Modifying a variable through a pointer of an incompatible type (other than unsigned char) can lead to unpredictable results. Subclause 6.2.7 of the C standard mentions that two types may be distinct yet compatible, and addresses precisely when two distinct types are compatible.

This problem is often caused by a violation of aliasing rules. The C Standard, subclause 6.5, paragraph 7 [ISO/IEC 9899:2011], 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 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.

Accessing an object by means of any other lvalue expression (other than unsigned char) results in undefined behavior 34.

Noncompliant Code Example

In this noncompliant example, an object of type float is incremented through an int *. Using the maximum units in last position is often done as a way to get the next representable value for a floating-point number. However, accessing an object through a pointer of incompatible types is undefined behavior.

#include <stdio.h>
 
void f(void) {
  if (sizeof(int) == sizeof(float)) {
    float f = 0.0f;
    int *ip = (int *)&f;
 
    printf("float is %f\n", f);
 
    (*ip)++;
 
    printf("float is %f\n", f);
  }
}

Compliant Solution

In this compliant solution, the standard C function nextafterf() is used to round towards the highest representable floating-point value:

#include <float.h>
#include <math.h>
#include <stdio.h>
 
void f(void) {
  float f = 0.0f;
  f = nextafterf(f, FLT_MAX);
  printf("float is %f\n", f);
}

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.

#include <stdio.h>
 
void func(void) {
  short a[2];
  a[0]=0x1111;
  a[1]=0x1111;

  *(int *)a = 0x22222222;

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

When translating this code, an implementation can 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].

Implementation Details

Recent versions of GCC turn on the option -fstrict-aliasing (which allows alias-based optimizations) by default with -O2. Some architectures then print "1111 1111" as a result. Without optimization, the executable generates the expected output "2222 2222."

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

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:

#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]);

  /* ... */
}

This code example now reliably outputs "2222 2222."

Noncompliant Code Example

In this example, a gadget object is allocated, then realloc() is called to create a widget object using the memory from the gadget object. Although reusing memory to change types is acceptable, accessing the memory copied from the original object is undefined behavior.

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

Compliant Solution

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

#include <stdlib.h>
#include <string.h>
 
struct gadget {
  int i;
  double d;
  char *p;
};
 
struct widget {
  char *q;
  int j;
  double e;
};
 
struct gadget *gp;
struct widget *wp;
 
gp = (struct gadget *)malloc(sizeof (struct gadget));
if (!gp) {
  /* Handle error */
}
/* ... */
wp = (struct widget *)realloc(gp, sizeof(struct widget));
if (!wp) {
  free(gp);
  /* Handle error */
}
memset(wp, 0, sizeof(struct widget));
if (wp->j == 12) {
  /* ... */
}

Noncompliant Code Example

According to subclause 6.7.6.2 of the C Standard [ISO/IEC 9899:2011], using two or more incompatible arrays in an expression results in undefined behavior. See also undefined behavior 76 of Appendix J.

For two array types to be compatible, both should have compatible underlying element types, and both size specifiers should have the same constant value. If either of these properties is violated, the resulting behavior is undefined.

In this noncompliant code example, the two arrays a and b fail to satisfy the equal size specifier criterion for array compatibility. Because a and b are not equal, writing to what is believed to be a valid member of a might exceed its defined memory boundary, resulting in an arbitrary memory overwrite.

enum { ROWS = 10, COLS = 15 };
 
void func(void) {
  int a[ROWS][COLS];
  int (*b)[ROWS] = a;
}

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:

enum { ROWS = 10, COLS = 15 };
 
void func(void) {
  int a[ROWS][COLS];
  int (*b)[COLS] = a;
}

Risk Assessment

Optimizing for performance can lead to aliasing errors that can be quite difficult to detect. Furthermore, as in the preceding example, unexpected results can lead to buffer overflow attacks and/or bypassing security checks and/or unexpected execution.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography