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diff --git a/share/doc/psd/15.yacc/ss7 b/share/doc/psd/15.yacc/ss7 new file mode 100644 index 000000000000..b6440c793857 --- /dev/null +++ b/share/doc/psd/15.yacc/ss7 @@ -0,0 +1,161 @@ +.\" Copyright (C) Caldera International Inc. 2001-2002. All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions are +.\" met: +.\" +.\" Redistributions of source code and documentation must retain the above +.\" copyright notice, this list of conditions and the following +.\" disclaimer. +.\" +.\" Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" +.\" All advertising materials mentioning features or use of this software +.\" must display the following acknowledgement: +.\" +.\" This product includes software developed or owned by Caldera +.\" International, Inc. Neither the name of Caldera International, Inc. +.\" nor the names of other contributors may be used to endorse or promote +.\" products derived from this software without specific prior written +.\" permission. +.\" +.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA +.\" INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR +.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED +.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE +.\" DISCLAIMED. IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE +.\" FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR +.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF +.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR +.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, +.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE +.\" OR OTHERWISE) RISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN +.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. +.\" +.\" @(#)ss7 8.1 (Berkeley) 6/8/93 +.\" +.\" $FreeBSD$ +.SH +7: Error Handling +.PP +Error handling is an extremely difficult area, and many of the problems are semantic ones. +When an error is found, for example, it may be necessary to reclaim parse tree storage, +delete or alter symbol table entries, and, typically, set switches to avoid generating any further output. +.PP +It is seldom acceptable to stop all processing when an error is found; it is more useful to continue +scanning the input to find further syntax errors. +This leads to the problem of getting the parser ``restarted'' after an error. +A general class of algorithms to do this involves discarding a number of tokens +from the input string, and attempting to adjust the parser so that input can continue. +.PP +To allow the user some control over this process, +Yacc provides a simple, but reasonably general, feature. +The token name ``error'' is reserved for error handling. +This name can be used in grammar rules; +in effect, it suggests places where errors are expected, and recovery might take place. +The parser pops its stack until it enters a state where the token ``error'' is legal. +It then behaves as if the token ``error'' were the current lookahead token, +and performs the action encountered. +The lookahead token is then reset to the token that caused the error. +If no special error rules have been specified, the processing halts when an error is detected. +.PP +In order to prevent a cascade of error messages, the parser, after +detecting an error, remains in error state until three tokens have been successfully +read and shifted. +If an error is detected when the parser is already in error state, +no message is given, and the input token is quietly deleted. +.PP +As an example, a rule of the form +.DS +stat : error +.DE +would, in effect, mean that on a syntax error the parser would attempt to skip over the statement +in which the error was seen. +More precisely, the parser will +scan ahead, looking for three tokens that might legally follow +a statement, and start processing at the first of these; if +the beginnings of statements are not sufficiently distinctive, it may make a +false start in the middle of a statement, and end up reporting a +second error where there is in fact no error. +.PP +Actions may be used with these special error rules. +These actions might attempt to reinitialize tables, reclaim symbol table space, etc. +.PP +Error rules such as the above are very general, but difficult to control. +Somewhat easier are rules such as +.DS +stat : error \';\' +.DE +Here, when there is an error, the parser attempts to skip over the statement, but +will do so by skipping to the next \';\'. +All tokens after the error and before the next \';\' cannot be shifted, and are discarded. +When the \';\' is seen, this rule will be reduced, and any ``cleanup'' +action associated with it performed. +.PP +Another form of error rule arises in interactive applications, where +it may be desirable to permit a line to be reentered after an error. +A possible error rule might be +.DS +input : error \'\en\' { printf( "Reenter last line: " ); } input + { $$ = $4; } +.DE +There is one potential difficulty with this approach; +the parser must correctly process three input tokens before it +admits that it has correctly resynchronized after the error. +If the reentered line contains an error +in the first two tokens, the parser deletes the offending tokens, +and gives no message; this is clearly unacceptable. +For this reason, there is a mechanism that +can be used to force the parser +to believe that an error has been fully recovered from. +The statement +.DS +yyerrok ; +.DE +in an action +resets the parser to its normal mode. +The last example is better written +.DS +input : error \'\en\' + { yyerrok; + printf( "Reenter last line: " ); } + input + { $$ = $4; } + ; +.DE +.PP +As mentioned above, the token seen immediately +after the ``error'' symbol is the input token at which the +error was discovered. +Sometimes, this is inappropriate; for example, an +error recovery action might +take upon itself the job of finding the correct place to resume input. +In this case, +the previous lookahead token must be cleared. +The statement +.DS +yyclearin ; +.DE +in an action will have this effect. +For example, suppose the action after error +were to call some sophisticated resynchronization routine, +supplied by the user, that attempted to advance the input to the +beginning of the next valid statement. +After this routine was called, the next token returned by yylex would presumably +be the first token in a legal statement; +the old, illegal token must be discarded, and the error state reset. +This could be done by a rule like +.DS +stat : error + { resynch(); + yyerrok ; + yyclearin ; } + ; +.DE +.PP +These mechanisms are admittedly crude, but do allow for a simple, fairly effective recovery of the parser +from many errors; +moreover, the user can get control to deal with +the error actions required by other portions of the program. |