It is often useful to merge two tokens into one while expanding macros. This is called token pasting or token concatenation. The ## preprocessing operator performs token pasting. 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.
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 to [[PRE01-A. Use parentheses within macros around parameter names]].
Risk Assessment
Recommendation |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
DCL03-A |
1 (low) |
1 (unlikely) |
1 (high) |
P1 |
L3 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
[[FSF 05]] Section 3.5, "Concatenation
"
[Saks 08] Dan Saks, Stephen C. Dewhurst. Presentation. Sooner Rather Than Later: Static Programming Techniques for C++.
[[ISO/IEC 9899-1999]] Section 6.10.3, "Macro replacement," Section 6.10.3.3, "The ## operator," and Section 6.10.3.4, "Rescanning and further replacement"
PRE04-A. Do not reuse a standard header file name 01. Preprocessor (PRE) PRE06-A. Enclose header file in an inclusion sandwich