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7. An object shall have its stored value accessed only by an lvalue expression that has one of
the following types: (78)—

• 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 the effective type of the
  object,

—

• a type that is the signed or unsigned type corresponding to a qualified version 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), or

—

• a character type.

/78) The intent of this list is to specify those circumstances in which an object may or may not be aliased./

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Noncompliant Code Example

In this 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.

#include <stdio.h>

int main()
{
    short a[2];

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

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

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

Optimized code produced by GCC 3.4.6.
.

#include <stdio.h>

int main()
{
    short a[2];

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

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

Implementation Details

In the case above, the compiler may assume that no access through an integer pointer can change the array a, consisting of shorts. Thus, {{printf}} may be called with the original values of a\[0\] and a\[1\]. What really happens is up to the compiler and may change with architecture and optimization level.

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To disable optimizations based on alias-analysis for faulty legacy code, the option -fno-strict-aliasing can be used as a work-around. The option -Wstrict-aliasing (which is included in -Wall) warns about some - but not all - cases of violation of aliasing rules when -fstrict-aliasing is active.

When GCC 3.4.6 compiles this code with optimization, it yeilds an executable that behaves like the following code:

...

#include <stdio.h>

int main()
{
    short a[2];

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

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

In other words, GCC 3.4.6 effectively removes the assignment.

Compliant Solution

To fix the code above, you can use a union instead of a cast.

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