IJG JPEG LIBRARY:  CODING RULES

This file was part of the Independent JPEG Group's software:

Copyright (C) 1991-1996, Thomas G. Lane.

It was modified by The libjpeg-turbo Project to include only information

relevant to libjpeg-turbo.

For conditions of distribution and use, see the accompanying README.ijg 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.

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.)

A similar solution is used for external function declarations (see the EXTERN

macro.)

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_".

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 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.