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3.7.11 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 ‘%define api.pure’ says that you want the parser to be reentrant. It looks like this:

%define api.pure full

The result is that the communication variables yylval and yylloc become local variables in yyparse, and a different calling convention is used for the lexical analyzer function yylex. See Calling Conventions for Pure Parsers, for the details of this. The variable yynerrs becomes local in yyparse in pull mode but it becomes a member of yypstate in push mode. (see The Error Reporting Function yyerror). The convention for calling yyparse itself is unchanged.

Whether the parser is pure has nothing to do with the grammar rules. You can generate either a pure parser or a nonreentrant parser from any valid grammar.