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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

Wiki MarkupThe {{\##}} 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 \[ [FSF 2005|AA. Bibliography#FSF 05]\].

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

Wiki MarkupParameters 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|AA. Bibliography#FSF 05]\].

Noncompliant Code Example

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#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|AA. Bibliography#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 noncompliant:

...

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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"

Bibliography

Wiki Markup\[[FSF 2005|AA. Bibliography#FSF 05]\] Section 3.4, "[Stringification|http://gcc.gnu.org/onlinedocs/cpp/Stringification.html]" and Section 3.5, "[Concatenation|http://gcc.gnu.org/onlinedocs/gcc-4.3.0/cpp/Concatenation.html#Concatenation]" \
[[Saks 2008|AA. Bibliography#Saks 08]\]

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      01. Preprocessor (PRE)      PRE06-C. Enclose header files in an inclusion guard