There are a handful more preprocessor commands which can largely be ignored by the beginner. They are commonly used in "include" files to make sure that things are not defined twice.

NOTE : ‘true’ has any non zero value in C. ‘false’ is zero. 

#undef         This undefines a macro, leaving the name free.

#if                 This is followed by some expression on the same line. It allows
                      conditional compilation. It is an advanced feature which can be

                      used to say: only compile the code between ‘#if’ and ‘#endif’
                       if the value following ‘#if’ is true, else leave out that code altogether. This is      Different from not executing code—the code will not even be compiled.

#ifdef     This is followed by a macro name. If that macro is defined then
                this is true.



#ifndef   This is followed by a macro name. If that name is not defined
                then this is true.

#else      This is part of an #if, #ifdef, #ifndef preprocessor statement.

#endif     This marks the end of a preprocessor statement.

#line        Has the form:

#line         constant ‘filename’This is for debugging mainly. This statement causes the com-
                  piler to believe that the next line is line number (constant) and
                  is part of the file (filename).

#error        This is a part of the proposed ANSI standard. It is intended for
                   debugging. It forces the compiler to abort compilation.

Read more about Prerocessor :-  Preprocessor

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