Why does #define INVALID_VALUE -999; give a syntax error when used?

I am trying to compare to a defined constants in C, and I have simplified my program to the following:

#include "stdio.h"
#include "stdlib.h"
#define INVALID_VALUE -999;

int main(void)
{
    int test=0;
    if(test==INVALID_VALUE) //The error line..
        return INVALID_VALUE;
    return 0;
}

And when I use gcc to compile, it gives out error "error: expected ‘)’ before ‘;’ token".

开发者_开发知识库

Is there any reason that this cannot be done?

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Remove the semicolon from your INVALID_VALUE definition.

Macros are replaced lexically (character-by-character) with no understanding of the syntax around them. Your macro INVALID_VALUE is set to -999;, so your if line expands the macro to:

if (test==-999;)

which is invalid C syntax.

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You need to remove the ; in #define INVALID_VALUE -999;. See the fixed code.

You could have worked towards this conclusion by understanding what the error message expected ‘)’ before ‘;’ token was telling you. It's telling you that it expected to find a ) before the ; token, but from inspecting the line alone you don't see a ;. So maybe there's one in the definition of INVALID_VALUE? Look up at #define INVALID_VALUE -999; and there it is! Think it should be there, but not sure? So let's try remove it and see if it works. Success!

This page goes and explains why you shouldn't conclude a #define with a semicolon, even if it is needed in the use of the macro. It's good to learn as much as possible from your mistake so that you don't make it again. Quote:

Macro definitions, regardless of whether they expand to a single or multiple statements should not conclude with a semicolon. If required, the semicolon should be included following the macro expansion. Inadvertently inserting a semicolon at the end of the macro definition can unexpectedly change the control flow of the program.

Another way to avoid this problem is to prefer inline or static functions over function-like macros.

In general, the programmer should ensure that there is no semicolon at the end of a macro definition. The responsibility for having a semicolon where needed during the use of such a macro should be delegated to the person invoking the macro.

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The C Preprocessor Macro Language is Distinct from C

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The ; in the macro definition should be removed.

This is an understandable mistake. If C were designed today, the macro language might be more integrated with the rest of C.

But on 16-bit machines in the early 1970's when C was invented, it was unwise to write an overly complicated program. It would end up useless as there would be no memory remaining to actually run the big masterpiece program, and even simple programs ran slowly.

So C was split into a rather simple macro preprocessor that was, originally, a completely separate program, and the compiler proper. The preprocessor program made no attempt to parse C beyond understanding the lexical analysis model.

When 32-bit machines took over, the preprocessor was typically integrated into the parser, but naturally the language needed to remain the same.

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The semi colon at the end of

#define INVALID_VALUE -999;

Classic.

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You do not need a semicolon after defining something. #define is actually a macro, and it will do an inline expansion on compile.

Thus,

#define IDENTIFIER 10;
int j = IDENTIFIER;

will expand as:

int j = 10;;

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Change macro

from:

#define INVALID_VALUE -999;

to

#define INVALID_VALUE -999

Bye