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