Preprocessor error when defining =

Question

I was trying some awkward preprocessing and came up with something like this:

#include <stdio.h>

#define SIX =6

int main(void)
{
  int x=6;
  int y=2;

  if(x=SIX)
    printf("X == 6\n");
  if(y=SIX)
    printf("Y==6\n");

  return 0;
}

gcc gives me the errors:

test.c: In function ‘main’:
test.c:10:8: error: expected expression before ‘=’ token
test.c:12:8: error: expected expression before ‘=’ token

Why is that?

Possible duplicate of What is the worst real-world macros/pre-processor abuse you've ever come across? Just kidding, good question. — 1''
– 1'', Commented Aug 23, 2013 at 15:59
Running this through the preprocessor, the tests turn into if(x= =6). I'm not sure why the space is inserted... presumably someone who knows one of the C specifications much better than me will come along... — cdhowie
– cdhowie, Commented Aug 23, 2013 at 16:00
The preprocessor deals in tokens, logically with spaces separating the tokens. When it tokenizes if(x=SIX), it has if, (, x, =, and SIX. When it macro expands SIX, it has extra tokens = and 6. But two adjacent tokens = are not the same as one token == (and are in fact invalid C syntax) — hence the compilation error. — Jonathan Leffler
– Jonathan Leffler, Commented Aug 23, 2013 at 16:01
Because the result of macro expansion is never rescanned for possible token pasting. — Jens
– Jens, Commented Aug 23, 2013 at 19:22

Antti Haapala · Accepted Answer · 2013-08-23 16:17:51Z

15

The == is a single token, it cannot be split in half. You should run gcc -E on your code

From GCC manual pages:

-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

Input files that don't require preprocessing are ignored.

For your code gcc -E gives the following output

  if(x= =6)
    printf("X == 6\n");

  if(y= =6)
    printf("Y==6\n");

The second = is what causes the error message expected expression before ‘=’ token

edited Aug 23, 2013 at 16:17

answered Aug 23, 2013 at 15:59

Antti Haapala

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Comments

Barmar · Accepted Answer · 2013-08-23 16:01:16Z

5

The preprocessor doesn't work at the character level, it operates at the token level. So when it performs the substitution, you get something equivalent to:

if (x = = 6)

rather than your desired:

if (x==6)

There are some specific exceptions to this, like the # stringification operator.

answered Aug 23, 2013 at 16:01

Barmar

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2 Comments

alk Over a year ago

Where does the blank between = and 6 come from?

Barmar Over a year ago

@alk I'm just using the spaces to show token boundaries. As far as the C grammar is concerned, there's no difference between =6 and = 6.

Hrishi · Accepted Answer · 2013-08-23 16:29:51Z

4

if(x=SIX)

is parsed as

if (x= =6).

So you get the error.

edited Aug 23, 2013 at 16:29

answered Aug 23, 2013 at 16:05

Hrishi

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Comments

Balau · Accepted Answer · 2013-08-23 16:02:57Z

0

What toolchain are you using? If you are using GCC, you can add the -save-temps option and check the test.i intermediate result to troubleshoot your problem.

I suspect you have a space between the x= and the =6.

answered Aug 23, 2013 at 16:02

Balau

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