You can trivially add printf-like statements on the fly with debuggers; with GDB...

You can trivially add printf-like statements on the fly with debuggers; with GDB, one method I often use is the ability to attach commands to breakpoints, to be executed when the breakpoint is hit.

  break SourceFile.cpp:123
  command
    pp var1
    pp var2
    continue
  end

With that `continue` at the end there, this breakpoint will not pause execution (except to run the commands).