It is necessary to understand how macro replacement works in C, particularly in the context of concatenating tokens using the ## operator and converting macro parameters to strings using the # operator.

Concatenating Tokens

The ## preprocessing operator is used to merge two tokens into one while expanding macros. This , which is called token pasting or token concatenation. When a macro is expanded, the two tokens on either side of each ## operator are combined into a single token , which that replaces the ## and the two original tokens in the macro expansion [FSF 2005].

Token pasting is most useful when one or both of the tokens come from a macro argument. If either of the tokens next to an a ## is a parameter name, it is replaced by its actual argument before ## executes. The actual argument is not macro expanded first.

Stringification

Parameters are not replaced inside string constants, but you can use the # preprocessing operator can be used instead. When a macro parameter is used with a leading #, the preprocessor replaces it with the literal text of the actual argument , converted to a string constant [FSF 2005].

...

The following definition for static_assert() from recommendation DCL03-C. Use a static assertion to test the value of a constant expression uses the JOIN() macro to concatenate the token assertion_failed_at_line_ with the value of __LINE__.:

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

__LINE__ is a predefined macro names which expands name that expands to an integer constant representing the presumed line number of the current source line within the current source file [ISO/IEC 9899:1999]. If the intention is to expand the __LINE__ macro, which is likely the case here, the following definition for JOIN() is noncompliant :

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

Because because the __LINE__ is not expanded, and the character array is subsequently named assertion_failed_at_line___LINE__.:

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

 Compliant Compliant Solution

To get the macro to expand, a second level of indirection is required, as shown by this compliant solution:

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

JOIN(x, y) calls JOIN_AGAIN(x, y) so that , if x or y is a macro, it is expanded before the ## operator pastes them together.

Note also that macro parameters cannot be individually parenthesized when concatenating tokens using the ## operator, converting macro parameters to strings using the # operator, or concatenating adjacent string literals. This is an exception, PRE01-C-EX2, to recommendation PRE01-C. Use parentheses within macros around parameter names.

...

#define str(s) #s
#define foo 4

str(foo)

...

To stringify the result of expansion of a macro argument, you must use two levels of macros must be used:

#define xstr(s) str(s)
#define str(s) #s
#define foo 4

The macro invocation xstr(foo) expands to 4. This is because s is stringified when it is used in str(), so it is not macro expanded first. However, s is an ordinary argument to xstr(), so it is completely macro expanded before xstr() is expanded. Consequently, by the time str() gets to its argument, it has already been macro expanded.

Risk Assessment

Automated Detection

Related Vulnerabilities

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

Related Guidelines

...

...

ISO/IEC 9899:1999 Section 6.10.3, "Macro replacement," Section 6.10.3.3, "The ## operator," Section 6.10.3.2, "The # operator," Section 6.10.3.4, "Rescanning and further replacement," and Section 6.10.8, "Predefined macro names"

Bibliography

...

...

Image Removed      01. Preprocessor (PRE)      PRE06-C. Enclose header files in an inclusion guardImage Added Image Added Image Added