Edit report at https://bugs.php.net/bug.php?id=62240&edit=1
ID: 62240 Updated by: [email protected] 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: Well, you weren't rounding. When you do (int)2.9 that's truncation, not rounding. So your fix would be to round it. Floating point numbers can't always be represented accurately, so you have to account for that in your code. There is nothing we can do in PHP to fix this short of not using the built-in floating point feature, and if we go around that it will be really slow. You can choose to do that yourself by using bcmath/gmp yourself if you prefer, of course. You will find that any other language you try on that machine will give you the same "incorrect" result. Previous Comments: ------------------------------------------------------------------------ [2012-06-08 22:29:34] zuallauz at gmail dot com I ran that new bit of C code: First machine (that works properly) came back with: 3.0000000000000000000000000000000000000000000000000000000000000000 Second machine (not working machine) came back with: 2.9999999999999995559107901499373838305473327636718750000000000000 I think we found the problem! In PHP I think I read in the docs that 'When converting from float to integer, the number will be rounded towards zero.' so that must be why PHP returns a 2 on this machine. Hmm so what's the best way to fix? ------------------------------------------------------------------------ [2012-06-08 09:54:44] [email protected] Strange. The PHP log function just looks like this (from ext/standard/math.c): PHP_FUNCTION(log) { double num, base = 0; if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "d|d", &num, &base) == FAILURE) { return; } if (ZEND_NUM_ARGS() == 1) { RETURN_DOUBLE(log(num)); } if (base <= 0.0) { php_error_docref(NULL TSRMLS_CC, E_WARNING, "base must be greater than 0"); RETURN_FALSE; } if (base == 1) { RETURN_DOUBLE(php_get_nan()); } else { RETURN_DOUBLE(log(num) / log(base)); } } Since you are calling it as log(8,2) you are hitting the last case there. So the only code executed is: RETURN_DOUBLE(log(num) / log(base)); And the RETURN_DOUBLE macro just sets the return value to the double returned by dividing those two log calls. There should be no difference between the little test program and PHP here. Although.. I think gcc might be playing tricks on us here because I used a constant. Try this instead: #include <stdio.h> #include <math.h> int main(char *argv[], int argc) { double base = 2.0; double num = 8.0; printf("%.64f\n",log(num)/log(base)); } Then compile it using: gcc a.c -o a -lm Do you get the same result on both machines? ------------------------------------------------------------------------ [2012-06-08 09:16:26] zuallauz at gmail dot com Yeah Memtest came back with no errors. The test C program returns the same result on both machines. Difference between the machines: Working machine: Intel Pentium 4 single core @2Ghz desktop Ubuntu 10.04 32 bit glibc 2.11.1 gcc 4.4 Not working machine: Intel dual core T2300 @1.66Ghz laptop Ubuntu 12.04 32 bit glibc 2.15 gcc 4.6 ------------------------------------------------------------------------ [2012-06-07 14:08:56] [email protected] I doubt it is bad memory. What's the difference between the two machines? Same architecture? Intel vs. AMD perhaps? Different glibc versions? Different compiler versions? It would be interesting to know what would cause this on some machines but not others. What about the little test C program? Does that return the same result on both machines? ------------------------------------------------------------------------ [2012-06-07 10:38:18] zuallauz at gmail dot com 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! ------------------------------------------------------------------------ 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
