INT11-C. Take care when converting from pointer to integer or integer to pointer

While it has been common practice to use integers and pointers interchangeably in C, pointer-to-integer and integer-to-pointer conversions are implementation-defined. 

Conversions between integers and pointers can have undesired consequences depending on the implementation. According to the C Standard, Section 6.3.2.3 [ISO/IEC 9899:2011],

An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation.

Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.

See also undefined behavior 24 of Annex J.

Do not convert an integer type to a pointer type if the resulting pointer is incorrectly aligned, does not point to an entity of the referenced type, or is a trap representation.

Do not convert a pointer type to an integer type if the result cannot be represented in the integer type.

These issues arise because the mapping functions for converting a pointer to an integer or an integer to a pointer must be consistent with the addressing structure of the execution environment. For example, not all machines have a flat memory model.

Noncompliant Code Example

In this noncompliant code example, the pointer ptr is converted to an integer value. Both a pointer and an int are assumed to be 32 bits. The high-order 9 bits of the number are used to hold a flag value, and the result is converted back into a pointer.

char *ptr;
unsigned int flag;
/* ... */
unsigned int number = (unsigned int)ptr;
number = (number & 0x7fffff) | (flag << 23);
ptr = (char *)number;

A similar scheme was used in early versions of Emacs, limiting its portability and preventing the ability to edit files larger than 8MB.

Please note that this noncompliant code example also violates EXP11-C. Do not make assumptions regarding the layout of structures with bit-fields.

Compliant Solution

Saving a few bits of storage is generally not as important as writing portable code. A struct can be used to provide room for both the pointer and the flag value. This is portable to machines of different word sizes, both smaller and larger than 32 bits, working even when pointers cannot be represented in any integer type.

struct ptrflag {
  char *pointer;
  unsigned int flag :9;
} ptrflag;

char *ptr;
unsigned int flag;
/* ... */
ptrflag.pointer = ptr;
ptrflag.flag = flag;

Noncompliant Code Example

It is sometimes necessary in low-level kernel or graphics code to access memory at a specific location, requiring a literal integer to pointer conversion. In this noncompliant code, a pointer is set directly to an integer constant, where it is unknown whether the result will be as intended.

unsigned int *ptr = 0xcfcfcfcf;

The result of this assignment is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation.

Compliant Solution

Adding an explicit cast may help the compiler convert the integer value into a valid pointer.

unsigned int *ptr = (unsigned int *) 0xcfcfcfcf;

Exceptions

INT11-EX1: A null pointer can be converted to an integer; it takes on the value 0. Likewise, a 0 integer can be converted to a pointer; it becomes the null pointer.

INT11-EX2: Any valid pointer to void can be converted to intptr_t or uintptr_t and back with no change in value. (This includes the underlying types if intptr_t and uintptr_t are typedefs, and any typedefs that denote the same types as intptr_t and uintptr_t.)

void h(void) {
  intptr_t i = (intptr_t)(void *)&i;
  uintptr_t j = (uintptr_t)(void *)&j;
 
  void *ip = (void *)i;
  void *jp = (void *)j;
 
  assert(ip == &i);
  assert(jp == &j);
}

Risk Assessment

Converting from pointer to integer or vice versa results in unportable code and may create unexpected pointers to invalid memory locations.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography