#pragma pack()

[Null]

Hello, i have few questions about #pragma pack().

1.can anyone tell how #pragma pack() works? I mean how it changes data structs?
2. Why some data structures (for reading MBR and boot sectors) must be packed?

[guestgulkan]

(Very ) Basically the compiler unless otherwise directed will line up structure members on
2 byte or 4 byte boundaries - this makes it easier and faster for the processor to handle.
So the structure contains secret padding bytes to make this happen.

The pragma pack directive (in MS Compiler) allows you to change this alignment scheme.

Some things (particularly in relation to hardware) do not have the luxury to waste bytes like
this and they send their data in an exact fit.
This means that it is not wise to read data from a hardware device directly into a normal structure.

If you have want to read data that is an exact fit into a structure - you can tell the compiler
to make the structure an exact fit like this (microsoft compiler);

#pragma pack(push, 1) // exact fit - no padding
struct MyStruct
{
  char b; 
  int a; 
  int array[2];
};
#pragma pack(pop) //back to whatever the previous packing mode was 

Without the pragma directive, the size of the structure is 16 bytes - with the packing of 1 - the size is 13 bytes