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

do you mean something like this ?

union {
   long long a;
   unsigned char b[sizeof(long long)];
} long_long_to_single_bytes;

ADDED:

I have recently used this on our AIX machine to transform the 64bit machine-indentifier into a byte-array.

std::string getHardwareUUID(void) {
#ifdef AIX
   struct xutsname m; // aix specific struct to hold the 64bit machine id
   unamex(&b);        // aix specific call to get the 64bit machine id
   long_long_to_single_bytes.a = m.longnid;
   return convertToHexString(long_long_to_single_bytes.b, sizeof(long long));
#else // Windows or Linux or Solaris or ...
   ... get a 6byte ethernet MAC address somehow and put it into mac_buf
   return convertToHexString(mac_buf, 6);
#endif

Answer 2

Example:

When using different socket types, but you want a comon type to refer.

Answer 3

struct cat_info
{
int legs;
int tailLen;
};

struct fish_info
{
bool hasSpikes;
};


union 
{
fish_info fish;
cat_info cat;
} animal_data;

struct animal
{
char* name;
int animal_type;
animal_data data;
};

Answer 4

Another example more: to save doing castings.

typedef union {
  long int_v;
  float float_v;
} int_float;

void foo(float v) {
  int_float i;
  i.float_v = v;
  printf("sign=%d exp=%d fraction=%d", (i.int_v>>31)&1, ((i.int_v>>22)&0xff)-128, i.int_v&((1<<22)-1));
}

instead of:

void foo(float v) {
  long i = *((long*)&v);
  printf("sign=%d exp=%d fraction=%d", (i>>31)&1, ((i>>22)&0xff)-128, i&((1<<22)-1));
}

Answer 5

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;
}

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->dataType){
       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.

Answer 6

For accessing registers or I/O ports bytewise as well as bitwise by mapping that particular port to memory, see the example below:

    typedef Union
{
  unsigned int a;
struct {
  unsigned bit0 : 1,
           bit1 : 1,
           bit2 : 1,
           bit3 : 1,
           bit4 : 1,
           bit5 : 1,
           bit6 : 1,
           bit7 : 1,
           bit8 : 1,
           bit9 : 1,
           bit10 : 1,
           bit11 : 1,
           bit12 : 1,
           bit13 : 1,
           bit14 : 1,
           bit15 : 1
} bits;
} IOREG;

# define PORTA (*(IOREG *) 0x3B)
...
unsigned int i = PORTA.a;//read bytewise
int j = PORTA.bits.bit0;//read bitwise
...
PORTA.bits.bit0 = 1;//write operation

Answer 7

I've typically used unions where you want to have different views of the data e.g. a 32-bit colour value where you want both the 32-bit val and the red,green,blue and alpha components

struct rgba
{
  unsigned char r;
  unsigned char g;
  unsigned char b;
  unsigned char a;
};

union  
{
  unsigned int val;
  rgba components;
}colorval32;

NB You could also achieve the same thing with bit-masking and shifting i.e

#define GETR(val) ((val&0xFF000000) >> 24)

but I find the union approach more elegant

Answer 8

For convenience, I use unions to let me use the same class to store xyzw and rgba values

#ifndef VERTEX4DH
    #define VERTEX4DH

    struct Vertex4d{

     union {
      double x;
      double r;
     };
     union {
      double y;
      double g;
     };
     union {
      double z;
      double b;
     };
     union {
      double w;
      double a;
     };

     Vertex4d(double x=0, double y=0,double z=0,double w=0) : x(x), y(y),z(z),w(w){}
    };

#endif

Answer 9

Unions are useful if you have different kinds of messages, in which case you don't have to know in any intermediate levels the exact type. Only the sender and receiver need to parse the message actual message. Any other levels only really need to know the size and possibly sender and/or receiver info.

Answer 10

Many examples of unions can be found in <X11/Xlib.h>. Few others are in some IP stacks (in BSD <netinet/ip.h> for instance).

As a general rule, protocol implementations use union construct.

Answer 11

I've used sometimes unions this way

//Define type of structure
typedef enum { ANALOG, BOOLEAN, UNKNOWN } typeValue_t;
//Define the union
typedef struct  {
  typeValue_t typeValue;
  /*On this structure you will access the correct type of
    data according to its type*/
  union {
    float ParamAnalog;
    char  ParamBool;
  };
} Value_t;

Then you could declare arrays of different kind of values, storing more or less efficiently the data, and make some "polimorph" operations like:

 void printValue ( Value_t value ) {
    switch (value.typeValue) {
       case BOOL:
          printf("Bolean: %c\n", value.ParamBool?'T':'F');
          break;
       case ANALOG:
          printf("Analog: %f\n", value.ParamAnalog);
          break;
       case UNKNOWN:
          printf("Error, value UNKNOWN\n");
          break;
    }
 }

Answer 12

Here is another example where a union could be useful.

(not my own idea, I have found this on a document discussing c++ optimizations)

begin-quote

.... Unions can also be used to save space, e.g.

first the non-union approach:

void F3(bool useInt) {
    if (y) {
        int a[1000];
        F1(a);  // call a function which expects an array of int as parameter
    }
    else {
        float b[1000];
        F2(b);  // call a function which expects an array of float as parameter
    }
}

Here it is possible to use the same memory area for a and b because their live ranges do not overlap. You can save a lot of cpu-cache space by joining a and b in a union:

void F3(bool useInt) {

    union {
        int a[1000];
        float b[1000];
    };

    if (y) {
        F1(a);  // call a function which expects an array of int as parameter
    }
    else {
        F2(b);  // call a function which expects an array of float as parameter
    }
}

Using a union is not a safe programming practice, of course, because you will get no warning from the compiler if the uses of a and b overlap. You should use this method only for big objects that take a lot of cache space. ...

end-qoute

Answer 13

Unions can also be useful when type punning, which is desirable in a select few places (such as some techniques for floating-point comparison algorithms).

Answer 14

In the Windows world, unions are commonly used to implement tagged variants, which are (or were, before .NET?) one standard way of passing data between COM objects.

The idea is that a union type can provide a single natural interface for passing arbitrary data between two objects. Some COM object could pass you a variant (e.g. type VARIANT or _variant_t) which could contain either a double, float, int, or whatever.

If you have to deal with COM objects in Windows C++ code, you'll see variant types all over the place.

• VARIANTs, SAFEARRAYs, and BSTRs, Oh My!
• Boost variant

Answer 15

• When reading serialized data that needs to be coerced into specific types.
• When returning semantic values from lex to yacc. (yylval)
• When implementing a polymorphic type, especially one that reads a DSL or general language
• When implementing a dispatcher that specifically calls functions intended to take different types.

Answer 16

Recently I think I saw some union used in vector programming. vector programming is used in intel MMX technology, GPU hardware, IBM's Cell Broadband Engine, and others.

a vector may correspond to a 128 bit register. It is very commonly used for SIMD architecture. since the hardware has 128-bit registers, you can store 4 single-precision-floating points in a register/variable. an easy way to construct, convert, extract individual elements of a vector is to use the union.

typedef union {
    vector4f vec; // processor-specific built-in type
    struct { // human-friendly access for transformations, etc
        float x;
        float y;
        float z;
        float w;
    };
    struct { // human-friendly access for color processing, lighting, etc
        float r;
        float g;
        float b;
        float a;
    };
    float arr[4]; // yet another convenience access
} Vector4f;

int main()
{
    Vector4f position, normal, color;
    // human-friendly access
    position.x = 12.3f;
    position.y = 2.f;
    position.z = 3.f;
    position.w = 1.f;

    // computer friendly access
    //some_processor_specific_operation(position.vec,normal.vec,color.vec);
    return 0;
}

if you take a path in PlayStation 3 Multi-core Programming, or graphics programming, a good chance you'll face more of these stuffs.

Answer 17

SDL uses an union for representing events: http://www.libsdl.org/cgi/docwiki.cgi/SDL_Event.