Rules for cascading macros in C

Question

I cannot find rules for C preprocessor that handle the case below. If you assume a straight substitution this case should not even compile because a=--1 . But if you assume substitution with parenthesis then a=-(-1) = 1 .

#include <stdio.h>
#define  ALPHA -1
#define  BETA -ALPHA
int main(void) {
    int a = BETA;
    int b = -ALPHA; // this compiles too, why? b = --1
    printf("%d\n",a); // 1
    return 0;
}

Answer 1

Preprocessing is the phase which precede compilation it generates a preprocessing translation unit which will be the input for compiler.

Some of those steps in its process are ( From C99 5.1.1.2 ):

3. The source file is decomposed into preprocessing tokens . (...)

4. Preprocessing directives are executed, macro invocations are expanded , (...)

As you can see Tokenization precede macro invocation so -ALPHA will be considered as 2 separate preprocessing tokens (*ie - and ALPHA ¹ ) and not as you thought by 2 preprocessing token --1 ( -- and 1 )

After that in rule 7:

7. White-space characters separating tokens are no longer significant . Each preprocessing token is converted into a token.

So the compiler will get tokens and ignore whitespaces.

If you generate the preprocessing file with gcc -E whitespaces have no significance and those you are seeing are only for formatting to user and cannot reflect the real internal behavior of CPP.

From GCC CPP 5.4.0 1.3 :

Tokens do not have to be separated by white space , but it is often necessary to avoid ambiguities (...)

Once the input file is broken into tokens, the token boundaries never change , except when the '##' preprocessing operator is used to paste tokens together

The compiler does not re-tokenize the preprocessor's output . Each preprocessing token becomes one compiler token.

Summarizing:

If you write -ALPHA => tokens are:

• - punctuator - binary minus sign

• - punctuator - binary minus sign

• 1 constant - integer constant

If you write --1 => tokens are:

• -- punctuator - Decrement operator

• 1 constant - integer constant

Decrement operator should not be used with constant that's why you get error in this case during compilation.

---

_{1 : ALPHA identifier will be replaced (in phase 4) by two preprocessing tokens - and 1 already identified in macro definition}

Answer 2

This is how your program looks after pre-processing is completed.

int main(void) {
    int a = - -1;
    int b = - -1;
    printf("%d\n",a);
    return 0;
}

You can view this output using -E option with gcc .

It is definitely not --1 . Notice the extra space.

Answer 3

First thing you have to understand that C Preprocessors is not a part of compilation process. It's just the substitution process which tells the compiler to do required pre-processing before the actual compilation.

To understand much you have to read the Macro Pitfalls .