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

Compliant Solution (union)

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

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This function typically outputs "2222 2222." However, there is no guarantee that this will be true, even on implementations that defined the unspecified behavior; values of type short need not have the same representation as values of type int.

Compliant Solution (memcpy)

This compliant solution uses memcpy to convert the shorts to an int:

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

void func(void) {
  short a[2];
  a[0]=0x1111;
  a[1]=0x1111;

  int i = 0x22222222;
  memcpy(a, &i, sizeof a);

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

Using memcpy for type punning avoids the unspecified behavior of the union. A good optimizing compiler will recognize the use of memcpy and will emit code just as fast as the union approach.

Noncompliant Code Example

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#include <stdlib.h>
 
struct gadget {
  int i;
  double d;
  char *p;
};
 
struct widget {
  char *q;
  int j;
  double e;
};
 
void func(void) {
  struct gadget *gp;
  struct widget *wp;
 
  gp = (struct gadget *)malloc(sizeof(struct gadget));
  if (!gp) {
    /* Handle error */
  }
  /* ... Initialize gadget ... */
  wp = (struct widget *)realloc(gp, sizeof(struct widget));
  if (!wp) {
    free(gp);
    /* Handle error */
  }
  if (wp->j == 12) {
    /* ... */
  }
  /* ... */
  free(wp);
}

Compliant Solution

This compliant solution reuses the memory from the gadget object but reinitializes the memory to a consistent state before reading from it:

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Most compilers will produce a warning diagnostic if the two array types used in an expression are incompatible.

Compliant Solution

In this compliant solution, b is declared to point to an array with the same number of elements as a, satisfying the size specifier criterion for array compatibility:

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