index<--

C Compiler directives

Directive Keywords

Directive	Description
#define	create a macro
#undef	remove a macro
#include	Inserts a header file
#ifdef	Check if a macro is defined
#ifndef	Check if a macro is not defined
#if	Check a compile time condition
#else	The alternative path for #if directive
#elif	Ladder directive, used with #if
#endif	Ends conditional directive
#error	Prints error message on stderr
#pragma	Special compiler directives

#Include

C is a modular language. You can combine smaller files into a larger program using #include directive followed by the header file name. There are two kind of header files: System headers and project local headers.

//system header
#include <stdio.h>
//local header
#include "my_file.h"

The C 90 standard headers list:

Headers added since C 90:

Project header files:

You can create your own header files and store them with your program files. A good practice is to store in header files: constants, macros, system wide global variables, and function prototypes.

Note: Function prototype is the function signature without its body. Functions can be implemented in other *.c source files that do not have same name as the header file. 

#define

Constants: Using #define you can create constants. These constants must use uppercase names.

#include <stdio.h>
#define MY_MESSAGE "Hello!"
int main() {
    #ifdef MY_MESSAGE
    printf(MY_MESSAGE);
    #endif
    return 0;
} 

Macros: Using #define you can create macros. These are expressions or reusable fragments of code:

#include <stdio.h>
#define AVG(x,y) ((x + y)/2)
int main(void) {
    printf("AVG = %d\n", AVG(10, 20)); //integers
    printf("AVG = %lf\n", AVG(1, 1.5)); //float
    return 0;
}

Note: AVG in previous example looks like a function but is not. It is a "macro" so it uses uppercase letters for identifier. The real difference is that macros are converted into literal expressions before compilation. Since this is not a function call, macro parameters do not have types, they are text literals that are replaced in expressions before compilation.

Warning: There is a danger that macro can produce unexpected results if is not enclosed in extra parentheses. It is a bad practice to overuse macros. It may decrease program readability and maintainability.

#error

This directive stop the compilation and display a message defined after #error:

#error message

#warning

This directive do not stop the compilation but display a warning message:

#warning message

#if

This is a conditional compiler directive. It uses a logical expression to make a decision block executed before compilation. The conditional expression can contain normal relation and logic operators in addition it can use operator defined() that take as argument a defined constant or macro.

#include <stdio.h>
#define SUM(x,y) (x + y)
int main(void) {
    #if defined(SUM)
    printf("SUM = %d\n", SUM(10, 20));
    #endif;
    return 0;
}

Predefined macros

ANSI C defines some useful macros and variables:

__FILE__ => The name of the current file, as a string literal
__LINE__ => Current line of the source file, as a numeric literal
__DATE__ => Current system date, as a string
__TIME__ => Current system time, as a string

Note: Some compilers have special #pragma directives and other operators. For more details about processor directives you must consult the compiler specific documentation. These details are out of scope from my introductory tutorial.

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