ID: 47168
User updated by: exploringbinary at gmail dot com
Reported By: exploringbinary at gmail dot com
Status: Assigned
Bug Type: Math related
Operating System: *
PHP Version: 5.2.9
Assigned To: rasmus
New Comment:
A reader of my blog pointed out that serialize and unserialize have
similar issues. I ran some tests and discovered that 2^-143 is the
smallest negative power of two that PHP can serialize, and 2^-40 is the
smallest it can unserialize.
Here's are the testcases:
This code works it prints
d:8.9683101716788292539118693330554632401936764280097009392452370
16894662929189507849514484405517578125E-44;
<?php
//Serialize 2^-143
$dyadic = //Compute as (2^-50)^2 * 2^-43 to break up
0.00000000000000088817841970012523233890533447265625*
0.00000000000000088817841970012523233890533447265625*
0.0000000000001136868377216160297393798828125;
echo serialize ($dyadic);
?>
This code fails it prints
d:4.4841550858394146269559346665277316200968382140048504696226185
08447331464594753924757242202758789062E-44;
<?php
//Serialize 2^-144
$dyadic = //Compute as (2^-50)^2 * 2^-44 to break up
0.00000000000000088817841970012523233890533447265625*
0.00000000000000088817841970012523233890533447265625*
0.00000000000005684341886080801486968994140625;
echo serialize ($dyadic);
?>
This code works it prints
0.0000000000009094947017729282379150390625
<?php
//Serialize and unserialize 2^-40
$dyadic = 0.0000000000009094947017729282379150390625;
printf ("%1.40f",unserialize(serialize($dyadic)));
?>
This code fails it prints
Fails, prints 0.0000000000004547473508864641189575195312
<?php
//Serialize and unserialize 2^-41
$dyadic = 0.00000000000045474735088646411895751953125;
printf ("%1.41f",unserialize(serialize($dyadic)));
?>
Previous Comments:
------------------------------------------------------------------------
[2009-04-30 03:48:24] [email protected]
There are a couple of other places that need to be changed. I have a
patch but I haven't had a chance to go through and fix the test cases
yet. I'll get to it before RC2 next week.
------------------------------------------------------------------------
[2009-04-30 01:23:58] ivan dot rey at inpltda dot com
I agree also.
Full precision should be available for the developer.
I wonder if recompiling php with a different CAP in In
php-5.2.8\ext\standard\formatted_print.c I found this:
#define MAX_FLOAT_PRECISION 40 is the actual solution or if it has some
further implications.
------------------------------------------------------------------------
[2009-04-29 05:21:05] [email protected]
I agree with Rick here. The 40-digit limit is strangely arbitrary and
doesn't match the size of the double. We are hiding available precision
here.
------------------------------------------------------------------------
[2009-01-30 21:51:56] exploringbinary at gmail dot com
Since I didn't find anything in the documentation, I poked around in
the source code. In php-5.2.8\ext\standard\formatted_print.c I found
this: #define MAX_FLOAT_PRECISION 40
There is no explanation as to why it's capped at 40.
What's interesting is that this code prints only 40 decimal places
(0.0000000000004547473508864641189575195312), without a warning:
<?php
/* Print 2^-41 */
$dyadic = 0.00000000000045474735088646411895751953125;
printf ("%1.2147483646f",$dyadic);
?>
While this code prints ``Warning: printf() [function.printf]: Precision
must be greater than zero and less than 2147483647 ...''
<?php
/* Print 2^-41 */
$dyadic = 0.00000000000045474735088646411895751953125;
printf ("%1.2147483647f",$dyadic);
?>
If you're going to warn about a maximum, why not warn about the real
maximum -- 40, not 2147483646?
I wrote an article about how this issue is handled in PHP and 7 other
languages at
http://www.exploringbinary.com/print-precision-of-dyadic-fractions-varies-by-language/
------------------------------------------------------------------------
[2009-01-20 22:26:19] exploringbinary at gmail dot com
Hi,
I read the documentation and the "how to submit a bug report" and
googled as well before (and now again after) submitting the bug report.
I cannot see discussion of this limit. Could you please give me a more
specific link?
I would also like to know the rationale for the limit. Why doesn't
javascript or gcc have this limit? Why did Microsoft accept my bug
report for a similar problem in VS? It seems like something PHP should
consider, or at least give rationale for why not.
Thanks.
------------------------------------------------------------------------
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the rest of the comments, please view the bug report online at
http://bugs.php.net/47168
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