Manual

Chapter 3: Bison Grammar Files 57
3.7.7 Freeing Discarded Symbols
During error recovery (see Chapter 6 [Error Recovery], page 83), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded until the parser falls
on its feet. If the parser runs out of memory, or if it returns via YYABORT or YYACCEPT, all
the symbols on the stack must be discarded. Even if the parser succeeds, it must discard
the start symbol.
When discarded symbols convey heap based information, this memory is lost. While
this behavior can be tolerable for batch parsers, such as in traditional compilers, it is unacceptable
for programs like shells or protocol implementations that may parse and execute
indefinitely.
The %destructor directive defines code that is called when a symbol is automatically
discarded.
%destructor { code } symbols
[Directive]
Invoke the braced code whenever the parser discards one of the symbols. Within
code, $$ designates the semantic value associated with the discarded symbol. The
additional parser parameters are also available (see Section 4.1 [The Parser Function
yyparse], page 63).
For instance:
%union
{
char *string;
}
%token STRING
%type string
%destructor { free ($$); } STRING string
guarantees that when a STRING or a string is discarded, its associated memory will be
freed.
Discarded symbols are the following:
• stacked symbols popped during the first phase of error recovery,
• incoming terminals during the second phase of error recovery,
• the current look-ahead and the entire stack (except the current right-hand side symbols)
when the parser returns immediately, and
• the start symbol, when the parser succeeds.
The parser can return immediately because of an explicit call to YYABORT or YYACCEPT,
or failed error recovery, or memory exhaustion.
Right-hand size symbols of a rule that explicitly triggers a syntax error via YYERROR are
not discarded automatically. As a rule of thumb, destructors are invoked only when user
actions cannot manage the memory.
3.7.8 Suppressing Conflict Warnings
Bison normally warns if there are any conflicts in the grammar (see Section 5.2 [Shift/Reduce
Conflicts], page 72), but most real grammars have harmless shift/reduce conflicts which are
resolved in a predictable way and would be difficult to eliminate. It is desirable to suppress