C Programming Preprocessor Directives

C Preprocessor Directives: Purpose and Usage

• Preprocessor directives are commands processed before code compilation.
• They improve code flexibility, readability, and maintainability.
• Enhance code reusability.
• Enable conditional compilation for different environments.
• Allow efficient handling of compile-time errors and warnings.
• Contribute to improved line control for debug and error reporting.

Common Preprocessor Directives

• #include: Inserts the content of a file into the current file.
  • #include <stdio.h>: Includes the standard input/output library
  • #include "myheader.h": Includes a user-defined header file
• #define: Defines macros for constants or function-like macros.
  • #define PI 3.14159: Defines a constant macro for π.
  • #define SQUARE(x) ((x) * (x)): Defines a function-like macro to square a value.
• #undef: Removes a previously defined macro.
  • #undef PI: Undefines the macro PI.
• #if, #elif, #else, #endif: Conditionality for code compilation based on defined conditions.
  • #define DEBUG 1
  • #if DEBUG
    • printf("Debug mode\n");
  • #elif RELEASE
    • printf("Release mode\n");
  • #else
    • printf("Unknown mode\n");
  • #endif
• #ifdef, #ifndef: Compiles code based on whether a macro is defined or not.
  • #define FEATURE_ENABLED
  • #ifdef FEATURE_ENABLED
    • printf("Feature is enabled\n");
  • #endif
  • #ifndef FEATURE_DISABLED
    • printf("Feature is not disabled\n");
  • #endif
• #error: Produces a compile-time error message.
  • #ifdef WRONG_CONDITION
    • #error "This condition is not allowed"
  • #endif

Example in a C Program

• Demonstrates practical use of preprocessor directives.
• Includes the standard library header file (stdio.h).
• Defines constant and function-like macros for calculations (PI and SQUARE(x)).
• Example demonstrates conditional compilation using the DEBUG_MODE macro.

Keywords Used in C Programming

• These are predefined words in C language that cannot be used as variable, function, or label names.
• Keywords are reserved words in C, they have special meaning for the compiler and they cannot be used for anything else.
• Some common keywords:
  • auto, double, int, struct, break, else, long, switch, case, enum, register, typedef, char, extern, return, union, const, float, short, unsigned, continue, for, signed, void, default, goto, sizeof, volatile, do, if, static, while

C++ Keywords

• C++ has 31 additional key words compared to C.

Identifiers in C Programming

• Used for naming variables, functions, and arrays.
• Must begin with a letter or an underscore (_).
• Can consist of letters, digits, and underscores.
• Cannot contain whitespace.
• Maximum length is 31 characters.
• Cannot be the same as keywords.
• Statement labels are a special type, used in goto statements.

Constants in C

• Similar to variables but with a fixed value that cannot be changed during program execution.

• Different types of constants:

  • Integer Constants: 0, 1, 2586, 201987.
  • Real (or) Floating point constants: 0.0, 153.56, 24896.32415.
  • Octal & Hexadecimal Constants: (octal: (013)8 = (11)10 ), (hexadecimal: (013)16 = (19)10).
  • Character constants: 'a', 'A', 'd'.
  • String constants: "Freshersnow".

• Two Ways to Define Constants in C:

  • Using the const keyword: const float PI = 3.14;
  • Using the #define preprocessor: #define PI 3.14