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 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 ## 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 2005].
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The following definition for static_assert() from 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__.:
| Code Block |
|---|
#define static_assert(e) \
typedef char JOIN(assertion_failed_at_line_, __LINE__) \
[(e) ? 1 : -1]
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Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
| CERT C++ Secure Coding Standard | PRE05-CPP. Understand macro replacement when concatenating tokens or performing stringification |
Bibliography
| [FSF 2005] | Section 3.4, "Stringification" Section 3.5, "Concatenation" |
| [Saks 2008] |
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