Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!mintaka!spdcc!ima!esegue!compilers-sender
From: bliss@sp64.csrd.uiuc.edu (Brian Bliss)
Newsgroups: comp.compilers
Subject: Re: Question About YACC's Memory Management
Keywords: yacc
Message-ID: <1990Oct18.213224.21195@csrd.uiuc.edu>
Date: 18 Oct 90 21:32:24 GMT
References: <2371@kiwi.mpr.ca>
Sender: compilers-sender@esegue.segue.boston.ma.us
Reply-To: bliss@sp64.csrd.uiuc.edu (Brian Bliss)
Organization: Center for Supercomputing Research and Development
Lines: 58
Approved: compilers@esegue.segue.boston.ma.us

In article <2371@kiwi.mpr.ca>, andrea@mprgate.mpr.ca (Jennitta Andrea) writes:
|> 
|>   I have a parser which is intended to be called repeatedly from a driver
|> program.  ...
|> 
|>   I have typed the value stack to contain pointers to characters.  I
|> malloc the memory required for each token in lex (assigning yylval to
|> point to that block of memory) before returning the token to yacc.  It
|> appears that this memory is not cleaned up when yacc frees the value stack.
|> 
|>   Am I required to explicitly free each token once the parser has reduced
|> a rule?  ...

3 ways around this:

1) modify yyparse, the file that is included with the yacc-generated
   tables to form the parser, to free the tokens when popping the stack.

2) don't use the yacc value stack, make your own, and free the tokens
   when popping it.

3) merge the lexer into the parser and use alloca () to allocate the
   memory.  instead of having lex recognize "xyz", allocate the string
   "xyz", and return the token XYZ, include the yacc rule:

   XYZ:    'x' 'y' 'z' {
              char *a = alloca (4);
              sprintf (a, "xyz");
              push (a);
            }

    all strings will be automatically freed when yyparse () returns.
    this can get messy if the lex rules are complicated, though.

bb
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