119 lines
5.2 KiB
Plaintext
119 lines
5.2 KiB
Plaintext
IJG JPEG LIBRARY: CODING RULES
|
|
|
|
Copyright (C) 1991-1996, Thomas G. Lane.
|
|
This file is part of the Independent JPEG Group's software.
|
|
For conditions of distribution and use, see the accompanying README file.
|
|
|
|
|
|
Since numerous people will be contributing code and bug fixes, it's important
|
|
to establish a common coding style. The goal of using similar coding styles
|
|
is much more important than the details of just what that style is.
|
|
|
|
In general we follow the recommendations of "Recommended C Style and Coding
|
|
Standards" revision 6.1 (Cannon et al. as modified by Spencer, Keppel and
|
|
Brader). This document is available in the IJG FTP archive (see
|
|
jpeg/doc/cstyle.ms.tbl.Z, or cstyle.txt.Z for those without nroff/tbl).
|
|
|
|
Block comments should be laid out thusly:
|
|
|
|
/*
|
|
* Block comments in this style.
|
|
*/
|
|
|
|
We indent statements in K&R style, e.g.,
|
|
if (test) {
|
|
then-part;
|
|
} else {
|
|
else-part;
|
|
}
|
|
with two spaces per indentation level. (This indentation convention is
|
|
handled automatically by GNU Emacs and many other text editors.)
|
|
|
|
Multi-word names should be written in lower case with underscores, e.g.,
|
|
multi_word_name (not multiWordName). Preprocessor symbols and enum constants
|
|
are similar but upper case (MULTI_WORD_NAME). Names should be unique within
|
|
the first fifteen characters. (On some older systems, global names must be
|
|
unique within six characters. We accommodate this without cluttering the
|
|
source code by using macros to substitute shorter names.)
|
|
|
|
We use function prototypes everywhere; we rely on automatic source code
|
|
transformation to feed prototype-less C compilers. Transformation is done
|
|
by the simple and portable tool 'ansi2knr.c' (courtesy of Ghostscript).
|
|
ansi2knr is not very bright, so it imposes a format requirement on function
|
|
declarations: the function name MUST BEGIN IN COLUMN 1. Thus all functions
|
|
should be written in the following style:
|
|
|
|
LOCAL(int *)
|
|
function_name (int a, char *b)
|
|
{
|
|
code...
|
|
}
|
|
|
|
Note that each function definition must begin with GLOBAL(type), LOCAL(type),
|
|
or METHODDEF(type). These macros expand to "static type" or just "type" as
|
|
appropriate. They provide a readable indication of the routine's usage and
|
|
can readily be changed for special needs. (For instance, special linkage
|
|
keywords can be inserted for use in Windows DLLs.)
|
|
|
|
ansi2knr does not transform method declarations (function pointers in
|
|
structs). We handle these with a macro JMETHOD, defined as
|
|
#ifdef HAVE_PROTOTYPES
|
|
#define JMETHOD(type,methodname,arglist) type (*methodname) arglist
|
|
#else
|
|
#define JMETHOD(type,methodname,arglist) type (*methodname) ()
|
|
#endif
|
|
which is used like this:
|
|
struct function_pointers {
|
|
JMETHOD(void, init_entropy_encoder, (int somearg, jparms *jp));
|
|
JMETHOD(void, term_entropy_encoder, (void));
|
|
};
|
|
Note the set of parentheses surrounding the parameter list.
|
|
|
|
A similar solution is used for forward and external function declarations
|
|
(see the EXTERN and JPP macros).
|
|
|
|
If the code is to work on non-ANSI compilers, we cannot rely on a prototype
|
|
declaration to coerce actual parameters into the right types. Therefore, use
|
|
explicit casts on actual parameters whenever the actual parameter type is not
|
|
identical to the formal parameter. Beware of implicit conversions to "int".
|
|
|
|
It seems there are some non-ANSI compilers in which the sizeof() operator
|
|
is defined to return int, yet size_t is defined as long. Needless to say,
|
|
this is brain-damaged. Always use the SIZEOF() macro in place of sizeof(),
|
|
so that the result is guaranteed to be of type size_t.
|
|
|
|
|
|
The JPEG library is intended to be used within larger programs. Furthermore,
|
|
we want it to be reentrant so that it can be used by applications that process
|
|
multiple images concurrently. The following rules support these requirements:
|
|
|
|
1. Avoid direct use of file I/O, "malloc", error report printouts, etc;
|
|
pass these through the common routines provided.
|
|
|
|
2. Minimize global namespace pollution. Functions should be declared static
|
|
wherever possible. (Note that our method-based calling conventions help this
|
|
a lot: in many modules only the initialization function will ever need to be
|
|
called directly, so only that function need be externally visible.) All
|
|
global function names should begin with "jpeg_", and should have an
|
|
abbreviated name (unique in the first six characters) substituted by macro
|
|
when NEED_SHORT_EXTERNAL_NAMES is set.
|
|
|
|
3. Don't use global variables; anything that must be used in another module
|
|
should be in the common data structures.
|
|
|
|
4. Don't use static variables except for read-only constant tables. Variables
|
|
that should be private to a module can be placed into private structures (see
|
|
the system architecture document, structure.txt).
|
|
|
|
5. Source file names should begin with "j" for files that are part of the
|
|
library proper; source files that are not part of the library, such as cjpeg.c
|
|
and djpeg.c, do not begin with "j". Keep source file names to eight
|
|
characters (plus ".c" or ".h", etc) to make life easy for MS-DOSers. Keep
|
|
compression and decompression code in separate source files --- some
|
|
applications may want only one half of the library.
|
|
|
|
Note: these rules (particularly #4) are not followed religiously in the
|
|
modules that are used in cjpeg/djpeg but are not part of the JPEG library
|
|
proper. Those modules are not really intended to be used in other
|
|
applications.
|