What purpose do #if, #else, #elif, #endif, #ifdef, #ifndef serve?

The following preprocessor directives are used for conditional compilation. Conditional compilation allows statements to be included or omitted based on conditions at compile time.

if
#else
#elif
#endif
#ifdef
#ifndef


In the following example, the printf statements are compiled when the symbol DEBUG is defined, but not compiled otherwise
/* remove to suppress debug printf's*/
#define DEBUG
...
x = ....

#ifdef DEBUG
printf( "x=%d\n" );
#endif...

y = ....;

#ifdef DEBUG
printf( "y=%d\n" );
#endif...



#if, #else, #elif statements

#if directive

* #if is followed by a intger constant expression.
* If the expression is not zero, the statement(s) following the #if are compiled, otherwise they are ignored.
* #if statements are bounded by a matching #endif, #else or #elif
* Macros, if any, are expanded, and any undefined tokens are replaced with 0 before the constant expression is evaluated
* Relational operators and integer operators may be used
Expression examples


#if 1
#if 0
#if ABE == 3
#if ZOO < 12
#if ZIP == 'g'
#if (ABE + 2 - 3 * ZIP) > (ZIP - 2)


In most uses, expression is simple relational, often equality test


#if SPARKY == '7'



#else directive
* #else marks the beginning of statement(s) to be compiled if the preceding #if or #elif expression is zero (false)
* Statements following #else are bounded by matching #endif



Examples


#if OS = 'A'
system( "clear" );
#else
system( "cls" );
#endif
#elif directive


* #elif adds an else-if branch to a previous #if
* A series of #elif's provides a case-select type of structure
* Statement(s) following the #elif are compiled if the expression is not zero, ignored otherwise
* Expression is evaluated just like for #if
Examples


#if TST == 1
z = fn1( y );
#elif TST == 2
z = fn2( y, x );
#elif TST == 3
z = fn3( y, z, w );
#endif

...
#if ZIP == 'g'
rc = gzip( fn );
#elif ZIP == 'q'
rc = qzip( fn );
#else
rc = zip( fn );
#endif

ifdef and #ifndef directives

Testing for defined macros with #ifdef, #ifndef, and defined()



* #ifdef is used to include or omit statements from compilation depending of whether a macro name is defined or not.
* Often used to allow the same source module to be compiled in different environments (UNIX/ DOS/MVS), or with different options (development/production).
* #ifndef similar, but includes code when macro name is not defined.

Examples


#ifdef TESTENV
printf( "%d ", i );
#endif
#ifndef DOS
#define LOGFL "/tmp/loga.b";
#else
#define LOGFL "c:\\tmp\\log.b";
#endif

efined() operator



* defined(mac), operator is used with #if and #elif and gives 1 (true) if macro name mac is defined, 0 (false) otherwise.
* Equivalent to using #ifdef and #ifndef, but many shops prefer #if with defined(mac) or !defined(mac)
Examples


#if defined(TESTENV)
printf( "%d ", i );
#endif
#if !defined(DOS)
#define LOGFL "/tmp/loga.b";
#else
#define LOGFL "c:\\tmp\\log.b";
#endif
Nesting conditional statements

Conditional compilation structures may be nested:

#if defined(UNIX)
#if LOGGING == 'y'
#define LOGFL "/tmp/err.log"
#else
#define LOGFL "/dev/null"
#endif
#elif defined( MVS )
#if LOGGING == 'y'
#define LOGFL "TAP.AVS.LOG"
#else
#define LOGFL "NULLFILE"
#endif
#elif defined( DOS )
#if LOGGING == 'y'
#define LOGFL "C:\\tmp\\err.log"
#else
#define LOGFL "nul"
#endif
#endif