These preprocessing directives create conditional compiling parameters that control thecompiling of the source code. They must begin on a separate line.
Syntax:
#ifconstant_expression#else
#endif
or
#if constant_expression
#elifconstant_expression#endif
The compiler only compiles the code after the #if expression if the constant_expressionevaluates to a non-zero value (true). If the value is 0 (false), then the compiler skips the linesuntil the next #else, #elif, or #endif. If there is a matching #else, and the constant_expressionevaluated to 0 (false), then the lines between the #else and the #endif are compiled. If there is amatching #elif, and the preceding #if evaluated to false, then the constant_expression after thatis evaluated and the code between the #elif and the #endif is compiled only if this expressionevaluates 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" wouldbe 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.
The preprocessing directives #define and #undef allow the definition of identifiers whichhold a certain value. These identifiers can simply be constants or a macro function. Thedirectives #ifdef and #ifndef allow conditional compiling of certain lines of code based onwhether or not an identifier has been defined.
Syntax:
#defineidentifier replacement-code
#undefidentifier
#ifdefidentifier#elseor#elif#endif
#ifndefidentifier#elseor#elif#endif
#ifdefidentifier is the same is#if defined(identifier).#ifndefidentifier is the same as#if!defined(identifier).
An identifier defined with#defineis available anywhere in the source code until a#undefis reached.
A function macro can be defined with#definein the following manner:
#defineidentifier(parameter-list) (replacement-text)The values in the parameter-list are replaced in the replacement-text.
Examples:
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 whichtakes 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 knownheader directories for the file (which is implementation-defined) and processes it. Whenenclosed with double quotation marks, then the entire contents of the source-file is replaced atthis 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 currentsourcecode 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 thecurrent 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 thespecified 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 areimplementation-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 currentsource file. It is in the format "mmm dd yyyy", the same as what is generated bythe asctime function. |
| __TIME__ | A string literal representing the current time when cimpiling began for the currentsource file. It is in the format "hh:mm:ss", the same as what is generated by theasctime function. |
| __STDC__ | The decimal constant 1. Used to indicate if this is a standard C compiler. |