Bug #62240 [Nab]: 5.4.3 regression, converting from float to int gives incorrect output

Edit report at https://bugs.php.net/bug.php?id=62240&edit=1 ID: 62240 User updated by: zuallauz at gmail dot com Reported by: zuallauz at gmail dot com Summary: 5.4.3 regression, converting from float to int gives incorrect output Status: Not a bug Type: Bug Package: *General Issues Operating System: Ubuntu 12.04 32-bit PHP Version: 5.4.3 Block user comment: N Private report: N New Comment: Yeah originally I had compiled my own PHP using the flags in the first post. I re-downloaded php-5.4.3.tar.bz2 from PHP.net and just did a basic ./configure && make then ran log(8,2) using sapi/cli/php test.php and the output was still the same incorrect result: float(3) int(2) However I have just tried the same thing on my other machine running 32bit Ubuntu 10.04 with 5.4.3 and it outputs correctly: float(3) int(3) So maybe there's a screw loose/bad memory in the first machine or something. I don't have an explanation for it. Probably not a bug after all, sorry! Previous Comments: ------------------------------------------------------------------------ [2012-06-07 09:14:24] rasmus@php.net That's interesting because that is exactly what is happening when you call PHP's log() function. log(8,2) in PHP ends up being log(8)/log(2) in C. Did you compile your PHP yourself? If not, could you try grabbing the 5.4.3 tarball and doing a simple: ./configure && make Building just the cli version is enough. Then run your log(8,2) test with sapi/cli/php test.php ------------------------------------------------------------------------ [2012-06-07 08:48:37] zuallauz at gmail dot com Hmm, I get the same output with 64 zeros: 3.0000000000000000000000000000000000000000000000000000000000000000 ------------------------------------------------------------------------ [2012-06-07 08:08:25] rasmus@php.net Well, float to int conversion issues isn't a PHP-specific thing. Try this little C program on your machine, for example: #include <stdio.h> #include <math.h> int main(char *argv[], int argc) { printf("%.64f\n",log(8)/log(2)); } If you name the file, "a.c" you can just type: make a then run it with: ./a My output on 64-bit Ubuntu: 10:07am x220:~> ./a 3.0000000000000000000000000000000000000000000000000000000000000000 It would be interesting to see your output. ------------------------------------------------------------------------ [2012-06-07 07:50:15] zuallauz at gmail dot com Actually I may just use the code below now that I've been reading up on the perils of converting from float to int in PHP: $num = log(8, 2); $num = (int) number_format($num, 0, '.', ''); ------------------------------------------------------------------------ [2012-06-07 07:25:04] zuallauz at gmail dot com I have put in a quick hack for now by converting from float to string first then converting to int. This seems to get the correct output: $num = (int) (string) log(8, 2); var_dump($num); Output: int(3) I don't think the problem lies in the log() function at all as that correctly retrieves the float(3) result. It's the conversion from the float to integer that is doing the incorrect calculation. Likely the log(8, 2) does not result in a perfect 3, it might actually be 2.9999999999999999 so there could be a conversion error happening. My test machine is running Ubuntu server 32-bit, so maybe the 64-bit edition has enough precision to do the integer conversion properly. I was talking to a colleague today and he mentioned there were some float to integer optimisations done in 5.3.11 that changed things? ------------------------------------------------------------------------ The remainder of the comments for this report are too long. To view the rest of the comments, please view the bug report online at https://bugs.php.net/bug.php?id=62240 -- Edit this bug report at https://bugs.php.net/bug.php?id=62240&edit=1