GDB Internals - Language Support

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# Language Support

GDB's language support is mainly driven by the symbol reader, although it is possible for the user to set the source language manually.

GDB chooses the source language by looking at the extension of the file recorded in the debug info; `.c` means C, `.f` means Fortran, etc. It may also use a special-purpose language identifier if the debug format supports it, such as DWARF.

## Adding a Source Language to GDB

To add other languages to GDB's expression parser, follow the following steps:

Create the expression parser.
  This should reside in a file ``lang-exp.y'`. Routines for building parsed expressions into a ``union exp_element'` list are in ``parse.c'`. Since we can't depend upon everyone having Bison, and YACC produces parsers that define a bunch of global names, the following lines must be included at the top of the YACC parser, to prevent the various parsers from defining the same global names:

      #define yyparse 	lang_parse
      #define yylex 	lang_lex
      #define yyerror 	lang_error
      #define yylval 	lang_lval
      #define yychar 	lang_char
      #define yydebug 	lang_debug
      #define yypact  	lang_pact
      #define yyr1		lang_r1
      #define yyr2		lang_r2
      #define yydef		lang_def
      #define yychk		lang_chk
      #define yypgo		lang_pgo
      #define yyact  	lang_act
      #define yyexca  	lang_exca
      #define yyerrflag  	lang_errflag
      #define yynerrs  	lang_nerrs

  At the bottom of your parser, define a `struct language_defn` and initialize it with the right values for your language. Define an `initialize_lang` routine and have it call ``add_language(lang_language_defn)'` to tell the rest of GDB that your language exists. You'll need some other supporting variables and functions, which will be used via pointers from your `lang_language_defn`. See the declaration of `struct language_defn` in ``language.h'`, and the other ``*-exp.y'` files, for more information.
Add any evaluation routines, if necessary
  If you need new opcodes (that represent the operations of the language), add them to the enumerated type in ``expression.h'`. Add support code for these operations in `eval.c:evaluate_subexp()`. Add cases for new opcodes in two functions from ``parse.c'`: `prefixify_subexp()` and `length_of_subexp()`. These compute the number of `exp_element`s that a given operation takes up.
Update some existing code
  Add an enumerated identifier for your language to the enumerated type `enum language` in ``defs.h'`. Update the routines in ``language.c'` so your language is included. These routines include type predicates and such, which (in some cases) are language dependent. If your language does not appear in the switch statement, an error is reported. Also included in ``language.c'` is the code that updates the variable `current_language`, and the routines that translate the `language_lang` enumerated identifier into a printable string. Update the function `_initialize_language` to include your language. This function picks the default language upon startup, so is dependent upon which languages that GDB is built for. Update `allocate_symtab` in ``symfile.c'` and/or symbol-reading code so that the language of each symtab (source file) is set properly. This is used to determine the language to use at each stack frame level. Currently, the language is set based upon the extension of the source file. If the language can be better inferred from the symbol information, please set the language of the symtab in the symbol-reading code. Add helper code to `expprint.c:print_subexp()` to handle any new expression opcodes you have added to ``expression.h'`. Also, add the printed representations of your operators to `op_print_tab`.
Add a place of call
  Add a call to `lang_parse()` and `lang_error` in `parse.c:parse_exp_1()`.
Use macros to trim code
  The user has the option of building GDB for some or all of the languages. If the user decides to build GDB for the language lang, then every file dependent on ``language.h'` will have the macro `_LANG_lang` defined in it. Use `#ifdef`s to leave out large routines that the user won't need if he or she is not using your language. Note that you do not need to do this in your YACC parser, since if GDB is not build for lang, then ``lang-exp.tab.o'` (the compiled form of your parser) is not linked into GDB at all. See the file ``configure.in'` for how GDB is configured for different languages.
Edit ``Makefile.in'`
  Add dependencies in ``Makefile.in'`. Make sure you update the macro variables such as `HFILES` and `OBJS`, otherwise your code may not get linked in, or, worse yet, it may not get `tar`red into the distribution!

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