What is structure padding?

Answer 1

When you are using C, structure padding is used to pad the data in such a way that it can be sent to external devices. Sometimes, the data is padded in such a way that it can be used on little endian vs big endian processors.

Padding is done to fast access of data from memory. Structure members are aligned to based on the memory pointer size, which is normally 32bit in Win32 systems. So a character variable defined may take 4 bytes. Hence the size of structure is not really the sum of size individual members.

Extra bits between or after the structure's defined fields, that do not belong to any of those fields. The purpose of the padding is to position the fields on byte boundaries or even word boundaries, which is convenient for the computer. And the purpose of that is faster reading / writing / searching.

Answer:

Most processors require specific memory alignment on variables certain types. Normally the minimum alignment is the size of the basic type in question, for 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 boundaries 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 correct 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 appears 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.