Manual

58 Bison 2.3
the warning about these conflicts unless the number of conflicts changes. You can do this
with the %expect declaration.
The declaration looks like this:
%expect n
Here n is a decimal integer. The declaration says there should be n shift/reduce conflicts
and no reduce/reduce conflicts. Bison reports an error if the number of shift/reduce conflicts
differs from n, or if there are any reduce/reduce conflicts.
For normal LALR(1) parsers, reduce/reduce conflicts are more serious, and should be
eliminated entirely. Bison will always report reduce/reduce conflicts for these parsers. With
GLR parsers, however, both kinds of conflicts are routine; otherwise, there would be no
need to use GLR parsing. Therefore, it is also possible to specify an expected number of
reduce/reduce conflicts in GLR parsers, using the declaration:
%expect-rr n
In general, using %expect involves these steps:
• Compile your grammar without %expect. Use the ‘-v’ option to get a verbose list of
where the conflicts occur. Bison will also print the number of conflicts.
• Check each of the conflicts to make sure that Bison’s default resolution is what you
really want. If not, rewrite the grammar and go back to the beginning.
• Add an %expect declaration, copying the number n from the number which Bison
printed. With GLR parsers, add an %expect-rr declaration as well.
Now Bison will warn you if you introduce an unexpected conflict, but will keep silent
otherwise.
3.7.9 The Start-Symbol
Bison assumes by default that the start symbol for the grammar is the first nonterminal
specified in the grammar specification section. The programmer may override this restriction
with the %start declaration as follows:
%start symbol
3.7.10 A Pure (Reentrant) Parser
A reentrant program is one which does not alter in the course of execution; in other words, it
consists entirely of pure (read-only) code. Reentrancy is important whenever asynchronous
execution is possible; for example, a nonreentrant program may not be safe to call from a
signal handler. In systems with multiple threads of control, a nonreentrant program must
be called only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is suitable for most
uses, and it permits compatibility with Yacc. (The standard Yacc interfaces are inherently
nonreentrant, because they use statically allocated variables for communication with yylex,
including yylval and yylloc.)
Alternatively, you can generate a pure, reentrant parser. The Bison declaration %pureparser
says that you want the parser to be reentrant. It looks like this:
%pure-parser