Title:       GNU Flymake -- an on-the-fly syntax checker for GNU Emacs
Version:     0.3
Environment: GNU emacs, GNU make
Description: Performs on-the-fly syntax checks of the files being edited
             using the external syntax check tool (usually the compiler).
             Highlights erroneous lines and displays associated
             error messages.


Switching to the GNU Emacs/GNU Make environment from the Microsoft Visual Studio 6.0/Visual Assist 6.0 pair had both positive and negative consequences. On one hand, memory consumption was dramatically reduced, making many things work much faster then they used to. On the other hand, such nice IDE features as context aware auto-completion and auto parameter info were no longer available. This resulted in an increased amount of typos and errors, not to be revealed until compilation time.

Furthermore, even Visual Assist was not able to correctly parse all of the C++ constructs I've been using. This is especially true for non-trivial templates as well as macros. In the end, the ultimate tool for checking syntax is the compiler, as it can easily handle any valid constructs and report errors if any. So, having compiler make on-the-fly syntax checks of the source code being edited seems like a good idea.

Users of some modern Java IDEs, like IntelliJ IDEA, enjoy this feature for a relatively long time. I don't know of any C++ IDE with this feature, and Visual C++ 6.0/.NET lack it as well. So came the idea of developing of an on-the-fly syntax checker for Emacs -- flymake.

Flymake overview

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 patterns. 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.

flymake in cpp file

File Func.c here has 2 errors and 1 warning. Using flymake-goto-next-error, it is posible to jump to erroneous line 7, highlighted in red, and calling flymake-display-err-menu-for-current-line will popup a menu containing error text provided by the compiler. Line 4 is highlighted in blue, as it only has a warning attached to it. Line 1 is red, but as calling flymake-display-err-menu-for-current-line shows, the error really belongs to line 5 of Func.h.

flymake in cpp file

Selecting the menu item will jump to line 5 of Func.h, which has the same error, as shown on the next figure.

flymake in h file

Notice that line 1 of Func.h is highlighted in red, as Func.c, which was actually used to syntax check the .h file, still contains 2 errors and 1 warning.

Syntax check is done 'on-the-fly', using some external check tool, which is most commonly a compiler (gcc for the pictures above).

Syntax check is started whenever a) buffer is loaded, b) a newline character is added to the buffer, and c) some changes were made to the buffer more than t seconds ago (t is configurable).

How flymake works

When flymake mode is active, any of the 3 conditions stated above will cause flymake to try to syntax check the current buffer. Flymake first determines whether it is able to do syntax check. It then saves a copy of the buffer in a temporary file in the buffer's directory (or in the system temp directory -- for java files), creates a syntax check command and launches a process with this command. The output is parsed using known error message patterns, and error information (file name, line number, type and text) is saved. After the process has finished, flymake highlights erroneous lines in the buffer using the accumulated error information.

Determining whether syntax check is possible

A simple algorithm based of buffer filename is used to determine whether a buffer can be syntax checked. There are 3 possible options:

Syntax check is configured by mapping a regular expression for a filename to the following 3 functions:

Flymake contains implementations of all functionality required to support different syntax check modes described above (making temporary copies, finding master files, etc.), as well as some tool-specific code.

Mapping of filename regexp to syntax check functions is contained in flymake-allowed-file-name-masks. For example, the following entry is used for c sources:

(".+\\.c$" flymake-simple-make-init flymake-simple-cleanup flymake-get-real-file-name)

flymake-simple-make-init creates a temporary copy and configures make command line, flymake-simple-cleanup deletes the temporary copy, and flymake-get-real-file-name is a universal function for resolving file names.

Syntax check is considered possible for a buffer if there's a corresponding entry in flymake-allowed-file-name-masks.

Making a temporary copy

After the possibility of the syntax check has been determined, a temporary copy of the current buffer is made so that the most recent unsaved changes could be seen by the syntax check tool. Making a copy is quite straightforward in a standalone case (mode 1), as it's just saving buffer contents in a file. Things get trickier, however, when master file is involved, as it requires to a) locate master file and b) patch it to include the current file using its new (temporary) name. Locating master file is discussed in the following section.

Patching just changes all appropriate lines of the master file so that they use the new (temporary) name of the current file. For example, suppose current file name is File.h, the master file is File.cpp, and it includes current file via #include "File.h". Current file's copy is saved to file File_flymake.h, so the include line must be changed to #include "File_flymake.h". Finally, patched master file is saved to File_flymk_master.cpp, and the last one is passed to the syntax check tool.

Locating a master file

Master file is located in two steps.

First, a list of possible master files is built. Again, a simple name matching is used to find the files. For a C++ header File.h, flymake searches for all .cpp files in the directories whose relative paths are stored in a customizable variable flymake-master-file-dirs, which usually contains something like ("." "./src"). No more than flymake-master-file-count-limit entries is added to the master file list. The list is then sorted to move files with names File.cpp to the top.

Next, each master file in a list is checked to contain the appropriate include directives. No more than flymake-check-file-limit of each file are parsed.

For File.h, the include directives to look for are #include "File.h", #include "../File.h", etc. Each include is checked via the include directories (next section) to be sure it points to the correct File.h.

First matching master file found stops the search. The master file is then patched and saved to disk. In case no master file was found, syntax check is aborted, and corresponding status (!) is reported in the mode line.

Getting the include directories

Two sets of include directories are distinguished: system include directories and project include directories. The former is just the contents of the INCLUDE environment variable. The latter is not so easy to obtain, and the way it can be obtained can vary greatly for different project. Therefore, a customizable variable flymake-get-project-include-dirs-function is used to provide the way to implement the desired behavior.

The default implementation, flymake-get-project-include-dirs-imp, uses a make call. This requires a correct base directory, that is, a directory containing a correct Makefile, be determined.

As obtaining the project include directories might be a costly operation, its return value is cached in the hash table. The cache is cleared in the beginning of every syntax check attempt.

Locating the buildfile

Flymake can be configured to use different tools for performing syntax checks. For example, direct compiler call to syntax check a perl script or a call to make for a more complicated case of a C++ source. The general idea is that simple files, like perl scripts and html pages, can be checked by directly invoking a corresponding tool. Files that are usually more complex and generally used as part of larger projects, might require non-trivial options be passed to the syntax check tool, like include directories for C++. The latter files are syntax checked via a call to make.

All make configuration data is usually stored in a file called Makefile. To allow for future extensions, flymake uses a notion of buildfile to reference the 'project configuration' file.

Special function, flymake-find-buildfile is provided for locating buildfiles. Searching for a buildfile is done in a manner similar to that of searching for possible master files. A customizable variable flymake-buildfile-dirs holds a list of relative paths to the buildfile. They are checked sequentially until a buildfile is found. In case there's no build file, syntax check is aborted. Buildfile values are also cached.

Starting the syntax check process

The command line for launching a process is returned by the initialization function. Flymake then just calls start-process to start an asynchronous process and configures process filter and sentinel which is used for processing the output of the syntax check tool.

Parsing the output

The output generated by the syntax check tool is parsed in the process filter using the error message patterns stored in the flymake-err-line-patterns variable. This variable contains a list of items of the form (regexp file-idx line-idx err-text-idx), used to determine whether a particular line is an error message and extract file name, line number and error text, respectively. Error type (error/warning) is also guessed by matching error text with the '^[wW]arning' pattern. Anything that was not classified as a warning is considered an error. Type is then used to sort error menu items, which shows error messages first.

Flymake is also able to interpret error message patterns missing err-text-idx information. This is done by merely taking the rest of the matched line ((substring line (match-end 0))) as error text. This trick allows to make use of a huge collection of error message line patterns from compile.el. All these error patterns are appended to the end of flymake-err-line-patterns.

The error information obtained is saved in a buffer local variable. The buffer for which the process output belongs is determined from the process-id -- buffer mapping updated after every process launch/exit.

Highlighting erroneous lines

Highlighting is implemented with overlays and happens in the process sentinel, after calling the cleanup function. 2 customizable faces are used: flymake-errline-face and flymake-warnline-face. Errors belonging outside the current buffer are considered to belong to line 1 of the current buffer.

Displaying flymake status in the mode line

After syntax check is finished, its results are displayed in the mode line. The following statuses are defined.

Flymake* or Flymake:E/W* Flymake is currently running. For the second case, E/W contains the error and warning count for the previous run.
Flymake Syntax check is not running. Usually this means syntax check was successfully passed (no errors, no warnings). Other possibilities are: syntax check was killed as a result of executing flymake-compile, or syntax check cannot start as compilation is currently in progress.
Flymake:E/W Number of errors/warnings found by the syntax check process.
Flymake:! Flymake was unable to find master file for the current buffer.

The following errors cause a warning message and switch flymake mode OFF for the buffer.

CFGERR Syntax check process returned nonzero exit code, but no errors/warnings were reported. This indicates a possible configuration error.
NOMASTER Flymake was unable to find master file for the current buffer.
NOMK Flymake was unable to find buildfile for the current buffer.
PROCERR Flymake was unable to launch syntax check process.

Error menu

For each erroneous line, a popup menu with all errors reported for that line can be displayed. It is built from the error information stored in the buffer local variables. Selecting the item whose error belongs to another file brings forward that file with the help of the flymake-goto-file-and-line function.

Interaction with other modes

The only mode flymake currently knows about is compile.

Flymake can be configured to not start syntax check if it thinks the compilation is in progress. The check is made by the flymake-compilation-is-running, which tests the compilation-in-progress variable. The reason why this might be useful is saving CPU time in case both syntax check and compilation are very CPU intensive. The original reason for adding this feature, though, was working around a locking problem with Visual C++ compiler.

Flymake also provides an alternative command for starting compilation, flymake-compile:

(defun flymake-compile()
    "kill all flymake syntax checks, start compilation"
    (call-interactively 'compile)

It just kills all the active syntax check processes before calling compile.

Performance issues

The whole idea of the on-the-fly syntax check will be compromised in case any single syntax check will require a significant amount of time to complete. So flymake is certainly meant to be used on a fast machine. Also, any possible optimizations, like switching on precompiled headers generation, will be of great help.

To give you some numbers, here's the check times for several C++ source files compiled on a Pentium-IV 1.7 GHz machine.

Number of lines in a file Precompiled headers Syntax check time, in seconds
1000 ON 1.0
1000 OFF 3.6
500 ON 0.7
500 OFF 3.1
10 ON 0.5
10 OFF 0.1

1 second delay is barely noticeable, and even with a 2-3 seconds delay on-the-fly syntax check might still be very useful.

Supported languages/compilers

One of ideas behind Flymake was to create a universal syntax checker. Basically, all that is necessary to use flymake with a certain syntax check tool is to provide the error message patterns for that tool (in case they aren't already there) and write the code to create the command line to be used to run the tool.

The following table provides a summary of languages/compilers flymake was tested with.

Language Compiler OS
C/C++ Microsoft Visual C++ 6.0 Windows 2000
C/C++ gcc 2.9 Windows 2000/XP/Linux
MS IDL midl 5.01 Windows 2000
Java IBM Jikes 1.17 Windows 2000
Latex MikTeX 2.2 Windows 2000
HTML HTML Tidy Windows 2000/XP
XML XMLStarlet Command Line XML Toolkit 0.7.0 Windows 2000/XP
Perl Perl 5.8 Windows 2000/XP

Known problems

Flymake makes syntax check attempts quite often. It might be unable to launch syntax check process for some reasons (no master file, etc.). Nevertheless, the failed attempts themselves might take some time (getting include directories, looking for a master file), interfering with the editing process, which is sometimes annoying. The possible solution is to switch off flymake-mode for such buffers.

Algorithms used for locating buildfiles and master files are quite simple, and won't cover all the use cases.

Flymake was primarily tested in Windows (Windows 2000, XP) environment, with only a few tests made on Linux (KNOPPIX 3.2 RE). So, switching to other operating system/build tool/compiler might require some customizations or even fixes.


Flymake might be of great help for editing the source code. When seen immediately, errors and warnings are much easier to fix. Warnings issued during compilation are sometimes left alone, as the project still 'builds OK' (unless 'treat warnings as errors' option is used). With flymake, those warnings will be displayed immediately, and so more likely be fixed.

Flymake requires quite a fast machine to run, but fast machines are not so rare nowadays.

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