PRE05-A. Understand macro replacement when concatenating tokens or performing stringification

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 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 replaces the ## and the two original tokens in the macro expansion [[FSF 05]].

Token pasting is most useful when one or both of the tokens comes from a macro argument. If either of the tokens next to an ## 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 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 05]].

Non-Compliant Code Example

The following definition for static_assert() from DCL03-A. 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 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 non-compliant:

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

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

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, they are 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-EX2 to PRE01-A. Use parentheses within macros around parameter names.

Non-Compliant Code Example

This example is non-compliant if the programmer's intent is to expand the macro before stringification:

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

str(foo)

The macro invocation str(foo) expands to "foo".

Compliant Solution

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

#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

Related Vulnerabilities

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

References

[[FSF 05]] Section 3.4, "Stringification; and Section 3.5, "Concatenation"
[[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"
[[Saks 08]] Dan Saks, Stephen C. Dewhurst. Presentation. Sooner Rather Than Later: Static Programming Techniques for C++.

PRE04-A. Do not reuse a standard header file name      01. Preprocessor (PRE)       PRE06-A. Enclose header files in an inclusion guard