Exercise 5.18 - recover from input errors¶

/* DCL:  A Recursive Descent Parser */

/* dcl: parse a declarator */

#include<stdio.h>
#include<string.h>
#include<ctype.h>

#define MAXTOKEN 100

enum {NAME,PARENS,BRACKETS};
enum { NO, YES};

void dcl(void);
void dirdcl(void);
void errmsg(char *);

int gettoken(void);
int tokentype;          /* type of last token */
char token[MAXTOKEN];       /* last token string */
char name[MAXTOKEN];        /* identifier name */
char out[1000];
char datatype[MAXTOKEN];
extern int prevtoken;
extern int tokentype;
extern char token[];
int prevtoken = NO;

int main(void)
{
    int i;

    if(gettoken()!=EOF)
    {
        strcpy(datatype,token);
        out[0]='\0';
        dcl();

        if(tokentype != '\n')
            printf("syntax error \n");

        printf(" %s %s %s \n",name,out,datatype);
        for(i=0;i<MAXTOKEN;i++)
        {
            datatype[i] = '\0'; //overwrites name
            out[i] = '\0';
            name[i] = '\0';
        }
    }
    else
    {
        return 0;
    }
}

int gettoken(void)
{
    int c, getch(void);
    void ungetch(int);
    char *p = token;

    if(prevtoken == YES)
    {
        prevtoken = NO;
        return tokentype;
    }

    while((c=getch()) == ' ' || c == '\t')
        ;

    if( c == '(')
    {
        if((c=getch()) == ')')
        {
            strcpy(token,"()");
            return tokentype = PARENS;
        }
        else
        {
            ungetch(c);
            return tokentype = '(';
        }
    }
    else if ( c == '[')
    {
        for(*p++ = c; (*p++ = getch()) != ']';)
            ;
        *p = '\0';
        return tokentype = BRACKETS;
    }
    else if ( isalpha(c))
    {
        for(*p++ =c; isalnum(c=getch());)
            *p++ = c;
        *p = '\0';
        ungetch(c);
        return tokentype = NAME;
    }
    else
        return tokentype =c;
}

/* dcl: parse a declarator */

void dcl(void)
{
    int ns;

    for(ns=0;gettoken()=='*';) /* count *'s */
        ns++;

    dirdcl();
    while(ns-- > 0)
        strcat(out," pointer to");
}

/* dirdcl: parse a direct declarator */

void dirdcl(void)
{
    int type;

    if(tokentype == '(')    /* dcl */
    {
        dcl();

        if(tokentype != ')')
            errmsg("error: missing ) \n");
    }
    else if(tokentype == NAME)  /* variable name */
        strcpy(name,token);
    else
        errmsg("error: expected name or (dcl) \n");

    while((type=gettoken()) == PARENS || type == BRACKETS )
        if(type == PARENS)
            strcat(out,"function returning");
        else
        {
            strcat(out," arg");
            strcat(out,token);
            strcat(out," of");
        }
}

void errmsg(char *msg)
{
    printf("%s", msg);
    prevtoken = YES;
}
#define BUFSIZE  100

char buf[BUFSIZE];          /* buffer for ungetch */
int bufp = 0;               /* next free position in buf */

int getch(void)             /* get a (possibly pushed back) character */
{
    return (bufp > 0) ? buf[--bufp]:getchar();
}

void ungetch(int c)     /* push a character back on input */
{
    if(bufp >= BUFSIZE)
        printf("ungetch: too many characters \n");
    else
        buf[bufp++] = c;
}