What is the use of yywrap

Return 1 if you are done or 0 if more processing is required. Every C program requires a main function. In this case we simply call yylex that is the main entry-point for lex. Some implementations of lex include copies of main and yywrap in a library thus eliminating the need to code them explicitly. This is why our first example, the shortest lex program, functioned properly. Here is a program that does nothing at all. All input is matched but no action is associated with any pattern so there will be no output.

The following example prepends line numbers to each line in a file. Some implementations of lex predefine and calculate yylineno. The input file for lex is yyin and defaults to stdin.

The definitions section is composed of substitutions, code, and start states. The Dragon Book describes Lex, a tool for creating lexical analyzers. We will use Flex, which is compatible with Lex, but is considerably faster. Flex takes a program written in a combination of Flex and C, and it writes out a file called lex.

When yylex is finished, it call function yywrap. If yywrap returns 1, then yylex returns 0 to its caller. The function yymore , by contrast, ensures that the succeeding characters recognized are appended to those already in yytext[].

This lets you do things sequentially, such as when one string of characters is significant and a longer one that includes the first is significant as well. Consider a language that defines a string as a set of characters between double quotation marks and specifies that to include a double quotation mark in a string, it must be preceded by a backslash. The regular expression matching that is somewhat confusing, so it might be preferable to write:.

Then the call to yymore causes the next part of the string "def to be tacked on the end. With the function yyless n you can specify the number of matched characters on which an action is to be performed: only the first n characters of the expression are retained in yytext[].

Suppose you are deciphering code, and working with only half the characters in a sequence that ends with a certain one, say an upper or lowercase Z.

You could write:. What would be the outputs of the lexical analyzer generated by the example LEX programs under section 3. If not, explain why. LEX declares the function yywrap of return-type int in the file lex.

LEX does not provide any definition for yywrap. If yywrap returns zero indicating false yylex assumes there is more input and it continues scanning from the location pointed to by yyin.

If yywrap returns a non-zero value indicating true , yylex terminates the scanning process and returns 0 i. If the programmer wishes to scan more than one input file using the generated lexical analyzer, it can be simply done by setting yyin to a new input file in yywrap and return 0.

As LEX does not define yywrap in lex. This options removes the call to yywrap in the lex. If not, LEX will flag an error.

Note the use of disambiguation rules here. Conceptually, LEX constructs a finite state machine to recognize all the regular expression patterns specified in the LEX program file. The code written by the programmer in the action part is executed when the machine is in accept state. The lex. LEX makes it's decision table visible if we compile the program with the -T flag. The finite state machine used by LEX is deterministic finite state automaton.

In this program, the main function obtains the tokens returned by yylex and checks if the input contains a valid identifier. As a result yylex returns the token ID, and the main function prints 'Acceptable' on the screen. This section explains how LEX converts a reqular expression to a finite automaton. An understanding of the contents of this section is not necessary to proceed to the next section.

Consider the first rule in the token simulator program in section 8. It consists of the following regular expression :. The syntax tree constructed for the above regular expression would look like :. In the syntax tree the inner nodes are operators while the leaves are the operands. The subscript assigned to every leaf is called the position of the leaf.