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Assertions are a valuable diagnostic tool for finding and eliminating software defects that may result in vulnerabilities (see MSC11-C. Incorporate diagnostic tests using assertions). 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 useful for identifying incorrect assumptions and not for runtime error checking. As a result, runtime assertions are generally unsuitable for server programs or embedded systems.unmigrated-wiki-markup

Static assertion is a new facility in the C+\+ 0X draft standard \[[Becker 08|AA. C References#Becker 08]\] and takes the form: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-expression is true, nothing will happen. However, if constant-expression is false, an error message containing string-literal) is issued will be output at compile time. For example

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

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

Static assertion is not available in C99, but the facility is being considered for inclusion in C1X by the ISO/IEC WG14 international standardization working group.

Noncompliant Code Example

This 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 {
  unsigned uint8_tchar MODE;
  uint32_tunsigned int DATA;
  uint32_tunsigned int COUNT;
};
 
int func(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 . This means that it is usually far away from the definition of the actual structure to which it refers. The diagnostic occurs only at runtime and only if the code path containing the assertion is executed.

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For assertions involving only constant expressions, some implementations allow the use of a preprocessor conditional statement may be used, as in this examplecompliant solution:

struct timer {
  uint8_tunsigned char MODE;
  uint32_tunsigned int DATA;
  uint32_tunsigned int COUNT;
};

#if (offsetofsizeof(struct timer, DATA) != 4 (sizeof(unsigned char) + sizeof(unsigned int) + sizeof(unsigned int)))
  #error "DATAStructure 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 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) \
typedef char JOIN(assertion_failed_at_line_, __LINE__) [(e) ? 1 : -1]

struct timer {
  unsigned uint8_tchar MODE;
  unsigned uint32_tint DATA;
  uint32_tunsigned int COUNT;
};

static_assert(offsetofsizeof(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.

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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 TR 24731 for remediation of existing the bounds-checking interfaces for string manipulation code and FIO35FIO34-C. Use feof() and ferror() to detect end-of-file and file errors when sizeof(int) == sizeof(char)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.

Automated Detection

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Related Vulnerabilities

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

Other Languages

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Related Guidelines

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 Bibliography

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References

\[[Becker 08|AA. C References#Becker 08]\] 
\[[Eckel 07|AA. C References#Eckel 07]\]
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10.1, "Conditional inclusion," and Section 6.10.3.3, "The ## operator," and Section 7.2.1, "Program diagnostics"
\[[Klarer 04|AA. C References#Klarer 04]\]
\[[Saks 05|AA. C References#Saks 05]\]
\[[Saks 08|AA. C References#Saks 08]\]

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