Modifying a variable through a pointer of an incompatible type can lead to unpredictable results. This is often caused by a violation of aliasing

 rules. C99 Section 6.5 Para. 7 \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] specifies those circumstances in which an object may or may not be aliased.

...

Noncompliant Code Example

The programmer in this noncompliant code example is attempting to read from a different union member than the one most recently written to (called "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;
}

However, instead of reading directly from union member, it assigns a pointer ip to reference the integer value and returns the value referenced by the pointer. Unfortunately, this is a violation of the strict aliasing rules and in this case the compiler may determine that ip refers to some other value than the value stored by t.i and return a value other than the expected value.

Noncompliant Code Example

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;
}

Because of optimization, the function f() may return something other than the expected value.

Compliant Solution

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;
} 

Noncompliant Code Example

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]);

When translating this code, an implementation may assume that no access through an integer pointer can change the array {{a}}, consisting of shorts. Consequently, {{printf()}} may be called with the original values of {{a\[0\]}} and {{a\[1\]}}. The actual behavior is implementation-defined and may change with optimization level.

...

When GCC 3.4.6 compiles this code with optimization, the assignment through the aliased pointer is effectively eliminated.

Compliant Solution

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

...

GCC Known Bugs C bugs, Aliasing issues while casting to incompatible types

GCC Manual

\[Walfridsson \03] Krister Walfridsson. [Aliasing, pointer casts and gcc 3.3|http://mail-index.netbsd.org/tech-kern/2003/08/11/0001.html] Aliasing issue. August, 2003.

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.5, "Expressions"

\[Acton 06\] Mike Acton. [Understanding Strict Aliasing|http://www.cellperformance.com/mike_acton/2006/06/understanding_strict_aliasing.html]. June 01, 2006.