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====== C Macro Expansion Rules ======

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|desc=Introduce macro expansion rules in **C**.}}

  * ''#''  convert macro parameter to string.
  * ''##''  combine multiple tokens into one token.
  * Nested macro expansion rules
      * The preprocessor will first process macro parameters until they are expanded to non-macro parameters or blocked by ''# ''or ''##''.
      * The expansion of macro parameters will be blocked by ''#''  or ''##''. That is, the expansion of macro parameters will stop and then the pre-processor will use the expanded macro parameters to expand the macro.

There are some examples to help understanding.

==== Example 1 ====

<codeprism lang=c>
#define str(a) #a #define foo(a, b) a ## b

str(x)
   > "x"

foo(x, y)
   > xy
</codeprism>

{{code:c:screenshot_from_2023-02-22_22-49-32.png?direct&615x436}}

==== Example 2 ====

<codeprism lang=c>
#define str(_a) #_a
#define A a
#define B b
#define foo(_a, _b) _a##_b
#define foo2(_a, _b) foo(_a, _b)

foo2(A, B)
   > foo2(a, b)
       > ab

foo(A, B)
    # Because A and B are arguments to foo, the extension of A and B are blocked by ##.
   > AB

str(foo2(A, B))
    # Because foo2 is an argument to foo, the extension of foo2 is blocked by #.
   > "foo2(A, B)"


</codeprism>

{{code:c:screenshot_from_2023-02-22_22-42-02.png?direct&624x468}}

==== Example 3 ====

<codeprism lang=c>
#define A a
#define B b
#define foo(_a, _b) _a##_b _a

foo(A, B)
   > AB A
       > AB a


</codeprism>

{{code:c:screenshot_from_2023-02-22_22-59-46.png?direct&634x251}}