Red Hat Bugzilla – Bug 141848
Weird rounding of floats
Last modified: 2007-11-30 17:10:56 EST
gcc-3.4.2-6.fc3 Also happens with gcc 3.2 and 4.0-pre (as shipped with FC3) $ cat test.c void test (float zoom) { printf ("%d %f\n", (int) (zoom * 100.0), (zoom * 100)); } int main (int argc, char **argv) { test (1.01); return 0; } $ gcc -o test test.c $ ./test 100 100.999999 It's supposed to print: 100 101.000000
There is nothing wrong on this, that's how floating point works. printf does no rounding. 1.01f is rounded when stored in float and the result is inexact too. E.g. 1.01 rounded to fit into float and double is: float 0x1.028f5cp+0 double 0x1.028f5c28f5c29p+0 and the exact number is actually 0x1.028f5c with period 0x28f5c. As you can see, when rounded to float it is slightly smaller and double is slightly bigger than the exact value. Therefore it is no surpise if this 0x0.0000028f5c28f5c28f5c28f5c28f5c28f5c28f5c28f5c... difference multiplied by 100 already shows up among the important bits.