Macros

There are three basic phases for C programming. preprocessing, compiling, and linking. C input file is first passed to a preprocessing program called the preprocessor. The output of the preprocessor is passed to the C compiler. The output of the C compiler is then fed into a linker program. The linker finally produces an executable form of your program.

Macros C Preprocessor is just a text substitution tool and they instruct compiler to do required pre- processing before actual compilation. Preprocessing is done before actual compilation begins. All preprocessor commands begin with a pound symbol (#).

Macros There are essentially three uses of the preprocessor--directives, constants, and macros. Directives are commands that tell the preprocessor to skip part of a file, include another file, or define a constant or macro. All other uses of the preprocessor involve processing #define'd constants or macros. Typically, constants and macros are written in ALL CAPS to indicate they are special.

Macros Line that begin with # are called preprocessing directives. Major kinds of preprocessor directives: File inclusion Macro definition Conditional compilation

Macros File inclusion Used to include a file Syntax # include # include “filename” The difference between <> and “”  looks firstly in the header path for the header file  whereas "" looks in the current directory of the file for the header.

Macros Macro Definition A macro is a fragment of code which has been given a name. Whenever the name is used, it is replaced by the contents of the macro. The #define directive specifies a macro identifier and a replacement list, and terminates with a new-line character. The #undef directive is used to cancel a definition for a macro.

Macros Macro Definition syntax #define identifier replacement-list Eg #include #define MACRO 25 Void main() { printf("\nMACRO Defined with value %d", MACRO); } Output MACRO Defined with value 25 (MACRO will be replaced to 25)

Macros Macro Definition Examples of simple macros: #define N 100 #define PI #define WARNING "Warning: nonstandard feature" #define BEGIN { #define END } void main() BEGIN // replace { int a[N];// replace int a[100] printf(“\nvalue of Pie %f”,PI);// print printf(“%s”,WARNING); END // replace }

Macros Parameterized Macro Function-like macros can take arguments, just like functions. Insert parameters between the pair of parentheses in the macro definition. To invoke a macro that takes arguments, ie the name of the macro followed by a list of actual arguments in parentheses, separated by commas. The number of arguments give must match the number of parameters in the macro definition. When the macro is expanded, each use of a parameter in its body is replaced by the corresponding argument.

Macros Eg #define MIN(X, Y) ((X) < (Y) ? (X) : (Y)) printf(“minimum %d”,MIN(a, b)); ==> x = ((a) < (b) ? (a) : (b)); # define EVEN(N) (N%2==0) if(EVEN(2)==1) { printf(“even”); }

Macros Conditional Compilation The conditional directives are: – #if - Test if a compile time condition is true – #else - The alternative for #if – #ifdef - If this macro is defined – #ifndef - If this macro is not defined – #elif - #else an #if in one statement – #endif - End preprocessor conditional

Macros Conditional Compilation #if defined(CREDIT) credit(); #elif defined(DEBIT) debit(); #else printerror(); #endif The function call to credit is compiled if the identifier CREDIT is defined. If the identifier DEBIT is defined, the function call to debit is compiled. If neither identifier is defined, the call to printerror is compiled. Note thatCREDIT and credit are distinct identifiers in C and C++ because their cases are different.