using '__' in names

[Roman Mashak]

Hello, All!

I often meet that '_' or '__' is used as prefix to
functions/macros/variables names. I wonder does it have some strict meaning?
I didn't find any distinct explanation in C standard or local FAQ.

With best regards, Roman Mashak. E-mail: mr*@tusur.ru

[Jack Klein]

On 16 Aug 2005 20:09:54 -0700, "Robert Gamble" <rg*******@gmai l.com>
wrote in comp.lang.c:

Daniele Benegiamo wrote:

Roman Mashak wrote:

I often meet that '_' or '__' is used as prefix to
functions/macros/variables names. I wonder does it have some strict meaning?

If I remember: all symbols that starts with an underscore or contains
two consecutive underscores are reserved. This means that no portable
programs can uses this sort of identifiers.

Identifiers starting with an underscore are reserved, there is no such
restriction on identifiers containing consecutive underscores.

Your statement is too broad.

Every identifier starting with an underscore in the following snippet
is valid in the application's namespace in the context where it is
declared:

#include <stdio.h>

int main(void)
{
int _123 = 0;
int _abc;
double _;
typedef struct _xyz
{
int _456;
int _all;
} _qrs;
_qrs _def = { 3, 4 };
_abc = _def._456;
_abc *= _def._all;
_ = _abc / 2.0;
printf("%f\n", _);
return _123;
}

And it must produce the output "6.000000" on any conforming C
implementation.

Here is the actual text of the standard for the two cases where the
underscore makes identifiers reserved:

"— All identifiers that begin with an underscore and either an
uppercase letter or another underscore are always reserved for any
use.

— All identifiers that begin with an underscore are always reserved
for use as identifiers with file scope in both the ordinary and tag
name spaces."

Note that leading underscores are specifically not reserved at block
scope as long as they are not followed by a second underscore or an
upper case letter.

--
Jack Klein
Home: http://JK-Technology.Com
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[Jack Klein]

On Wed, 17 Aug 2005 13:15:21 +0900, "Roman Mashak" <mr*@tusur.ru >
wrote in comp.lang.c:

Hello, Jack!
You wrote on Tue, 16 Aug 2005 22:19:54 -0500:

JK> All symbols beginning with an underscore followed by a lower case
JK> letter ("_a" through "_z") are also reserved for the implementation at
JK> file scope.
Actually, this one is wrong, my mistake in quoting from memory instead
of looking it up in the standard. All identifiers beginning with an
'_' are reserved at file scope, so "_0" through "_9" and just plain
"_" are reserved outside of a block.
If I unerstand you right, standard also doesn't recommend these type of
symbols in your own libraries, application etc.? Also I met this in many
SDK, developed by many respected companies.
When is says "reserved for the implementation" it means exactly that,
and your applications and libraries are not part of the
implementation.

Third parties who develop libraries for distribution certainly don't
want their users complaining that including one of their header files
or linking to one of their libraries causes the compile or link to
break, or even worse, the program builds but suddenly has "strange"
errors. Such libraries are often written by the compiler implementer,
in which case they have a perfect right to use those symbols, or for a
particular compiler and the vendor has tested it to make sure there
are no conflicts.
JK> If you look at a header supplied by your compiler, you might see that
JK> it includes something like this at the top:

With best regards, Roman Mashak. E-mail: mr*@tusur.ru

If you do you violate the implementation name space, the C standard
doesn't define what will happen. Most likely any such identifier that
you use won't match one used by your implementation and the program
will work just fine. If the name does clash, you may get the compiler
or linker errors I mentioned above, or the strange problems.

Here are three lines from the stdio.h standard header supplied with
Microsoft Visual C++ 6.0:

#ifndef _INC_STDIO
#define _INC_STDIO

....and:

#endif /* _INC_STDIO */

So what do you think will happen if I use that implementation on this
source file:

#include <stdio.h>

int main(void)
{
int _INC_STDIO = 0;
return _INC_STDIO;
}

???

Replace the string "_INC_STDIO " with nothing, and see what the
compiler has to work with after the preprocessor phase.

The point is, while there are a few cases where you can use a leading
underscore, it's not worth remembering the rules. Just never define
any identifier, including macros, in your programs that begin with an
underscore and you'll never have that particular problem. There is
absolutely no compelling need or any possible gain to doing so.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html

[Emmanuel Delahaye]

Roman Mashak wrote on 17/08/05 :

I often meet that '_' or '__' is used as prefix to functions/macros/variables
names. I wonder does it have some strict meaning? I didn't find any distinct
explanation in C standard or local FAQ.

It has no peculiar meaning, but it belongs to the implementation
namespace. Users should not have names starting with with _[_A-Z]

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

"C is a sharp tool"

[Mark McIntyre]

On 16 Aug 2005 20:09:54 -0700, in comp.lang.c , "Robert Gamble"
<rg*******@gmai l.com> wrote:

Identifiers starting with an underscore are reserved, there is no such
restriction on identifiers containing consecutive underscores.

Yes there is:
All identifiers that begin with an underscore and either an
uppercase letter or another underscore are always reserved for any
use

..
--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

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[Robert Gamble]

Mark McIntyre wrote:

On 16 Aug 2005 20:09:54 -0700, in comp.lang.c , "Robert Gamble"
<rg*******@gmai l.com> wrote:

Identifiers starting with an underscore are reserved, there is no such
restriction on identifiers containing consecutive underscores.

Yes there is:
All identifiers that begin with an underscore and either an
uppercase letter or another underscore are always reserved for any
use

See my response to Daniele.

Robert Gamble

[Keith Thompson]

Mark McIntyre <ma**********@s pamcop.net> writes:

On 16 Aug 2005 20:09:54 -0700, in comp.lang.c , "Robert Gamble"
<rg*******@gmai l.com> wrote:

Identifiers starting with an underscore are reserved, there is no such
restriction on identifiers containing consecutive underscores.

Yes there is:
All identifiers that begin with an underscore and either an
uppercase letter or another underscore are always reserved for any
use

I think the reference was to identifiers like foo__bar.

--
Keith Thompson (The_Other_Keit h) ks***@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.

[Robert Gamble]

Keith Thompson wrote:

Mark McIntyre <ma**********@s pamcop.net> writes:

On 16 Aug 2005 20:09:54 -0700, in comp.lang.c , "Robert Gamble"
<rg*******@gmai l.com> wrote:

Identifiers starting with an underscore are reserved, there is no such
restriction on identifiers containing consecutive underscores.

Yes there is:
All identifiers that begin with an underscore and either an
uppercase letter or another underscore are always reserved for any
use

I think the reference was to identifiers like foo__bar.

Exactly. I was trying to say that having consecutive underscores alone
doesn't put it into the reserved category, I thought my original
wording was pretty clear, I guess I was wrong.

Robert Gamble

[Mark McIntyre]

On 17 Aug 2005 16:13:15 -0700, in comp.lang.c , "Robert Gamble"
<rg*******@gmai l.com> wrote:

Mark McIntyre wrote:

On 16 Aug 2005 20:09:54 -0700, in comp.lang.c , "Robert Gamble"
<rg*******@gmai l.com> wrote:

>Identifiers starting with an underscore are reserved, there is no such
>restriction on identifiers containing consecutive underscores.

Yes there is:

See my response to Daniele.

I read your original post as relating to identifiers starting with
underscores, since thats how the sentence begins. If you meant it to
relate to having two underscores elsewhere in the macro name, that
wasn't at all clear.
--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

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[Robert Gamble]

Mark McIntyre wrote:

On 17 Aug 2005 16:13:15 -0700, in comp.lang.c , "Robert Gamble"
<rg*******@gmai l.com> wrote:

Mark McIntyre wrote:

On 16 Aug 2005 20:09:54 -0700, in comp.lang.c , "Robert Gamble"
<rg*******@gmai l.com> wrote:

>Identifiers starting with an underscore are reserved, there is no such
>restriction on identifiers containing consecutive underscores.

Yes there is:

See my response to Daniele.

I read your original post as relating to identifiers starting with
underscores, since thats how the sentence begins. If you meant it to
relate to having two underscores elsewhere in the macro name, that
wasn't at all clear.

In the first part I used "starting with", in the second I used
"containing ". If I meant identifiers starting with consecutive
underscores, that is what I would have said. I assumed that one could
easily deduct that an identifier beginning with multiple underscores
would be reserved as per the first part of my statement. I don't think
the statement was really that poorly worded but I will try to be more
clear going forward.

Robert Gamble

[Cool Guy]

Robert Gamble <rg*******@gmai l.com> wrote:

I don't think
the statement was really that poorly worded but I will try to be more
clear going forward.

Had you not misused the comma, it might've been clearer, IMO.