How a Preprocessor works in C? Last Updated : 08 Feb, 2018 Comments Improve Suggest changes Like Article Like Report Compiling a C program - Behind the Scene A Preprocessor is a system software (a computer program that is designed to run on computer's hardware and application programs). It performs preprocessing of the High Level Language(HLL). Preprocessing is the first step of the language processing system. Language processing system translates the high level language to machine level language or absolute machine code(i.e. to the form that can be understood by machine). The preprocessor doesn't know about the scope rules of C. Preprocessor directives like #define come into effect as soon as they are seen and remain in effect until the end of the file that contains them; the program's block structure is irrelevant. A Preprocessor mainly performs three tasks on the HLL code : Removing comments : It removes all the comments. A comment is written only for the humans to understand the code. So, it is obvious that they are of no use to a machine. So, preprocessor removes all of them as they are not required in the execution and won't be executed as well. This is how to see a file with removed comments in linux) : Write a C code (let the file name be prog.c). Preprocess it using the command gcc -E prog.c You will see the output with the code having no comments. This file is saved with a '.i' extension (prog.i) which will be input to the compiler. File inclusion : Including all the files from library that our program needs. In HLL we write #include which is a directive for the preprocessor that tells it to include the contents of the library file specified. For example, #include will tell the preprocessor to include all the contents in the library file stdio.h. This can also be written using double quotes - #include "stdio.h" Note: If the filename is enclosed within angle brackets, the file is searched for in the standard compiler include paths. If the filename is enclosed within double quotes, the search path is expanded to include the current source directory. Macro expansion : Macros can be called as small functions that are not as overhead to process. If we have to write a function (having a small definition) that needs to be called recursively (again and again), then we should prefer a macro over a function. So, defining these macros is done by preprocessor. #define SI 1000is a simple example of a macro. There are two types of macros: Object-like (do not take parameters) and function-like (Can take parameters) // object-like macro #define // function-like macro #define () You can delete a macro definition with #undef: // delete the macro # undef We can write multi-line macro same like function, but each statement ends with “\”. CPP #include <stdio.h> #define MACRO(num, str) {\ printf("%d", num);\ printf(" is");\ printf(" %s number", str);\ printf("\n");\ } Comment More infoAdvertise with us Next Article Header Files in C D diksha Improve Article Tags : C Language Similar Reads C Preprocessors Preprocessors are programs that process the source code before the actual compilation begins. They are not part of the compilation process but operate separately, allowing programmers to modify the code before compilation. It is the first step that the C source code goes through when being converted 8 min read C Preprocessor Directives In C programming, the preprocessor is a program that process the source code before the actual compilation begins. It uses preprocessor directives are commands that instruct the preprocessor to perform specific actions. These directives start with the # symbol.List of Preprocessor DirectivesThe foll 6 min read How a Preprocessor works in C? Compiling a C program - Behind the Scene A Preprocessor is a system software (a computer program that is designed to run on computer's hardware and application programs). It performs preprocessing of the High Level Language(HLL). Preprocessing is the first step of the language processing system. Lan 3 min read Header Files in C In C programming, a header file is a file that ends with the .h extension and contains features like functions, data types, macros, etc that can be used by any other C program by including that particular header file using "#include" preprocessor.C language uses header files to provide the standard 5 min read What’s difference between header files "stdio.h" and "stdlib.h" ? In C programming, standard header files provide various inbuilt functionalities and two of the most commonly used standard header files are stdio.h and stdlib.h. The <stdio.h> provides Standard Input Output tools such as printf(), scanf(), etc while <stdlib.h> provides some commonly used 4 min read How to write your own header file in C? As we all know that files with .h extension are called header files in C. These header files generally contain function declarations which we can be used in our main C program, like for e.g. there is need to include stdio.h in our C program to use function printf() in the program. So the question ar 4 min read Macros and its types in C In C programming, a macro is a symbolic name or constant that represents a value, expression, or code snippet. They are defined using the #define directive, and when encountered, the preprocessor substitutes it with its defined content.ExampleC#include <stdio.h> // Macro definition #define LIM 4 min read Interesting Facts about Macros and Preprocessors in C In a C program, all lines that start with # are processed by preprocessor which is a special program invoked by the compiler. by this we mean to say that the ‘#’ symbol is used to process the functionality prior than other statements in the program, that is, which means it processes some code before 5 min read # and ## Operators in C In C, # and ## operators are preprocessor operators using in macros for token manipulation. They are known as stringizing and token pasting operators and are used in macro definition with #define preprocessor. In this article, we will learn about these operators and how to use them in C programs.Str 3 min read How to print a variable name in C? Printing a variable name means printing the identifier that is assigned to the variable. To print it, it should be in the form of string. This can be done by using stringification.Stringification in C is the method to convert the argument of a function like macro to a string. This can be done with t 1 min read Like