Difference between a Structure and a Union?

Question

Is there any good example to give the difference between a `struct` and a `union`?Basically I know that `struct` uses all the memory of its member and `union` uses the largest members memory space. Is there any other OS level difference?

Answer 1

/*******************************

declares a structure type and a union type.

then use sizeof() function to see if there is a difference of their sizes on the heap.

*********************************************/

#include <stdio.h>

typedef struct _TDataRec

{

int x;

double flAmt;

char str[25];

long longIntnum;

char boolSwitchFlag;

} * _TDataRec;

#define DATA_REC_SIZE ( (sizeof( struct _TDataRec))

typedef union _TDataNode

{

int x;

double flAmt;

char str[25];

long longIntnum;

char boolSwitchFlag;

} * _TDataNode;

#define DATA_NODE_SIZE ( (sizeof( struct _TDataNode))

int main()

{

printf(" size of data rec structure is %d \n", DATA_REC_SIZE);

printf(" size of data node union is %d \n", DATA_NODE_SIZE);

return 0;

}