Union: Unions like structure contain members whose individual data types may differ from one another. However the members that compose a union all share the same storage area within the computers memory where as each member within a structure is assigned its own unique storage area. Thus unions are used to conserve memory. They are useful for application involving multiple members. Where values need not be assigned to all the members at any one time. Like structures union can be declared using the keyword union as follows: union { int float char } code;
item m; p; c;
this declares a variable code of type union item. The union contains three members each with a different data type. However we can use only one of them at a time. This is because if only one location is allocated for union variable irrespective of size. The compiler allocates a piece of storage that is large enough to access a union member we can use the same syntax that we use to access structure members. That is code.m code.p code.c are all valid member variables. During accessing we should make sure that we are accessing the member whose value is currently stored. For example a statement such as code.m=456; code.p=456.78; printf(“%d”,code.m); Would produce erroneous result. In effect a union creates a storage location that can be used by one of its members at a time. When a different number is assigned a new value the new value supercedes the previous members value. Unions may be used in all places where a structure is allowed. The notation for accessing a union member that is nested inside a structure remains the same as for the nested structure.
What are the differences between a union and a structure in C? :
A union is a way of providing an alternate way of describing the same memory area. In this way, you could have a struct that contains a union, so that the "static", or similar portion of the data is described first, and the portion that changes is described by the union. The idea of a union could be handled in a different way by having 2 different structs defined, and making a pointer to each kind of struct. The pointer to struct "a" could be assigned to the value of a buffer, and the pointer to struct "b" could be assigned to the same buffer, but now a>somefield and b>someotherfield are both located in the same buffer. That is the idea behind a union. It gives different ways to break down the same buffer area. 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 dependentif 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 abovementioned "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; }