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(bison.info)Pure Decl

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A Pure (Reentrant) Parser
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   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 non-reentrant program may not be safe to call
from a signal handler.  In systems with multiple threads of control, a
non-reentrant 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 `%pure_parser' says that you want the parser to be
reentrant.  It looks like this:

     %pure_parser

   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'.  Note: Calling
Conventions for Pure Parsers, for the details of this.
The variable `yynerrs' also becomes local in `yyparse' (Note: 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.