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C

C Preprocessor

Parameterized macros

int square(int x) {
  return x * x;
}

The macro rewrites the above code as follows:

#define square(x) ( (x) \* (x) )

No spaces are allowed between the macro name and the opening parenthesis

#include <stdio.h>
#define MAX(x,y) ( (x) > (y) ? (x) : (y) )
int main(void) {
  printf("Max between 20 and 10 is %d\n", MAX(10, 20));
  return 0;
}

defined() operator

#include <stdio.h>
#if !defined (MESSAGE)
   #define MESSAGE "You wish!"
#endif
int main(void) {
  printf("Here is the message: %s\n", MESSAGE);
  return 0;
}

tag paste operator ()

#include <stdio.h>
#define tokenpaster(n) printf ("token" #n " = %d", token##n)
int main(void) {
  int token34 = 40;
  tokenpaster(34);
  return 0;
}

String Constantization Operator ()

#include <stdio.h>
#define message\_for(a, b) \
 printf(#a " and " #b ": We love you!\n")
int main(void) {
  message_for(Carole, Debra);
  return 0;
}

When the above code is compiled and executed, it produces the following result:

Carole and Debra: We love you!

When you need to convert a macro parameter to a string constant, use the string constant operator #

Macro continuation operator ()

A macro is usually written on a single line.

#define message\_for(a, b) \
 printf(#a " and " #b ": We love you!\n")

If the macro is too long to fit on a single line, use the macro continuation operator \

Predefined macros

macro description
__DATE__ The current date, a character constant in the format "MMM DD YYYY"
__TIME__ The current time, a character constant in the format "HH:MM:SS"
__FILE__ This will contain the current filename, a string constant
__LINE__ This will contain the current line number, a decimal constant
__STDC__ Defined as 1 when the compiler compiles against the ANSI standard
ANSI C defines a number of macros that you can use, but you cannot directly modify these predefined macros

#Predefined macro example

#include <stdio.h>
int main() {
  printf("File :%s\n", __FILE__);
  printf("Date :%s\n", __DATE__);
  printf("Time :%s\n", __TIME__);
  printf("Line :%d\n", __LINE__);
  printf("ANSI :%d\n", __STDC__);
}

Preprocessor Directives

command description
#define define a macro
#include include a source code file
#undef undefined macro
#ifdef Returns true if the macro is defined
#ifndef Returns true if the macro is not defined
#if Compile the following code if the given condition is true
#else Alternative to #if
#elif If the #if condition is false, the current condition is true
#endif End a #if...#else conditional compilation block
#error Print an error message when standard error is encountered
#pragma Issue special commands to the compiler using the standardized method 
// replace all MAX\_ARRAY\_LENGTH with 20
#define MAX\_ARRAY\_LENGTH 20
// Get stdio.h from the system library
#include <stdio.h>
// Get myheader.h in the local directory
#include "myheader.h"
#undef FILE\_SIZE
#define FILE\_SIZE 42 // undefine and define to 42
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