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 , which 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 that 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 a ## 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 can be used 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__.:

#define static_assert(e) \
  typedef char JOIN(assertion_failed_at_line_, __LINE__) \
    [(e) ? 1 : -1]

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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-C-EX2, to PRE01-C. Use parentheses within macros around parameter names.

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

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

The macro invocation xstr(foo) expands to 4 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.

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