Using Bison you can free discarded symbols using %destructor directive:
http://www.gnu.org/software/bison/manual/html_node/Destructor-Decl.html
typically the action is to delete a node, or to free a string.
Note that bison is able to generate also a C++ based parser, and not only C.
Anyway, if the parser is a single batch process, when the process exits
all the memory is freed,
so you don't have to care about memory leaks (in this situation
free/delete will only slow down your compiler) .
Memory leaks are important if you're developing an interactive parser,
(eg a shell parser),
or if the parser is always running as a background service (eg visual
studio intellisense)
Luca
On 12/06/2012 14:12, Fabricio Pellegrini wrote:
Hello,
First of all, I'm using:
- Window 7 Enterprise 32bits
- Visual Studio 2010
- Bison++ Version 1.21-8, adapted from GNU bison by
<mailto:[email protected]> [email protected]
Currently we are using Bison++, to parse our proprietary script. During the
parsing we are just building a tree-structured, based on the Interpreter
Pattern, that later will be saved was a proprietary format and loaded by our
server. So we make use of dynamic allocation inside the action's blocks. The
problem is that recently, I found out, if the parser gets any syntax error
after not being able to reduce anymore, we get memory leaks due its
allocation during the parser.
For example, let's say that I have the following grammar:
stmt:
ref '=' expr ';'
{
$$ = Create(ASSIGN); //The factory will create and return a new
"ASSIGN" object
$$->SetOp1($1);
$$->SetOp2($3);
}
;
expr :
expr '+' expr
{
$$ = Create(ADD); //The factory will create and return a new "ADD"
object
$$->SetOp1($1);
$$->SetOp2($3);
}
| ref
{
$$ = $1;
}
;
ref :
REF
{
$$ = Create(REF); //The factory will return a new "REF" object
$$->SetName($1);
}
;
Assuming that we insert the following input:
x = a + b //The ';' isn't present
The parse will rise an error, because it can't reduce using the "stmt -> ref
'=' expr ';' " rule.
So, after it exits, We can't delete the memory that was allocated during the
reducing of expr and ref.
Reading the O'Reilly's book "YACC& Flex", I found out that I could use
"error recovering" to get back the control of the parser and release the
memory when it finds an error.
Then, I tried to add the rule "stmt -> ref '=' expr error';'" to be able to
delete the memory allocated during the reducing of expr and ref, like you
can see below:
stmt:
ref '=' expr ';'
{
$$ = Create(ASSIGN); //The factory will create and return a new
"ASSIGN" object
$$->SetOp1($1);
$$->SetOp2($3);
}
| ref '=' expr error';'
{
$$ = NULL;
delete $1;
delete $3;
}
It will work for the preview input, x = a + b, but won't for, x = + b;
So I would like to know, if adding more recovery points is the right way to
avoid memory leaks on my case. If no, is there a better approach that I
could use?
Thank you for your time
Fabricio G. Pellegrini
Software Developer
EDiSPHERE Software Private Limited | 215, Congress Nagar, Nagpur 440012
Email:<mailto:[email protected]>
[email protected] | Website:<http://www.edisphere.com/>
http://www.edisphere.com
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