Preprocessor examples in C language

Question

I want some examples of C preprocessor directives, such as:

#define pi 3.14
#define MAX 100

I know only this. I want to know more than this, more about preprocessor directives.

Answer 1

The biggest example would be

 #include<stdio.h>

But there are a fair amount. You can also define macros:

 #define MAX(X,Y) (((X) > (Y)) ? (X) : (Y))

And use header guards

#ifndef A_H
#define A_H

// code

#endif

There are proprietary extensions that compilers define to let you give processing directives:

#ifdef WIN32 // WIN32 is defined by all Windows 32 compilers, but not by others.
#include <windows.h>
#else
#include <unistd.h>
#endif

And the if statement cause also be used for commenting:

#if 0

int notrealcode = 0;

#endif

I often use the preprocessor to make debug builds:

#ifdef EBUG
printf("Debug Info");
#endif

$ gcc -DEBUG file.c //debug build
$ gcc file.c //normal build

And as everyone else has pointed out there are a lot of places to get more information:

• http://predef.sourceforge.net/
• http://en.wikipedia.org/wiki/C%5Fpreprocessor
• http://gcc.gnu.org/onlinedocs/cpp/

Answer 2

Are you entirely familiar with the fundamentals, e.g. as covered on wikipedia? Or do you need an elementary tutorial? or what?

Answer 3

Maybe one of

• Wikipedia on C preprocessor
• GNU C preprocessor docs

can be of help.

Answer 4

There's a lot more to the processor than #define. Don't forget #include and conditional compilation.

http://en.wikipedia.org/wiki/C%5Fpreprocessor

Answer 5

This will stop the compile if the conditional fails.

#define WIDTH 10
#define HEIGHT 20

#if WIDTH < HEIGHT
#    error "WIDTH must be greater than or equal to HEIGHT"
#endif

C Preprocessor Man pages http://gcc.gnu.org/onlinedocs/cpp/index.html

Answer 6

The most important one:

#ifndef THIS_HEADER_H
#define THIS_HEADER_H

// Declarations go here    

#endif //THIS_HEADER_H

This keeps a header file from being included in a single C file multiple times.

For gcc sources I like to use __LINE__, as in:

printf("%s: %d: Some debug info\n", __func__, __LINE__);

For debugging purposes.