"MaybeExtern" Keyword in C

w_bwr · Oct 16, 2012

This is a C code for an Embedded application. What does the MaybeExtern implies? It is not given any value.

This Code is written in a Main.c file:

Code:

#define EpanelMain
#include "globals.h"
#undef EpanelMain

This code is written in globals.h .

Code:

#ifdef EpanelMain
    #define MaybeExtern 
#else
    #define MaybeExtern extern
#endif


MaybeExtern _far float fGenCurrent[MaxGeneric], fGenK[MaxGeneric], fGenOff[MaxGeneric], fGenTmp[MaxGeneric];
MaybeExtern _far char cGenNdx, cGenAddr[MaxGeneric], cGenAttr[MaxGeneric], cGenCol[MaxGeneric], cGenDec[MaxGeneric], cGenType[MaxGeneric];
MaybeExtern _far char cGenLbl[MaxGeneric][CPDLabelLen+1], cGenUnit[MaxGeneric][CPDUnitLen];


MaybeExtern _far unsigned char ucGenericNum;
.
.
// Some More Variables
.
.
#undef MaybeExtern

pedroromanvr · Oct 17, 2012

If you think about it. The macro checking the EpanelMain is making that code to define local or extern variables.

Code:

#ifdef EpanelMain  //If EpanelMain is defined
    #define MaybeExtern  //Then the "boolean" MaybeExtern will be defined as nothing
#else
    #define MaybeExtern extern  //Else define MaybeExtern as extern
#endif

So this is a "game" of declare/define depending on EpanelMain.

w_bwr · Oct 17, 2012

pedroromanvr said:
If you think about it. The macro checking the EpanelMain is making that code to define local or extern variables.

Code:

#ifdef EpanelMain //If EpanelMain is defined #define MaybeExtern //Then the "boolean" MaybeExtern will be defined as nothing. .[/QUOTE] The Macro name does show any arguments. Is this Macro Syntax? If EpanelMain is set. This makes this "MaybeExtern _far float fGenCurrent[MaxGeneric]" variable only this "_far float fGenCurrent[MaxGeneric]".

bigdogguru · Oct 17, 2012

w_bwr said:
The Macro name does show any arguments. Is this Macro Syntax?

Actually, the technical term is preprocessor/compiler directives and are used to "prep" the source before the actual process of compiling.

Essentially, any valid keyword preceded by the "#" symbol is a preprocessor/compiler directive, including predefined macros, includes, etc.

BigDog

w_bwr · Oct 17, 2012

Preprocessor Driective are commonly used for three purporses:
1: Header Filer like this

Code:

#include <stdio.h>

2: Macro

Code:

#define [Function Name with arguments]    [condition]
#define Max(int a, int b)  (a>b) ? a :b

3: Defining Constants

Code:

#include [Name]  [Constant Value]
#include T4_Trap 0x27

But what does this means:

Code:

#ifdef EpanelMain
    #define MaybeExtern 
#else
    #define MaybeExtern extern
#endif

EPanel is defined in the main file. so the "if" condition true.

Code:

#define MaybeExtern

What is it exactly _ a Macro or boolean constant as pedroromanvr mentioned? It doesn't show any arguments neither show a constant value.

pedroromanvr · Oct 17, 2012

bigdogguru is correct, the proper term is compiler or preprocessor directives.
In this case

Code:

#if
#ifdef
#ifndef
#else
#elif
#endif

Those are directives that can be used for conditional compilation. So basically in

Code:

#ifdef EpanelMain
    #define MaybeExtern 
#else
    #define MaybeExtern extern
#endif

You are asking, is EpanelMain defined?, if not all variables will be declared as extern in compilation time.

bigdogguru · Oct 17, 2012

Here are a few tutorials concerning preprocessor/compiler directives:

Preprocessor directives

The GNU C Preprocessor

THE C/C++ PREPROCESSOR DIRECTIVES 1

Keep in mind, many directives are compiler dependent and may or may not exist depending on the compiler implementation.

Always consult your compilers documentation before attempting to utilize some of the more esoteric directives.

BigDog

alexan_e · Oct 17, 2012

If EpanelMain is defined then the resulting code equals

Code:

_far float fGenCurrent[MaxGeneric], fGenK[MaxGeneric], fGenOff[MaxGeneric], fGenTmp[MaxGeneric];
_far char cGenNdx, cGenAddr[MaxGeneric], cGenAttr[MaxGeneric], cGenCol[MaxGeneric], cGenDec[MaxGeneric], cGenType[MaxGeneric];
_far char cGenLbl[MaxGeneric][CPDLabelLen+1], cGenUnit[MaxGeneric][CPDUnitLen];

If EpanelMain is not defined then the code results to

Code:

extern _far float fGenCurrent[MaxGeneric], fGenK[MaxGeneric], fGenOff[MaxGeneric], fGenTmp[MaxGeneric];
extern _far char cGenNdx, cGenAddr[MaxGeneric], cGenAttr[MaxGeneric], cGenCol[MaxGeneric], cGenDec[MaxGeneric], cGenType[MaxGeneric];
extern _far char cGenLbl[MaxGeneric][CPDLabelLen+1], cGenUnit[MaxGeneric][CPDUnitLen];

"MaybeExtern" Keyword in C

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