lexer.c

简单编译器

char lexbuf[BSIZE];
int  lineno = 1;
int  tokenval = NONE;

int lexan()
{
	int t;
	while(1) {
		t = getchar();
		if (t == ' '|| t == '\t')
			;  
		else if (t == '\n')
			lineno = lineno + 1;
		else if (isdigit(t)) {
			ungetc(t,stdin);
			scanf("%d",&tokenval);
			return NUM;
		}
		else if (isalpha(t)) {
			int p,b=0;
			while (isalnum(t)) {
				lexbuf[b] = t;
				t = getchar();
				b = b + 1;
				if(b >= BSIZE)
					error("compiler error");
			}
			lexbuf[b] = EOF;             
			if (t != EOF)
				 ungetc(t,stdin);
			p = lookup(lexbuf);                   
			if(p == 0)
				p = insert(lexbuf,ID);
			tokenval = p;                          
			return symtable[p].token;
		}
		else if (t == EOF)
			return DONE;
		else {
			tokenval = NONE;
			return t;
		}
	}
}