#if, #elif, #else, #endif

These preprocessing directives create conditional compiling parameters that control the compiling of the source code. They must begin on a separate line.

Syntax:

#if constant_expression
#else
#endif

or

#if constant_expression
#elif constant_expression
#endif

The compiler only compiles the code after the #if expression if the constant_expression evaluates to a non-zero value (true). If the value is 0 (false), then the compiler skips the lines until the next #else,#elif, or #endif. If there is a matching #else, and the constant_expression evaluated to 0 (false), then the lines between the #else and the #endif are compiled. If there is a matching #elif, and the preceding#if evaluated to false, then the constant_expression after that is evaluated and the code between the #elif and the #endif is compiled only if this expression evaluates to a non-zero value (true).

Examples:

int main(void)
 {
  #if 1
    printf("Yabba Dabba Do!\n");
  #else
    printf("Zip-Bang!\n");
  #endif
  return 0;
}

Only "Yabba Dabba Do!" is printed.

int main(void)
 {
  #if 1
    printf("Checkpoint1\n");
  #elif 1
    printf("Checkpoint2\n");
  #endif
  return 0;
}

Only "Checkpoint1" is printed. Note that if the first line is #if 0, then only "Checkpoint2" would be printed.

#if OS==1
  printf("Version 1.0");
#elif OS==2
  printf("Version 2.0");
#else
  printf("Version unknown");
#endif

Prints according to the setting of OS which is defined with a #define. 

1.7.2 #define, #undef, #ifdef, #ifndef

The preprocessing directives #define and #undef allow the definition of identifiers which hold a certain value. These identifiers can simply be constants or a macro function. The directives #ifdef and #ifndefallow conditional compiling of certain lines of code based on whether or not an identifier has been defined.

Syntax:

#define identifier replacement-code

#undef identifier

#ifdef identifier
#else or #elif
#endif

#ifndef identifier
#else or #elif
#endif

#ifdef identifier is the same is #if defined( identifier).
#ifndef identifier is the same as #if !defined(identifier).
An identifier defined with #define is available anywhere in the source code until a #undef is reached.
A function macro can be defined with #define in the following manner:

#define identifier(parameter-list) (replacement-text)

The values in the parameter-list are replaced in the replacement-text.

Examples:

#define PI 3.141
printf("%f",PI);

#define DEBUG
#ifdef DEBUG
  printf("This is a debug message.");
#endif

#define QUICK(x) printf("%s\n",x);
QUICK("Hi!")

#define ADD(x, y) x + y
z=3 * ADD(5,6)

This evaluates to 21 due to the fact that multiplication takes precedence over addition.

#define ADD(x,y) (x + y)
z=3 * ADD(5,6)

This evaluates to 33 due to the fact that the summation is encapsulated in parenthesis which takes precedence over multiplication.

1.7.3 #include

The #include directive allows external header files to be processed by the compiler.

Syntax:

#include <header-file>

or

#include "source-file"

When enclosing the file with < and >, then the implementation searches the known header directories for the file (which is implementation-defined) and processes it. When enclosed with double quotation marks, then the entire contents of the source-file is replaced at this point. The searching manner for the file is implementation-specific.

Examples:

#include
#include "my_header.h"


1.7.4 #line

The #line directive allows the current line number and the apparent name of the current sourcecode filename to be changed.

Syntax:

#line line-number filename

Note that if the filename is not given, then it stays the same. The line number on the current line is one greater than the number of new-line characters (so the first line number is 1). 
Examples:

#line 50 user.c

#line 23


1.7.5 #error

The #error directive will cause the compiler to halt compiling and return with the specified error message.

Syntax:

#error message

Examples:

#ifndef VERSION
#error Version number not specified.
#endif


1.7.6 #pragma

This #pragma directive allows a directive to be defined. Its effects are implementation-defined. If the pragma is not supported, then it is ignored.

Syntax:

#pragma directive

1.7.7 Predefined Macros

The following macros are already defined by the compiler and cannot be changed.

__LINE__	A decimal constant representing the current line number.
__FILE__	A string representing the current name of the source code file.
__DATE__	A string representing the current date when compiling began for the current source file. It is in the format "mmm dd yyyy", the same as what is generated by the asctime function.
__TIME__	A string literal representing the current time when cimpiling began for the current source file. It is in the format "hh:mm:ss", the same as what is generated by the asctime function.
__STDC__	The decimal constant 1. Used to indicate if this is a standard C compiler.