Flymake is implemented as an Emacs minor mode. It runs the syntax check tool (the compiler for C++ files, perl for perl files, etc.) in the background, passing it a temporary copy of the current buffer and parses the output for known error/warning message atterns. Flymake then highlights erroneous lines (that is, lines for which at least one error or warning has been reported), and displays an overall buffer status in the mode line, as shown on the figure below.