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This standard recommends the inclusion of diagnostic tests into your program using the assert() macro or other mechanisms Assertions are a valuable diagnostic tool for finding and eliminating software defects that may result in vulnerabilities (see MSC11-AC. Incorporate diagnostic tests using assertions). Static assertion is a new facility in the C++ 0X draft standard. This facility gives the ability to make assertions 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 rather is diagnosed at compile time

Static assertions take the form:

The runtime assert() macro has some limitations, however, in that it incurs a runtime overhead and because it calls abort(). Consequently, the runtime assert() macro is useful only for identifying incorrect assumptions and not for runtime error checking. As a result, runtime assertions are generally unsuitable for server programs or embedded systems.

Static assertion is a new facility in the C Standard. It takes the form

static_assert(constant-expression, string-literal);

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Subclause 6.7.10 of the C Standard [ISO/IEC 9899:2011] states:

The constant expression shall be an integer constant expression. If the value of

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the constant expression compares unequal to 0, the declaration has no effect. Otherwise

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, the constraint is violated and the implementation shall produce a diagnostic message

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that includes the text of the string literal, except that characters not in the basic source character set are not required to appear in the message.

It means that if constant-literal is issued expression is true, nothing will happen. However, if constant-expression is false, an error message containing string-literal will be output at compile time. For example:

/* Passes */
static_assert(
  sizeof(int) <= sizeof(longvoid*), 
  "sizeof(int) <= sizeof(longvoid*)"
); 

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

...

Static assertion is not available in C99.

Noncompliant Code Example

This non-compliant noncompliant 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.:

#include <assert.h>
 
struct timer {
  uint8_tunsigned char MODE;
  unsigned uint32_tint DATA;
  unsigned uint32_tint COUNT;
};
 
int mainfunc(void) {
  assert(offsetofsizeof(struct timer, DATA) == 4 sizeof(unsigned char) + sizeof(unsigned int) + sizeof(unsigned int));
}

While the Although the use of the runtime assertion is better than nothing, it needs to be placed in a function and executed, typically removed from . This means that it is usually far away from the definition of the actual structure to which it refers. The diagnostic occurs 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 a preprocessor conditional statement may be used, as in this compliant solution:

struct timer {
  unsigned uint8_tchar MODE;
  unsigned uint32_tint DATA;
  unsigned uint32_tint COUNT;
};

#if (offsetofsizeof(struct timer, DATA) != 4(sizeof(unsigned char) + sizeof(unsigned int) + sizeof(unsigned int)))
  #error DATA"Structure must benot athave offsetany 4padding"
#endif

Using #error directives allows for clear diagnostic messages. Because this approach evaluates assertions at compile time, there is no run-time runtime 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 portable compliant solution mimics the behavior of uses static_assert in a portable manner.:

#include <assert.h>
 
#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 {
  unsigned uint8_tchar MODE;
  unsigned uint32_tint DATA;
  unsigned uint32_tint COUNT;
};

static_assert(offsetofsizeof(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 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] to concatenate tokens. See \[[PRE05-A. Understand macro replacement when concatenating tokens]\] to understand how macro replacement behaves in C when using the {{\##}} operator.

sizeof(unsigned char) + sizeof(unsigned int) + sizeof(unsigned int),
              "Structure must not have any padding");

Static assertions allow incorrect assumptions to be diagnosed at compile time instead of resulting in a silent malfunction or runtime error. Because the assertion is performed at compile time, no runtime cost in space or time is incurred. An assertion can be used at file or block scope, and failure results in a meaningful and informative diagnostic error message.

Other uses of static assertion are shown in STR07-C. Use the bounds-checking interfaces for string manipulation and FIO34-C. Distinguish between characters read from a file and EOF or WEOF.

Risk Assessment

Static assertion is a valuable diagnostic tool for finding and eliminating software defects that may result in vulnerabilities at compile time. The absence of static assertions, however, does not mean that code is incorrect.

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Automated Detection

Related Vulnerabilities

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

References

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

Related Guidelines

 Bibliography

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Image Added Image Added Image AddedDCL02-A. Use visually distinct identifiers       02. Declarations and Initialization (DCL)       DCL04-A. Take care when declaring more than one variable per declaration