C Preprocessor

Preprocessor Directives

Directive	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

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(void) {
  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__);
}

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 \

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 #

tag paste operator (##)

#include <stdio.h>

#define tokenpaster(n) printf ("Token " #n " = %d\n", token##n)

int main(void) {
  int token34 = 40;
  tokenpaster(34);

  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;
}

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;
}