re-using variables

[Mitesh]

Hi all,

I have the following code:

$req = "SELECT * FROM table1";
$res = mysql_query($re q);

if(!$res)
return;

while($line = mysql_fetch_arr ay($res))
{
.............
.............
}

$req = "SELECT * FROM table2";
$res = mysql_query($re q);

if(!$res)
return;

while($line = mysql_fetch_arr ay($res))
{
.............
.............
}

What I have discovered is re-using the $req variable is fine. While re-
using the $res produces undefined result (sometimes the second query
works fine and sometimes it doesn't. When I use different variables
like $res1 and $res2 the mysql queries always succeed. Is the initial
approach of re-using variable wrong if variables contain a
resource(refere nce type)?

In other words since PHP uses garbage collection re-using the same
variable can cause problems. Is this true?

[Ivan Marsh]

Re: re-using variables

On Tue, 27 Feb 2007 09:10:28 -0800, Mitesh wrote:

What I have discovered is re-using the $req variable is fine. While re-
using the $res produces undefined result (sometimes the second query
works fine and sometimes it doesn't. When I use different variables like
$res1 and $res2 the mysql queries always succeed. Is the initial
approach of re-using variable wrong if variables contain a
resource(refere nce type)?
>
In other words since PHP uses garbage collection re-using the same
variable can cause problems. Is this true?

Google: mysql free result

[Mitesh]

Re: re-using variables


Ivan Marsh wrote:

On Tue, 27 Feb 2007 09:10:28 -0800, Mitesh wrote:
>

What I have discovered is re-using the $req variable is fine. While re-
using the $res produces undefined result (sometimes the second query
works fine and sometimes it doesn't. When I use different variables like
$res1 and $res2 the mysql queries always succeed. Is the initial
approach of re-using variable wrong if variables contain a
resource(refere nce type)?

In other words since PHP uses garbage collection re-using the same
variable can cause problems. Is this true?

>
Google: mysql free result

Thanx, but that doesn't address my question. I may have another type
of resource other than mysql result. So, should I be concerned about
re-using reference variables in PHP bound to some resource?

[Sjoerd]

Re: re-using variables

Mitesh wrote:

$res = mysql_query($re q);
.............
$res = mysql_query($re q);
In other words since PHP uses garbage collection re-using the same
variable can cause problems. Is this true?

Using the same variable twice is fine. Don't bother too much about the
garbage collector. Since it removes variables when you don't use them
anymore, it never gets in the way.

You probably have another problem which causes things to fail sometimes.

Sjoerd

[Ivan Marsh]

Re: re-using variables

On Tue, 27 Feb 2007 09:57:27 -0800, Mitesh wrote:

>
Ivan Marsh wrote:

>On Tue, 27 Feb 2007 09:10:28 -0800, Mitesh wrote:
>>

What I have discovered is re-using the $req variable is fine. While re-
using the $res produces undefined result (sometimes the second query
works fine and sometimes it doesn't. When I use different variables like
$res1 and $res2 the mysql queries always succeed. Is the initial
approach of re-using variable wrong if variables contain a
resource(refere nce type)?
>
In other words since PHP uses garbage collection re-using the same
variable can cause problems. Is this true?

>>
>Google: mysql free result

>
Thanx, but that doesn't address my question. I may have another type
of resource other than mysql result. So, should I be concerned about
re-using reference variables in PHP bound to some resource?

I haven't had any trouble with it... but then I tend to destroy/create
anything I'm going to re-use.

[Tim Streater]

Re: re-using variables

In article <1172596225.876 668.42750@k78g2 000cwa.googlegr oups.com>,
"Mitesh" <oopsbabies@hot mail.comwrote:

Hi all,
>
I have the following code:
>
$req = "SELECT * FROM table1";
$res = mysql_query($re q);
>
if(!$res)
return;
>
while($line = mysql_fetch_arr ay($res))
{
.............
.............
}
>
$req = "SELECT * FROM table2";
$res = mysql_query($re q);
>
if(!$res)
return;
>
while($line = mysql_fetch_arr ay($res))
{
.............
.............
}
>
What I have discovered is re-using the $req variable is fine. While re-
using the $res produces undefined result (sometimes the second query
works fine and sometimes it doesn't. When I use different variables
like $res1 and $res2 the mysql queries always succeed. Is the initial
approach of re-using variable wrong if variables contain a
resource(refere nce type)?
>
In other words since PHP uses garbage collection re-using the same
variable can cause problems. Is this true?

Pass. But I systematically do mysql_free_resu lt($res) whether I need to
or not, not just before a re-use of $res in another query, but also at
the end of the script. I don't know how it works internally or how much
one needs to worry about this.

[Ivan Marsh]

Re: re-using variables

On Tue, 27 Feb 2007 20:44:47 +0000, Tim Streater wrote:

In article <1172596225.876 668.42750@k78g2 000cwa.googlegr oups.com>,
"Mitesh" <oopsbabies@hot mail.comwrote:
>

>>
>What I have discovered is re-using the $req variable is fine. While re-
>using the $res produces undefined result (sometimes the second query
>works fine and sometimes it doesn't. When I use different variables
>like $res1 and $res2 the mysql queries always succeed. Is the initial
>approach of re-using variable wrong if variables contain a
>resource(refer ence type)?
>>
>In other words since PHP uses garbage collection re-using the same
>variable can cause problems. Is this true?

>
Pass. But I systematically do mysql_free_resu lt($res) whether I need to
or not, not just before a re-use of $res in another query, but also at
the end of the script. I don't know how it works internally or how much
one needs to worry about this.

The resource gets freed automatically at the end of the script. It's
always a good idea to free something you're going to re-use in case your
code isn't working properly you could be left with old data still in the
resource that's hiding where your real issue is.

[Toby A Inkster]

Re: re-using variables

Mitesh wrote:

What I have discovered is re-using the $req variable is fine. While re-
using the $res produces undefined result (sometimes the second query
works fine and sometimes it doesn't. When I use different variables
like $res1 and $res2 the mysql queries always succeed.

Well, obviously.

You have:

$res = mysql_query("SE LECT * FROM table1");
...
$res = mysql_query("SE LECT * FROM table2");
if(!$res)
return;

$res is still going to be set as a result of your first query.

Use unset($res) between each query.

--
Toby A Inkster BSc (Hons) ARCS
Contact Me ~ http://tobyinkster.co.uk/contact
Geek of ~ HTML/SQL/Perl/PHP/Python*/Apache/Linux

* = I'm getting there!

[Jerry Stuckle]

Re: re-using variables

Ivan Marsh wrote:

On Tue, 27 Feb 2007 20:44:47 +0000, Tim Streater wrote:
>

>In article <1172596225.876 668.42750@k78g2 000cwa.googlegr oups.com>,
> "Mitesh" <oopsbabies@hot mail.comwrote:
>>

>>What I have discovered is re-using the $req variable is fine. While re-
>>using the $res produces undefined result (sometimes the second query
>>works fine and sometimes it doesn't. When I use different variables
>>like $res1 and $res2 the mysql queries always succeed. Is the initial
>>approach of re-using variable wrong if variables contain a
>>resource(refe rence type)?
>>>
>>In other words since PHP uses garbage collection re-using the same
>>variable can cause problems. Is this true?

>Pass. But I systematically do mysql_free_resu lt($res) whether I need to
>or not, not just before a re-use of $res in another query, but also at
>the end of the script. I don't know how it works internally or how much
>one needs to worry about this.

>
The resource gets freed automatically at the end of the script. It's
always a good idea to free something you're going to re-use in case your
code isn't working properly you could be left with old data still in the
resource that's hiding where your real issue is.
>
>

One correction. It gets freed when the garbage collector runs. This is
sometime after the script ends, but may not be immediately.

I always free my results, close connections, and generally clean up for
myself. It's just good programming practice.

--
=============== ===
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attgl obal.net
=============== ===

[Jerry Stuckle]

Re: re-using variables

Toby A Inkster wrote:

Mitesh wrote:
>

>What I have discovered is re-using the $req variable is fine. While re-
>using the $res produces undefined result (sometimes the second query
>works fine and sometimes it doesn't. When I use different variables
>like $res1 and $res2 the mysql queries always succeed.

>
Well, obviously.
>
You have:
>
$res = mysql_query("SE LECT * FROM table1");
...
$res = mysql_query("SE LECT * FROM table2");
if(!$res)
return;
>
$res is still going to be set as a result of your first query.
>
Use unset($res) between each query.
>

No, Toby.

$res will have the results of the second query. The resource returned
by the second query (or false) will overwrite what was in $res.

But it's still not good programming practice.


--
=============== ===
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attgl obal.net
=============== ===

[Michael Fesser]

Re: re-using variables

..oO(Jerry Stuckle)

>Toby A Inkster wrote:
>

>You have:
>>
> $res = mysql_query("SE LECT * FROM table1");
> ...
> $res = mysql_query("SE LECT * FROM table2");
> if(!$res)
> return;
>>
>$res is still going to be set as a result of your first query.
>>
>Use unset($res) between each query.

>
>No, Toby.
>
>$res will have the results of the second query. The resource returned
>by the second query (or false) will overwrite what was in $res.

Try that with PDO and it will crash most likely. Overwriting $res will
not necessarily free the previous result set, which might then lead to
MySQL complaining about an unbuffered query or something like that.

| You cannot use the same variable for a PDOStatement object twice. As
| others have pointed out it works when you set this variable to null in
| between.

http://bugs.php.net/bug.php?id=35793

Micha

[Jerry Stuckle]

Re: re-using variables

Michael Fesser wrote:

.oO(Jerry Stuckle)
>

>Toby A Inkster wrote:
>>

>>You have:
>>>
>> $res = mysql_query("SE LECT * FROM table1");
>> ...
>> $res = mysql_query("SE LECT * FROM table2");
>> if(!$res)
>> return;
>>>
>>$res is still going to be set as a result of your first query.
>>>
>>Use unset($res) between each query.

>No, Toby.
>>
>$res will have the results of the second query. The resource returned
>by the second query (or false) will overwrite what was in $res.

>
Try that with PDO and it will crash most likely. Overwriting $res will
not necessarily free the previous result set, which might then lead to
MySQL complaining about an unbuffered query or something like that.
>
| You cannot use the same variable for a PDOStatement object twice. As
| others have pointed out it works when you set this variable to null in
| between.
>

http://bugs.php.net/bug.php?id=35793

>
Micha

We're not talking PDO , Micha.

--
=============== ===
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attgl obal.net
=============== ===

[Mitesh]

Re: re-using variables


Jerry Stuckle wrote:

Michael Fesser wrote:

.oO(Jerry Stuckle)

Toby A Inkster wrote:
>
>You have:
>>
> $res = mysql_query("SE LECT * FROM table1");
> ...
> $res = mysql_query("SE LECT * FROM table2");
> if(!$res)
> return;
>>
>$res is still going to be set as a result of your first query.
>>
>Use unset($res) between each query.
No, Toby.
>
$res will have the results of the second query. The resource returned
by the second query (or false) will overwrite what was in $res.

Try that with PDO and it will crash most likely. Overwriting $res will
not necessarily free the previous result set, which might then lead to
MySQL complaining about an unbuffered query or something like that.

| You cannot use the same variable for a PDOStatement object twice. As
| others have pointed out it works when you set this variable to null in
| between.

http://bugs.php.net/bug.php?id=35793

Micha

>
We're not talking PDO , Micha.
>
--
=============== ===
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attgl obal.net
=============== ===

Ok if we need to call, mysql_free_resu lt for mysql resources doesn't
that mean every other type of resources bound this way to a variable
must have a freeing function that has to be called before the variable
can be re-used? So doesn't it coincide with what I am saying that
variables bound to resources when re-used may cause problems. (and
according to the others post the variables can be re-used if the
previously allocated resource is freed)

[Tim Streater]

Re: re-using variables

In article <1172668483.913 253.134510@h3g2 000cwc.googlegr oups.com>,
"Mitesh" <oopsbabies@hot mail.comwrote:

Jerry Stuckle wrote:

Michael Fesser wrote:

.oO(Jerry Stuckle)
>
>Toby A Inkster wrote:
>>
>>You have:
>>>
>> $res = mysql_query("SE LECT * FROM table1");
>> ...
>> $res = mysql_query("SE LECT * FROM table2");
>> if(!$res)
>> return;
>>>
>>$res is still going to be set as a result of your first query.
>>>
>>Use unset($res) between each query.
>No, Toby.
>>
>$res will have the results of the second query. The resource returned
>by the second query (or false) will overwrite what was in $res.
>
Try that with PDO and it will crash most likely. Overwriting $res will
not necessarily free the previous result set, which might then lead to
MySQL complaining about an unbuffered query or something like that.
>
| You cannot use the same variable for a PDOStatement object twice. As
| others have pointed out it works when you set this variable to null in
| between.
>

http://bugs.php.net/bug.php?id=35793

>
Micha

We're not talking PDO , Micha.

--
=============== ===
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attgl obal.net
=============== ===

>
Ok if we need to call, mysql_free_resu lt for mysql resources doesn't
that mean every other type of resources bound this way to a variable
must have a freeing function that has to be called before the variable
can be re-used? So doesn't it coincide with what I am saying that
variables bound to resources when re-used may cause problems. (and
according to the others post the variables can be re-used if the
previously allocated resource is freed)

My understanding was that, if you re-use $res without a call to
mysql_free_resu lt, it re-uses what $res is, which is a pointer to a
result set, but doing that does *not* free up the space used by the
result set. At least that's how it used to be. I don't know if they have
improved the internals such that mysql_free_resu lt is now a dummy call,
or whether that is inherently impossible because mysql itself holds the
result set.

Perhaps a guru can enlighten us.

-- tim

[Mitesh]

Re: re-using variables


Tim Streater wrote:

In article <1172668483.913 253.134510@h3g2 000cwc.googlegr oups.com>,
"Mitesh" <oopsbabies@hot mail.comwrote:
>

Jerry Stuckle wrote:

Michael Fesser wrote:
.oO(Jerry Stuckle)

Toby A Inkster wrote:
>
>You have:
>>
> $res = mysql_query("SE LECT * FROM table1");
> ...
> $res = mysql_query("SE LECT * FROM table2");
> if(!$res)
> return;
>>
>$res is still going to be set as a result of your first query.
>>
>Use unset($res) between each query.
No, Toby.
>
$res will have the results of the second query. The resource returned
by the second query (or false) will overwrite what was in $res.

Try that with PDO and it will crash most likely. Overwriting $res will
not necessarily free the previous result set, which might then lead to
MySQL complaining about an unbuffered query or something like that.

| You cannot use the same variable for a PDOStatement object twice. As
| others have pointed out it works when you set this variable to null in
| between.

http://bugs.php.net/bug.php?id=35793

Micha
>
We're not talking PDO , Micha.
>
--
=============== ===
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attgl obal.net
=============== ===

Ok if we need to call, mysql_free_resu lt for mysql resources doesn't
that mean every other type of resources bound this way to a variable
must have a freeing function that has to be called before the variable
can be re-used? So doesn't it coincide with what I am saying that
variables bound to resources when re-used may cause problems. (and
according to the others post the variables can be re-used if the
previously allocated resource is freed)

>
My understanding was that, if you re-use $res without a call to
mysql_free_resu lt, it re-uses what $res is, which is a pointer to a
result set, but doing that does *not* free up the space used by the
result set. At least that's how it used to be. I don't know if they have
improved the internals such that mysql_free_resu lt is now a dummy call,
or whether that is inherently impossible because mysql itself holds the
result set.
>
Perhaps a guru can enlighten us.
>
-- tim

My concern is not if the previous resource is deallocated or not. I
don't have a huge result set returned. It will eventually be freed.

What I am really afraid is the working of my current resource pointed
to by $res which I have experienced to be not working occasionally as
I stated that my second fetch array function suceeds some times and
fails other times.

Can the following happen?

When I overwrite $res variable (whatever the variable maybe a pointer
or a reference). Suppose the garbage collector kicks in and finds that
the previous resource isn't owned by any variable and tries to free
it. When it tries to free, it accesses the $res variable. But now the
$res variable owns another resource i.e. my second mysql result set
and wrongly frees the second resource hence my second
mysql_fetch_arr ay fails.

I am sure that I have read that in PHP variables that contain
resources are reference variables (similar to pointers) unlike other
variables like the $req variable storing a string. We can indded make
a variable act like a reference using an ampersand i think.

Yes it seems we need a guru here.

So my would be question to our guru is can we safely re-use variables
that are references?

Right now I resolved the situation using two different variable names
before I got the suggestion on using mysql_free_resu lt. The product is
already shipped and it seems to be working.