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Explain better why to make a pure parser.

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Richard M. Stallman
1998-05-05 22:18:45 +00:00
parent e0672a6170
commit 70811b851d
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@@ -3098,24 +3098,32 @@ 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.
The Bison parser is not normally a reentrant program, because it uses
statically allocated variables for communication with @code{yylex}. These
variables include @code{yylval} and @code{yylloc}.
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 @code{yylex},
including @code{yylval} and @code{yylloc}.)
The Bison declaration @code{%pure_parser} says that you want the parser
to be reentrant. It looks like this:
Alternatively, you can generate a pure, reentrant parser. The Bison
declaration @code{%pure_parser} says that you want the parser to be
reentrant. It looks like this:
@example
%pure_parser
@end example
The effect is that the two communication variables become local
variables in @code{yyparse}, and a different calling convention is used
for the lexical analyzer function @code{yylex}. @xref{Pure Calling,
,Calling Conventions for Pure Parsers}, for the details of this. The
variable @code{yynerrs} also becomes local in @code{yyparse}
(@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}).
The convention for calling @code{yyparse} itself is unchanged.
The result is that the communication variables @code{yylval} and
@code{yylloc} become local variables in @code{yyparse}, and a different
calling convention is used for the lexical analyzer function
@code{yylex}. @xref{Pure Calling, ,Calling Conventions for Pure
Parsers}, for the details of this. The variable @code{yynerrs} also
becomes local in @code{yyparse} (@pxref{Error Reporting, ,The Error
Reporting Function @code{yyerror}}). The convention for calling
@code{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.
@node Decl Summary, , Pure Decl, Declarations
@subsection Bison Declaration Summary