Programming in C

9.0 The Preprocessor 463

produces

printf ("x10 = %i\n", x10);

after substitution and concatenation of the character strings.

Spaces are not required around the #and ##operators.

9.2.2 The #errorDirective

The general format for declaring the #errordirective is as follows:

#error text

...

The specified textis written as an error message by the preprocessor.

9.2.3 The #ifDirective

One general format for declaring the #ifdirective is as follows:

#if constant_expression

...

#endif

The value of constant_expressionis evaluated. If the result is nonzero, all program

lines up until the #endifdirective are processed; otherwise, they are automatically

skipped and are not processed by the preprocessor or by the compiler.

Another general format for declaring the #ifdirective is as follows:

#if constant_expression_1

...

#elif constant_expression_2

...

#elif constant_expression_n

...

#else

...

#endif

If constant_expression_1is nonzero, all program lines up until the #elifare

processed, and the remaining lines up to the #endifare skipped. Otherwise, if

constant_expression_2is nonzero, all program lines up until the next #elifare

processed, and the remaining lines up to the #endif are skipped. If none of the constant

expressions evaluates to nonzero, the lines after the #else(if included) are processed.

The special operator definedcan be used as part of the constant expression, so

#if defined (DEBUG)

...

#endif

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