| Aladdin's C coding guidelines |
|---|
For other information, see the Ghostscript overview.
gx_device_fubar * const fdev = (gx_device_fubar *)dev;
rather than
#define fdev ((gx_device_fubar *)dev)
The many rules that Ghostscript's code follows almost everywhere are meant to produce code that is easy to read. It's important to observe them as much as possible in order to maintain a consistent style, but if you find a rule getting in your way or producing ugly-looking results once in a while, it's OK to break it.
Using preprocessor conditionals can easily lead to unreadable code, since the eye really wants to read linearly rather than having to parse the conditionals just to figure out what code is relevant. It's OK to use conditionals that have small scope and that don't change the structure or logic of the program (typically, they select between different sets of values depending on some configuration parameter), but where possible, break up source modules rather than use conditionals that affect the structure or logic.
In .c files don't use preprocessor conditionals that test for individual platforms or compilers. Use them only in header files named xxx_.h.
Ghostscript code uses macros heavily to effectively extend the rather weak abstraction capabilities of the C language, specifically in the area of memory management and garbage collection: in order to read Ghostscript code effectively, you simply have to learn some of these macros as though they were part of the language. The current code also uses macros heavily for other purposes, but we are trying to phase these out as rapidly as possible, because they make the code harder to read and debug, and to use the rules that follow consistently in new code.
Define macros in the smallest scope you can manage (procedure, file, or .h file), and #undef them at the end of that scope: that way, someone reading the code can see the definitions easily when reading the uses. If that isn't appropriate, define them in as large a scope as possible, so that they effectively become part of the language. This places an additional burden on the reader, but it can be amortized over reading larger amounts of code.
Try hard to use procedures instead of macros. Use "inline" if you really think the extra speed is needed, but only within a .c file: don't put inline procedures in .h files, because most compilers don't honor "inline" and you'll wind up with a copy of the procedure in every .c file that includes the .h file.
Don't use macros to define shorthands for casted pointers. For instance, avoid
#define fdev ((gx_device_fubar *)dev)
and instead use
gx_device_fubar * const fdev = (gx_device_fubar *)dev;
The use of const alerts the reader that this is effectively a synonym.
If you define a macro that looks like a procedure, make sure it will work wherever a procedure will work. In particular, put parentheses around every use of an argument within the macro body, so that the macro will parse correctly if some of the arguments are expressions, and put parentheses around the entire macro body. (This is still subject to the problem that an argument may be evaluated more than once, but there is no way around this in C, since C doesn't provide for local variables within expressions.)
If a macro generates anything larger than a single expression (that is, one or more statements), surround it with BEGIN and END. These work around the fact that simple statements and compound statements in C can't be substituted for each other syntactically.
If you define macros for special loop control structures, make their uses look somewhat like ordinary loops, for instance:
BEGIN_RECT(xx, yy) {
... body indented one position ...
} END_RECT(xx, yy);
If at all possible, don't use free variables in macros -- that is, variables that aren't apparent as arguments of the macro. If you must use free variables, list them all in a comment at the point where the macro is defined.
Preferably CAPITALIZE macro names.
Use const for pointer referents (that is, const T *) wherever possible and appropriate. Don't bother using it for anything else, except as described above for casted pointers.
If you find yourself wanting to use void *, try to find an alternative using unions or (in the case of super- and subclasses) casts, unless you're writing something like a memory manager that really treats memory as opaque.
Don't use anonymous structures if you can possibly avoid it, except perhaps as components of other structures. Ideally, use the "struct" keyword only for declaring named structure types, like this:
typedef struct xxx_yyy_s {
... members ...
} xxx_yyy_t;
Many older structure names don't have _t on the end, but it should be used in all new code. (The _s name is needed only to satisfy some debuggers. No code should use it.)
If a procedure parameter is itself a procedure, do list its parameter types rather than just using (). For example,
int foo(P1(int (*callback)(P2(int, int))));
rather than just
int foo(P1(int (*callback)()));
Don't declare parameters as being of type float, short, or char. If you do this and forget to include the prototype at a call site, ANSI compilers will generate incompatible calling sequences. Use floatp (a synonym for double, mnemonic for "float parameter") instead of float, and use int or uint instead of short or char.
ANSI compilers in their default mode do all floating point computations in double precision, so never cast a float to a double explicitly.
Unless there's a good reason for doing otherwise, return floatp (double) rather than float values. Many floating point units do everything in double internally and have to do extra work to convert between double and float.
Use "private" instead of "static" for constructs (procedures and variables) declared at the outermost scope of a file. This allows making such constructs either visible or invisible to profilers with a single changed #define.
Don't create any new non-const static variables (whether exported or local to a file): they are incompatible with reentrancy, and we're in the process of eliminating all of them.
Avoid static const data, but don't knock yourself out over it, especially if it's local to a file.
Avoid extern in .c files: put it in header files.
The most important descriptive comments are ones in header files that describe structures, including invariants; but every procedure or structure declaration, or group of other declarations, should have a comment. Don't spend a lot of time commenting executable code unless something unusual or subtle is going on.
In older code, calling a variable or parameter procedure always used explicit indirection, for instance, (*ptr->func)(...) rather than ptr->func(...). Since all current compilers accept the latter form, use it in new code.
Every code file should start with comments containing
/*Id$: filename.c $*/
(using the comment convention appropriate to the language of the file), and
If you create a file by copying the beginning of another file, be sure to update the copyright year and change the file name.
For each
#include "xxx.h"
make sure there is a dependency on $(xxx_h) in the makefile. If xxx ends with a "_", this rule still holds, so that if you code
#include "math_.h"
the makefile must contain a dependency on "$(math__h)" (note the two underscores "__").
List the dependencies bottom-to-top, like the #include statements themselves; within each level, list them alphabetically. Do this also with #include statements themselves whenever possible (but sometimes there are inter-header dependencies that require bending this rule).
List #include statements from "bottom" to "top", that is, in the following order:
- System includes ("xxx_.h")
- gs*.h
- gx*.h (yes, gs and gx are in the wrong order.)
- s*.h
- i*.h (or other interpreter headers that don't start with "i")
In header files, always use the following at the beginning of a header file to prevent double inclusion:
{{ Copyright notice etc. }}
#ifndef <filename>_INCLUDED
#define <filename>_INCLUDED
{{ The contents of the file }}
#endif /* <filename>_INCLUDED */
The header file is the first place that a reader goes for information about procedures, structures, constants, etc., so ensure that every procedure and structure has a comment that says what it does. Divide procedures into meaningful groups set off by some distinguished form of comment.
After the initial comments, arrange C files in the following order:
- #include statements
- Exported data declarations
- Explicit externs (if necessary)
- Forward declarations of procedures
- Private data declarations
- Exported procedures
- Private procedures
Be flexible about the order of the declarations if necessary to improve readability. Many older files don't follow this order, often without good reason.
We've formatted all of our code using the GNU indent program.
indent -bad -nbap -nsob -br -ce -cli4 -npcs -ncs \
-i4 -di0 -psl -lp -lps somefile.c
does a 98% accurate job of producing our preferred style. Unfortunately, there are bugs in all versions of GNU indent, requiring both pre- and post-processing of the code. The gsindent script in the Ghostscript fileset contains the necessary workarounds.
Put indentation points every 4 spaces, with 8 spaces = 1 tab stop.
Indent in-line blocks thus:
{
... declarations ...
{{ blank line if any declarations above }}
... statements ...
}
Similarly, indent procedures thus:
return_type
proc_name(... arguments ...)
{
... declarations ...
{{ blank line if any declarations above }}
... statements ...
}
If a control construct (if, do, while, or for) has a one-line body, use this:
... control construct ...
... subordinate simple statement ...
If it has a multi-line body, use this:
... control construct ... {
... subordinate code ... }
If the subordinate code has declarations, see blocks above.
For if-else statements, do this:
if ( ... ) {
... subordinate code ...
} else if ( ... ) {
... subordinate code ...
} else {
... subordinate code ...
}
Similarly, for do-while statements, do this:
do {
... body ...
} while ( ... condition ... );
Do put a space:
Don't put a space:
Parentheses are important in only a few places:
(xx && yy) || zz
(x << 3) | (y >> 5)
((x) * (x) + (y) * (y))
Anywhere else, given the choice, use fewer parentheses.
For stylistic consistency with the existing Ghostscript code, put parentheses around conditional expressions even if they aren't syntactically required, unless you really dislike doing this. Note that the parentheses should go around the entire expression, not the condition. For instance, instead of
hpgl_add_point_to_path(pgls, arccoord_x, arccoord_y,
(pgls->g.pen_down) ? gs_lineto : gs_moveto);
use
hpgl_add_point_to_path(pgls, arccoord_x, arccoord_y,
(pgls->g.pen_down ? gs_lineto : gs_moveto));
Use fully spelled-out English words in names, rather than contractions. This is most important for procedure and macro names, global variables and constants, values of #define and enum, structure and other typedef names, and structure member names, and for argument and variable names which have uninformative types like int. It's not very important for arguments or local variables of distinctive types, or for local index or count variables.
Avoid names that run English words together: "hpgl_compute_arc_center" is better than "hpgl_compute_arccenter". However, for terms drawn from some predefined source, like the names of PostScript operators, use a term in its well-known form (for instance, gs_setlinewidth rather than gs_set_line_width).
Procedures, variables, and structures visible outside a single .c file should generally have prefixes that indicate what subsystem they belong to (in the case of Ghostscript, gs_ or gx_). This rule isn't followed very consistently.
The Ghostscript code uses certain names consistently for certain kinds of values. Some of the commonest and least obvious are these two:
A value to be returned from a procedure:
< 0 An error code defined in gserrors.h (or errors.h) 0 Normal return > 0 A non-standard but successful return (which must be documented, preferably with the procedure's prototype)
A value returned from a stream procedure:
< 0 An exceptional condition as defined in scommon.h 0 Normal return (or, from the "process" procedure, means that more input is needed) 1 More output space is needed (from the "process" procedure)
In general, don't create procedures that are private and only called from one place. However, if a compound statement (especially an arm of a conditional) is too long for the eye easily to match its enclosing braces "{...}" -- that is, longer than 10 or 15 lines -- and it doesn't use or set a lot of state held in outer local variables, it may be more readable if you put it in a procedure.
Don't assign new values to procedure parameters. It makes debugging very confusing when the parameter values printed for a procedure are not the ones actually supplied by the caller. Instead use a separate local variable initialized to the value of the parameter.
If a local variable is only assigned a value once, assign it that value at its declaration, if possible. For example,
int x = some expression ;
rather than
int x;
...
x = some expression ;
Use a local pointer variable like this to "narrow" pointer types:
int
someproc(... gx_device *dev ...)
{
gx_device_printer *const pdev = (gx_device_printer *)dev;
...
}
Every caller should check for error returns and, in general, propagate them to its callers. By convention, nearly every procedure returns an int to indicate the outcome of the call:
< 0 Error return 0 Normal return > 0 Non-error return other than the normal case
Copyright © 1996, 1997, 1998 Aladdin Enterprises. All rights reserved.
This file is part of Aladdin Ghostscript. See the Aladdin Free Public License (the "License") for full details of the terms of using, copying, modifying, and redistributing Aladdin Ghostscript.
Ghostscript version 5.50, 16 September 1998