Notice that the statement motion commands have the same key bindings as backward-sentence and forward-sentence, respectively. In fact, they act as sentence commands if you use them within a C comment.
Similarly, M-q is normally the fill-paragraph command; C mode augments it with the ability to preserve indentations and decorative characters at the beginnings of lines. For example, if your cursor is anywhere in this comment:
/* This is
* a
* comment paragraph with wildly differing right
* margins.
* It goes on for a while,
* then stops. */
typing M-q has this result:
/* This is a comment paragraph with wildly differing right margins.
* It goes on for a while, then stops. */
You will find that the preprocessor conditional motion commands are a godsend if you have to slog through someone else's voluminous code. Especially if you're faced with code built to run on a variety of systems—like Emacs itself—often the most important question you need answered is, "What code is actually compiled?"
With C-c C-u, you can tell instantly what preprocessor conditional governs the code in question. Consider this code block:
#define LUCYX
#define BADEXIT -1
#ifdef LUCYX
...
*ptyv = open ("/dev/ptc", O_RDWR | O_NDELAY, 0);
if (fd < 0)
return BADEXIT;
...
#else
...
fprintf (stderr, "You can't do that on this system!");
...
#endif
Imagine that the ellipses (...) represent hundreds of lines of code. Now suppose you are trying to determine under what conditions the file /dev/ptc is opened. If your cursor is on that line of code, you can type C-c C-u, and the cursor moves to the line #ifdef LUCYX—telling you that the code is compiled if you're on a LUCYX system. If you want to skip the code that would not be compiled and go directly to the end of the conditional, type C-c C-n. We will see another command that is useful for dealing with C preprocessor code later in this section.
C statement and statement block delimiter characters are bound to commands that, in addition to inserting the appropriate character, also provide proper indentation. These characters are {, }, ;, and : (for labels and switch cases). For example, if you are closing out a statement block or function body, you can press C-j (or Enter) and type }, and Emacs lines it up with its matching {. This eliminates the need for you to scroll back through the code to find out what column the { is in.
Because } is a parenthesis-type character, Emacs attempts to "flash" a matching { when you type }. If the matching { is outside of the text displayed in your window, Emacs instead prints the line containing the { in the minibuffer. Furthermore, if only whitespace (blanks or tabs) follows the { on its line, Emacs also prints a ^J (for C-j) followed by the next line, thus giving a better idea of the context of the {.
Recall the "times" example earlier in this chapter. Let's say you are typing in a } to end the function, and the { that begins the function body is off-screen. There is no code on the line following the beginning {, so you see the following in the minibuffer after you type }:
Matches {^J int i;
9.3.2 Customizing Code Indentation Style
Coding style in C—or any programming language for that matter—is a very personal thing. C programmers learn from various books or by referring to various different pieces of other people's code; eventually they evolve a personal style that may or may not conform to those that they learned from.
C mode provides a rich set of features for customizing its indentation behavior that mirrors this way of learning the language. At the simplest level, you can choose a coding style by name. Then, if you're not satisfied, you can customize your chosen style or even create your own from scratch. The latter tasks, however, require a fair amount of Emacs Lisp programming knowledge (see Chapter 11) and perhaps a bit of bravery.
You can choose a named coding style with the command M-x c-set-style. This command prompts you for the name of the style you want. The easiest thing to do at this point is to type Tab, the completion character (see Chapter 14), which brings up a *Completions* window that lists all of the choices. Type one of them and press Enter to select it.
By default, Emacs comes loaded with the styles shown in Table 9-4.
Table 9-4. Built-in cc-mode indentation styles
| Style | Description |
|---|---|
| bsd | Style used in code for BSD-derived versions of Unix. |
| cc-mode | The default coding style, from which all others are derived. |
| ellemtel | Style used in C++ documentation from Ellemtel Telecommunication Systems Laboratories in Sweden. |
| gnu | Style used in C code for Emacs itself and other GNU-related programs. |
| java | Style used in Java code (the default for Java mode). |
| k&r | Style of the classic text on C, Kernighan and Ritchie's The C Programming Language. |
| linux | Style used in C code that is part of the Linux kernel. |
| python | Style used in python extensions. |
| stroustrup | C++ coding style of the standard reference work, Bjarne Stroustrup's The C++ Programming Language. |
| user | Customizations you make to .emacs or via Custom (see Chapter 10). All other styles inherit these customizations if you set them. |
| whitesmith | Style used in Whitesmith Ltd.'s documentation for their C and C++ compilers. |