LLGEN
Generator of syntax analyzier (parser)
GENERATOR L LGEN
The main task of generator LLGEN is generate a
parser (in C), which use the recursive go down
method without recurrence;
The source code is generated by LLgen'a on the basis
of the file containing the specification;
In the file with specification we can use the extended
specification of simple LL(1) grammars.
Because LLgen includes a built-in mechanism of
static and dynamic conflict resolution, it allows the
use of ambiguous grammars;
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GENERATOR LLGEN
Diagram the organization of LLgen:
scan.l
gram.g
LEX
LLgen
scan.c
gram.c
Lpars.c
Lpars.h
GCC
scane.exe
file.txt
RESULT
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GENERATOR LLGEN
flex –l scan.l (use of generator LEX)
result - lex.yy.c
LLgen gram.g (use of generator LLgen for file with
the speciticafion gram.g)
result - Lpars.c and Lpars.h
gcc lex.yy.c Lpars.c gram.c (compilation in C)
./a.out < file.in (analisi of file)
4 4
GENERATOR LLGEN
LLgen generator uses (default) an external
lexical analyzer (generated by Lex). For this
purpose is used the function yylex();
The file Lpars.h which is generated during
generator
operation
LLgen,
contains
definitions that assigned numeric constants
declared the names of the token-s;
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GNENRATOR LLGEN
Ways to use another analyzer are as follows:
 Put the implementation of the scanner directly
in the specification grammar (in the ’{’ ’}’or in
an external file);
 In the specifications, we have to indicate the
name of the function which is use by LLgen;
%lexical name_function;
 If it is necessary, we have to incorporate into
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the lexical analyzer of file Lpars.h;
GENERATOR LLGEN
LLgen is a tool row. The specification file for
LLgen we create in a form of plain text file,
sometimes in several files;
Each of the generated source code, contains
productions, directive of generator LLgen and
declarations and code in C.
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CREATE SPECIFICATION
Each production from the specifications for the
program LLgen comprises: nonterminal, the
character ":" and the right hand side of the
production. Ends with a semicolon;
The right side of alternative production are
separated by a "|";
The right side of the production can be of
terminals, nonterminals and semantic actions;
nonterminal : the right hand side of the
production ;
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CREATE SPECIFICATION
The rules create specifications:
 White spaces are ignored, but can not occur
within name;
 Comments are introducing after the character
„//”;
 Comments can not be nested;
 Comments may occur at any place where it is
allowed to occurrence of names;
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CREATE SPECIFICATION
The rules create specifications:
 The names of terminal and non-terminal
symbols can be any length. They have a syntax
such as C language identifiers;
 Symbol names must not conflict with
keywords in C;
 Capitalization is distinguishable;
10
CREATE SPECIFICATION
The rules create specifications:
 The names of symbols can be any length, but
in LLgen significant is 50 characters;
 All names generated and used by LLgen begin
with the prefix LL;
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DECLARATION OF TERMINA
Terminals that are not letters, we declare:
%token ken;
If you have multiple terminals to declaration,
we can do this:
%token name1, name2, name3;
Any use of the terminal must be preceded his
declaration;
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DEClARATION OF TERMINAL







The terminals, which are the letters are included in
quotes;
LLgen also recognizes (as C) a set of special
literals, i.e.:
new line
‘\n’
tab
‘\t’
carriage return
‘\r’
apostrophe
‘\’’
withdrawal character
‘\b’
backslash
‘\\’
octal number
‘\xxx’
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DECLRATION OF TERMINAL
REMEMBER!!!
Assume that the parser encountered in the test
file, name that has not been declared as a
token. This name will be treated by LLgen as
a symbol nonterminal;
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DECLRATION OF TERMINAL
Nonterminals is implemented like a function
of the C language;
In LLgen we can use local variables. The
generator enables them to declare, in
brackets, only the left side of production as a
nonterminal symbol, eg .:
A {int ken;} : S ken T ;
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DECLRATION OF TERMINAL
Through the semantic action, we mean any
single instruction (a group of instructions)
written in C, which are enclosed in braces;
In LLgen the semantic actions can insert only
the right side of the production, eg .:
A {int counter} : S ken {counter=1;} T ;
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STARTER NONTERMINAL
Analyzers generated by LLgen may have
multiple terminals not boot;
Declaration
of
starter
nonterminals
(otherwise axiom) is as follows:
%start function , name_of_nonterminal;
example:
%start parse, S;
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COMMANDS OF COMPILATION
The command, which is used to start of the generator
is LLgen. This command is invoked for pilku
specifications (extension g), for example,.: LLgen
gram.g
LLgen on the output produces three files:
 gram.c – file in C, which contains implementation of
parser;
 Lpars.h – file containing the syntax analyzer
interface;
 Lpars.c – parser skeleton and board control;
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OPTION -V
Sometimes it's helpful for startup and testing
parser, use the -v option;
Thanks to the -v option, will be generated file
LL.output, which contains information about
the unresolved conflicts that have arisen in the
grammar;
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EXTENSION OF GRAMMAR OF
SYNTAX
Extensions of syntax context-free grammars:
 * (*quantity) – feedback clouser;
 + (+quantity) – positive clouser;
 ? – optionality operator;
 [...] – The possibility of grouping of symbols;
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Example
Let ∑={a,b}. Let consider the following
regular language L=L(b*a). Then:
S:BA
;
B:
| ‘b’ B
;
A : ‘a’
;
S:BA
;
B : ‘b’ *
;
A : ‘a’
;
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Example
Let ∑={a,b}. Let consider the following
language L={b, ab, aab, aaab}. Then:
S:AB
;
A:
| ‘a’ C
;
C:
| ‘a’
;
B : ‘b’
;
S:AB
;
A : ‘a’ *3
;
B : ‘b’
;
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Example
Let ∑={a,b}. Let consider the following
language L={ab, aab, aaab}. Then:
S:AB
;
A : ‘a’ C
;
C:
| ‘a’
;
B : ‘b’
;
S:AB
;
A : ‘a’ +3
;
B : ‘b’
;
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Example
Let ∑={a,b}. Let consider the following
landuade L={b, ab}. Then:
S:AB
;
A:
| ‘a’
;
B : ‘b’
;
S:AB
;
A : ‘a’ ?
;
B : ‘b’
;
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Example
Let ∑={a,b}. Let consider the following
language L={A ∈∑* : |A|=2}. Then:
S : ‘a’ B
| ‘b’ B
;
B : ‘a’
| ‘b’
;
S : [ ‘a’ | ‘b’ ] +2
;
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COMPARISON
Consider the grammar, which is not a simple
LL(1) grammar.
Compare the effort of procedure for
adjusting the grammar and implementation
of grammar in generator LLgen;
Let ∑=[a,b}. We will write a program that
accepts context-free language L={A∈∑* :
A=an bn ; n ∈Ν};
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COMPARISON
{
int quan_a, quan_b;
}
%start parse , S;
S:AB
{ if (quan_a= = quan_b) puts(’’OK.’’);
else puts(’’Blad’’); }
;
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COMPARISON
We remove the leftmost recursion;
A : ’a’ { quan_a=1; }
| A ’a’ { quan_a++; }
;
B : ‘b’ { quan_b=1; }
| B ‘b’ { quan_b++; }
;
A : ’a’ { quan_a=1; }
| ’a’ A { quan_a++; }
;
B : ‘b’ { quan_b=1; }
| ‘b’ B { quan_b++; }
;
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COMPARISON
A : ’a’ C
;
C:
| ’a’ C
;
B : ’b’ D
;
D:
| ’b’ D
;
{ quan_a++; }
{ quan_a=0; }
{ quan_a++; }
{ quan_b++; }
{ quan_b=0; }
{ quan_b++; }
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COMPARISON
S : {quan_a=quan_b=0} A B
{ if (quan_a= = quan_b) puts(’’OK.’’);
else puts(’’Blad’’); }
;
A : [ ’a’ {quan_a++} ] +
;
B : [ ’b’ {quan_b++} ] +
;
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LLSYMB
LLsymb is a global integer variable that can
take on different values. What value will be
accepted, depending on the position of the
read head on the right side of the production:
Possible values:
 If the parser read the token, then in variable
LLsymb we have this token;
 After grouping and alternative, in variable is
remembered token;
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CREATE SPECIFICATION
In the file with the specifications for the
generator LLgen, should be included
implementation of the main function;
%start parse, S ;
int main(){
parse();
return 0;
}
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CREATE SPECIFICATION
The file with the specifications for the
generator LLgen should also be included
function LLmessage;
This function is automatically called by the
parser when an error occurs syntax;
void LLmessage ( int tk );
Do not
return any
value
It has one integer parameter
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CREATE SPECIFICATION
The variable tk accepts the following values:
 when he was expected token „tk” – tk > 0;
 when loaded is an unexpected token and it has
been removed – tk = 0;
 if not encountered the expected end of the file
and other input will be ignored – tk = - 1;
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Example
The operation of LLgen generator is best seen in
the example. At the entrance there is a string of
words made up of natural alphabet, words end
with a colon and are separated by a comma. Given
the input string contains at least one word....
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CONFLICTS
During working of the syntax generator, can
occur the following conflicts:
- We are not able to determine which of the right
sides should to be developed - conflict of
alternatives;
- The structure that is currently in progress,
includes the closure and it is difficult to
determine, whether the input is its
continuation, or start another construction - a
conflict of repetitions;
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CONFLICTS
Conflict of alternatives can be resolved in two
ways:
 dynamic settlement of the alternatives
conflict:
%if (condition)
 static settlement of the alternatives conflict:
%prefer ⇔ %if(1)
%avoid ⇔ %if(0)
37
Example
Consider the task of testing whether the binary
number is even number;
Lexical analyzer identifies and returns the
binary numbers;
%%
[01] { return yytext[0]; }
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Example
{
int read_number;
}
%start parse, S;
S : ’0’ { read_number = 0; } R
| ’1’ { read_number = 1; } R
;
R : %if (read_number ==0 ) {puts(”even
number”);}
|
{puts(”odd number”);}
|S
;
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SOLVING OF THE CONFLICT
Example use of mechanism of static
resolution of alternatives conflict is the socalled problem: "dangling else";
This issue will be discussed in detail during
the lecture devoted to the generator YACC;
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SOLVING OF THE CONFLICT
To resolve of the conflict of repetition we may
use the keyword %while;
%while ( condition )
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THE END
END OF THE SIXTH LECTURE