parser.doc - C parser for Bison tables {This file is similar to parserclass.doc in the AUIS C++ sources.} May, 1994 parser - object for parsing WJHansen, Andrew Consortium A parser object represents a grammar and the state of a parse according to that grammar. After parsing one text, the object can be reused to parse another. Unlike yacc and other systems, the grammar is represented by uniquely named tables, so there are no name conflicts and multiple parsers are possible. Grammar tables are generated with a version of the Bison package from the Free Software Foundation. An awk script removes the Bison parser, so the resulting parser and application is not tainted with FSF's General Public License. In the descriptions below, it is assumed that the grammar is gggg and is described in file gggg.y. (The grammar description is upward compatible from that of yacc, but there are differences in the treatment of errors.) Overview The gggg.y file is processed through Bison to produce file, which is then processed through the 'mkparser' script This file defines the function gggg_New() which the application code calls to allocate a parser This is then passed as an argument to parser_Parse to stream. the gggg.tab.c to produce gggg.c. object for gggg. parse a lexeme The .y files for parser are not completely compatible with yacc and Bison. The AUIS version of Bison supports an upward-compatible extension to the grammar language. Grammars would be compatible if they avoided this extension. However, error handling is different in how errors are reported and how error conditions are signaled from action routines. These differences require some conversion. Another difference is that the AUIS Bison supports the -k switch which is needed to support token translation. Grammar (.y file) The AUIS version of Bison supports one additional token type: multi-character tokens. These are written in the grammar surrounded by quotation marks, as in "<=". Thus one rule of the grammar might be expression : expression "<=" factor ; In other words it is not necessary to define LE as a token and then teach the token analyzer that a less-than followed by an equal-sign is the token LE. (AUIS's 'tlex' token analyzer determines the token list from the tables generated by Bison.) Semantic action routines {specified in braces in the grammar} may refer to value stack locations with $$, $1, $2, and so on, as in yacc/Bison. In addition, the variable 'parser' points to the parser object for the parse in progress. One use for this is to access the associated 'rock' value: struct whatever *info = (struct whatever *)parser_GetRock(parser); The result of the compilation can be passed back to the application program by storing it into a component of the rock. Suppose the value is in $1 and the target field of the rock is 'value', then the code is info->value = $1; Note that no rockvalue will exist unless one has been stored. For instance, struct whatever parserinfo; ... parser_SetRock(gparser, &parserinfo); Compilation errors are reported by calling parser_Error. Its default action is to call parser_ErrorGuts, which prints an error message. If an application wishes some other error action, it should override ErrorGuts. To do so, it must include at least a declaration for ErrorGuts in the .y file; this is the signal for mkparser to include the appropriate declaration in gggg.h. The declaration in the .y file should be void gggg_ErrorGuts(int severity, char *severityname, char *msg); This function will be called by parse_Error whenever an error is detected. At that time, (severity&~parser_FREEMSG) will be one of the values parser_WARNING, parser_SERIOUS, parser_SYNTAX, or parser_FATAL, as defined in cparser.h and severityname will be the corresponding string. The parameter msg will be a character string; if the severity value is or'ed with parser_FREEMSG, ErrorGuts must free the character string. Applications should call Error instead of ErrorGuts because the former computes the maximum severity and counts the number of errors. In a context where there is no pointer to the parser object for the current compilation, it can be retrieved via parser_GetCurrentparser(). In yacc and Bison, special action macros are available to control parse termination and error processing: yyclearin, yyerrok, YYACCEPT, and YYERROR. However, since the semantic actions are in a function rather than embedded in the parser itself, these macros are no longer appropriate. Instead, a semantic action routine can terminate with one of the following macros: none - parsing continues normally parser_ACCEPT parser_ABORT parser_ERROR parser_CLEARIN parser_CLINERR parser_ERROROK parser_CLINEROK - parsing terminates and succeeds - parsing terminates and fails - syntax error; parser enters error state - pending input token is discarded - pending input token is discarded and parser enters error state - parser leaves error state and continues - pending input token is discarded and parser leaves error state In yacc/Bison, YYSTYPE defaults to int. Mkparser removes this default; the .y file must have a type named YYSTYPE by one of: include a %union section in the grammar header in gggg.y,or #define YYSTYPE in gggg.y or a file it #includes, or declare YYSTYPE with a typedef in gggg.y or a file it #includes. Application Code In general, an application creates a parser object for a given grammar by calling gggg_New: struct parser *gparser = gggg_New(); The parse itself is done by calling parser_Parse with this object as the first argument and a lexeme stream as two more arguments: parser_Parse(gparser, lexer, lexrock); A complete program might look like this . . . #include <gggg.h> /* include header file created by Bison and mkparser from gggg.y */ . . . struct parser *gparser = gggg_New(); . . . /* modify gparser object. For instance: */ . . . parser_SetRock(gparser, xxxxx); /* now do the parse */ if (parser_Parse(gparser, lexer, lexer_rock) == parser_OK) { /* action for successful parse */ } else { /* action for failed parse */ } The files gggg.h is generated by mkparser and declares gggg_New(). Tokens are acquired by the parser by calling the lexer provided as the second argument. The third argument, lexer_rock, is supplied as one of the arguments to the lexer. The full type expected of the lexer function is int lexer(void *lexrock, void *yylval); Lexer routines can copy semantic values into *yylval, which will have space for a value of type YYSTYPE. Note that, if the value is a pointer to an object, the pointer should be stored in *yylval and not in yylval (which will disappear as the function returns). The lexer must return Bison token numbers rather than yacc numbers; yacc uses the first 256 values to indicate distinct ASCII characters, but Bison does not. In 'tlex', the Bison token numbers are acquired from the gggg.tab.c file generated by Bison; other lexers can generate yacc token numbers and translate them with parser_TranslateTokenNumber: if the yacc token number is t, the Bison token number is parser_TranslateTokenNumber(gparser, t) Between the gggg_New() and the call to parser_Parse, the application can apply other functions to the object such as parser_EnumerateTokens to enter reserved words in a symbol table, parser_SetKillVal to handle error cleanup, or parser_SetRock to store a pointer for use by semantic action routines. The parser returns the maximum severity from among the severity values passed to parser_Error. These values are parser_OK no error parser_WARNING there was some minor problem parser_SERIOUS compilation aborted, but scan continued parser_SYNTAX same as SERIOUS, but due to a syntax error parser_FATAL compilation could not continue When a syntax error occurs, the value stack is popped without calling semantic routines. This can remove pointers to allocated memory which ought to be freed. To allow the application to deal with this, the application can specify a 'killval' function which will be called for each value that is discarded from the stack without calling a semantic routine. See the function parser_SetKillVal. Parse-time stacks A parser object has two stacks which are initially allocated at 500 elements, but grow as needed. Use left recursion in grammars to avoid requiring great stack depth. Note that stack depth reflects the amount of information a program reader needs to interpret the program and a grammar requiring a large stack is too complex. The value stack contains copies of objects as returned by the lexer and set in the action routines. If these objects contain pointers--to "pointee" objects, the client is responsible for the memory occupied by the pointees. If the parser terminates early for a syntax error or ABORT, these values can be deleted by supplying a KillVal function. parser_SetKillVal(gparser, f) will establish f as the killval function. After a syntax error and before discarding the stack, this function is called for each value on the stack. The killval function is also called as states are popped for error recovery. The call is (killvalfunction)(parseobject, value-pointer-from-stack) Mkparser The mkparser script is invoked to produce gggg.c and gggg.h from the gggg.tab.c file generated by Bison. It is possible to have Bison generate the additional file gggg.tab.h by specifying the -d switch. (For tlex, the Andrew version of Bison must be used and the -r switch specified in addition to -k.) At minimum, mkparser has one argument, the prefix of the file names: mkparser gggg where the input files are gggg.tab.c and gggg.tab.h (if any). The output will be the two files gggg.c and gggg.h. Mkparser may have one or two additional arguments, the name of the Bison output .c file and the name of the .h file. If specified, these files are used instead of the files named by concatenating the given prefix to .tab.c and .tab.h. In any case, the prefix is used to generate the name gggg_New(). Compilation In the Imakefile, the grammar is processed with a rule like: gggg.c gggg.h: gggg.y ExecuteFromDESTDIR(bison -k gggg.y) ExecuteFromDESTDIR(mkparser gggg) (The -k switch in the AUIS version of bison generates causes output of a few additonal declarations.) The .c file resulting from mkparser is compiled as a normal .c file and linked together with other source files for the application. Bison's -l switch should NOT be used; mkparserclass depends on the #line directives in the file. If necessary, these can be removed from gggg.C with sed: sed '/^#line/d' gggg.C > ,gggg.C; mv ,gggg.C gggg.C Linking The application must be linked with cparser.o or a library containing it. cparser.o is the result of compiling cparser.c, from the same source directory as mkparser. In AUIS, cparser.o is installed in $ANDREWDIR/lib/atk/libcparser.a. Functions provided by parser object: int parser_Parse(struct parser *self, parser_lexerfptr, void *lexerrock) Causes the parser to run to completion using the lexeme stream supplied as the second and third arguments. Returns one of the severity values, indicating the highest severity error encountered. int parser_ParseNumber(char *buf, long *plen, long *intval, double *dvlval) Parses a number from buf and sets *plen its length in buf. If intval is non-null, *intval is set to the number's integer value. Similarly, if dblval is non-null, *dblval is set to the number's value as a double. Returns 1 if syntactically an integer, 2 for a double, and 0 for a syntax error. An integer is a zero followed by a string of octal digits, a non-zero digit followed by decimal digits, 0x followed by a string of hexadecimal digits, or a character within apostrophes, possibly \-escaped. A real is of the form [ddd][.][ddd][Epddd] where [...] indicates an optional part except that the complete number must have either . or Epddd ddd is a digit sequence (one or more digits) p (sign) may be empty or + or E (exponent indicator) may be 'e' or 'E' int parser_TransEscape(char *buf, int *plen) buf holds a character sequence (at least three chars) that occurred after a backslash in a string. The translation is returned as an int. The number of characters used is returned in *plen. (plen may be NULL) The translations are a superset of C: escape seq : translation --------------- : -----------\\ \' \" \b \t : as in C \n \v \f \r : as in C \ddd : octal digits, as in C \? : \177 (DEL) \e : \033 (ESC, ctl-[) \^@ : \000 (NUL) \^a ... \^z : \001 ... \032 (ctl-a ... ctl-z) \^[ \^\ \^] : \033 \034 \035 \^^ \^_ : \036 \037 void parser_Error(struct parser *self, int severity, char *msg) Call this function to report an error. It counts the number of errors, records the maximum severity, and then calls ErrorGuts for disposition. void parser_EnumerateReservedWords(struct parser *self, parser_enumresfptr handler, void *rock) The handler is called for each alphabetic reserved word: handler(rock, char *word, int tokennumber) It is not called for names beginning with "set"; for names beginning with "tok", only the rest of the name is passed. Uppercase letters are converted to lower, and vice versa. int parser_TokenNumberFromName(struct parser *self, char *name) Returns the token number corresponding to the string. Typical strings: "function", "setID", "tokNULL", "'a'", "\":=\"" (Note the quotes around special character tokens.) If the name is not found, returns 0. char parser_TranslateTokenNumber(struct parser *self, int x) Bison numbers tokens differently than yacc; in particular, the first 256 do not correspond to the ASCII characters. This function converts a yacc token number, x, into the token number required by Bison. void parser_SetRock(struct parser *self, void *r) Sets the 'rock' value associated with the parser. This value is then available in any context--lexical analysis, semantic action routine, or other--which has a pointer to the parser object. void * parser_GetRock(struct parser *self) Returns the 'rock' value. void parser_SetKillVal(struct parser *self, parser_killfptr kv) This function sets the killval function to kv. The latter is called when value stack items are popped for errors. See above. parser_killfptr parser_GetKillVal(struct parser *self) Returns the killval function. struct parser *parser_GetCurrentparser() During any call to parser_Parse, this function returns the current parser object. This can be supplied as the object for parser_Error. int parser_SetDebug(int value) Sets the debug flag to the given value; value must be 0 or 1. Returns the prior value. int parser_GetErrorState(struct parser *self) The error-state is an integer indicating how many tokens must be successfully parsed before resuming correct parsing. Usually this value is zero; when a syntax error is detected, the value is set to three. To change errorstate, an action concludes with parser_ERROR to indicate an error or parser_ERROROK to reset the error state to zero. void parser_SetMaxSeverity(struct parser *self, int s) Sets the value remembered as the maximum severity encountered. It is preferable to do so by calling parser_Error. int parser_GetMaxSeverity(struct parser *self) Returns the current maximum severity value. void parser_SetNErrors(struct parser *self, int n) Allows the application to set the number of errors encountered. It is usually incorrect to call this function. int parser_GetNErrors(struct parser *self) Returns the number of errors that have been encountered in the current compilation. char **parser_GetTokenNames(struct parser *self) Returns a pointer to an array of all token names in order by token number. short GetNTokens(struct parser *self) Returns the number of tokens in the grammar. Copyright 1992, 1994 Carnegie Mellon University. All rights Reserved. $Disclaimer: # Permission to use, copy, modify, and distribute this software and its # documentation for any purpose is hereby granted without fee, # provided that the above copyright notice appear in all copies and that # both that copyright notice, this permission notice, and the following # disclaimer appear in supporting documentation, and that the names of # IBM, Carnegie Mellon University, and other copyright holders, not be # used in advertising or publicity pertaining to distribution of the software # without specific, written prior permission. # # IBM, CARNEGIE MELLON UNIVERSITY, AND THE OTHER COPYRIGHT HOLDERS # DISCLAIM ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING # ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. 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