Re: a question about yyrestart & reusing a scanner

[Aaron Jackson]

On Apr 21, 2005, at 8:29 AM, Cheng Po-wen wrote:

> Hi,
>
> I have a a question about how to reuse a scanner,
> The document (which is flex.pdf -- Flex, version 2.5.31) says
>
> there is no need to destroy a scanner if you plan to reuse it. A flex
> scanner (both reentrant
> and non-reentrant) may be restarted by calling yyrestart
>
> my question is :
>
> 1. Is it safe, for a long lived applicaion to reuse a scanner all the
> time by calling
> yyrestart(..) ?
>
> will there be any dangling resources, which are not released or freed 
> by
> doing so???

Looking at the man page:

      Essentially there is no difference
       between just assigning yyin to a new input file or using 
yyrestart() to
       do so; the latter is available for compatibility with previous 
versions
       of flex, and because it can be used to switch input files in the 
middle
       of  scanning.   It  can  also  be  used to throw away the 
current input
       buffer, by calling it with an argument of yyin; but better  is  
to  use
       YY_FLUSH_BUFFER  (see above).  Note that yyrestart() does not 
reset the
       start condition to INITIAL (see Start Conditions, below).

I read that to say yyrestart just points yyin to point to the next 
input file, and flushes the current input buffer.  Other than the 
current input buffer, I guess it doesn't free any resources...  I also 
read that to say yyrestart is not the preferred way to do this.

> 2. Does yy_scan_buffer (or yy_scan_string) internally do the "yyrestart
> things" ?
>
> what I mean is my scanner scans "strings", instead of "stdin".
> so, if i want to reuse a scanner,
> do I have to call yyrestart(NULL , my_scanner), and then call
> yy_scan_buffer(.....) ???
>
> or I can just call yy_scan_buffer(.....), and it internally does the
> "yyrestart things" ?

I have had success doing the following in a loop (while also using 
yacc/bison):

struct  yy_buffer_state* bufferState;
s="some string";
...
bufferState = yy_scan_string(*s);
yyparse();
 yy_delete_buffer(bufferState);
...

This seems to work as I expect it to.  I am just learning flex myself, 
so everything above may be totally wrong...

Aaron

> THANX
>
> Sting
>
>
>
> The function yylex_destroy should be called to free resources used by
> the scanner.
> After yylex_destroy is called, the contents of yyscanner should not be
> used. Of course,
> there is no need to destroy a scanner if you plan to reuse it. A flex
> scanner (both reentrant
> and non-reentrant) may be restarted by calling yyrestart
>
> --
> Sting, Cheng Po-wen(鄭博文)
> Phone :  886-3-5914545  Fax : 886-3-5820085
> E-Mail : address@hidden
>
> Internet Software Technology Division(W100)
> Computer & Communication Research Lab. / ITRI
>
>
>
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