Difference between union and structures in C

union and structures in C

The difference between structure and union in c are: 1. union allocates the memory equal to the maximum memory required by the member of the union but structure allocates the memory equal to the total memory required by the members. 2. In union, one block is used by all the member of the union but in case of structure, each member have their own memory space  

Difference in their Usage:

While structure enables us treat a number of different variables stored at different in memory , a union enables us to treat the same space in memory as a number of different variables. That is a Union offers a way for a section of memory to be treated as a variable of one type on one occasion and as a different variable of a different type on another occasion.
There is frequent rwquirement while interacting with hardware to access access a byte or group of bytes simultaneously and sometimes each byte individually. Usually union is the answer.
=======Difference With example***** Lets say a structure containing an int,char and float is created and a union containing int char float are declared. struct TT{ int a; float b; char c; } Union UU{ int a; float b; char c; }
sizeof TT(struct) would be >9 bytes (compiler dependent-if int,float, char are taken as 4,4,1)
sizeof UU(Union) would be 4 bytes as supposed from above.If a variable in double exists in union then the size of union and struct would be 8 bytes and cumulative size of all variables in struct.  

Detailed Example:

struct foo 
{ 
char c; 
long l; 
char *p; 
}; 

union bar 
{ 
char c; 
long l; 
char *p; 
}; 

A struct foo contains all of the elements c, l, and p. Each element is 
separate and distinct. 

A union bar contains only one of the elements c, l, and p at any given 
time. Each element is stored in the same memory location (well, they all 
start at the same memory location), and you can only refer to the element 
which was last stored. (ie: after "barptr->c = 2;" you cannot reference 
any of the other elements, such as "barptr->p" without invoking undefined 
behavior.) 

Try the following program. (Yes, I know it invokes the above-mentioned
"undefined behavior", but most likely will give some sort of output on 
most computers.) 

==========
#include 

struct foo 
{ 
char c; 
long l; 
char *p; 
}; 

union bar 
{ 
char c; 
long l; 
char *p; 
}; 

int main(int argc,char *argv[])
{ 
struct foo myfoo;
union bar mybar;

myfoo.c = 1; 
myfoo.l = 2L; 
myfoo.p = "This is myfoo";

mybar.c = 1; 
mybar.l = 2L; 
mybar.p = "This is mybar";

printf("myfoo: %d %ld %s\n",myfoo.c,myfoo.l,myfoo.p);
printf("mybar: %d %ld %s\n",mybar.c,mybar.l,mybar.p);

return 0; 
} 

==========

On my system, I get: 

myfoo: 1 2 This is myfoo
mybar: 100 4197476 This is mybar
==========
credit to original author. 

Structure: Structure is a combination elements, which can be predefined data types or other structure. The length/size of the structure is the sum of the length of its elements. 
In C, structures cannot containn functions. in C++ it can.

Union: Union is a combination elements, which can be predefined data types or other union . But, the size/length of union is the maximum of internal elements. 

the sizeof() operator returns the size slightly more than calculated size due to padding, which again depends on OS
== Answer == Union allocates the memory equal to the maximum memory required by the member of the union but structure allocates the memory equal to the total memory required by the members. In union,one block is used by all the member of the union but in case of structure, each member have their own memory space.
Union allocates the memory equal to the maximum memory requried by the member of the union but structure allocates the memory equal to sum of the memory allocated to its each individual members.
In Union, one block is used by all the member of union but in case of structure, each member have their own memory space.
Union allocates the memory equal to the maximum memory requried by the member of the union but structure allocates the memory equal to total memory requried by the members. In Union, one block is used by all the member of union but in case of structure, each member have their own memory space.