dir, err := filepath.Abs(filepath.Dir(os.Args[0]))
if err != nil {
     return err
}

logPath := filepath.Join(dir, "service.log")

f, err := os.OpenFile(logPath, os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)
if err != nil {
     return err
}

p.LogFile = f
log.SetOutput(f)