CertC-INT36

Converting a pointer to integer or integer to pointer

Required inputs: IR

Although programmers often 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, subclause 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.

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. (See undefined behavior 24.)

The mapping between pointers and integers must be consistent with the addressing structure of the execution environment. Issues may arise, for example, on architectures that have a segmented memory model.

Noncompliant Code Example

The size of a pointer can be greater than the size of an integer, such as in an implementation where pointers are 64 bits and unsigned integers are 32 bits. This code example is noncompliant on such implementations because the result of converting the 64-bit ptr cannot be represented in the 32-bit integer type: 

void f(void) {
  char *ptr;
  /* ... */
  unsigned int number = (unsigned int)ptr;
  /* ... */
}
Compliant Solution

Any valid pointer to void can be converted to intptr_t or uintptr_t and back with no change in value. (See INT36-EX2.) The C Standard guarantees that a pointer to void may be converted to or from a pointer to any object type and back again and that the result must compare equal to the original pointer. Consequently, converting directly from a char * pointer to a uintptr_t, as in this compliant solution, is allowed on implementations that support the uintptr_t type. 

#include <stdint.h>
 
void f(void) {
  char *ptr;
  /* ... */
  uintptr_t number = (uintptr_t)ptr;
  /* ... */
}
Noncompliant Code Example

In this noncompliant code example, the pointer ptr is converted to an integer value. The high-order 9 bits of the number are used to hold a flag value, and the result is converted back into a pointer. This example is noncompliant on an implementation where pointers are 64 bits and unsigned integers are 32 bits because the result of converting the 64-bit ptr cannot be represented in the 32-bit integer type. 

void func(unsigned int flag) {
  char *ptr;
  /* ... */
  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.

Compliant Solution

This compliant solution uses a struct to provide storage for both the pointer and the flag value. This solution 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;
 
void func(unsigned int flag) {
  char *ptr;
  /* ... */
  ptrflag.pointer = ptr;
  ptrflag.flag = flag;
}
Noncompliant Code Example

It is sometimes necessary 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 *g(void) {
  unsigned int *ptr = 0xdeadbeef;
  /* ... */
  return ptr;
} 

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

Unfortunately this code cannot be made safe while strictly conforming to ISO C.

A particular platform (that is, hardware, operating system, compiler, and Standard C library) might guarantee that a memory address is correctly aligned for the pointer type, and actually contains a value for that type. A common practice is to use addresses that are known to point to hardware that provides valid values.

Exceptions

INT36-C-EX1: The integer value 0 can be converted to a pointer; it becomes the null pointer.

INT36-C-EX2: Any valid pointer to void can be converted to intptr_t or uintptr_t or their underlying types and back again with no change in value. Use of underlying types instead of intptr_t or uintptr_t is discouraged, however, because it limits portability. 

#include <assert.h>
#include <stdint.h>
 
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 code that is not portable and may create unexpected pointers to invalid memory locations.

Rule Severity Likelihood Remediation Cost Priority Level
INT36-C Low Probable High P2 L3
Related Guidelines
Taxonomy Taxonomy item Relationship
CERT C INT11-CPP. Take care when converting from pointer to integer or integer to pointer Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013 Pointer Casting and Pointer Type Changes [HFC] Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961:2013 Converting a pointer to integer or integer to pointer [intptrconv] Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11 CWE-587, Assignment of a Fixed Address to a Pointer 2017-07-07: CERT: Partial overlap
CWE 2.11 CWE-704 2017-06-14: CERT: Rule subset of CWE
CWE 2.11 CWE-758 2017-07-07: CERT: Rule subset of CWE
CWE 3.1 CWE-119, Improper Restriction of Operations within the Bounds of a Memory Buffer 2018-10-19:CERT:None
CWE 3.1 CWE-466, Return of Pointer Value Outside of Expected Range 2018-10-19:CERT:None
Bibliography
[ ISO/IEC 9899:2011] 6.3.2.3, "Pointers"
Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/integers-int/int36-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

cafe_message

{}

None

False

Options

message_predicate

message_predicate : typing.Callable[[Cafe_Message], bool] | None = None

If provided, a custom predicate to filter relevant messages. Receives the message node and should return True for messages to report.
 

reported_messages

reported_messages : set[int] | None = {1053, 152, 767}

If provided, only messages of these types are reported.
 

reported_severities

reported_severities : set[str] = {'error', 'remark', 'warning'}

List of severities to display.
 

use_error_number

use_error_number : bool = False

Whether the error number from the frontend should be used.
 

use_rule_severity

use_rule_severity : bool = True

Whether the rule's severity or the compiler's severity should be used.