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 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
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 the constant expression compares unequal to 0, the declaration has no effect. Otherwise, the constraint is violated and the implementation shall produce a diagnostic message 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 will be output at compile time.
Static assertion is not available in C99.
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 to behave correctly:
Although the use of the runtime assertion is better than nothing, it needs to be placed in a function and executed. 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.
For assertions involving only constant expressions, a preprocessor conditional statement may be used, as in this compliant solution:
#error directives allows for clear diagnostic messages. Because this approach evaluates assertions at compile time, there is no runtime penalty.
This portable compliant solution uses
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.
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.
|Axivion Bauhaus Suite
|Users can implement a custom check that reports uses of the
Could detect violations of this rule merely by looking for calls to
|LDRA tool suite
|C++ Secure Coding Standard
|VOID DCL03-CPP. Use a static assertion to test the value of a constant expression
|Subclause 6.7.10, "Static Assertions"