Wiki MarkupModifying 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 states 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, 6.5, paragraph 7 [ ISO/IEC 9899:2024 ], specifies those circumstances in which an object may or may not be type can lead to unpredictable results. This is often caused by a violation of aliasing rules. C99 Section 6.5 Para. 7 \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] 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.

Noncompliant Code Example

The programmer in In this noncompliant code example is attempting to read from a different union member than the one most recently written to (called "type-punning"), an object of type float is incremented through an int *. The programmer can use the unit in the last place to get the next representable value for a floating-point type.  However, accessing an object through a pointer of an incompatible type is undefined behavior.

union a_union#include <stdio.h>
 
void f(void) {
  if (sizeof(int) i;
  double d;
};

int f(== sizeof(float)) {
  a_union t;
 float int *ip;
  t.d f = 30.00f;
  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.

 int *ip = (int *)&f;
    (*ip)++;
    printf("float is %f\n", f);
  }
}

Compliant Solution

In this compliant solution, the standard C function nextafterf() is used to round toward 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);
}

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 values of type short is treated as an integer and assigned an integer value. The resulting value of the two shorts is undefinedvalues are indeterminate.

#include <stdio.h>
 
void func(void) {
  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 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\[1\]}}. The actual behavior is implementation-defined and may change with optimization level.

Implementation Details

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

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() typically outputs "2222 2222". However, there is no guarantee that this will be true; 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

In this noncompliant code 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;
};
 
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:

#include <stdlib.h>
#include <string.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 */
  }
  /* ... */
  wp = (struct widget *)realloc(gp, sizeof(struct widget));
  if (!wp) {
    free(gp);
    /* Handle error */
  }
  memset(wp, 0, sizeof(struct widget));
  /* ... Initialize widget ... */

  if (wp->j == 12) {
    /* ... */
  }
  /* ... */
  free(wp);
}

Noncompliant Code Example

According to the C Standard, 6.7.7.3 [ISO/IEC 9899:2024], using two or more incompatible arrays in an expression is undefined behavior. (See also undefined behavior 76.)

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 which that can be quite difficult to detect. Furthermore, as in the case abovepreceding example, unexpected results can lead to buffer overflow attacks and/or , bypassing security checks and/, or unexpected execution.

References

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.

\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] 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.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-119 and EXP39-C

Independent( ARR30-C, ARR38-C, ARR32-C, INT30-C, INT31-C, EXP39-C, EXP33-C, FIO37-C) STR31-C = Subset( Union( ARR30-C, ARR38-C)) STR32-C = Subset( ARR38-C)

Intersection( EXP39-C, CWE-119) =

• Reading memory assigned to one type, but being accessed through a pointer to a larger type.

EXP39-C – CWE-119 =

• Writing to memory assigned to one type, but accessed through a pointer to a larger type

• Reading memory assigned to one type, but being accessed through a pointer to a smaller (or equal-sized) type

CWE-119 – EXP39-C =

• Reading beyond a buffer using a means other than accessing a variable through an incompatible pointer.

CWE-123 and EXP39-C

Intersection( CWE-123, EXP39-C) = Ø

EXP39-C allows overflowing a (small) buffer, but not arbitrary memory writes. (Possibly an arbitrary-memory write exploit could be devised using a “perfect storm” of incompatible types, but this would be uncommon in practice.)

CWE-125 and EXP39-C

Independent( ARR30-C, ARR38-C, EXP39-C, INT30-C) STR31-C = Subset( Union( ARR30-C, ARR38-C)) STR32-C = Subset( ARR38-C)

Intersection( EXP39-C, CWE-125) =

• Reading memory assigned to one type, but being accessed through a pointer to a larger type.

ESP39-C – CWE-125 =

• Reading memory assigned to one type, but being accessed through a pointer to a smaller (or equal-sized) type

CWE-125 – EXP39-C =

• Reading beyond a buffer using a means other than accessing a variable through an incompatible pointer.

CWE-188 and EXP39-C

Intersection( CWE-188, EXP39-C) = Ø

CWE-188 appears to be about making assumptions about the layout of memory between distinct variables (that are not part of a larger struct or array). Such assumptions typically involve pointer arithmetic (which violates ARR30-C). EXP39-C involves only one object in memory being (incorrectly) interpreted as if it were another object. EG a float being treated as an int (usually via pointers and typecasting)

CWE-704 and EXP39-C

CWE-704 = Union( EXP39-C, list) where list =

• Incorrect (?) typecast that is not incompatible

Bibliography

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Image Added Image Added Image Added EXP38-C. Do not call offsetof() on bit-field members or invalid types      03. Expressions (EXP)      04. Integers (INT)