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 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
Recommendation |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
PRE05-A |
low |
unlikely |
medium |
P1 |
L2 |
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