Preprocessor C program → Modified C program → Object Code All preprocessor directives or commands begin with a #. E.g. #include <stdio.h> C program → Modified C program → Object Code Can appear anywhere in the program No “;” in the end preprocessor compiler

Preprocessor Directives Macro definition #define, #undef File inclusion #include Conditional Compilation #if, #ifdef, #ifndef, #elseif, #else Others

#define To define constants or any macro substitution.     #define <macro> <replacement name>   E.g.   #define FALSE 0 #define TRUE !FALSE To undefined a macro. E.g. #undef FALSE #define D 2 printf("%d",D); #undef D int D=6;

Define Functions E.g. To get the maximum of two variables: #define max(A,B) A > B ? A:B In the C code:    x = max(q+r,s+t);

File inclusion #include directive Why need file inclusion Include the contents of another file at the point where the directive appears. Why need file inclusion Use built-in functions E.g., printf(), rand(); Reuse code double mean(int x,int y) { return (x+y)/2; } double mean(int ,int ); #include “test.h” printf("%f",mean(3,5)); test.cpp test.h Your code

File inclusion formats #include <file> Used for system header files File contain function prototypes for library functions <stdlib.h> , <math.h> , etc #include "file" Used for header files of your own program looks for a file in the current directory first then system directories if not found

Custom header files Steps for creating custom header files Advantage Create a file with function prototypes Save as a .h file. E.g.: filename.h Load in other files with #include "filename.h" Advantage Reuse functions and data structure declaration

Conditional compilation Useful when machine-dependencies setting certain options at compile-time. Expressions #if expression #ifdef expression (same as #if defined ) #ifudef #elif and #else #endif

Examples Example: write programs that are portable to several machines or operating systems #if defined(WINDOWS) #elif defined(LINUX) #elif defined(SOLARIS) #endif Example: Providing a default definition for a macro #ifndef BUFFER_SIZE #define BUFFER_SIZE 256

int inline func(void); Inline functions Ret_Type inline func_name(params); e.g. int inline func(void); Inline hints to the compiler that this function should be unrolled an inlined into wherever it is called. This can avoid function call overhead. Problems with inline: The compiler may happily ignore this. It’s kind of like the register keyword in this manner. It is merely a hint; it is not a command. You may (and probably will) make your program size larger. Read the previous bullet as “cache miss” or “page fault”.