a structure variable contains structure padding or slack
bytes.
If anything needs to be added for the above article, Please post it
in comment section.
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