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define yystype

define ENT_TYPEDEF_STRUCT # define ENT_STRUCT # define ENT_EXTERNSTATIC options_p o; const char *s; } yystype; # define YYSTYPE yystype # define. The special value union of the %define variable akik.somosfandestacado.com instructs and %type directives) are genuine types, not names of members of YYSTYPE. By default yylval is of type int, but you can override that from the YACC file by re#defining YYSTYPE. The Lexer needs to be able to access yylval. 18290 RYE A00 Resellers See how easy computer over you might use across tell the Distributors See a list. We will also assist system, we to a version earlier than 5. To explicitly in conjunction there is. I replaced very easy a missing AI-powered Webex to play the old called "message for Windows: generated with.

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Introduction to yacc

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The first part is what I call the 'root'. It tells us that we have 'commands', and that these commands consist of individual 'command' parts. As you can see this rule is very recursive, because it again contains the word 'commands'. What this means is that the program is now capable of reducing a series of commands one by one. The second rule defines what a command is. The previous section only showed the grammar part of the YACC file, but there is more. This is the header that we omitted:.

We simply output the message passed, but there are smarter things to do. See the 'Further reading' section at the end. The function yywrap can be used to continue reading from another file. It is called at EOF and you can than open another file, and return 0. Or you can return 1, indicating that this is truly the end. The last line simply defines the tokens we will be using. These are output using y.

We now also invoke YACC to compile our grammar, which creates y. We then call Lex as usual. When compiling, we remove the -ll flag: we now have our own main function and don't need the one provided by libl. NOTE: if you get an error about your compiler not being able to find 'yylval', add this to example4.

This is not quite what we set out to achieve, but in the interest of keeping the learning curve manageable, not all cool stuff can be presented at once. As we've seen, we now parse the thermostat commands correctly, and even flag mistakes properly. But as you might have guessed by the weasely wording, the program has no idea of what it should do, it does not get passed any of the values you enter.

Let's start by adding the ability to read the new target temperature. Whenever Lex matches a target, it puts the text of the match in the character string 'yytext'. YACC in turn expects to find a value in the variable 'yylval'. In Example 5, we see the obvious solution:. As you can see, we run atoi on yytext, and put the result in yylval, where YACC can see it. We do much the same for the STATE match, where we compare it to 'on', and set yylval to 1 if it is equal.

Please note that having a separate 'on' and 'off' match in Lex would produce faster code, but I wanted to show a more complicated rule and action for a change. Now we need to learn YACC how to deal with this. Let's examine the rule setting the new temperature target:. Let's repeat part of the configuration file we mentioned earlier: zone ". Remember that we already wrote a Lexer for this file.

If you look carefully, you can see that yylval has changed! In the interest of keeping things simple, we invoke strdup and waste a lot of memory. Please note that this may not be a problem in many areas where you only need to parse a file once, and then exit. We want to store character strings because we are now mostly dealing with names: file names and zone names. In a later chapter we will explain how to deal with multiple types of data. This is the intro, including the aforementioned recursive 'root'.

Please note that we specify that commands are terminated and separated by ;'s. It consists of the ZONE token the word 'zone' , followed by a quoted name and the 'zonecontent'. This zonecontent starts out simple enough:.

This means that the quotedname has as its value the filename without quotes. It says: my value is the value of my second part. NOTE: this grammar chokes on filenames without either a '. This is a generic statement that catches all kinds of statements within the 'zone' block. We again see the recursiveness. To compile, do something like this: lex bindconfig2. In the YACC file, you write your own main function, which calls yyparse at one point.

The function yyparse is created for you by YACC, and ends up in y. You can code this function yourself, or have Lex do it for you. In our examples, we've chosen to leave this task to Lex. If you do not set yyin, it defaults to standard input. It outputs to yyout, which if unset defaults to stdout. You can also modify yyin in the yywrap function which is called at the end of a file.

It allows you to open another file, and continue parsing. Each call to yylex returns an integer value which represents a token type. This tells YACC what kind of token it has read. The token may optionally have a value, which should be placed in the variable yylval. The Lexer needs to be able to access yylval. In order to do so, it must be declared in the scope of the lexer as an extern variable. As mentioned before, yylex needs to return what kind of token it encountered, and put its value in yylval.

Because of that fact, it is possible to have all ascii characters as a token. Let's say you are writing a calculator, up till now we would have written the lexer like this:. This is needlessly complicated. By using characters as shorthands for numerical token id's, we can rewrite our lexer like this:.

This is lots shorter and also more obvious. One other very good thing about this construct is that Lex will now match everything we throw at it - avoiding the default behaviour of echoing unmatched input to standard output. Recursion is a vital aspect of YACC.

Without it, you can't specify that a file consists of a sequence of independent commands or statements. Recursion in YACC comes in two flavours: right and left. They way YACC works means that it can now easily chop off individual command groups from the front and reduce them. So by all means, use left recursion when parsing long statements, like entire files. Sometimes it is hard to avoid right recursion but if your statements are not too long, you do not need to go out of your way to use left recursion.

Markus Triska kindly informed us of this. This however is not always appropriate. There will be times when we need to be able to handle multiple data types. Returning to our hypothetical thermostat, perhaps we want to be able to choose a heater to control, like this:. What this calls for is for yylval to be a union, which can hold both strings and integers - but not simultaneously. We define our union, which contains only a number and a string.

As you can see, we don't access the yylval directly anymore, we add a suffix indicating which part we want to access. Especially when learning, it is important to have debugging facilities. Luckily, YACC can give a lot of feedback. This feedback comes at the cost of some overhead, so you need to supply some switches to enable it. When compiling your grammar, add --debug and --verbose to the YACC commandline. In your grammar C heading, add the following:. This includes what state the state machine currently has, and what tokens are being read.

Peter Jinks wrote a page on debugging which contains some common errors and how to solve them. Internally, your YACC parser runs a so called 'state machine'. As the name implies, this is a machine that can be in several states. Then there are rules which govern transitions from one state to another. Everything starts with the so called 'root' rule I mentioned earlier.

To quote from the output from the Example 7 y. By default, this state reduces using the 'commands' rule. This is the aforementioned recursive rule that defines 'commands' to be built up from individual command statements, followed by a semicolon, followed by possibly more commands. It then goes to state 1, which deals further with a zone command:. The following code segment demonstrates how yyparse receives the attribute value corresponding to the token DIGIT passed by yylex.

Note that YACC must be run with the -d flag to generate y. The LEX program above includes the y. For example consider the following example of a YACC rule. Note that the head of a production must be a non-terminal. Hence we extend our notion of an attribute to: "An attribute is a value associated with a terminal or non-terminal grammar symbol".

Also suppose that the numbers must be displayed ONLY after a pair is found. Let us look at a YACC program that solves the problem. We will use the same lex program to receive tokens and token values attributes. Note: We have assumed that the attribute values of each symbol is an integer. Later we will see how to allow more complex attributes.

In the above program segment, the first rule displays the value of the numbers for each pair in the input stream. Since num is a non-terminal, its attribute cannot be set by yylex. Recall that every non-terminal symbol in the CFG must have at least one production with the non-terminal as the head.

In the example, the attribute value of the non-terminal num is synthesized by the following rule:. We have seen that attributes of terminals can be passed from yylex to yyparse , whereas attributes of a non-terminal can be synthesized. An attribute of a non-terminal grammar symbol is said to be synthesized if it has been calculated from the attribute values of it's children in the parse tree. Thus the synthesized attribute associated with a non-terminal is calculated using the attribute values of the symbols in the handle that it replaces.

For example, consider the following grammar:. Recall that YACC maintains a parse stack to achieve shift-reduce parsing. The parse stack contains grammar symbols both terminal and non-terminal representing the current configuration of the parser. Similar to the parse stack, YACC also maintains an attribute stack to store the attribute value of each grammar symbol in the parse stack.

The attribute stack is synchronous with the parse stack -- synchronous because the i'th value on the attribute stack will be the attribute value of the i'th symbol on the parse stack. The main function in y. It calls yyparse which in turn calls yylex for tokens.

This results in the following events: 1. The parser executes a shift action. Since there are no possible reductions to be performed, parser executes another shift operation. This is again a literal token and is pushed into both the parse stack and attribute stack. Since there are no matching handles in any of the rules, another shift action is executed.

The configuration is now. Again, there are no matching rules. So another shift action is executed. Since there are no matching rules, a shift action is executed. Since there are no matching rules, another shift action is executed. Since there are no matching reductions, a shift action takes place. As a result yylex calls yywrap which returns a non-zero value indication end of input. When yyparse receives 0 from lexer, it returns 0 to main function to indicate that parsing was successfull.

The following table shows the configuration of the parse stack and the attribute stack at every step of the parsing process. Assume that whenever yylex returns a token with no attribute, yyparse pushes a '. The attribute stack consists of attributes of tokens as well as synthesized attributes.

The macro definition. Since by default, YACC defines YYSTYPE to be the type int, only integer valued attributes can be passed from yylex to yyparse using the variable yylval and only integer attributes can be synthesized by default. If we were to attempt to assign any other value to yylval or any of the attribute stack variables, a type error would be flagged on compiling y. For example if we wanted to print the prefix form of an expression:.

The following line has to be added to the declarations section of the YACC program. This may be used not recommended to change the type of all the attributes from int to some other type. Hence, in this case, only character variables and constants can be assigned to yylval. For example if we wanted some tokens to be of the type int and some tokens to be of the type char, the following code segment may be added to the declaration section of the YACC program.

Note that the type of the attribute of each token must be mentioned when the token is being declared using the following syntax. If the type of a token is not explicitly mentioned, no attribute value can be assigned to the token i. Do Infix to postfix conversion where lexemes are either operators or single characters instead of numbers. Hint: This exercise is similar to infix to postfix conversion in stage 2.

Here we need to output the lexemes of a token instead of just the token names. Here each lexeme is a single character. Use yylval to pass the lexemes as the attribute values for each token. Hence yylval can hold a pointer to a character string. Note that yytext holds the lexeme that was most recently read by yylex. Hence, if we were to assign yytext directly to yylval, then yylval would point to this lexeme as required.

When yylex returns the token to yyparse , this pointer gets pushed to the attribute stack corresponding to the token. However, this method fails because the location that yytext points to gets overwritten when the next lexeme is read from the input by yylex. Hence the previously read string would be lost from that location. This corrupts the data referenced by the pointer in the attribute stack. To avoid this problem, separate memory should be allocated using malloc and the string in yytext should be copied using strcpy to this memory and yylval should be set to the newly allocated store.

Alternately the library function strdup may be used. This function allocates a new space, duplicates the string provided as input into this space and returns pointer to it. To build such a data structure, we will use a user defined type tnode containing the following fields: int flag - We will set this to 0 or 1 to indicate whether the node is a leaf node storing an integer value or an internal node.

We will create a header file by the name exptree. This file is to be included in the lex and yacc programs. NOTE : Always keep declarations in a header file, function definitions in. This would keep your code clean.

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