# SWIG C/C++ Scanning

David M. Beazley
dave-swig@dabeaz.com
January 11, 2007

## Introduction

This document describes functions that can be used to tokenize C/C++ input text. These functions are relatively low-level and are meant to be used in the implementation of scanners that can be plugged into yacc or used for other purposes. For instance, the preprocessor uses these functions to evaluate and test constant expressions.

All of these functions are declared in `Source/Swig/swigscan.h`. This API is considered to be stable.

## Creation and Deletion of Scanners

The following functions are used to create and destroy a scanner object. More than one scanner object can be created and used as necessary.

`Scanner *NewScanner()`

> Creates a new scanner object. The scanner contains initially contains no text. To feed text to the scanner use `Scanner_push()`.

`Scanner *DelScanner()`

> Deletes a scanner object.

## Scanner Functions

`void Scanner_clear(Scanner *s)`

> Clears all text from the scanner. This can be used to reset a scanner to its initial state, ready to receive new input text.

`void Scanner_push(Scanner *s, String *text)`

> Pushes an input string into the scanner. Subsequent tokens will be returned from the new string. If the scanner is already processing a string, the pushed string takes precedence--in effect, interrupting the scanning of the previous string. This behavior is used to implement certain SWIG features such as the `%inline` directive. Once the pushed string has been completely scanned, the scanner will return to scanning the previous string (if any). The scanning of text relies upon the DOH file interface to strings (`Getc()`, `Ungetc()`, etc.). Prior to calling this function, the input string should be set so that its file pointer is in the location where you want scanning to begin. You may have to use `Seek()` to set the file pointer back to the beginning of a string prior to using this function.

`void Scanner_pushtoken(Scanner *s, int tokvalue, String_or_char *val)`

> Pushes a token into the scanner. This exact token will be returned by the next call to `Scanner_token()`. `tokvalue` is the integer token value to return and `val` is the token text to return. This function is only used to handle very special parsing cases. For instance, if you need the scanner to return a fictitious token into order to enter a special parsing case.

`int Scanner_token(Scanner *s)`

> Returns the next token. An integer token code is returned (see table below) on success. If no more input text is available 0 is returned. If a scanning error occurred, -1 is returned. In this case, error information can be obtained using `Scanner_errinfo()`.

`String *Scanner_text(Scanner *s)`

> Returns the scanned text corresponding to the last token returned by `Scanner_token()`. The returned string is only valid until the next call to `Scanner_token()`. If you need to save it, make a copy.

`void Scanner_skip_line(Scanner *s)`

> Skips to the end of the current line. The text skipped can be obtained using `Scanner_text()` afterwards.

`void Scanner_skip_balanced(Scanner *s, int startchar, int endchar)`

> Skips to the end of a block of text denoted by starting and ending characters. For example, `{` and `}`. The function is smart about how it skips text. String literals and comments are ignored. The function also is aware of nesting. The skipped text can be obtained using `Scanner_text()` afterwards. Returns 0 on success, -1 if no matching `endchar` could be found.

`void Scanner_set_location(Scanner *s, int startchar, int endchar)`

> Changes the current filename and line number of the scanner.

`String *Scanner_file(Scanner *s)`

> Gets the current filename associated with text in the scanner.

`int Scanner_line(Scanner *s)`

> Gets the current line number associated with text in the scanner.

`int Scanner_start_line(Scanner *s)`

> Gets the starting line number of the last token returned by the scanner.

`void Scanner_idstart(Scanner *s, char *idchar)`

> Sets additional characters (other than the C default) that may be used to start C identifiers. `idchar` is a string containing the characters (e.g., "%@"). The purpose of this function is to up special keywords such as "%module" or "@directive" as simple identifiers.

`String *Scanner_errmsg(Scanner *s)`

> Returns the error message associated with the last scanner error (if any). This will only return a meaningful result if `Scanner_token()` returned -1.

`int Scanner_errline(Scanner *s)`

> Returns the line number associated with the last scanner error (if any). This will only return a meaningful result if `Scanner_token()` returned -1. The line number usually corresponds to the starting line number of a particular token (e.g., for unterminated strings, comments, etc.).

`int Scanner_isoperator(int tokval)`

> A convenience function that returns 0 or 1 depending on whether `tokval` is a valid C/C++ operator (i.e., a candidate for operator overloading).

`void Scanner_freeze_line(int val)`

> Freezes the current line number depending upon whether or not `val` is 1 or 0. When the line number is frozen, newline characters will not result in updates to the line number. This is sometimes useful in tracking line numbers through complicated macro expansions.

## Token Codes

The following table shows token codes returned by the scanner. These are integer codes returned by the `Scanner_token()` function.

>     Token code                   C Token
>     -------------------------    -------------
>     SWIG_TOKEN_LPAREN            (
>     SWIG_TOKEN_RPAREN            )
>     SWIG_TOKEN_SEMI              ;
>     SWIG_TOKEN_COMMA             ,
>     SWIG_TOKEN_STAR              *
>     SWIG_TOKEN_TIMES             *
>     SWIG_TOKEN_LBRACE            {
>     SWIG_TOKEN_RBRACE            }
>     SWIG_TOKEN_EQUAL             =
>     SWIG_TOKEN_EQUALTO           ==
>     SWIG_TOKEN_NOTEQUAL          !=
>     SWIG_TOKEN_PLUS              +
>     SWIG_TOKEN_MINUS             -
>     SWIG_TOKEN_AND               &
>     SWIG_TOKEN_LAND              &&
>     SWIG_TOKEN_OR                |
>     SWIG_TOKEN_LOR               ||
>     SWIG_TOKEN_XOR               ^
>     SWIG_TOKEN_LESSTHAN          <
>     SWIG_TOKEN_GREATERTHAN       >
>     SWIG_TOKEN_LTEQUAL           <=
>     SWIG_TOKEN_GTEQUAL           >=
>     SWIG_TOKEN_NOT               ~
>     SWIG_TOKEN_LNOT              !
>     SWIG_TOKEN_LBRACKET          [
>     SWIG_TOKEN_RBRACKET          ]
>     SWIG_TOKEN_SLASH             /
>     SWIG_TOKEN_DIVIDE            /
>     SWIG_TOKEN_BACKSLASH         \
>     SWIG_TOKEN_POUND             #
>     SWIG_TOKEN_PERCENT           %
>     SWIG_TOKEN_MODULO            %
>     SWIG_TOKEN_COLON             :
>     SWIG_TOKEN_DCOLON            ::
>     SWIG_TOKEN_DCOLONSTAR        ::*
>     SWIG_TOKEN_LSHIFT            <<
>     SWIG_TOKEN_RSHIFT            >>
>     SWIG_TOKEN_QUESTION          ?
>     SWIG_TOKEN_PLUSPLUS          ++
>     SWIG_TOKEN_MINUSMINUS        --
>     SWIG_TOKEN_PLUSEQUAL         +=
>     SWIG_TOKEN_MINUSEQUAL        -=
>     SWIG_TOKEN_TIMESEQUAL        *=
>     SWIG_TOKEN_DIVEQUAL          /=
>     SWIG_TOKEN_ANDEQUAL          &=
>     SWIG_TOKEN_OREQUAL           |=
>     SWIG_TOKEN_XOREQUAL          ^=
>     SWIG_TOKEN_LSEQUAL           <<=
>     SWIG_TOKEN_RSEQUAL           >>=
>     SWIG_TOKEN_MODEQUAL          %=
>     SWIG_TOKEN_ARROW             ->
>     SWIG_TOKEN_ARROWSTAR         ->*
>     SWIG_TOKEN_PERIOD            .
>     SWIG_TOKEN_AT                @
>     SWIG_TOKEN_DOLLAR            $
>     SWIG_TOKEN_ENDLINE           Literal newline
>     SWIG_TOKEN_ID                identifier
>     SWIG_TOKEN_FLOAT             Floating point with F suffix (e.g., 3.1415F)
>     SWIG_TOKEN_DOUBLE            Floating point (e.g., 3.1415 )
>     SWIG_TOKEN_INT               Integer (e.g., 314)
>     SWIG_TOKEN_UINT              Unsigned integer (e.g., 314U)
>     SWIG_TOKEN_LONG              Long integer (e.g., 314L)
>     SWIG_TOKEN_ULONG             Unsigned long integer (e.g., 314UL)
>     SWIG_TOKEN_LONGLONG          Long long integer (e.g., 314LL )
>     SWIG_TOKEN_ULONGLONG         Unsigned long long integer (e.g., 314ULL)
>     SWIG_TOKEN_CHAR              Character literal in single quotes ('c')
>     SWIG_TOKEN_STRING            String literal in double quotes ("str")
>     SWIG_TOKEN_RSTRING           Reverse quote string (`str`)
>     SWIG_TOKEN_CODEBLOCK         SWIG code literal block %{ ... %}
>     SWIG_TOKEN_COMMENT           C or C++ comment  (// or /* ... */)
>     SWIG_TOKEN_ILLEGAL           Illegal character

Notes

- When more than one token code exist for the same token text, those codes are identical (e.g., `SWIG_TOKEN_STAR` and `SWIG_TOKEN_TIMES`).

- String literals are returned in their exact representation in which escape codes (if any) have been interpreted.

- All C identifiers and keywords are simply returned as `SWIG_TOKEN_ID`. To check for specific keywords, you will need to add extra checking on the returned text.

- C and C++ comments include the comment starting and ending text (e.g., "//", "/*").

- The maximum token integer value is found in the constant `SWIG_MAXTOKENS`. This can be used if you wanted to create an array or table for the purposes of remapping tokens to a different set of codes. For instance, if you are using these functions to write a yacc-compatible lexer.