src/3rdparty/libjpeg/coderules.doc
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     1 IJG JPEG LIBRARY:  CODING RULES
       
     2 
       
     3 Copyright (C) 1991-1996, Thomas G. Lane.
       
     4 This file is part of the Independent JPEG Group's software.
       
     5 For conditions of distribution and use, see the accompanying README file.
       
     6 
       
     7 
       
     8 Since numerous people will be contributing code and bug fixes, it's important
       
     9 to establish a common coding style.  The goal of using similar coding styles
       
    10 is much more important than the details of just what that style is.
       
    11 
       
    12 In general we follow the recommendations of "Recommended C Style and Coding
       
    13 Standards" revision 6.1 (Cannon et al. as modified by Spencer, Keppel and
       
    14 Brader).  This document is available in the IJG FTP archive (see
       
    15 jpeg/doc/cstyle.ms.tbl.Z, or cstyle.txt.Z for those without nroff/tbl).
       
    16 
       
    17 Block comments should be laid out thusly:
       
    18 
       
    19 /*
       
    20  *  Block comments in this style.
       
    21  */
       
    22 
       
    23 We indent statements in K&R style, e.g.,
       
    24 	if (test) {
       
    25 	  then-part;
       
    26 	} else {
       
    27 	  else-part;
       
    28 	}
       
    29 with two spaces per indentation level.  (This indentation convention is
       
    30 handled automatically by GNU Emacs and many other text editors.)
       
    31 
       
    32 Multi-word names should be written in lower case with underscores, e.g.,
       
    33 multi_word_name (not multiWordName).  Preprocessor symbols and enum constants
       
    34 are similar but upper case (MULTI_WORD_NAME).  Names should be unique within
       
    35 the first fifteen characters.  (On some older systems, global names must be
       
    36 unique within six characters.  We accommodate this without cluttering the
       
    37 source code by using macros to substitute shorter names.)
       
    38 
       
    39 We use function prototypes everywhere; we rely on automatic source code
       
    40 transformation to feed prototype-less C compilers.  Transformation is done
       
    41 by the simple and portable tool 'ansi2knr.c' (courtesy of Ghostscript).
       
    42 ansi2knr is not very bright, so it imposes a format requirement on function
       
    43 declarations: the function name MUST BEGIN IN COLUMN 1.  Thus all functions
       
    44 should be written in the following style:
       
    45 
       
    46 LOCAL(int *)
       
    47 function_name (int a, char *b)
       
    48 {
       
    49     code...
       
    50 }
       
    51 
       
    52 Note that each function definition must begin with GLOBAL(type), LOCAL(type),
       
    53 or METHODDEF(type).  These macros expand to "static type" or just "type" as
       
    54 appropriate.  They provide a readable indication of the routine's usage and
       
    55 can readily be changed for special needs.  (For instance, special linkage
       
    56 keywords can be inserted for use in Windows DLLs.)
       
    57 
       
    58 ansi2knr does not transform method declarations (function pointers in
       
    59 structs).  We handle these with a macro JMETHOD, defined as
       
    60 	#ifdef HAVE_PROTOTYPES
       
    61 	#define JMETHOD(type,methodname,arglist)  type (*methodname) arglist
       
    62 	#else
       
    63 	#define JMETHOD(type,methodname,arglist)  type (*methodname) ()
       
    64 	#endif
       
    65 which is used like this:
       
    66 	struct function_pointers {
       
    67 	  JMETHOD(void, init_entropy_encoder, (int somearg, jparms *jp));
       
    68 	  JMETHOD(void, term_entropy_encoder, (void));
       
    69 	};
       
    70 Note the set of parentheses surrounding the parameter list.
       
    71 
       
    72 A similar solution is used for forward and external function declarations
       
    73 (see the EXTERN and JPP macros).
       
    74 
       
    75 If the code is to work on non-ANSI compilers, we cannot rely on a prototype
       
    76 declaration to coerce actual parameters into the right types.  Therefore, use
       
    77 explicit casts on actual parameters whenever the actual parameter type is not
       
    78 identical to the formal parameter.  Beware of implicit conversions to "int".
       
    79 
       
    80 It seems there are some non-ANSI compilers in which the sizeof() operator
       
    81 is defined to return int, yet size_t is defined as long.  Needless to say,
       
    82 this is brain-damaged.  Always use the SIZEOF() macro in place of sizeof(),
       
    83 so that the result is guaranteed to be of type size_t.
       
    84 
       
    85 
       
    86 The JPEG library is intended to be used within larger programs.  Furthermore,
       
    87 we want it to be reentrant so that it can be used by applications that process
       
    88 multiple images concurrently.  The following rules support these requirements:
       
    89 
       
    90 1. Avoid direct use of file I/O, "malloc", error report printouts, etc;
       
    91 pass these through the common routines provided.
       
    92 
       
    93 2. Minimize global namespace pollution.  Functions should be declared static
       
    94 wherever possible.  (Note that our method-based calling conventions help this
       
    95 a lot: in many modules only the initialization function will ever need to be
       
    96 called directly, so only that function need be externally visible.)  All
       
    97 global function names should begin with "jpeg_", and should have an
       
    98 abbreviated name (unique in the first six characters) substituted by macro
       
    99 when NEED_SHORT_EXTERNAL_NAMES is set.
       
   100 
       
   101 3. Don't use global variables; anything that must be used in another module
       
   102 should be in the common data structures.
       
   103 
       
   104 4. Don't use static variables except for read-only constant tables.  Variables
       
   105 that should be private to a module can be placed into private structures (see
       
   106 the system architecture document, structure.doc).
       
   107 
       
   108 5. Source file names should begin with "j" for files that are part of the
       
   109 library proper; source files that are not part of the library, such as cjpeg.c
       
   110 and djpeg.c, do not begin with "j".  Keep source file names to eight
       
   111 characters (plus ".c" or ".h", etc) to make life easy for MS-DOSers.  Keep
       
   112 compression and decompression code in separate source files --- some
       
   113 applications may want only one half of the library.
       
   114 
       
   115 Note: these rules (particularly #4) are not followed religiously in the
       
   116 modules that are used in cjpeg/djpeg but are not part of the JPEG library
       
   117 proper.  Those modules are not really intended to be used in other
       
   118 applications.