Help rounding-off numbers (not using printf)

[Michel Biedermann]

Is there a C function (user defined or primitive (?)) that will let me
truncate or round-off a number UP TO a certain decimal place.  For example
transforming .567 into .6 or -1.502 into -1.5 (i.e. N = round(.567) would
set N to .6.  I do not want to simply print a rounded-off number but be
able to use it as well.
 
Thanks a lot for the help...
 
Michel Biedermann
U. of Oregon