Tutorial: Emacs for Programmers

Many programs are accompanied by a ChangeLog, which describes updates and modifications to the source on a day-to-day basis. Emacs allows you to semi-automatically update the ChangeLog, using the command M-x add-change-log-entry (or, C-x 4 a, which will do the same in another window).

For example, let's say that we're editing the source file grover.c, and we add a bit of code to grover(). To document this change, we use C-x 4 a, which will open a window containing:

Sun Jul 24 14:39:50 1994 Matt Welsh (mdw@loomer.debian.org
)
        * grover.c (grover):

This command determines that we are within the function grover(), in the file grover.c, and indicates this at the beginning of the entry. We can now enter a new log entry and save the file in the usual way. Each source file that you add entries for will be given its own item.

You can compile programs, and even run a debugger, entirely within Emacs. The most basic compilation command is M-x compile, which will run make (or another command of your choice) in the directory of the current buffer.

The default compilation command is make -k.

(The -k switch will prevent make from halting on an error which has no bearing on other targets in the makefile.) When using M-x compile, you will be prompted for the compilation command to use, and also whether you wish to save any buffers that have changed. If you wish to change the default command, set the variable compile-command to another value. For example,

(setq compile-command "make")

will cause M-x compile to run make without the -k argument.

You can also set a value for compile-command for a particular source file. This is done by including “local variable definitions” within the source file itself. For example, we could include the following comments within a C source file:

/* Local Variables: */
/* mode: C */
/* compile-command: "make" */
/* End: */

These comments set the values of the mode and compile-command variables for the buffer containing this code. When the file is opened by Emacs, it recognizes the line containing Local Variables: and uses subsequent lines, until the line containing End:, to assign values to variables for this buffer alone. You can use this feature to set values for any Emacs variables specific to this buffer.

Now, when we use M-x compile, Emacs runs the given compilation command (here, make) in another window, with which we can monitor the progress of the compilation. To kill the compilation process, type C-c C-k in the compilation buffer.

Once the compilation completes, we can use the error messages printed (if any) to automatically visit the source which caused the errors. For example, let's say that we use M-x compile and the following errors result:

cd /amd/noon/c/mdw/test/lj/
make
gcc -O -O2 -I/usr/include -I. -c main.c -o main.o
In file included from main.c:12:
libpx.h:30: image.h: No such file or directory
libpx.h:31: misc.h: No such file or directory
make: *** [main.o] Error 1
Compilation exited abnormally with code 1 at Sun Jul 24 16:32:17

Instead of manually locating libpx.h and jumping to the line in question, you can move point to the error message in the compilation buffer, press C-c C-c, and the source corresponding to the error is automatically visited. You can then correct the bug, move to the next error, and repeat. Under X, pressing mouse-2 on an error message in the compilation buffer will jump to the corresponding source line.

If you have a large number of error messages, pressing M-n in the compilation buffer will move to the next error message (in addition to viewing the corresponding source). To cause M-n to have this behavior within the C source buffer as well, you can use the following command in your Emacs startup file: (define-key c-mode-map "\M-n" 'next-error)

To simplify the compilation process, I use the following code within my Emacs configuration file:

(defun my-save-and-compile ()
  (interactive "")
  (save-buffer 0)
  (compile "make -k"))
(define-key c-mode-map "\C-c\C-c' 'my-save-and-compile)

This defines a new function, my-save-and-compile, which will automatically save the current buffer and run make -k. This saves me the hassle of answering the various prompts given by M-x compile alone. Now, using C-c C-c within a C Mode buffer will save the source and compile it.

Once you get used to the above mechanism, fixing bugs and recompiling code becomes quite painless—you can concentrate on debugging and let Emacs locate the errors, run make, and so forth.