[Mohan Giri]

What is the difference between structure and union?? Where we use structure data type and where we use union data type?

[warrior1981]

a structure and a union differs in the memory allocation
a union takes the highest type of the variable allocated.
for example....
union u1{
int rolno;
char name;
};

then the maximum abt of memory will be that as the size of the integer....

but in the case of the structure,
struct s2{
int rollno;
char name;
};
the memory allocation will be that of a char and a int
thats the plain difference between the two.....

[Chankama]

Warrior is right. If it wasn't clear, with a structure, what he meant was that the total memory allocated will be sizeof(int) + sizeof(char)..

In anycase, why are unions useful?.. Well, one application would be if u want to easily "go through" a variable. For example, in the example above, if the "int" has a particular value that u had set, using a "char" array variable would allow u to obtain a byte from the int without having to worry about bit shifting/masking/pointers and such..