1.7.1 #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 #ifndef allow 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 <stdio.h>
#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.