The Preprocessor #include #define N 10 C program Preprocessor Modified C program Preprocessor Object codes

Caution The preprocessor has only a limited knowledge of C. As a result, it’s quite capable of creating illegal programs as it executes directives.

Preprocessing Directives Macro definition: #define to defines a macro; the #undefine to removes a macro definition. File inclusion: #include causes the content of a specified file to be included in a program. Conditional compilatoin: #if, #ifdef, #ifndef, #elif, #else and #endif allow blocks of text to be either included in or excluded from a program, depending on conditions that can be tested by the preprocessor.

Macro Definitions Simple Macros: #define identifier replacement-list #define N = 100 …. int a[N]; #define N 100; …. int a[N];

Simple Macro Simple macros are primarily used for defining “manifest constants”: giving names to numeric, character, and string values: #define STR_LEN 80 #define TRUE 1 #define FALSE 0 #define PI #define CR ‘\r’ #define EOS ‘\0’ #define MEM_ERR “Error: not enough memory”

Simple Macro---advantages It makes programs easier to read It makes programs easier to modify It helps avoid inconsistencies and typographical errors Making minor changes to the syntax of C Renaming types: #define BOOL int Controlling conditional compilation

Parameterized Macros #define identifier(x1, x2, …, xn ) replacement-list #define MAX(x, y) ((x) > (y) ? (x) : (y)) #define IS_EVEN(n) ((n)%2 == 0) #define TOUPPER(c) (‘a’<=(c)&&(c)<=‘z’?(c) –’a’+’A’:(c))

Parameterized Macros--- advantages The program may be slightly faster Macros are “generic”

Parameterized Macros--- disadvantages The compiled code will often be larger n = MAX(I, MAX(j, k)); n = ((i)>(((j)>(k)?(j):(k)))?(i):(((j)>(k)?(j):(k)))); Arguments aren’t type-checked It’s not possible to have a pointer to a macro A macro may evaluate its arguments more than once n = MAX(i++, j); n = ((i++) > (j) ? (i++) : (j))

Conditional Compilation The #if and #endif Directives #define DEBUG 1 #if DEBUG printf(“Value of i: %d\n”, i); #endif

Writing Large Programs Source file **.c Header file **.h #include : search the directory in which system header files reside(on UNIX, system header files are kept in the directory /usr/include #include “filename”: search the current directory #include “c:\cprogs\utils.h” /* windows path */ #include “/cprogs/utils.h” /* UNIX path */

Pointer as Arguments boolean.h #define BOOL int #define TRUE 1 #define FALSE 0 src1.c #include “boolean.h” src2.c #include “boolean.h”

Sharing Function Prototypes stack.h void make_empty(void); int is_empty(void); int is_full(void); void push(int i); int pop(void); stack.c int contents[100]; Int top = 0; void make_empty(void) { ……. } int is_empty(void); { ……. } int is_full(void); void push(int i); int pop(void); cal.c #include “stack.h” int main(void) { make_empty(); …….. }

Sharing Variable Declaration External variables can be shared among files in much the same way functions are. We can put its definition in one source file, then put declarations in other files that need to use the variable. int i; /* declares i and defines it as well */ extern int i; /* declares i without defining it */ extern works with variables of all types.

Be careful When declarations of the same variable appear in different files, the compiler can’t check that the declaration match the variable’s definition. int i; extern long i;

Protecting Header Files Header1.h #include “Header1.h” Header2.h Header3.h #include “Header2.h” #include “Header3.h”

Protecting Header Files stack.h #ifndef STACK_H #define STACK_H void make_empty(void); int is_empty(void); int is_full(void); void push(int i); int pop(void); #endif

Building a Multiple-File Program Compiling: each source file in the program must be compiled separately. the compiler generates a file containing object code:.o in UNIX or.obj in Windows Linking: the linker conbines the object files to produce an executable file. Most compiler supports a simpler way: gcc –o justify justify.c line.c word.c

Makefiles justify: justify.o word.o line.o gcc –o justify justify.o word.o line.o justify.o: justify.c word.h line.h gcc –c justify.c word.o: word.c word.h gcc –c word.c line.o: line.c line.h gcc –c line.c

Makefiles Each command in a makefile must be preceded by a tab character, not a space. A makefile is normally stored in a file called Makefile (or makefile) To invoke make, use the command make target If no target is specified when make is invoked, it will build the target of the first rule.