5.14.1 Preprocessor Directives

The XC8 accepts several specialized preprocessor directives, in addition to the standard directives. All of these are tabulated below.

Table 5-15. Preprocessor Directives
Directive	Meaning	Example
`#`	Preprocessor null directive, do nothing.	`#`
`#define`	Define preprocessor macro.	`#define SIZE (5) #define FLAG #define add(a,b) ((a)+(b))`
`#elif`	Short for `#else #if.`	see `#ifdef`
`#else`	Conditionally include source lines.	see `#if`
`#endif`	Terminate conditional source inclusion.	see `#if`
`#error`	Generate an error message.	`#error Size too big`
`#if`	Include source lines if constant expression true.	`#if SIZE < 10 c = process(10) #else skip(); #endif`
`#ifdef`	Include source lines if preprocessor symbol defined.	`#ifdef FLAG do_loop(); #elif SIZE == 5 skip_loop(); #endif`
`#ifndef`	Include source lines if preprocessor symbol not defined.	`#ifndef FLAG jump(); #endif`
`#include`	Include text file into source.	`#include <stdio.h> #include “project.h”`
`#line`	Specify line number and filename for listing	`#line 3 final`
`#nn filename`	(where `nn` is a number, and `filename` is the name of the source file) the following content originated from the specified file and line number.	`#20 init.c`
`#pragma`	Compiler specific options.	See the Pragma Directives section in this guide.
`#undef`	Undefines preprocessor symbol.	`#undef FLAG`
`#warning`	Generate a warning message.	`#warning Length not set`

Macro expansion using arguments can use the # operator to convert an argument to a string and the ## operator to concatenate arguments. If two expressions are being concatenated, consider using two macros in case either expression requires substitution itself; for example

#define __paste1(a,b)   a##b
#define __paste(a,b)    __paste1(a,b)

lets you use the paste macro to concatenate two expressions that themselves might require further expansion. Remember that once a macro identifier has been expanded, it will not be expanded again if it appears after concatenation.

Preprocessor macros can take a variable number of arguments, just like C functions. Use an ellipsis (…) after any named arguments to define a variable argument list. Use the special token __VA_ARGS__ to expand this argument list in the macro replacement text. For example, the following PIC18 code places a variable number of words into program memory. Note that the arguments were converted to strings using the # operator so as to be usable in the __asm() statement.

#define PLACE_WORDS(...) \
__asm("PSECT cdata,class=CODE,reloc=2,noexec"); \
__asm("DW " #__VA_ARGS__)

Call this macro in the usual way, passing it as many arguments as are required, for example:

PLACE_WORDS(0x100, 1, 223, 0x30F0);