How values are changed in C unions?

#include <stdio.h>

int main()
{
 typedef union{
     int a ;
     char c;
     float f;
 } myu;
 
 myu sam;
 sam.a = 10;
 sam.f=(float)5.99;
  sam.c= 'H';

 printf("%d\n %c\n %f\n",sam.a,sam.c,sam.f);
 return 0;
}

Output

1086303816

H

5.990025

How come the value of integer has changed so drastically while the float is almost the same.

>Solution :

Try this. Instead of using printf (which will mainly output nonesens) show the raw memory after each modification.

The code below assumes that int and float are 32 bit types and that your compiler does not add padding bytes in this union.

#include <string.h>
#include <stdio.h>
#include <assert.h>

void showmemory(void* myu)
{
  unsigned char memory[4];
  memcpy(memory, myu, 4);

  for (int i = 0; i < 4; i++)
  {
    printf("%02x ", memory[i]);
  }
  printf("\n");
}

int main()
{
  typedef union {
    int a;
    char c;
    float f;
  } myu;

  assert(sizeof(myu) == 4);  // assume size of the union is 4 bytes

  myu sam;
  sam.a = 10;
  showmemory(&sam);

  sam.f = (float)5.99;
  showmemory(&sam);
 
  sam.c = 'H';
  showmemory(&sam);
}

Possible output on a little endian system:

0a 00 00 00      // 0a is 10 in hexadecimal
14 ae bf 40      // 5.99 in float
48 ae bf 40      // 48 is 'H'