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This problem is often caused by a violation of aliasing rules. The C Standard, 6.5, paragraph 7 [ ISO/IEC 9899:20112024 ], specifies those circumstances in which an object may or may not be aliased.

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:

• a type compatible with the effective type of the object,
• a qualified version of a type compatible with the effective type of the object, 
• a type that is the signed or unsigned type corresponding to compatible with the underlying type of the effective type of the object, a type that is
• the signed or unsigned type corresponding to compatible with a qualified version of the underlying type of the effective type of the object, 
• an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), oror 
• a character type. 

Accessing an object by means of any other lvalue expression (other than unsigned char) is undefined behavior 37.

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#include <stdio.h>
 
void func(void) {
  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]);

  /* ... */
}

The C standard states:

If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called “type punning”). This might be a trap representation

The call to printf() printf() behavior in this compliant solution is unspecified, but it is commonly accepted as an implementation extension. (See unspecified behavior 11.)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.; the object representations of a and i are unspecified and need not be compatible in this way, despite this operation being commonly accepted as an implementation extension. (See unspecified behavior 11.)

Noncompliant Code Example

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According to the C Standard, 6.7.67.2 3 [ISO/IEC 9899:20112024], using two or more incompatible arrays in an expression is undefined behavior. (See also undefined behavior 76.)

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Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

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