sleep has a resolution of one second. It means it can trigger anywhere in that one second and therefore on the average it will be 0.5 seconds "off" the exact mark.
in reply to Re^2: RRD-Ping getting values on right interval
in thread RRD-Ping getting values on right interval
A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James