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lex: how to restart a lexer with string input

Posted on 1999-01-20
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Last Modified: 2006-11-17
(note: I'm using AT&T lex)
I'm writing a shared library that lexs string input sent by external programs. Problem is, when the function is called consecutively by any instance of an external program, the lexer -- yylex() --  does not get reset. Thus, subsequent return values are null (the first call works great though!). Below is part of this program:


Part of the header file:
    ...
    #undef input
    #undef unput
    #define input() (*instring++)
    #define unput(c) ( *--instring = c)
    ...
and later I define the main function:

    char * lexfunction(char *yourstring)
    {
      ...
       instring=cpy_of_yourstring;
       yylex();
      ...
       return parsed_string;
    }

THEN an external script which loads in this library
can call "lexfunction(somestring)".

QUESTION: how do I reset the lexer. I know it's something with either yywrap() or yylex() but I don't know how to do it!

Any suggestions would be GREATLY appreciated.
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Question by:greenhaze
9 Comments
 
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by:greenhaze
ID: 1258232
Edited text of question
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by:greenhaze
ID: 1258233
Edited text of question
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Expert Comment

by:thoellri
ID: 1258234
Try adding the following function to your source:

int yywrap() {
  return 0;
}

Hope this helps
  Tobias
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Author Comment

by:greenhaze
ID: 1258235
I had tried that. It causes the program to hang because yylex() needs a return 1 to shutdown. Modifying yywrap() seems to be best suited for applications that spin through a series of inputs with each execution.

As a shared library, it's not clear to me when execution really ends. Maybe that's the central issue here: how do I ensure a new instance of the function when new data comes in? This is very frustrating!!

Thanks for any help!
0
 
LVL 1

Author Comment

by:greenhaze
ID: 1258236
I had tried that. It causes the program to hang because yylex() needs a return 1 to shutdown. Modifying yywrap() seems to be best suited for applications that spin through a series of inputs with each execution.

As a shared library, it's not clear to me when execution really ends. Maybe that's the central issue here: how do I ensure a new instance of the function when new data comes in? This is very frustrating!!

Thanks for any help!
0
 
LVL 5

Expert Comment

by:thoellri
ID: 1258237
I'm sorry that it didn't fix your problem - I'm at a loss here :-(
  Tobias

0
 
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Expert Comment

by:_Scotch_
ID: 1258238
Have you done something with YY_INPUT to lex a buffer rather
than a file ??

If so have a look at yy_create_buffer() and yy_switch_to_buffer()

-- I use flex but imagine the interface is much the same as lex
0
 

Accepted Solution

by:
login0000 earned 200 total points
ID: 1258239
to force lex to go back to the initial state after each call ,you can use yyterminate.
you can add for example this whitch work eaven if lex work on a string an not a file.

<<EOF>>                        {
                              //      printf("end du fichier");
                                    BEGIN(0);
                                    yyterminate();
                        
                              }
just try it. good luck
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Author Comment

by:greenhaze
ID: 1258240
This is correct if using GNU flex; however, I'm using AT&T lex - its buggier cousin.

Likewise, the YY_INPUT macro is flex-specific. I had to use the #define input() statements for AT&T lex.

Alas, I got it running. Thanks for all the suggestions!


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