DCL03-A. Use a static assertion to test the value of a constant expression

This standard recommends the inclusion of diagnostic tests into your program using the assert() macro or other mechanisms (see MSC11-A. Incorporate diagnostic tests using assertions). Static assertion is a new facility in the C++ 0X draft standard. This facility enables assertions to be made at compile time rather than runtime, providing the following advantages:

• All processing must be performed during compile time - no runtime cost in space or time is tolerable.
• Assertion failure must result in a meaningful and informative diagnostic error message.
• It can be used at file or block scope.
• Misuse does not result in silent malfunction but is diagnosed at compile time.

Static assertions take the form

static_assert(constant-expression, string-literal);

In a static assert declaration, the constant-expression is a constant expression that can be contextually converted to bool. If the value of the expression when converted is true, the declaration has no effect. Otherwise the program is ill-formed, and a diagnostic message (which includes the text of the string-literal) is issued at compile time. For example:

static_assert(sizeof(int) <= sizeof(long), "sizeof(int) <= sizeof(long)"); /* Passes */
static_assert(sizeof(double) <= sizeof(int), "sizeof(double) <= sizeof(int)"); /* Fails */

Non-Compliant Code Example

This non-compliant code uses the assert() macro to assert a property concerning a memory-mapped structure that is essential for the code that uses this structure to behave correctly.

struct timer {
  uint8_t MODE;
  uint32_t DATA;
  uint32_t COUNT;
};

int main(void) {
  assert(offsetof(timer, DATA) == 4);
}

While the use of the runtime assertion is better than nothing, it needs to be placed in a function and executed, typically removed from the actual structure to which it refers. The diagnostic only occurs at runtime, and only if the code path containing the assertion is executed.

Compliant Solution

For assertions involving only constant expressions, some implementations allow the use of a preprocessor conditional statement, as in this example:

struct timer {
  uint8_t MODE;
  uint32_t DATA;
  uint32_t COUNT;
};

#if (offsetof(timer, DATA) != 4)
  #error DATA must be at offset 4
#endif

Using #error directives allows for clear diagnostic messages. Because this approach evaluates assertions at compile time, there is no run-time penalty.

Unfortunately, this solution is not portable. C99 does not require that implementations support sizeof, offsetof, or enumeration constants in #if conditions. According to Section 6.10.1, "Conditional inclusion," all identifiers in the expression that controls conditional inclusion either are or are not macro names. Some compilers allow these constructs in conditionals as an extension, but most do not.

Compliant Solution

This compliant solution mimics the behavior of static_assert in a portable manner.

#define JOIN(x, y) JOIN_AGAIN(x, y)
#define JOIN_AGAIN(x, y) x ## y

#define static_assert(e, s) \
  typedef char JOIN(assertion_failed_at_line_, __LINE__) [(e) ? 1 : -1]

struct timer {
  uint8_t MODE;
  uint32_t DATA;
  uint32_t COUNT;
};

static_assert(offsetof(struct timer, DATA) == 4, "offsetof(struct timer, DATA) == 4");

The static_assert() macro accepts a constant expression e, which is evaluated as the first operand to the conditional operator. If e evaluates to nonzero, an array type with a dimension of 1 is defined; otherwise an array type with a dimension of -1 is defined. Because it is invalid to declare an array with a negative dimension, the resulting type definition will be flagged by the compiler. The name of the array is used to indicate the location of the failed assertion.

The macro argument string-literal is ignored in this case; this is meant for future compatibility.

The JOIN() macro used the ## operator [[ISO/IEC 9899-1999]] to concatenate tokens. See [PRE05-A. Understand macro replacement when concatenating tokens]to understand how macro replacement behaves in C when using the ## operator.

Risk Assessment

Related Vulnerabilities

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

References

[[Eckel 07]]
[[ISO/IEC 9899-1999]] Section 6.10.1, "Conditional inclusion," and Section 6.10.3.3, "The ## operator"
[[Klarer 04]]
[[Saks 05]]
[[Saks 08]]

DCL02-A. Use visually distinct identifiers       02. Declarations and Initialization (DCL)       DCL04-A. Take care when declaring more than one variable per declaration