Mail Archives: cygwin/2009/12/24/16:19:22
Hi,
I have wondering why when I do floating-point math in Cygwin 1.7, Cygwin does not output the "correct" result, eg:
main () {
double qcb[4] = {0.41585680180318363, 0.70823637211274604, 0.089200955545759006, -0.56347808399291521};
double qba[4] = {0.09648538897087118, -0.37032037362514358, 0.89403246842889339, 0.23293633462432001};
double qca[4];
qca[0]= qcb[3]*qba[0] + qcb[2]*qba[1] - qcb[1]*qba[2] + qcb[0]*qba[3];
qca[1]= -qcb[2]*qba[0] + qcb[3]*qba[1] + qcb[0]*qba[2] + qcb[1]*qba[3];
qca[2]= qcb[1]*qba[0] - qcb[0]*qba[1] + qcb[3]*qba[2] + qcb[2]*qba[3];
qca[3]= -qcb[0]*qba[0] - qcb[1]*qba[1] - qcb[2]*qba[2] + qcb[3]*qba[3];
// I am expecting the result to be:
printf("qca = [-0.623718486146499718, 0.736824293298044886, -0.260654850643024127, 0.011147182658310384] CORRECT (LINUX/SOLARIS)\n");
// I get the above result on both Linux/Solaris AND even when running under Cygwin's gdb and using gdb's printf
// but when I compile and run on cygwin, I get the wrong result:
printf("qca = [%.18f, %.18f, %.18f, %.18f] NOT CORRECT (CYGWIN)\n",qca[0],qca[1],qca[2],qca[3]);
}
Here is the output when I compiled and run on cygwin:
qca = [-0.623718486146499718, 0.736824293298044886, -0.260654850643024127, 0.011147182658310384] CORRECT (LINUX/SOLARIS)
qca = [-0.623718486146499607, 0.736824293298044886, -0.260654850643024072, 0.011147182658310373] NOT CORRECT (CYGWIN)
Here's the output when running under cygwin's gdb (it matches Linux/Solaris outputs but not cygwin’s output):
(gdb) printf "%.18f\n", qcb[3]*qba[0] + qcb[2]*qba[1] - qcb[1]*qba[2] + qcb[0]*qba[3]
-0.623718486146499718
(gdb) printf "%.18f\n", -qcb[2]*qba[0] + qcb[3]*qba[1] + qcb[0]*qba[2] + qcb[1]*qba[3]
0.736824293298044886
(gdb) printf "%.18f\n", qcb[1]*qba[0] - qcb[0]*qba[1] + qcb[3]*qba[2] + qcb[2]*qba[3]
-0.260654850643024127
(gdb) printf "%.18f\n", -qcb[0]*qba[0] - qcb[1]*qba[1] - qcb[2]*qba[2] + qcb[3]*qba[3]
0.011147182658310384
(gdb)
These errors accumulate and eventually builds up so much that the my simulation result is totally wrong when running under cygwin.
Please help.
Thanks,
Linh
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