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 05|AA. Bibliography#FSF 05]\macros, 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 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

Wiki MarkupParameters 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 05|AA. Bibliography#FSF 05]\FSF 2005].

Noncompliant Code Example

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]

Wiki Markup{{\_\_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 \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\].macro name that expands to an integer constant representing the presumed line number of the current source line within the current source file. If the intention is to expand the __LINE__ macro, which is likely the case here, the following definition for JOIN() is noncompliant :

#define JOIN(x, y) x ## y

because the __LINE__ is not expanded, and the character array is subsequently named assertion_failed_at_line___LINE__.:

#define JOIN(x, y) x ## y

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

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This example is noncompliant if the programmer's intent is to expand the macro before stringification:

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

str(foo)

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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. 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.

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Image Added Image Added Image Added.html#Concatenation]" \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] 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" \[[Saks 08|AA. Bibliography#Saks 08]\]Image Removed      01. Preprocessor (PRE)      PRE06-C. Enclose header files in an inclusion guard