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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
1 changed files with 20 additions and 12 deletions
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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 handler. In systems with multiple threads of control, a nonreentrant
program must be called only within interlocks. program must be called only within interlocks.
The Bison parser is not normally a reentrant program, because it uses Normally, Bison generates a parser which is not reentrant. This is
statically allocated variables for communication with @code{yylex}. These suitable for most uses, and it permits compatibility with YACC. (The
variables include @code{yylval} and @code{yylloc}. 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 Alternatively, you can generate a pure, reentrant parser. The Bison
to be reentrant. It looks like this: declaration @code{%pure_parser} says that you want the parser to be
reentrant. It looks like this:
@example @example
%pure_parser %pure_parser
@end example @end example
The effect is that the two communication variables become local The result is that the communication variables @code{yylval} and
variables in @code{yyparse}, and a different calling convention is used @code{yylloc} become local variables in @code{yyparse}, and a different
for the lexical analyzer function @code{yylex}. @xref{Pure Calling, calling convention is used for the lexical analyzer function
,Calling Conventions for Pure Parsers}, for the details of this. The @code{yylex}. @xref{Pure Calling, ,Calling Conventions for Pure
variable @code{yynerrs} also becomes local in @code{yyparse} Parsers}, for the details of this. The variable @code{yynerrs} also
(@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}). becomes local in @code{yyparse} (@pxref{Error Reporting, ,The Error
The convention for calling @code{yyparse} itself is unchanged. 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 @node Decl Summary, , Pure Decl, Declarations
@subsection Bison Declaration Summary @subsection Bison Declaration Summary