C Preprocessor: Basics¶

The C Preprocessor: Why!

Dirty Hack!

Has nothing to do with the language itself
Invented to quickly solve problems in an ad hoc manner - completely ignoring the language
Brutal and stupid text replacement
⟶ Please use cautiously!

`#include`¶

Include the content of a file at the point in the source file where #include is written

Two incarnations, with a subtle difference

#include "file.h": search first in the directory where the “includer” is, and then along the search path
#include <file.h>: searches only along the include path

No rules, but …

Header files contain declarations and macros
Header files generally include header files: have to protect themselves against multiple inclusion ⟶ include guards
#include always comes near the beginning of a source file - never inside

Macro: definition of a token that is brutally replaced

#define forever for (;;)

...
    forever {
        sleep(1);
        printf("No way out!");
    }

#define LOWER 0
#define UPPER 300
#define STEP 20

for (i = LOWER; i < UPPER; i += STEP)
    ...

Better: C99 const keyword

const int LOWER = 0;

⟶ Typed immutable variable

Original problem

#define max(a, b) (((a) > (b))? (a) : (b))
...
x = max(1, 2);

⟶ Statement is expanded as if it were a function call

But …

Braces are necessary:

/* #define max(a, b) (((a) > (b))? (a) : (b)) */
#define max(a, b) ((a > b)? a : b)
...
x = max(p+q, r+s);

brutally expands to …

x = (p+q > r+s) ? p+q : r+s;

⟶ Operator precedence massacre!

Macros: Inline Replacement as Function Call (3)

C99: inline keyword

inline int max(int a, int b)
{
    return (a > b)? a : b;
}

Drawback: cannot use max() with different types

Macros: Inline Replacement as Function Call (4)

One more thing:

#define max(a, b) (((a) > (b))? (a) : (b))
...
x = max(i++, j++);

brutally expands to …

x = ((i++) > (j++)) ? (i++) : (j++);

⟶ Parameters are evaluated more than once!

a.h¶

#include "c.h"

b.h¶

#include "c.h"

c.h¶

extern int g_lobal;

main.h¶

#include "a.h"
#include "b.h"

⟶ Error: multiple declaration of g_lobal

Solution: define a “guard” macro by hand (!)

#ifndef HAVE_C_H
#define HAVE_C_H

extern int g_lobal;

#endif

OMG!