What is Union - C Data Type

C examples for Data Type:union

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Introduction

The union type is like the struct type, except that all fields share the same memory position.

The size of a union is the size of the largest field it contains.

In the following code creates a union type, which is 4 bytes large. 

union mix {
  char c;  /* 1 byte */
  short s; /* 2 bytes */
  int i;   /* 4 bytes */
};

Since the memory is sharing, the union type can only store one value at a time.

Changing one field will overwrite the value of the others.

Demo Code

#include <stdio.h>
union mix {// w  w  w.  j ava2  s  .co  m
  char c;  /* 1 byte */
  short s; /* 2 bytes */
  int i;   /* 4 bytes */
};

int main(void) {
  union mix m;

  printf("%d\n",m.c);
  printf("%d\n",m.s);

  m.c = 0xFF; /* set first 8 bits */

  printf("%d\n",m.c);
  printf("%d\n",m.s);


  m.s = 0;    /* reset first 16 bits */

  printf("%d\n",m.c);
  printf("%d\n",m.s);
}

union provides multiple ways of using the same memory location.

Demo Code

#include <stdio.h>
union mix {/*from  ww  w  .  j a v a 2s. c  o  m*/
  char c[4];                  /* 4 bytes */
  struct { short hi, lo; } s; /* 4 bytes */
  int i;                      /* 4 bytes */
} m;


int main(void) {
  union mix m;

    m.i=0xFF00F00F; //11111111 00000000 11110000 00001111
    m.s.lo;         //11111111 00000000                   
    m.s.hi;         //                  11110000 00001111 
    m.c[3];         //11111111                            
    m.c[2];         //         00000000                   
    m.c[1];         //                  11110000          
    m.c[0];         //                           00001111 

    printf("%d\n",m.i);
    printf("%d\n",m.s.lo);

}

Result


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