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The union data type

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• The union data type
  • Syntax to define a union data structure:

    union UnionName { datatype1 varName1; // List of variables datatype2 varName2; ... };

    Meaning:

    A union data structure is a number of memory cells used to store any one of the variables specified in the union structure In other words: The same memory cell(s) are used to store one of the variables in the list of variables at any one time It is not possible to store more than one of the variables given in the list of variable at the same time

    Defining a union typed variable:

    Just like a struct data type, you can define variables of a union data type after you have defined the structure of a union data type Example: union myExample // Union definition { int a; double b; short c; char d; }; union myExample x; // Define a variable of the type union myExample

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  • How the different variables in a union typed variable are stored in memory:

    Example union definition: union myExample { int a; double b; short c; char d; }; How the different variable in the type union myExample are stored in memory: Observe that: Every member variable in a union typed variable start at the same memory address The number of bytes used to store a member variable depends on the size (= data type) of the member variable, a uses 4 because it is an int type variable b uses 8 because it is an double type variable And so on. The size of a union typed variable is equal to the size of the largest component variable

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• A C program that shows the facts of the union data structure
  • We can easily show the above facts with the following C program:

    union myUnion // Union structure { int a; double b; short c; char d; }; struct myStruct // Struct with the same member variables { int a; double b; short c; char d; }; int main(int argc, char *argv[]) { struct myStruct s; // Define a struct union myUnion u; // and a union variable // Print the size and the address of each component printf("Structure variable:\n"); printf("sizeof(s) = %d\n", sizeof(s) ); printf("Address of s.a = %u\n", &(s.a) ); printf("Address of s.b = %u\n", &(s.b) ); printf("Address of s.c = %u\n", &(s.c) ); printf("Address of s.d = %u\n", &(s.d) ); putchar('\n'); printf("Union variable:\n"); printf("sizeof(u) = %d\n", sizeof(u) ); printf("Address of u.a = %u\n", &(u.a) ); printf("Address of u.b = %u\n", &(u.b) ); printf("Address of u.c = %u\n", &(u.c) ); printf("Address of u.d = %u\n", &(u.d) ); }

    Output:

    Structure variable: sizeof(s) = 24 Address of s.a = 4290768696 Address of s.b = 4290768704 Address of s.c = 4290768712 Address of s.d = 4290768714 Union variable: sizeof(u) = 8 Address of u.a = 4290768688 (Same location !!!) Address of u.b = 4290768688 Address of u.c = 4290768688 Address of u.d = 4290768688

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc union1.c To run:          ./a.out

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• The use of union variables
  • Common uses of union typed variables:

    As a buffer to store a value for an user-specified operation The user-specified operation can receive different types of values. You can use multiple struct variables of the same size inside a union variable to re-map the different bits to make accessing the bit information easier This technique is commonly used in network programming to process different message headers This topic is beyond the scope of this course....

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  • Personal anecdote:

    The only time in my life that I needed to use a union typed variable was when I was writing an interpreter for a new data base manipulation language I used a union typed variable to hold value from a parsed expression Example: The expression 3 + 4 contains 2 integer values as operands for the operation + While the expression (3 < 4) AND (4 < 5) contains 2 boolean values as operands for the operation AND I used a union typed variable to store the operands.

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  • Network programming:

    When you do network programming with different package formats, you will encounter unions of many different struct variables. This allow you to manipulate different network protocol headers easily.

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