Pre Processor Directives

#include

The #include "file" looks for file, first in same directory as of the source file.
If the file is not found there, then it is demoted to #include <file> which looks for the file according to the rules defined by the implementation.

#define

This directive is used to define macros.

Without Arguments

It is used to replace tokens by a replacement test.

#define TOKEN replacement text

Any of the following appearing in source code, won't be replaced

• "TOKEN" (token appearing as a string¹)
• TOKENIZATION (token appear as part of some identifier or keyword etc)

The replacement text can be carried to multiple lines by using \ in the end.

#define TEST for (int i = 0; i < 5; i++) {\
    printf("hi\n");\
}

int main () {
    TEST
    return 0;
}

It prints

hi
hi
hi
hi
hi

With Arguments

#define MACRO(arg1, arg2) some replacement text using arg1 and arg2

int main () {
    MACRO(1, 2)
}

There are some problems with this though

#define SQUARE1(x) x * x
#define SQUARE2(x) (x) * (x)

int main () {
    int x;
    int a = SQUARE1(5);
    int b = SQUARE1(5 + x);
    int c = SQUARE2(5 + x);
}

gcc -E main.c -o main.i
cat main.i

int main () {
    int x;
    int a = 5 * 5;
    int b = 5 + x * 5 + x; // (1)!
    int c = (5 + x) * (5 + x);
}

1. Problematic!

We can also use # to have the replacement text wrapped around double quotes.

#define MACRO(x) printf(#x)

int main () {
    MACRO(hello world); // (1)!
}

1. TOKEN(hello world) will be replaced by printf("hello world").

We can use ## to concatenate multiple arguments within the replacement text.

#define MACRO1(x, y) x y
#define MACRO2(x, y) x ## y

int main () {
    MACRO1(1, 2) // (1)!
    MACRO2(1, 2) // (2)!
}

1. Gets replaced by 1 2
2. Gets replaced by 12

#undef

Removes the definition of a macro which was defined using #define.

#define MACRO 420
#undef MACRO

int main () {
    int x = MACRO; // (1)!
}

1. After being pre-processed, the MACRO is not replaced by 420

#if

Similar to if statements.

#elif

Similar to else if statements.

#else

Similar to else statements.

#endif

Specifies the end of a block defined by #if and #ifdef.

#if !defined(HDR) // (1)!
#define HDR

// file content
#endif

1. defined() checks if a macro was defined or not.

#ifdef

Both #if defined(MACRO) and #ifdef MACRO are same.

#ifndef

Both #if !defined(MACRO) and #ifndef MACRO are same.

We can combine all these to conditionally include files too.

#if SYSTEM == SYSV
    #define HDR "sysv.h"
#elif SYSTEM == BSD
    #define HDR "bsd.h"
#elif SYSTEM == MSDOS
    #define HDR "msdos.h"
#else
    #define HDR "default.h"

#include HDR // (1)!

1. The HDR symbol contains the appropriate file as replacement text according to what the SYSTEM symbol contains and then it is included.

References

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1. Read more about strings. ↩