Chenfengldw / small_c_compiler

compile principle project

Implementing a Lexical-Syntax Analyzer

By Duowen Liu, student ID:5130309723

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Introduction

In this project, I implement a simple compiler front-end, including a lexical analyzer and a syntax analyzer, for Small-C , which is a C-like language containing a subset of the C programming language.

Tools and environment:

• Lex,a lexical analyzer which help to divide code into tokens
• Yacc,a syntax analyzer help to generate parse tree
• compiler g++ under ubuntu

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Implement

step 1: Lexical analyzer

In this step, we will write a lexical analyser. The lexical analyser reads in the Small-C source code, and recognize tokens according to regular definitions

In lex.l,we define several tokens:

SEMI		[;]
COMMA		[,]
DOT			[.]
UNDERSCORE	[_]
LP			[(]

After that,we use flex to compile lex.l for future use

####step 2: Syntax analyzer In this step,we use Yacc to build our analyzer.Yacc is an LALR parser generator, which stands for yet another compiler-compiler . In this project, we can use yacc/bison (bison is another version of yacc) to generate a parser.Here are syntax example:

EXTDEFS:
	EXTDEF EXTDEFS	{
		cout<<"deal with extdefs -> extdef extdefs\n";
		addNode("EXTDEFS");
		arr[counter-1].son[1] = stk.top();stk.pop();
		arr[counter-1].son[0] = stk.top();stk.pop();
		stk.push(&arr[counter-1]);
	}

	|/*EMPTY*/{
		cout<<"deal with extdefs -> empty\n";
		addNode("EXTDEFS");addNode("empty");arr[counter-2].son[0] =&arr[counter-1];
	stk.push(&arr[counter-2]);
	}
								
	;

step 3: Tree building

-PROGRAM
-----EXTDEFS
---------EXTDEF
-------------SPEC
-----------------TYPE
-------------FUNC
-----------------main()
-----------------LP
-----------------PARAS
---------------------empty
-----------------RP
-------------STMTBLOCK

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Error handling

When your code exists an error,the parser will deliver an error message:

YACC: syntax error YACC: line 2 YACC: at }

To implement that we need to write yyerror function like this:

int yyerror (const char *msg) {
	printFlag = false;
	fprintf (stderr, "YACC: %s\n", msg);
	fprintf(stderr, "YACC: line %d\n", yylineno);
	fprintf(stderr, "YACC: at %s\n", yytext);
	return -1;
}

Test cases

To test my project,please run the test.sh script and run ./a.out:

xxx@ubuntu: ./test.sh
xxx@ubuntu: ./a.out