ABL electronic PIC Microcontrollers PIC16 - Manual (Page 141)

Directives
#if
,
#elif
,
#else
, and
#endif
The conditional directives
#if
,
#elif
,
#else
, and
#endif
work very similar to
the common C conditional statements. If the expression you write after the
#if
has a nonzero value, the line group immediately following the
#if
directive is
retained in the translation unit.
Syntax is:
#if constant_expression_1
< section_1 >
[#elif constant_expression_2
< section_2 >]
...
[#elif constant_expression_n
< section_n >]
[#else
< final_section >]
#endif
Each
#if
directive in a source file must be matched by a closing
#endif
direc-
tive. Any number of
#elif
directives can appear between the
#if
and
#endif
directives, but at most one
#else
directive is allowed. The
#else
directive, if
present, must be the last directive before
#endif
.
The sections can be any program text that has meaning to the compiler or the pre-
processor. The preprocessor selects a single section by evaluating the
constant_expression
following each
#if
or
#elif
directive until it finds a
true (nonzero) constant expression. The
constant_expressions
are subject to
macro expansion.
If all occurrences of constant-expression are false, or if no
#elif
directives
appear, the preprocessor selects the text block after the
#else
clause. If the
#else
clause is omitted and all instances of
constant_expression
in the
#if
block are false, no section is selected for further processing.
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