In a C/C++ program, all lines that start with ‘#’ are processed by preporcessor which is a special program invoked by the compiler. In a very basic term, preprocessor takes a C/C++ program and produces another C program without any ‘#’.


When we use #include directive, the contents of included header file (after preprocessing) are copied to the current file.
Angular brackets < and > instruct the preprcessor to look in the standard folder where all header files are held. Double quotes “ and “ instruct the preprcessor to look into the current folder and if the file is not present in current folder, then in standard folder of all header files.


When we use #define for a constant, the preprocessor produces a C program where the defined constant is searched and matching tokens are replaced with the given expression. For example in the following program max is defined as 100.


Macro functions

The macros can take function like arguments, the arguments are not checked for data type. For example, the following macro INCREMENT(x) can be used for x of any data type.


Macro expansion

The macro arguments are not evaluated before macro expansion. For example consider the following program.


Macro concatenation

The tokens passed to macros can be concatenated using operator ## called Token-Pasting operator.


Macro strings

A token passed to macro can be converted to a sting literal by using # before it.


Multiline Macros

The macros can be written in multiple lines using ‘\’. The last line doesn’t need to have ‘\’.


Macro Argument Problems

The macros with arguments should be avoided as they cause problems sometimes. And Inline functions should be preferred as there is type checking parameter evaluation in inline functions. From C99 onward, inline functions are supported by C language also. For example consider the following program. From first look the output seems to be 1, but it produces 36 as output.


If we use inline functions, we get the expected output. Also the program given above can be corrected using inline functions.




Preprocessors also support if-else directives which are typically used for conditional compilation.



A header file may be included more than one time directly or indirectly, this leads to problems of redeclaration of same variables/functions. To avoid this problem, directives like defined, ifdef and ifndef are used.


Some standard MACROS

There are some standard macros which can be used to print program file (__FILE__), Date of compilation (__DATE__), Time of compilation (__TIME__) and Line Number in C code (__LINE__).


Suggested Reading

  1. Difference between macro and inline
  2. Difference between typedef and #define