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Use of "empty character constant?"

php.internals

Matt W

20 years ago

Hi all,

I'm a C newbie and just started experimenting with the PHP internals a
couple weeks ago...  A few days ago I noticed assignment/comparison of char
variables to '' (nothing between single quotes) in, for example, the
is_numeric_string function (zend_operators.h), snprintf.c, spprintf.c, etc.
I don't understand what it's doing. :-/  The only references I can find are
about "empty character constant" compiler errors, which is what happens if I
try to use '' in the same way (VC++ Express).  Though PHP compiles fine with
these "constructs," so something must be special there?

Sorry to bother the list with this if it's a general C thing that should be
known and not PHP-code specific.  But I've tried to figure it out on my own
(which I have with other stuff so far :-)) and can't, so I'd appreciate it
if someone can clue me in!


Thanks,
Matt

Pierre Joye

20 years ago

On 7/8/06, Matt W <php_lists@realplain.com> wrote:

> Hi all,
>
> I'm a C newbie and just started experimenting with the PHP internals a
> couple weeks ago...  A few days ago I noticed assignment/comparison of char
> variables to '' (nothing between single quotes) in, for example, the
> is_numeric_string function (zend_operators.h), snprintf.c, spprintf.c, etc.
> I don't understand what it's doing. :-/  The only references I can find are
> about "empty character constant" compiler errors, which is what happens if I
> try to use '' in the same way (VC++ Express).  Though PHP compiles fine with
> these "constructs," so something must be special there?

http://publications.gbdirect.co.uk/c_book/chapter5/character_handling.html

Cheers,
--Pierre

Matt W

20 years ago

Hi Pierre,

Thanks for the reply, but where does that page talk about an EMPTY character
constant?  I pretty much understand character handling otherwise. :-)


Thanks,
Matt


----- Original Message -----
From: "Pierre"
Subject: Re: [PHP-DEV] Use of "empty character constant?"

> On 7/8/06, Matt W wrote:
> > Hi all,
> >
> > I'm a C newbie and just started experimenting with the PHP internals a
> > couple weeks ago...  A few days ago I noticed assignment/comparison of

char

> > variables to '' (nothing between single quotes) in, for example, the
> > is_numeric_string function (zend_operators.h), snprintf.c, spprintf.c,

etc.

> > I don't understand what it's doing. :-/  The only references I can find

are

> > about "empty character constant" compiler errors, which is what happens

if I

> > try to use '' in the same way (VC++ Express).  Though PHP compiles fine

with

Pierre Joye

20 years ago

On 7/8/06, Matt W <php_lists@realplain.com> wrote:

> Hi Pierre,
>
> Thanks for the reply, but where does that page talk about an EMPTY character
> constant?  I pretty much understand character handling otherwise. :-)

Then I do not understand your question. Do you have the exact
error/warning/notice? And some example code?

--Pierre

Matt W

20 years ago

Hi Pierre,

:-O  I was just going to give you an example from zend_operators.h,
is_numeric_string() and guess what?  It's the "correct" way that I thought
it should be!  I thought PHP 6 changed something, to where

/* handle hex numbers */
if (length>=2 && str[0]=='0' && (str[1]=='x' || str[1]=='X')) {
conv_base=16;
}

which is easy to understand :-) became

if (length>=2 && str[0]=='' && (str[1]=='x' || str[1]=='X')) {
conv_base=16;
}

BUT, I recently started looking at http://lxr.php.net so I could
cross-reference, etc., instead of searching my local sources, and that's
where I first saw this "change."  It took you asking for an example for me
to realize that LXR is the problem, screwing up the display of '0' !
(Though I *think* I saw it correctly in some places?)  LOL, I can't believe
it.  I kinda thought, "Oh well, I just don't get it," but I'm glad I
e-mailed before spending more time than I have trying to figure out what the
empty char constant does!

http://lxr.php.net/ident?i=is_numeric_string -- check out 12 lines into the
function definition. :-/

Well, thanks for helping me get on the right track to "solving" this.  The
LXR thing needs fixing I guess.


Matt


----- Original Message -----
From: "Pierre"
Subject: Re: [PHP-DEV] Use of "empty character constant?"

> On 7/8/06, Matt W wrote:
> > Hi Pierre,
> >
> > Thanks for the reply, but where does that page talk about an EMPTY

character

Antony Dovgal

20 years ago

On 08.07.2006 14:48, Matt W wrote:

> Hi Pierre,
>
> :-O  I was just going to give you an example from zend_operators.h,
> is_numeric_string() and guess what?  It's the "correct" way that I thought
> it should be!  I thought PHP 6 changed something, to where
>
>  /* handle hex numbers */
>  if (length>=2 && str[0]=='0' && (str[1]=='x' || str[1]=='X')) {
>     conv_base=16;
>  }
>
> which is easy to understand :-) became
>
>  if (length>=2 && str[0]=='' && (str[1]=='x' || str[1]=='X')) {
>     conv_base=16;
>  }

'0' became '' ?
Why would _character_ 0 become nothing?

--
Wbr,
Antony Dovgal

Matt W

20 years ago

Hi Antony,

----- Original Message -----
From: "Antony Dovgal"

> On 08.07.2006 14:48, Matt W wrote:
> > Hi Pierre,
> >
> > :-O  I was just going to give you an example from zend_operators.h,
> > is_numeric_string() and guess what?  It's the "correct" way that I

thought

> > it should be!  I thought PHP 6 changed something, to where
> >
> >  /* handle hex numbers */
> >  if (length>=2 && str[0]=='0' && (str[1]=='x' || str[1]=='X')) {
> >     conv_base=16;
> >  }
> >
> > which is easy to understand :-) became
> >
> >  if (length>=2 && str[0]=='' && (str[1]=='x' || str[1]=='X')) {
> >     conv_base=16;
> >  }
>
> '0' became '' ?
> Why would _character_ 0 become nothing?

I don't know; was wondering the same thing. :-)  But, that is what I saw on
http://lxr.php.net  :-(


Matt

Antony Dovgal

20 years ago

On 08.07.2006 15:10, Matt W wrote:

> Hi Antony,
>
> ----- Original Message -----
> From: "Antony Dovgal"
>
>
>> On 08.07.2006 14:48, Matt W wrote:
>> > Hi Pierre,
>> >
>> > :-O  I was just going to give you an example from zend_operators.h,
>> > is_numeric_string() and guess what?  It's the "correct" way that I
> thought
>> > it should be!  I thought PHP 6 changed something, to where
>> >
>> >  /* handle hex numbers */
>> >  if (length>=2 && str[0]=='0' && (str[1]=='x' || str[1]=='X')) {
>> >     conv_base=16;
>> >  }
>> >
>> > which is easy to understand :-) became
>> >
>> >  if (length>=2 && str[0]=='' && (str[1]=='x' || str[1]=='X')) {
>> >     conv_base=16;
>> >  }
>>
>> '0' became '' ?
>> Why would _character_ 0 become nothing?
>
> I don't know; was wondering the same thing. :-)  But, that is what I saw on
> http://lxr.php.net  :-(

What do you mean "I saw on lxr"?
I still don't understand a word..

--
Wbr,
Antony Dovgal

Matt W

20 years ago

Hi Antony,

----- Original Message -----
From: "Antony Dovgal"

> On 08.07.2006 15:10, Matt W wrote:
> > Hi Antony,
> >
> > ----- Original Message -----
> > From: "Antony Dovgal"
> >>
> >> '0' became '' ?
> >> Why would _character_ 0 become nothing?
> >
> > I don't know; was wondering the same thing. :-)  But, that is what I saw

on

> > http://lxr.php.net  :-(
>
> What do you mean "I saw on lxr"?
> I still don't understand a word..

Didn't you see my explanation in the e-mail you first replied to? :-)
http://lxr.php.net/source/ZendEngine2/zend_operators.h#82

That's where I saw that " '0' became '' "   The LXR code-cross-referencing
thing wrongly displays '0' , which was the cause of my confusion.


Matt

Antony Dovgal

20 years ago

On 08.07.2006 15:28, Matt W wrote:

> Didn't you see my explanation in the e-mail you first replied to? :-)
> http://lxr.php.net/source/ZendEngine2/zend_operators.h#82
>
> That's where I saw that " '0' became '' "   The LXR code-cross-referencing
> thing wrongly displays '0' , which was the cause of my confusion.

Please report it to LXR developers: http://lxr.linux.no/

--
Wbr,
Antony Dovgal

Michael Wallner

20 years ago

Matt W wrote:

> Hi all,
>
> I'm a C newbie and just started experimenting with the PHP internals
> a couple weeks ago...  A few days ago I noticed assignment/comparison
> of char variables to '' (nothing between single quotes) in, for
> example, the is_numeric_string function (zend_operators.h),
> snprintf.c, spprintf.c, etc. I don't understand what it's doing. :-/
>

I don't see what you're seeing and cannot find a single occurrence of ''.

--
Michael

Mike Dransfield

20 years ago

Michael Wallner wrote:

> Matt W wrote:
>> Hi all,
>>
>> I'm a C newbie and just started experimenting with the PHP internals
>> a couple weeks ago...  A few days ago I noticed assignment/comparison
>> of char variables to '' (nothing between single quotes) in, for
>> example, the is_numeric_string function (zend_operators.h),
>> snprintf.c, spprintf.c, etc. I don't understand what it's doing. :-/
>>
> I don't see what you're seeing and cannot find a single occurrence of ''.
>

http://lxr.php.net/source/ZendEngine2/zend_operators.h#71

I think he is talking about this, it appears as '' on lxr