# Writing a parser in C

The section is based on the section “Integrating a new language parser” in “How to Add Support for a New Language to Exuberant Ctags (EXTENDING)” of Exuberant Ctags documents.

Now suppose that I want to truly integrate compiled-in support for Swine into ctags.

## Registering a parser

First, I create a new module, `swine.c`, and add one externally visible function to it, `extern parserDefinition *SwineParser(void)`, and add its name to the table in `parsers.h`. The job of this parser definition function is to create an instance of the `parserDefinition` structure (using `parserNew()`) and populate it with information defining how files of this language are recognized, what kinds of tags it can locate, and the function used to invoke the parser on the currently open file.

The structure `parserDefinition` allows assignment of the following fields:

    struct sParserDefinition {
            /* defined by parser */
            char* name;                    /* name of language */
            kindDefinition* kindTable;         /* 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 */
            const char *const *aliases;    /* list of default aliases (alternative names) */
            parserInitialize initialize;   /* initialization routine, if needed */
            parserFinalize finalize;       /* finalize routine, if needed */
            simpleParser parser;           /* simple parser (common case) */
            rescanParser parser2;          /* rescanning parser (unusual case) */
            selectLanguage* selectLanguage; /* may be used to resolve conflicts */
            unsigned int method;           /* See METHOD_ definitions above */
            unsigned int useCork;              /* bit fields of corkUsage */
            ...
    };

The `name` field must be set to a non-empty string. Also either `parser` or `parser2` must set to point to a parsing routine which will generate the tag entries. All other fields are optional.

## Reading input file stream

Now all that is left is to implement the parser. In order to do its job, the parser should read the file stream using using one of the two I/O interfaces: either the character-oriented `getcFromInputFile()`, or the line-oriented `readLineFromInputFile()`.

See “Input text stream” for more details.

## Parsing

How our Swine parser actually parses the contents of the file is entirely up to the writer of the parser--it can be as crude or elegant as desired. You will note a variety of examples from the most complex (`parsers/cxx/*.[hc]`) to the simplest (`parsers/make.[ch]`).

## Adding a tag to the tag file

When the Swine parser identifies an interesting token for which it wants to add a tag to the tag file, it should create a `tagEntryInfo` structure and initialize it by calling `initTagEntry()`, which initializes defaults and fills information about the current line number and the file position of the beginning of the line. After filling in information defining the current entry (and possibly overriding the file position or other defaults), the parser passes this structure to `makeTagEntry()`.

See “Output tag stream” for more details.

## Adding the parser to `ctags`

Lastly, be sure to add your the name of the file containing your parser (e.g. `parsers/swine.c`) to the macro `PARSER_SRCS` in the file `source.mak`, so that your new module will be compiled into the program.

## Misc.

This is all there is to it. All other details are specific to the parser and how it wants to do its job.

There are some support functions which can take care of some commonly needed parsing tasks, such as keyword table lookups (see `main/keyword.c`), which you can make use of if desired (examples of its use can be found in `parsers/c.c`, `parsers/eiffel.c`, and `parsers/fortran.c`).

Support functions can be found in `main/*.h` excluding `main/*_p.h`.

Almost everything is already taken care of automatically for you by the infrastructure. Writing the actual parsing algorithm is the hardest part, but is not constrained by any need to conform to anything in ctags other than that mentioned above.

There are several different approaches used in the parsers inside Universal Ctags and you can browse through these as examples of how to go about creating your own.