macros

[borophyll@gmail.com]

Hi

Can anyone explain why the output of the following code:

#define A 1
#define B 2
A.B

after preprocessing only, is:

1 . 2

Why does the preprocessor put spaces between each character, when the
macro invocation has none?

Regards, B

[Eric Sosman]

Re: macros

borophyll@gmail .com wrote:

Hi
>
Can anyone explain why the output of the following code:
>
#define A 1
#define B 2
A.B
>
after preprocessing only, is:
>
1 . 2
>
Why does the preprocessor put spaces between each character, when the
macro invocation has none?

The preprocessor is often -- and incorrectly -- said to
operate on the text of the program's source code. In fact,
it operates on tokens (formally, "preprocess ing tokens")
that have been derived from the text. These tokens simply
come one after another, pretty maids all in a row; they do
not need to be separated by spaces. Equally, though, they
do not magically combine with each other just because they
happen to be adjacent.

So: By the time the sample code reaches the preprocessor,
it has been divided into three tokens which we may represent as

A

[CBFalconer]

Re: macros

borophyll@gmail .com wrote:

>
Can anyone explain why the output of the following code:
>
#define A 1
#define B 2
A.B
>
after preprocessing only, is:
>
1 . 2
>
Why does the preprocessor put spaces between each character, when
the macro invocation has none?

Because that is specified by the C standard. As a purely practical
matter, it keeps individual words separated in macros.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>


--
Posted via a free Usenet account from http://www.teranews.com

[pete]

Re: macros

borophyll@gmail .com wrote:

>
Hi
>
Can anyone explain why the output of the following code:
>
#define A 1
#define B 2
A.B
>
after preprocessing only, is:
>
1 . 2
>
Why does the preprocessor put spaces between each character, when the
macro invocation has none?

Why did you put a dot in between A and B?

What should AB mean?

--
pete

[Keith Thompson]

Re: macros

Eric Sosman <esosman@ieee-dot-org.invalidwrit es:

borophyll@gmail .com wrote:

>Hi
>Can anyone explain why the output of the following code:
>#define A 1
>#define B 2
>A.B
>after preprocessing only, is:
>1 . 2
>Why does the preprocessor put spaces between each character, when the
>macro invocation has none?

>
The preprocessor is often -- and incorrectly -- said to
operate on the text of the program's source code. In fact,
it operates on tokens (formally, "preprocess ing tokens")
that have been derived from the text. These tokens simply
come one after another, pretty maids all in a row; they do
not need to be separated by spaces. Equally, though, they
do not magically combine with each other just because they
happen to be adjacent.

[...]

Quite correct.

However, some preprocessors are *implemented* as text-to-text
translators; they take input text, analyze it as a sequence of
preprocessor tokens, and generate output text that is then analyzed by
later compiler phases a a sequence of tokens.

In this case, the preprocessor implementation inserts blanks between
'1', '.', and '2' precisely so that the next phase will see them as
distinct tokens, rather than as a single token '1.2' (a floating
constant).

The fact that the preprocessor implementation allows you to see its
output is just an extra feature, not required by the language. It
could just as correctly have produced a sequence of tokens in some
binary form, as long as the next compilation phase is able to
interpret that form as tokens.

(Historically, of course, the preprocessor was a separate program that
did text processing, as it still is in some implementations . This
stuff about "preprocess or tokens" and "tokens" is a formalization of
what it does; text-to-text processing is now just one way to implement
that formalized process.)

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[borophyll@gmail.com]

Re: macros

On Aug 23, 12:01 pm, CBFalconer <cbfalco...@yah oo.comwrote:

Because that is specified by the C standard.

Care to quote which part?

regards, B

[Ivan Gotovchits]

Re: macros

borophyll@gmail .com wrote:

Hi
>
Can anyone explain why the output of the following code:
>
#define A 1
#define B 2
A.B
>
after preprocessing only, is:
>
1 . 2
>
Why does the preprocessor put spaces between each character, when the
macro invocation has none?
>
Regards, B

may be you want this:
A##.##B /* No dots will be */

[Laurent Deniau]

Re: macros

On 23 août, 08:39, Ivan Gotovchits <ivan.gotovch.. .@auriga.ruwrot e:

boroph...@gmail .com wrote:

Hi

>

Can anyone explain why the output of the following code:

>

#define A 1
#define B 2
A.B

>

after preprocessing only, is:

>

1 . 2

>

Why does the preprocessor put spaces between each character, when the
macro invocation has none?

>

Regards, B

>
may be you want this:
A##.##B /* No dots will be */

This doesn't work and gives 1##.##2

#define AB(a,b) AB_(a,b)
#define AB_(a,b) a ## . ## b

AB(A,B) -1.2

a+, ld.

[borophyll@gmail.com]

Re: macros

Are the # and ## operators only allowed within a macro definition?

B.

[Kenneth Brody]

Re: macros

borophyll@gmail .com wrote:

>
Hi
>
Can anyone explain why the output of the following code:
>
#define A 1
#define B 2
A.B
>
after preprocessing only, is:
>
1 . 2
>
Why does the preprocessor put spaces between each character, when the
macro invocation has none?

Strange... My compiler's preprocessor outputs "1.B". (No spaces,
but it failed to expand "B".) Is it broken?

Note that it does replace "A->B" with "1->2", which is why I may not
have noticed this before. I do have code with macros for struct
entries, but they're probably all within pointers.

--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer .h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:ThisIsA SpamTrap@gmail. com>

[Keith Thompson]

Re: macros

Kenneth Brody <kenbrody@spamc op.netwrites:

borophyll@gmail .com wrote:

>Can anyone explain why the output of the following code:
>>
>#define A 1
>#define B 2
>A.B
>>
>after preprocessing only, is:
>>
>1 . 2
>>
>Why does the preprocessor put spaces between each character, when the
>macro invocation has none?

>
Strange... My compiler's preprocessor outputs "1.B". (No spaces,
but it failed to expand "B".) Is it broken?

Probably, but it's hard to be certain by looking at the output of the
preprocessor, which is normally internal to the compiler. The token
sequence 1 . 2 can't appear in a legal program; if the behavior
doesn't affect any legal programs, and doesn't cause the compiler to
fail to issue a diagnostic for some program with a syntax error or
constraint violation, then strictly speaking it's not a bug as far as
the C standard is concerned. (If your preprocessor has an additional
requirement to generate textual output, it may be a bug if it doesn't
meet that requirement correctly.)

Try the following program. It should print "ok".

#define A a
#define B b
int main(void)
{
struct { int b; } a;
a.b = 42;
if (A.B == 42) {
puts("ok");
}
else {
puts("bug");
}
return 0;
}

But here A and B expand to identifiers, not integer constants, so if
there is a bug this may not trigger it.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Harald van =?UTF-8?B?RMSzaw==?=]

Re: macros

CBFalconer wrote:

borophyll@gmail .com wrote:

>>
>Can anyone explain why the output of the following code:
>>
>#define A 1
>#define B 2
>A.B
>>
>after preprocessing only, is:
>>
>1 . 2
>>
>Why does the preprocessor put spaces between each character, when
>the macro invocation has none?

>
Because that is specified by the C standard.

The C standard does not require or even acknowledge any externally visible
intermediate forms between translation phases.

[Eric Sosman]

Re: macros

Kenneth Brody wrote:

borophyll@gmail .com wrote:

>Hi
>>
>Can anyone explain why the output of the following code:
>>
>#define A 1
>#define B 2
>A.B
>>
>after preprocessing only, is:
>>
>1 . 2
>>
>Why does the preprocessor put spaces between each character, when the
>macro invocation has none?

>
Strange... My compiler's preprocessor outputs "1.B". (No spaces,
but it failed to expand "B".) Is it broken?

No. The only required output is a diagnostic; anything
else is outside the scope of the Standard, hence not "broken"
by appeal to the Standard's requirements.

--
Eric Sosman
esosman@ieee-dot-org.invalid

[borophyll@gmail.com]

Re: macros

I've just noted another funny thing. Why does

#define A stdio
#define B h
A.B

produce stdio.h and not stdio . h? Would it not see these as three
individual tokens just as in the 1 . 2 example? Why does it combine
them in this instance and not the other? I suspect it is because
there is no ambiguity with stdio.h, which cannot possibly be confused
as a single token in the main compilation phase, whereas 1.2 can??

regards, B.

[Keith Thompson]

Re: macros

borophyll@gmail .com writes:

I've just noted another funny thing. Why does
>
#define A stdio
#define B h
A.B
>
produce stdio.h and not stdio . h? Would it not see these as three
individual tokens just as in the 1 . 2 example? Why does it combine
them in this instance and not the other? I suspect it is because
there is no ambiguity with stdio.h, which cannot possibly be confused
as a single token in the main compilation phase, whereas 1.2 can??

Once again, the preprocessor (as far as the standard is concerned)
produces a sequence of tokens that are processed by later compilation
phases. The standard doesn't specify how this sequence of tokens is
represented.

One common implementation (but not the only one) is for the
preprocessor to generate text as output, and for the later compiler
phases to parse the preprocessor's output just as if it were C source
code (i.e., if the preprocessor's input is legal C, its output is also
legal C). In that representation,
stdio.h
and
stdio . h
are exactly equivalent. But given
#define A 1
#define B 2
A.B
the outputs
1.2
and
1 . 2
are different (one token vs. three), so a text-based preprocessor has
to insert blanks to ensure that later compiler phases see the correct
token sequences.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"