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.