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]]

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