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What is the purpose of the #ifndef preprocessor directive in C ?

  • A.To undefine a macro
  • B.To include the following code only if the named macro is not already defined
  • C.To include a header file conditionally at run time
  • D.To define a macro with no value

Answer: B

Correct answer: To include the following code only if the named macro is not already defined

#ifndef tests whether a macro is undefined and, if so, compiles the block up to the matching #endif. Its commonest use is the include guard, which stops a header being processed twice in the same translation unit.

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