C: Where is union practically used?

Question

I have a example with me where in which the alignment of a type is guaranteed, union max_align . I am looking for a even simpler example in which union is used practically, to explain my friend.

Answer 1

I usually use unions when parsing text. I use something like this:

typedef enum DataType { INTEGER, FLOAT_POINT, STRING } DataType ;

typedef union DataValue
{
    int v_int;
    float v_float;
    char* v_string;
}DataValue;

typedef struct DataNode
{
    DataType type;
    DataValue value;
}DataNode;

void myfunct()
{
    long long temp;
    DataNode inputData;

    inputData.type= read_some_input(&temp);

    switch(inputData.type)
    {
        case INTEGER: inputData.value.v_int = (int)temp; break;
        case FLOAT_POINT: inputData.value.v_float = (float)temp; break;
        case STRING: inputData.value.v_string = (char*)temp; break;
    }
}

void printDataNode(DataNode* ptr)
{
   printf("I am a ");
   switch(ptr->type){
       case INTEGER: printf("Integer with value %d", ptr->value.v_int); break;
       case FLOAT_POINT: printf("Float with value %f", ptr->value.v_float); break;
       case STRING: printf("String with value %s", ptr->value.v_string); break;
   }
}

If you want to see how unions are used HEAVILY, check any code using flex/bison. For example see splint, it contains TONS of unions.