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. 119
