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Preprocessor directives in c language
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- 2. Definition A preprocessoris a program that processes our source program before it is passed to the compiler. The preprocessor works on the source code and creates “expanded source code”. The preprocessor offers several features called preprocessor directives. Each of theses preprocessor directives begin with a # symbol. The directives can be placed anywhere in a program but most often placed at the beginning of the program,before the function definition.
- 3. Preprocessor directives areexecuted before compilation.
- 4. The variouspreprocessor directives are as follows: 1)Macro Expansion (#define) II)File Inclusion(#include) III)Conditional Compilation(#ifdef -#endif) IV)Miscellaneous Directives (#undef,#pragma-startup,exit,warn)
- 5. Macro Expansion A“macro” is a fragment of code which has been given a name. Wherever the name is used,it is replaced by the contents of the macro. There are two kinds of macros which are as follows: I)Object-like macros resemble data objects II)Function-like macros resemble function calls.
- 6. Object –like Macros The object-like macro is an identifier that is replaced by value. It is widely used to represent numeric constants. It is called object-like because it looks like a data object in code that uses it. Syntax: #define identifier replacement-text
- 7. Examples #define x 4x=identifier 4=replacement-text void main() { int a; a=x; } #define pi 3.14 pi=identifier 3.14=substitute void main() { float r=6.25,area; area=pi*r*r; printf(“Area of circle=%f”,area); }
- 8. Note: To createmacros with the #define directive,the syntax for macros is: #define macro macro expansion replacement list #define directive can be used as follows: I) It can be used to define operators like: #define AND && #define OR ||
- 9. II)it could beused even to replace a condition: # define AND && #define ARRANGE (a>25 AND a<50) III) It could be used to replace even an entire C statement like: # define FOUND printf(“Hello World”)
- 10. Function-like Macro Itlooks like a function call & can be defined with the # define directive but with a pair of braces or parenthesis immediately after the macro name. For Example: #define Area(x) 3.14* r*r
- 11. Macro with arguments Function –like macros can take arguments just like true functions.
- 12. Difference between Macro& Function I)In a macro call,the preprocessor replaces the macro template with its macro exapansion only.whereas, in a functional call the control is passed to a function along with certain arguments,some calculations are performed in the function& a useful value is returned back from the function.
- 13. II) ifwe use a macro hundred times in a program,the macro expansion goes into our source code at hundred different palaces,thus increasing the program size. On the other hand,if a function is used,then even if it is called from hundred different places in the program,it would take the same amount of space in the program,thus making the prigram smaller and compact.
- 14. File Inclusion Usedto include one file into another file or it simply causes the entire content of one file to be inserted into the source code of another file. We use file inclusions for the following reasons: I) if we have a very large program, the code is best divided into several different files,each containing a set of related functions.These files are then included at the beginning of main program file.
- 15. II)There are somecommonly needed functions & macro definitions that can be stored in a file &that file can be included in every program we write which would add all the statements in this file to our program as if we have typed in. The two ways to write include statement: #include “filename”
- 16. #include “goto.h”:It searches for a file named “goto.h” in the directory containing the current file. #include<goto.h>: it searches for a file named “goto.h” in a standard list of system directories.
- 17. Undefining a Macro The #undef preprocessor directive is used to undefine the constant or macro defined by #define. Syntax: Example:
- 18. Preprocessor Operators TheC preprocessor offers the following operators to help create macros − I)The Macro Continuation () Operator A macro is normally confined to a single line. The macro continuation operator () is used to continue a macro that is too long for a single line. For example −
- 19. II)The Stringize (#)Operator The stringize or number-sign operator ('#'), when used within a macro definition, converts a macro parameter into a string constant. This operator may be used only in a macro having a specified argument or parameter list. For example − # define say(m) printf(#m) void main() { clrscr(); Output: say(Hello); Hello getche(); }
- 20. III)The Token Pasting(##) Operator The token-pasting operator (##) within a macro definition combines two arguments. It permits two separate tokens in the macro definition to be joined into a single token. For example −
- 21. When the abovecode is compiled and executed, it produces the following result − It happened so because this example results in the following actual output from the preprocessor −
- 22. Conditional Compilation Theseallow us to include certain portion of the code depending upon the output of constant expression. The #ifdef preprocessor directive checks if macro is defined by #define. If yes, it executes the code otherwise #else code is executed, if present. Syntax:
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- 24. #ifndef directive The#ifndef preprocessor directive checks if macro is not defined by #define. If yes, it executes the code otherwise #else code is executed, if present. Syntax:
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- 26. #pragma directive The#pragma preprocessor directive is used to provide additional information to the compiler. The #pragma directive is used by the compiler to offer machine or operating-system feature. Syntax:
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- 28. Syntax: #pragma startup<function_name> startup pragma allow the program to specify function(s) that should be called upon program startup Function is called before main(). Syntax : #pragma exit <function_name> exit pragma allow the program to specify function(s) that should be called just before the program terminates through _exit. Function is called after main(). <function_name> must be a previously declared function that takes no arguments and returns void. It should be declared as – void func(void); The function name must be defined (or declared) before the pragma line is reached.
- 29. # error directive The #error preprocessor directive indicates error. The compiler gives fatal error if #error directive is found and skips further compilation process.
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