Macro definition

I have a question about the macro shown as follows:

Code:

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#define _MUXSELf(a)              0##a << 30

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What does 0##a mean here?

Thanks for the help,

aayu

It basically joins the tokens on its right and left, e.g., if a was 123, you would end up with 0123 << 30.

Thanks.

aayu

I try to test the macro by using the following code:

Code:

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#include <stdlib.h>
#include <stdio.h>

#define ttoken(a)    0##a << 2

int main()
{
  int a = 8;
  int b;
  b = ttoken(a);
  printf("b = %d\n", b);

  return 0;
}

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after using:

Code:

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gcc -std=c99 token1.c

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the following is the compiler error:

Code:

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token1.c:10:1: error: invalid suffix "a" on integer constant

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Anything wrong here?

Thanks again,

aayu

Yes, the fact that you wrote ttoken(a). You should be using octal digits instead, e.g., ttoken(71).

You are correct. But why I have to use octal digits, I tryied to use hex or dec, the compiler complained except using octal digits. I must missed something here.

Thanks,

aayu

In C, a number that starts with a 0 is in octal, just as a number that starts 0x is in hex.

And the "##" preprocessor operators are used for string catenation.

Quote:

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Originally Posted by itCbitC

And the "##" preprocessor operators are used for string catenation.

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what it has to do with anything? preprocessor in any case just replaces one character sequence with another...

Quote:

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Originally Posted by vart

what it has to do with anything? preprocessor in any case just replaces one character sequence with another...

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Not if you try to catenate a number and a variable ie 0 ## a produces 0a not 07 if a=7.

Quote:

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Originally Posted by itCbitC

Not if you try to catenate a number and a variable ie 0 ## a produces 0a not 07 if a=7.

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Why not? It exactly what it does...
as I said it replaces char sequence ttoken(a) with char sequence 0a

or char sequence ttoken(71) with char sequence 071

I'm not sure using word strings here is helpful as it conflicts with C-term... there are no C-strings involved here

Quote:

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Originally Posted by vart

Why not? It exactly what it does...
as I said it replaces char sequence ttoken(a) with char sequence 0a

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And is 0a an octal number?

Quote:

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Originally Posted by vart

or char sequence ttoken(71) with char sequence 071

I'm not sure using word strings here is helpful as it conflicts with C-term... there are no C-strings involved here

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Alrighty! char sequence it is then.