C++: Some programs need to exercise precise control over the memory areas where data is placed.
Some programs need to exercise precise control over the memory areas where data is placed. For example, suppose we wish to read the contents of the boot sector into a structure. For this the byte arrangement of the structure elements must match the arrangement of various fields in the boot sector of the disk.
The #pragma pack directives offer a way to fulfill this requirement. The #pragma pack directive specifies packing alignment for structure and union members. The #pragma takes effect at the first structure or union declaration after the #pragma is seen. Consider the following structure:
#pragma pack (1)
struct emp
{
int a ;
float s ;
char ch ;
} ;
#pragma pack( )
Here, #pragma pack ( 1 ) lets each structure element to begin on a 1-byte boundary. Hence the size of the structure will be 9. (int - 4, float - 4, char - 1). If we use #pragma pack ( 2 ) each structure element can begin on a 2-byte boundary. Hence the size of the structure will be 10. (int - 4, float - 4, char - 2)
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