How to avoid structure padding in C?

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Question Posted / xertu

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How to avoid structure padding in C?..

Those are 3 different things.

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.

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.

Is This Answer Correct ?

59 Yes

2 No

How to avoid structure padding in C?..

Answer / ravi

by using #pragma you can avoid structure padding. and that
to u can use it in linux or unix if i m not wrong.

Is This Answer Correct ?

55 Yes

4 No

How to avoid structure padding in C?..

Answer / lokesh mogra

yes u can use pragma to change change byte alignment.
for e.g.
typedef struct _s1{
unsigned int i;
unsigned char c;
unsigned long a;
unsigned short e;
} s1;
Size of this structure is of 11 Bytes. but due to default
byte alignment(8 byte) which is different for different
compilers. The size of structure would be 16 Bytes.
In order to change the alignment, we will have to do
something like this.

#pragma pack(push,1)
typedef struct _s1{
unsigned int i;
unsigned char c;
unsigned long a;
unsigned short e;
//unsigned char b;
} s1;
#pragma pack(pop)

This will change the byte alignment to 1 Byte. and thus size
of structure will be exactly 11 bytes

Is This Answer Correct ?

49 Yes

11 No

How to avoid structure padding in C?..

Answer / vishnu948923

by using #pragma pack

Is This Answer Correct ?

31 Yes

1 No

How to avoid structure padding in C?..

Answer / t swain

by using #pragma you can avoid structure padding.

Is This Answer Correct ?

18 Yes

2 No

How to avoid structure padding in C?..

Answer / lakshman naganoor

By #pragma

Example:
#pragma pack(push,1)
struct mystruct_A {
int b;
short d;
double c;
char m;
};
#pragma pack(pop)

main()

{
printf("size of structure mystruct_Ad is %d\n",sizeof
(struct mystruct_A));
}

size of structure mystruct_Ad is 15

Note:size of structure mystruct_Ad without using #pragma

Is This Answer Correct ?

3 Yes

0 No

How to avoid structure padding in C?..

Answer / akshay singh

i think for avoid structure padding you use only the #pragma pack(1) its sufficient for structure padding
i am not sure about windows application this concept i used in linux or unix

Is This Answer Correct ?

0 Yes

0 No

How to avoid structure padding in C?..

Answer / santosh

Packing, on the other hand prevents compiler from doing padding - this has to be explicitly requested - under GCC it's __attribute__((__packed__)), so the following:

struct __attribute__((__packed__)) mystruct_A {
char a;
int b;
char c;
};

Is This Answer Correct ?

0 Yes

1 No

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