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The programmer in this noncompliant code example is attempting to read from a different union member than the one most recently written to; this is known as "type-punning."

union a_union {
  int i;
  double d;
};

int f() {
  a_union t;
  int *ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
}

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In this noncompliant code example, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type.

union a_union {
  int i;
  double d;
};

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

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Type-punning is allowed provided the memory is accessed through the union type. This compliant solution returns the expected value.

union a_union {
  int i;
  double d;
};
          
int f() {
  a_union t;
  t.d = 3.0;
  return t.i;
} 

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In this noncompliant code example, an array of two shorts is treated as an integer and assigned an integer value. The resulting value of the two shorts is undefined.

short a[2];

a[0]=0x1111;
a[1]=0x1111;

*(int *)a = 0x22222222;  /* violation of aliasing rules */

printf("%x %x\n", a[0], a[1]);

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This compliant solution uses a union type that includes a type compatible with the effective type of the object.

union {
  short a[2];
  int i;
} u;

u.a[0]=0x1111;
u.a[1]=0x1111;

u.i = 0x22222222;

printf("%x %x\n", u.a[0], u.a[1]);

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