(flex.info)User Values

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14 Values Available To the User
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This chapter summarizes the various values available to the user in the
rule actions.

'char *yytext'
     holds the text of the current token.  It may be modified but not
     lengthened (you cannot append characters to the end).

     If the special directive '%array' appears in the first section of
     the scanner description, then 'yytext' is instead declared 'char
     yytext[YYLMAX]', where 'YYLMAX' is a macro definition that you can
     redefine in the first section if you don't like the default value
     (generally 8KB). Using '%array' results in somewhat slower
     scanners, but the value of 'yytext' becomes immune to calls to
     'unput()', which potentially destroy its value when 'yytext' is a
     character pointer.  The opposite of '%array' is '%pointer', which
     is the default.

     You cannot use '%array' when generating C++ scanner classes (the
     '-+' flag).

'int yyleng'
     holds the length of the current token.

'FILE *yyin'
     is the file which by default 'flex' reads from.  It may be
     redefined but doing so only makes sense before scanning begins or
     after an EOF has been encountered.  Changing it in the midst of
     scanning will have unexpected results since 'flex' buffers its
     input; use 'yyrestart()' instead.  Once scanning terminates because
     an end-of-file has been seen, you can assign 'yyin' at the new
     input file and then call the scanner again to continue scanning.

'void yyrestart( FILE *new_file )'
     may be called to point 'yyin' at the new input file.  The
     switch-over to the new file is immediate (any previously
     buffered-up input is lost).  Note that calling 'yyrestart()' with
     'yyin' as an argument thus throws away the current input buffer and
     continues scanning the same input file.

'FILE *yyout'
     is the file to which 'ECHO' actions are done.  It can be reassigned
     by the user.

'YY_CURRENT_BUFFER'
     returns a 'YY_BUFFER_STATE' handle to the current buffer.

'YY_START'
     returns an integer value corresponding to the current start
     condition.  You can subsequently use this value with 'BEGIN' to
     return to that start condition.