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diff --git a/share/doc/psd/15.yacc/ss2 b/share/doc/psd/15.yacc/ss2 new file mode 100644 index 000000000000..f1fdb4407581 --- /dev/null +++ b/share/doc/psd/15.yacc/ss2 @@ -0,0 +1,190 @@ +.\" 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. +.\" +.\" @(#)ss2 8.1 (Berkeley) 6/8/93 +.\" +.\" $FreeBSD$ +.SH +2: Actions +.PP +With each grammar rule, the user may associate actions to be performed each time +the rule is recognized in the input process. +These actions may return values, and may obtain the values returned by previous +actions. +Moreover, the lexical analyzer can return values +for tokens, if desired. +.PP +An action is an arbitrary C statement, and as such can do +input and output, call subprograms, and alter +external vectors and variables. +An action is specified by +one or more statements, enclosed in curly braces ``{'' and ``}''. +For example, +.DS +A : \'(\' B \')\' + { hello( 1, "abc" ); } +.DE +and +.DS +XXX : YYY ZZZ + { printf("a message\en"); + flag = 25; } +.DE +are grammar rules with actions. +.PP +To facilitate easy communication between the actions and the parser, the action statements are altered +slightly. +The symbol ``dollar sign'' ``$'' is used as a signal to Yacc in this context. +.PP +To return a value, the action normally sets the +pseudo-variable ``$$'' to some value. +For example, an action that does nothing but return the value 1 is +.DS + { $$ = 1; } +.DE +.PP +To obtain the values returned by previous actions and the lexical analyzer, the +action may use the pseudo-variables $1, $2, . . ., +which refer to the values returned by the +components of the right side of a rule, reading from left to right. +Thus, if the rule is +.DS +A : B C D ; +.DE +for example, then $2 has the value returned by C, and $3 the value returned by D. +.PP +As a more concrete example, consider the rule +.DS +expr : \'(\' expr \')\' ; +.DE +The value returned by this rule is usually the value of the +.I expr +in parentheses. +This can be indicated by +.DS +expr : \'(\' expr \')\' { $$ = $2 ; } +.DE +.PP +By default, the value of a rule is the value of the first element in it ($1). +Thus, grammar rules of the form +.DS +A : B ; +.DE +frequently need not have an explicit action. +.PP +In the examples above, all the actions came at the end of their rules. +Sometimes, it is desirable to get control before a rule is fully parsed. +Yacc permits an action to be written in the middle of a rule as well +as at the end. +This rule is assumed to return a value, accessible +.\" XXX What does this mean? Nobody seems to understand it. +.\" through the usual \$ mechanism by the actions to +through the usual mechanism by the actions to +the right of it. +In turn, it may access the values +returned by the symbols to its left. +Thus, in the rule +.DS +A : B + { $$ = 1; } + C + { x = $2; y = $3; } + ; +.DE +the effect is to set +.I x +to 1, and +.I y +to the value returned by C. +.PP +Actions that do not terminate a rule are actually +handled by Yacc by manufacturing a new nonterminal +symbol name, and a new rule matching this +name to the empty string. +The interior action is the action triggered off by recognizing +this added rule. +Yacc actually treats the above example as if +it had been written: +.DS +$ACT : /* empty */ + { $$ = 1; } + ; + +A : B $ACT C + { x = $2; y = $3; } + ; +.DE +.PP +In many applications, output is not done directly by the actions; +rather, a data structure, such as a parse tree, is constructed in memory, +and transformations are applied to it before output is generated. +Parse trees are particularly easy to +construct, given routines to build and maintain the tree +structure desired. +For example, suppose there is a C function +.I node , +written so that the call +.DS +node( L, n1, n2 ) +.DE +creates a node with label L, and descendants n1 and n2, and returns the index of +the newly created node. +Then parse tree can be built by supplying actions such as: +.DS +expr : expr \'+\' expr + { $$ = node( \'+\', $1, $3 ); } +.DE +in the specification. +.PP +The user may define other variables to be used by the actions. +Declarations and definitions can appear in +the declarations section, +enclosed in the marks ``%{'' and ``%}''. +These declarations and definitions have global scope, +so they are known to the action statements and the lexical analyzer. +For example, +.DS +%{ int variable = 0; %} +.DE +could be placed in the declarations section, +making +.I variable +accessible to all of the actions. +The Yacc parser uses only names beginning in ``yy''; +the user should avoid such names. +.PP +In these examples, all the values are integers: a discussion of +values of other types will be found in Section 10. |