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<!-- $Id: EXTENDING.html 198 2002-09-04 01:17:32Z darren $ --><html><head><title>Exuberant Ctags: Adding support for a new language</title></head><body><h1>How to Add Support for a New Language to Exuberant Ctags</h1><p><b>Exuberant Ctags</b> has been designed to make it very easy to add your owncustom language parser. As an exercise, let us assume that I want to addsupport for my new language, <em>Swine</em>, the successor to Perl (i.e. Perlbefore Swine <wince>). This language consists of simple definitions oflabels in the form "<code>def my_label</code>". Let us now examine the variousways to do this.</p><h2>Operational background</h2><p>As ctags considers each file name, it tries to determine the language of thefile by applying the following three tests in order: if the file extension hasbeen mapped to a language, if the file name matches a shell pattern mapped toa language, and finally if the file is executable and its first line specifiesan interpreter using the Unix-style "#!" specification (if supported on theplatform). If a language was identified, the file is opened and then theappropriate language parser is called to operate on the currently open file.The parser parses through the file and whenever it finds some interestingtoken, calls a function to define a tag entry.</p><h2>Creating a user-defined language</h2><p>The quickest and easiest way to do this is by defining a new language usingthe program options. In order to have Swine support available every time Istart ctags, I will place the following lines into the file<code>$HOME/.ctags</code>, which is read in every time ctags starts:<code><pre> --langdef=swine --langmap=swine:.swn --regex-swine=/^def[ \t]*([a-zA-Z0-9_]+)/\1/d,definition/</pre></code>The first line defines the new language, the second maps a file extension toit, and the third defines a regular expression to identify a languagedefinition and generate a tag file entry for it.</p><h2>Integrating a new language parser</h2><p>Now suppose that I want to truly integrate compiled-in support for Swine intoctags. First, I create a new module, <code>swine.c</code>, and add oneexternally visible function to it, <code>extern parserDefinition*SwineParser(void)</code>, and add its name to the table in<code>parsers.h</code>. The job of this parser definition function is tocreate an instance of the <code>parserDefinition</code> structure (using<code>parserNew()</code>) and populate it with information defining how filesof this language are recognized, what kinds of tags it can locate, and thefunction used to invoke the parser on the currently open file.</p><p>The structure <code>parserDefinition</code> allows assignment of the followingfields:<code><pre> const char *name; /* name of language */ kindOption *kinds; /* tag kinds handled by parser */ unsigned int kindCount; /* size of `kinds' list */ const char *const *extensions; /* list of default extensions */ const char *const *patterns; /* list of default file name patterns */ parserInitialize initialize; /* initialization routine, if needed */ simpleParser parser; /* simple parser (common case) */ rescanParser parser2; /* rescanning parser (unusual case) */ boolean regex; /* is this a regex parser? */</pre></code></p><p>The <code>name</code> field must be set to a non-empty string. Also, unless<code>regex</code> is set true (see below), either <code>parser</code> or<code>parser2</code> must set to point to a parsing routine which willgenerate the tag entries. All other fields are optional.<p>Now all that is left is to implement the parser. In order to do its job, theparser should read the file stream using using one of the two I/O interfaces:either the character-oriented <code>fileGetc()</code>, or the line-oriented<code>fileReadLine()</code>. When using <code>fileGetc()</code>, the parsercan put back a character using <code>fileUngetc()</code>. How our Swine parseractually parses the contents of the file is entirely up to the writer of theparser--it can be as crude or elegant as desired. You will note a variety ofexamples from the most complex (c.c) to the simplest (make.c).</p><p>When the Swine parser identifies an interesting token for which it wants toadd a tag to the tag file, it should create a <code>tagEntryInfo</code>structure and initialize it by calling <code>initTagEntry()</code>, whichinitializes defaults and fills information about the current line number andthe file position of the beginning of the line. After filling in informationdefining the current entry (and possibly overriding the file position or otherdefaults), the parser passes this structure to <code>makeTagEntry()</code>.</p><p>Instead of writing a character-oriented parser, it may be possible to specifyregular expressions which define the tags. In this case, instead of defining aparsing function, <code>SwineParser()</code>, sets <code>regex</code> to true,and points <code>initialize</code> to a function which calls<code>addTagRegex()</code> to install the regular expressions which define itstags. The regular expressions thus installed are compared against each line of the input file and generate a specified tag when matched. It is usuallymuch easier to write a regex-based parser, although they can be slower (oneparser example was 4 times slower). Whether the speed difference matters toyou depends upon how much code you have to parse. It is probably a goodstrategy to implement a regex-based parser first, and if it is too slow foryou, then invest the time and effort to write a character-based parser.</p><p>A regex-based parser is inherently line-oriented (i.e. the entire tag must berecognizable from looking at a single line) and context-insensitive (i.e thegeneration of the tag is entirely based upon when the regular expressionmatches a single line). However, a regex-based callback mechanism is alsoavailable, installed via the function <code>addCallbackRegex()</code>. Thisallows a specified function to be invoked whenever a specific regularexpression is matched. This allows a character-oriented parser to operatebased upon context of what happened on a previous line (e.g. the start or endof a multi-line comment). Note that regex callbacks are called just before thefirst character of that line can is read via either <code>fileGetc()</code> orusing <code>fileGetc()</code>. The effect of this is that before either ofthese routines return, a callback routine may be invoked because the linematched a regex callback. A callback function to be installed is defined bythese types:<code><pre> typedef void (*regexCallback) (const char *line, const regexMatch *matches, unsigned int count); typedef struct { size_t start; /* character index in line where match starts */ size_t length; /* length of match */ } regexMatch;</pre></code></p><p>The callback function is passed the line matching the regular expression andan array of <code>count</code> structures defining the subexpression matchesof the regular expression, starting from \0 (the entire line).</p><p>Lastly, be sure to add your the name of the file containing your parser (e.g.swine.c) to the macro <code>SOURCES</code> in the file <code>source.mak</code>and an entry for the object file to the macro <code>OBJECTS</code> in the samefile, so that your new module will be compiled into the program.</p><p>This is all there is to it. All other details are specific to the parser andhow it wants to do its job. There are some support functions which can takecare of some commonly needed parsing tasks, such as keyword table lookups (seekeyword.c), which you can make use of if desired (examples of its use can befound in c.c, eiffel.c, and fortran.c). Almost everything is already taken careof automatically for you by the infrastructure. Writing the actual parsingalgorithm is the hardest part, but is not constrained by any need to conformto anything in ctags other than that mentioned above.</p><p>There are several different approaches used in the parsers inside <b>ExuberantCtags</b> and you can browse through these as examples of how to go aboutcreating your own.</p><h2>Examples</h2><p>Below you will find several example parsers demonstrating most of thefacilities available. These include three alternative implementationsof a Swine parser, which generate tags for lines beginning with"<CODE>def</CODE>" followed by some name.</p><code><pre>/*************************************************************************** * swine.c * Character-based parser for Swine definitions **************************************************************************//* INCLUDE FILES */#include "general.h" /* always include first */#include <string.h> /* to declare strxxx() functions */#include <ctype.h> /* to define isxxx() macros */#include "parse.h" /* always include */#include "read.h" /* to define file fileReadLine() *//* DATA DEFINITIONS */typedef enum eSwineKinds { K_DEFINE} swineKind;static kindOption SwineKinds [] = { { TRUE, 'd', "definition", "pig definition" }};/* FUNCTION DEFINITIONS */static void findSwineTags (void){ vString *name = vStringNew (); const unsigned char *line; while ((line = fileReadLine ()) != NULL) { /* Look for a line beginning with "def" followed by name */ if (strncmp ((const char*) line, "def", (size_t) 3) == 0 && isspace ((int) line [3])) { const unsigned char *cp = line + 4; while (isspace ((int) *cp)) ++cp; while (isalnum ((int) *cp) || *cp == '_') { vStringPut (name, (int) *cp); ++cp; } vStringTerminate (name); makeSimpleTag (name, SwineKinds, K_DEFINE); vStringClear (name); } } vStringDelete (name);}/* Create parser definition stucture */extern parserDefinition* SwineParser (void){ static const char *const extensions [] = { "swn", NULL }; parserDefinition* def = parserNew ("Swine"); def->kinds = SwineKinds; def->kindCount = KIND_COUNT (SwineKinds); def->extensions = extensions; def->parser = findSwineTags; return def;}</pre></code><p><pre><code>/*************************************************************************** * swine.c * Regex-based parser for Swine **************************************************************************//* INCLUDE FILES */#include "general.h" /* always include first */#include "parse.h" /* always include *//* FUNCTION DEFINITIONS */static void installSwineRegex (const langType language){ addTagRegex (language, "^def[ \t]*([a-zA-Z0-9_]+)", "\\1", "d,definition", NULL);}/* Create parser definition stucture */extern parserDefinition* SwineParser (void){ static const char *const extensions [] = { "swn", NULL }; parserDefinition* def = parserNew ("Swine"); parserDefinition* const def = parserNew ("Makefile"); def->patterns = patterns; def->extensions = extensions; def->initialize = installMakefileRegex; def->regex = TRUE; return def;}</code></pre><p><pre>/*************************************************************************** * swine.c * Regex callback-based parser for Swine definitions **************************************************************************//* INCLUDE FILES */#include "general.h" /* always include first */#include "parse.h" /* always include */#include "read.h" /* to define file fileReadLine() *//* DATA DEFINITIONS */typedef enum eSwineKinds { K_DEFINE} swineKind;static kindOption SwineKinds [] = { { TRUE, 'd', "definition", "pig definition" }};/* FUNCTION DEFINITIONS */static void definition (const char *const line, const regexMatch *const matches, const unsigned int count){ if (count > 1) /* should always be true per regex */ { vString *const name = vStringNew (); vStringNCopyS (name, line + matches [1].start, matches [1].length); makeSimpleTag (name, SwineKinds, K_DEFINE); }}static void findSwineTags (void){ while (fileReadLine () != NULL) ; /* don't need to do anything here since callback is sufficient */}static void installSwine (const langType language){ addCallbackRegex (language, "^def[ \t]+([a-zA-Z0-9_]+)", NULL, definition);}/* Create parser definition stucture */extern parserDefinition* SwineParser (void){ static const char *const extensions [] = { "swn", NULL }; parserDefinition* def = parserNew ("Swine"); def->kinds = SwineKinds; def->kindCount = KIND_COUNT (SwineKinds); def->extensions = extensions; def->parser = findSwineTags; def->initialize = installSwine; return def;}</pre><p><pre>/*************************************************************************** * make.c * Regex-based parser for makefile macros **************************************************************************//* INCLUDE FILES */#include "general.h" /* always include first */#include "parse.h" /* always include *//* FUNCTION DEFINITIONS */static void installMakefileRegex (const langType language){ addTagRegex (language, "(^|[ \t])([A-Z0-9_]+)[ \t]*:?=", "\\2", "m,macro", "i");}/* Create parser definition stucture */extern parserDefinition* MakefileParser (void){ static const char *const patterns [] = { "[Mm]akefile", NULL }; static const char *const extensions [] = { "mak", NULL }; parserDefinition* const def = parserNew ("Makefile"); def->patterns = patterns; def->extensions = extensions; def->initialize = installMakefileRegex; def->regex = TRUE; return def;}</pre></body></html>?? 快捷鍵說明
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