@cindex position, textual
Many applications, like interpreters or compilers, have to produce verbose
-and useful error messages. To achieve this, one must be able to keep track of
+and useful error messages. To achieve this, one must be able to keep track of
the @dfn{textual position}, or @dfn{location}, of each syntactic construct.
Bison provides a mechanism for handling these locations.
-Each token has a semantic value. In a similar fashion, each token has an
+Each token has a semantic value. In a similar fashion, each token has an
associated location, but the type of locations is the same for all tokens and
-groupings. Moreover, the output parser is equipped with a default data
+groupings. Moreover, the output parser is equipped with a default data
structure for storing locations (@pxref{Locations}, for more details).
Like semantic values, locations can be reached in actions using a dedicated
-set of constructs. In the example above, the location of the whole grouping
+set of constructs. In the example above, the location of the whole grouping
is @code{@@$}, while the locations of the subexpressions are @code{@@1} and
@code{@@3}.
When a rule is matched, a default action is used to compute the semantic value
-of its left hand side (@pxref{Actions}). In the same way, another default
-action is used for locations. However, the action for locations is general
+of its left hand side (@pxref{Actions}). In the same way, another default
+action is used for locations. However, the action for locations is general
enough for most cases, meaning there is usually no need to describe for each
-rule how @code{@@$} should be formed. When building a new location for a given
+rule how @code{@@$} should be formed. When building a new location for a given
grouping, the default behavior of the output parser is to take the beginning
of the first symbol, and the end of the last symbol.
The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears
in every Bison grammar file to separate the sections.
-The prologue may define types and variables used in the actions. You can
+The prologue may define types and variables used in the actions. You can
also use preprocessor commands to define macros used there, and use
@code{#include} to include header files that do any of these things.
The grammar rules define how to construct each nonterminal symbol from its
parts.
-The epilogue can contain any code you want to use. Often the definition of
+The epilogue can contain any code you want to use. Often the definition of
the lexical analyzer @code{yylex} goes here, plus subroutines called by the
actions in the grammar rules. In a simple program, all the rest of the
program can go here.
calculator. As in C, comments are placed between @samp{/*@dots{}*/}.
@example
-/* Reverse polish notation calculator. */
+/* Reverse polish notation calculator. */
%@{
#define YYSTYPE double
%token NUM
-%% /* Grammar rules and actions follow */
+%% /* Grammar rules and actions follow. */
@end example
The declarations section (@pxref{Prologue, , The prologue}) contains two
The parser function @code{yyparse} continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
-input tokens; we will arrange for the latter to happen at end of file.
+input tokens; we will arrange for the latter to happen at end-of-input.
@node Rpcalc Line
@subsubsection Explanation of @code{line}
This is what happens in the first rule (the one that uses @code{NUM}).
The formatting shown here is the recommended convention, but Bison does
-not require it. You can add or change whitespace as much as you wish.
+not require it. You can add or change white space as much as you wish.
For example, this:
@example
defined at the beginning of the grammar; @pxref{Rpcalc Decls,
,Declarations for @code{rpcalc}}.)
-A token type code of zero is returned if the end-of-file is encountered.
-(Bison recognizes any nonpositive value as indicating the end of the
-input.)
+A token type code of zero is returned if the end-of-input is encountered.
+(Bison recognizes any nonpositive value as indicating end-of-input.)
Here is the code for the lexical analyzer:
@example
@group
-/* Lexical analyzer returns a double floating point
+/* The lexical analyzer returns a double floating point
number on the stack and the token NUM, or the numeric code
- of the character read if not a number. Skips all blanks
- and tabs, returns 0 for EOF. */
+ of the character read if not a number. It skips all blanks
+ and tabs, and returns 0 for end-of-input. */
#include <ctype.h>
@end group
@{
int c;
- /* skip white space */
+ /* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
;
@end group
@group
- /* process numbers */
+ /* Process numbers. */
if (c == '.' || isdigit (c))
@{
ungetc (c, stdin);
@}
@end group
@group
- /* return end-of-file */
+ /* Return end-of-input. */
if (c == EOF)
return 0;
- /* return single chars */
+ /* Return a single char. */
return c;
@}
@end group
#include <stdio.h>
void
-yyerror (const char *s) /* Called by yyparse on error */
+yyerror (const char *s) /* called by yyparse on error */
@{
printf ("%s\n", s);
@}
@noindent
In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
CALCulator''). Bison produces a file named @file{@var{file_name}.tab.c},
-removing the @samp{.y} from the original file name. The file output by
+removing the @samp{.y} from the original file name. The file output by
Bison contains the source code for @code{yyparse}. The additional
functions in the input file (@code{yylex}, @code{yyerror} and @code{main})
are copied verbatim to the output.
tracking. This feature will be used to improve the error messages. For
the sake of clarity, this example is a simple integer calculator, since
most of the work needed to use locations will be done in the lexical
-analyser.
+analyzer.
@menu
* Decls: Ltcalc Decls. Bison and C declarations for ltcalc.
@subsection The @code{ltcalc} Lexical Analyzer.
Until now, we relied on Bison's defaults to enable location
-tracking. The next step is to rewrite the lexical analyser, and make it
+tracking. The next step is to rewrite the lexical analyzer, and make it
able to feed the parser with the token locations, as it already does for
semantic values.
@{
int c;
- /* skip white space */
+ /* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
++yylloc.last_column;
- /* step */
+ /* Step. */
yylloc.first_line = yylloc.last_line;
yylloc.first_column = yylloc.last_column;
@end group
@group
- /* process numbers */
+ /* Process numbers. */
if (isdigit (c))
@{
yylval = c - '0';
@}
@end group
- /* return end-of-file */
+ /* Return end-of-input. */
if (c == EOF)
return 0;
- /* return single chars and update location */
+ /* Return a single char, and update location. */
if (c == '\n')
@{
++yylloc.last_line;
@code{YYLTYPE}) containing the token's location.
Now, each time this function returns a token, the parser has its number
-as well as its semantic value, and its location in the text. The last
+as well as its semantic value, and its location in the text. The last
needed change is to initialize @code{yylloc}, for example in the
controlling function:
@smallexample
%@{
-#include <math.h> /* For math functions, cos(), sin(), etc. */
+#include <math.h> /* For math functions, cos(), sin(), etc. */
#include "calc.h" /* Contains definition of `symrec' */
%@}
%union @{
@smallexample
@group
-/* Fonctions type. */
+/* Function type. */
typedef double (*func_t) (double);
/* Data type for links in the chain of symbols. */
@end group
@group
-/* Put arithmetic functions in table. */
+/* Put arithmetic functions in table. */
void
init_table (void)
@{
ptr->name = (char *) malloc (strlen (sym_name) + 1);
strcpy (ptr->name,sym_name);
ptr->type = sym_type;
- ptr->value.var = 0; /* set value to 0 even if fctn. */
+ ptr->value.var = 0; /* Set value to 0 even if fctn. */
ptr->next = (struct symrec *)sym_table;
sym_table = ptr;
return ptr;
@{
int c;
- /* Ignore whitespace, get first nonwhite character. */
+ /* Ignore white space, get first nonwhite character. */
while ((c = getchar ()) == ' ' || c == '\t');
if (c == EOF)
@}
@end group
@group
- while (c != EOF && isalnum (c));
+ while (isalnum (c));
ungetc (c, stdin);
symbuf[i] = '\0';
@end group
@end smallexample
-This program is both powerful and flexible. You may easily add new
+This program is both powerful and flexible. You may easily add new
functions, and it is a simple job to modify this code to install
predefined variables such as @code{pi} or @code{e} as well.
All the usual escape sequences used in character literals in C can be
used in Bison as well, but you must not use the null character as a
-character literal because its numeric code, zero, is the code @code{yylex}
-returns for end-of-input (@pxref{Calling Convention, ,Calling Convention
+character literal because its numeric code, zero, signifies
+end-of-input (@pxref{Calling Convention, ,Calling Convention
for @code{yylex}}).
@item
grammatical meaning. That depends only on where it appears in rules and
on when the parser function returns that symbol.
-The value returned by @code{yylex} is always one of the terminal symbols
-(or 0 for end-of-input). Whichever way you write the token type in the
-grammar rules, you write it the same way in the definition of @code{yylex}.
-The numeric code for a character token type is simply the numeric code of
-the character, so @code{yylex} can use the identical character constant to
-generate the requisite code. Each named token type becomes a C macro in
+The value returned by @code{yylex} is always one of the terminal
+symbols, except that a zero or negative value signifies end-of-input.
+Whichever way you write the token type in the grammar rules, you write
+it the same way in the definition of @code{yylex}. The numeric code
+for a character token type is simply the positive numeric code of the
+character, so @code{yylex} can use the identical value to generate the
+requisite code, though you may need to convert it to @code{unsigned
+char} to avoid sign-extension on hosts where @code{char} is signed.
+Each named token type becomes a C macro in
the parser file, so @code{yylex} can use the name to stand for the code.
(This is why periods don't make sense in terminal symbols.)
@xref{Calling Convention, ,Calling Convention for @code{yylex}}.
into a separate header file @file{@var{name}.tab.h} which you can include
in the other source files that need it. @xref{Invocation, ,Invoking Bison}.
-The @code{yylex} function must use the same character set and encoding
-that was used by Bison. For example, if you run Bison in an
+If you want to write a grammar that is portable to any Standard C
+host, you must use only non-null character tokens taken from the basic
+execution character set of Standard C. This set consists of the ten
+digits, the 52 lower- and upper-case English letters, and the
+characters in the following C-language string:
+
+@example
+"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~"
+@end example
+
+The @code{yylex} function and Bison must use a consistent character
+set and encoding for character tokens. For example, if you run Bison in an
@sc{ascii} environment, but then compile and run the resulting program
in an environment that uses an incompatible character set like
-@sc{ebcdic}, the resulting program will probably not work because the
+@sc{ebcdic}, the resulting program may not work because the
tables generated by Bison will assume @sc{ascii} numeric values for
-character tokens. Portable grammars should avoid non-@sc{ascii}
-character tokens, as implementations in practice often use different
-and incompatible extensions in this area. However, it is standard
+character tokens. It is standard
practice for software distributions to contain C source files that
were generated by Bison in an @sc{ascii} environment, so installers on
platforms that are incompatible with @sc{ascii} must rebuild those
says that two groupings of type @code{exp}, with a @samp{+} token in between,
can be combined into a larger grouping of type @code{exp}.
-Whitespace in rules is significant only to separate symbols. You can add
-extra whitespace as you wish.
+White space in rules is significant only to separate symbols. You can add
+extra white space as you wish.
Scattered among the components can be @var{actions} that determine
the semantics of the rule. An action looks like this:
@cindex position, textual
Though grammar rules and semantic actions are enough to write a fully
-functional parser, it can be useful to process some additionnal informations,
+functional parser, it can be useful to process some additional information,
especially symbol locations.
@c (terminal or not) ?
describing the behavior of the output parser with locations.
The most obvious way for building locations of syntactic groupings is very
-similar to the way semantic values are computed. In a given rule, several
+similar to the way semantic values are computed. In a given rule, several
constructs can be used to access the locations of the elements being matched.
The location of the @var{n}th component of the right hand side is
@code{@@@var{n}}, while the location of the left hand side grouping is
@end example
As for semantic values, there is a default action for locations that is
-run each time a rule is matched. It sets the beginning of @code{@@$} to the
+run each time a rule is matched. It sets the beginning of @code{@@$} to the
beginning of the first symbol, and the end of @code{@@$} to the end of the
last symbol.
-With this default action, the location tracking can be fully automatic. The
+With this default action, the location tracking can be fully automatic. The
example above simply rewrites this way:
@example
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT
-Actually, actions are not the best place to compute locations. Since
+Actually, actions are not the best place to compute locations. Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each
-rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
+rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
matched, before the associated action is run.
Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.
-The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is
-the location of the grouping (the result of the computation). The second one
+The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is
+the location of the grouping (the result of the computation). The second one
is an array holding locations of all right hand side elements of the rule
-being matched. The last one is the size of the right hand side rule.
+being matched. The last one is the size of the right hand side rule.
By default, it is defined this way for simple LALR(1) parsers:
@itemize @bullet
@item
-All arguments are free of side-effects. However, only the first one (the
+All arguments are free of side-effects. However, only the first one (the
result) should be modified by @code{YYLLOC_DEFAULT}.
@item
@item %verbose
Write an extra output file containing verbose descriptions of the
parser states and what is done for each type of look-ahead token in
-that state. @xref{Understanding, , Understanding Your Parser}, for more
+that state. @xref{Understanding, , Understanding Your Parser}, for more
information.
@node Calling Convention
@subsection Calling Convention for @code{yylex}
-The value that @code{yylex} returns must be the numeric code for the type
-of token it has just found, or 0 for end-of-input.
+The value that @code{yylex} returns must be the positive numeric code
+for the type of token it has just found; a zero or negative value
+signifies end-of-input.
When a token is referred to in the grammar rules by a name, that name
in the parser file becomes a C macro whose definition is the proper
When a token is referred to in the grammar rules by a character literal,
the numeric code for that character is also the code for the token type.
-So @code{yylex} can simply return that character code. The null character
-must not be used this way, because its code is zero and that is what
+So @code{yylex} can simply return that character code, possibly converted
+to @code{unsigned char} to avoid sign-extension. The null character
+must not be used this way, because its code is zero and that
signifies end-of-input.
Here is an example showing these things:
yylex (void)
@{
@dots{}
- if (c == EOF) /* Detect end of file. */
+ if (c == EOF) /* Detect end-of-input. */
return 0;
@dots{}
if (c == '+' || c == '-')
- return c; /* Assume token type for `+' is '+'. */
+ return c; /* Assume token type for `+' is '+'. */
@dots{}
- return INT; /* Return the type of the token. */
+ return INT; /* Return the type of the token. */
@dots{}
@}
@end example
@example
@group
@dots{}
- yylval = value; /* Put value onto Bison stack. */
- return INT; /* Return the type of the token. */
+ yylval = value; /* Put value onto Bison stack. */
+ return INT; /* Return the type of the token. */
@dots{}
@end group
@end example
@example
@group
@dots{}
- yylval.intval = value; /* Put value onto Bison stack. */
- return INT; /* Return the type of the token. */
+ yylval.intval = value; /* Put value onto Bison stack. */
+ return INT; /* Return the type of the token. */
@dots{}
@end group
@end example
the current input line or current statement if an error is detected:
@example
-stmnt: error ';' /* on error, skip until ';' is read */
+stmnt: error ';' /* On error, skip until ';' is read. */
@end example
It is also useful to recover to the matching close-delimiter of an
@samp{.y}. The parser file's name is made by replacing the @samp{.y}
with @samp{.tab.c}. Thus, the @samp{bison foo.y} filename yields
@file{foo.tab.c}, and the @samp{bison hack/foo.y} filename yields
-@file{hack/foo.tab.c}. It's is also possible, in case you are writing
+@file{hack/foo.tab.c}. It's also possible, in case you are writing
C++ code instead of C in your grammar file, to name it @file{foo.ypp}
-or @file{foo.y++}. Then, the output files will take an extention like
-the given one as input (repectively @file{foo.tab.cpp} and @file{foo.tab.c++}).
+or @file{foo.y++}. Then, the output files will take an extension like
+the given one as input (respectively @file{foo.tab.cpp} and
+@file{foo.tab.c++}).
This feature takes effect with all options that manipulate filenames like
@samp{-o} or @samp{-d}.
bison -d @var{infile.yxx}
@end example
@noindent
-will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}. and
+will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}, and
@example
bison -d @var{infile.y} -o @var{output.c++}
@item -b @var{file-prefix}
@itemx --file-prefix=@var{prefix}
Pretend that @code{%verbose} was specified, i.e, specify prefix to use
-for all Bison output file names. @xref{Decl Summary}.
+for all Bison output file names. @xref{Decl Summary}.
@item -r @var{things}
@itemx --report=@var{things}
@itemx --verbose
Pretend that @code{%verbose} was specified, i.e, write an extra output
file containing verbose descriptions of the grammar and
-parser. @xref{Decl Summary}.
+parser. @xref{Decl Summary}.
@item -o @var{filename}
@itemx --output=@var{filename}
@item -g
Output a VCG definition of the LALR(1) grammar automaton computed by
-Bison. If the grammar file is @file{foo.y}, the VCG output file will
+Bison. If the grammar file is @file{foo.y}, the VCG output file will
be @file{foo.vcg}.
@item --graph=@var{graph-file}
-The behaviour of @var{--graph} is the same than @samp{-g}. The only
-difference is that it has an optionnal argument which is the name of
+The behavior of @var{--graph} is the same than @samp{-g}. The only
+difference is that it has an optional argument which is the name of
the output graph filename.
@end table
token. @xref{Action Features, ,Special Features for Use in Actions}.
@item YYDEBUG
-Macro to define to equip the parser with tracing code. @xref{Tracing,
+Macro to define to equip the parser with tracing code. @xref{Tracing,
,Tracing Your Parser}.
@item YYERROR
syntax error. @xref{Action Features, ,Special Features for Use in Actions}.
@item YYSTACK_USE_ALLOCA
-Macro used to control the use of @code{alloca}. If defined to @samp{0},
+Macro used to control the use of @code{alloca}. If defined to @samp{0},
the parser will not use @code{alloca} but @code{malloc} when trying to
-grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA}
+grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA}
to anything else.
@item YYSTYPE
time to resolve reduce/reduce conflicts. @xref{GLR Parsers}.
@item %file-prefix="@var{prefix}"
-Bison declaration to set the prefix of the output files. @xref{Decl
+Bison declaration to set the prefix of the output files. @xref{Decl
Summary}.
@item %glr-parser
@c
@c @item %header-extension
@c Bison declaration to specify the generated parser header file extension
-@c if required. @xref{Decl Summary}.
+@c if required. @xref{Decl Summary}.
@item %left
Bison declaration to assign left associativity to token(s).
@xref{GLR Parsers}.
@item %name-prefix="@var{prefix}"
-Bison declaration to rename the external symbols. @xref{Decl Summary}.
+Bison declaration to rename the external symbols. @xref{Decl Summary}.
@item %no-lines
Bison declaration to avoid generating @code{#line} directives in the
@xref{Precedence Decl, ,Operator Precedence}.
@item %output="@var{filename}"
-Bison declaration to set the name of the parser file. @xref{Decl
+Bison declaration to set the name of the parser file. @xref{Decl
Summary}.
@item %prec
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