38e505c3e5
Add Caldera license. Approved by: David Taylor <davidt@caldera.com> Make roughly buildable under FreeBSD. The results are not perfect: the original Makefile referred to a refer file papers/Ind, which doesn't seem to have been kept, so the references to other publications are missing. In addition, the pagination is not correct, with the result that some .DS/.DE blocks leave large amounts of white space empty before them. Possibly this could be fixed by putting the (blank) footnotes at the end. PR: 35345 Requested by: Tony Finch <fanf@dotat.at>
162 lines
6.7 KiB
Plaintext
162 lines
6.7 KiB
Plaintext
.\" Copyright (C) Caldera International Inc. 2001-2002. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions are
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.\" met:
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.\"
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.\" Redistributions of source code and documentation must retain the above
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.\" copyright notice, this list of conditions and the following
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.\" disclaimer.
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.\"
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.\" Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\"
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.\" This product includes software developed or owned by Caldera
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.\" International, Inc. Neither the name of Caldera International, Inc.
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.\" nor the names of other contributors may be used to endorse or promote
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.\" products derived from this software without specific prior written
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.\" permission.
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.\"
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.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA
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.\" INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
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.\" BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
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.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
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.\" OR OTHERWISE) RISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
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.\" IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" @(#)ss7 8.1 (Berkeley) 6/8/93
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.\"
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.\" $FreeBSD$
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.SH
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7: Error Handling
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.PP
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Error handling is an extremely difficult area, and many of the problems are semantic ones.
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When an error is found, for example, it may be necessary to reclaim parse tree storage,
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delete or alter symbol table entries, and, typically, set switches to avoid generating any further output.
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.PP
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It is seldom acceptable to stop all processing when an error is found; it is more useful to continue
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scanning the input to find further syntax errors.
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This leads to the problem of getting the parser ``restarted'' after an error.
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A general class of algorithms to do this involves discarding a number of tokens
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from the input string, and attempting to adjust the parser so that input can continue.
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.PP
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To allow the user some control over this process,
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Yacc provides a simple, but reasonably general, feature.
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The token name ``error'' is reserved for error handling.
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This name can be used in grammar rules;
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in effect, it suggests places where errors are expected, and recovery might take place.
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The parser pops its stack until it enters a state where the token ``error'' is legal.
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It then behaves as if the token ``error'' were the current lookahead token,
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and performs the action encountered.
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The lookahead token is then reset to the token that caused the error.
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If no special error rules have been specified, the processing halts when an error is detected.
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.PP
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In order to prevent a cascade of error messages, the parser, after
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detecting an error, remains in error state until three tokens have been successfully
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read and shifted.
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If an error is detected when the parser is already in error state,
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no message is given, and the input token is quietly deleted.
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.PP
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As an example, a rule of the form
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.DS
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stat : error
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.DE
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would, in effect, mean that on a syntax error the parser would attempt to skip over the statement
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in which the error was seen.
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More precisely, the parser will
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scan ahead, looking for three tokens that might legally follow
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a statement, and start processing at the first of these; if
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the beginnings of statements are not sufficiently distinctive, it may make a
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false start in the middle of a statement, and end up reporting a
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second error where there is in fact no error.
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.PP
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Actions may be used with these special error rules.
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These actions might attempt to reinitialize tables, reclaim symbol table space, etc.
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.PP
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Error rules such as the above are very general, but difficult to control.
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Somewhat easier are rules such as
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.DS
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stat : error \';\'
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.DE
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Here, when there is an error, the parser attempts to skip over the statement, but
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will do so by skipping to the next \';\'.
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All tokens after the error and before the next \';\' cannot be shifted, and are discarded.
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When the \';\' is seen, this rule will be reduced, and any ``cleanup''
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action associated with it performed.
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.PP
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Another form of error rule arises in interactive applications, where
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it may be desirable to permit a line to be reentered after an error.
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A possible error rule might be
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.DS
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input : error \'\en\' { printf( "Reenter last line: " ); } input
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{ $$ = $4; }
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.DE
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There is one potential difficulty with this approach;
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the parser must correctly process three input tokens before it
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admits that it has correctly resynchronized after the error.
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If the reentered line contains an error
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in the first two tokens, the parser deletes the offending tokens,
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and gives no message; this is clearly unacceptable.
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For this reason, there is a mechanism that
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can be used to force the parser
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to believe that an error has been fully recovered from.
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The statement
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.DS
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yyerrok ;
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.DE
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in an action
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resets the parser to its normal mode.
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The last example is better written
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.DS
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input : error \'\en\'
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{ yyerrok;
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printf( "Reenter last line: " ); }
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input
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{ $$ = $4; }
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;
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.DE
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.PP
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As mentioned above, the token seen immediately
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after the ``error'' symbol is the input token at which the
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error was discovered.
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Sometimes, this is inappropriate; for example, an
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error recovery action might
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take upon itself the job of finding the correct place to resume input.
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In this case,
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the previous lookahead token must be cleared.
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The statement
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.DS
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yyclearin ;
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.DE
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in an action will have this effect.
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For example, suppose the action after error
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were to call some sophisticated resynchronization routine,
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supplied by the user, that attempted to advance the input to the
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beginning of the next valid statement.
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After this routine was called, the next token returned by yylex would presumably
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be the first token in a legal statement;
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the old, illegal token must be discarded, and the error state reset.
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This could be done by a rule like
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.DS
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stat : error
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{ resynch();
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yyerrok ;
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yyclearin ; }
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;
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.DE
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.PP
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These mechanisms are admittedly crude, but do allow for a simple, fairly effective recovery of the parser
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from many errors;
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moreover, the user can get control to deal with
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the error actions required by other portions of the program.
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