Exercise 5-18: dcl that recovers from errors

Make dcl recover from input errors.

Each input line holds one declaration, and may have errors: anything that doesn't fit the grammar, such as a missing ) or ], a missing name, extra tokens at the end, a data type that isn't a name, or a blank line. For a line with an error, print syntax error once instead of a description, and carry on with the next line as if nothing had happened. Print the other lines as dcl does, and treat the end of the input like the end of a line.

The book's dcl program, which converts a declaration such as char **argv to words (argv: pointer to pointer to char):

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

#define MAXTOKEN  100

enum { NAME, PARENS, BRACKETS };

void dcl(void);
void dirdcl(void);
int gettoken(void);

int tokentype;           /* type of last token */
char token[MAXTOKEN];    /* last token string */
char name[MAXTOKEN];     /* identifier name */
char datatype[MAXTOKEN]; /* data type = char, int, etc. */
char out[1000];

int main(void)  /* convert declaration to words */
{
    while (gettoken() != EOF) {   /* 1st token on line */
        strcpy(datatype, token);  /* is the datatype */
        out[0] = '\0';
        dcl();       /* parse rest of line */
        if (tokentype != '\n')
            printf("syntax error\n");
        printf("%s: %s %s\n", name, out, datatype);
    }
    return 0;
}

/* 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 != ')')
            printf("error: missing )\n");
    } else if (tokentype == NAME)  /* variable name */
        strcpy(name, token);
    else
        printf("error: expected name or (dcl)\n");
    while ((type=gettoken()) == PARENS || type == BRACKETS)
        if (type == PARENS)
            strcat(out, " function returning");
        else {
            strcat(out, " array");
            strcat(out, token);
            strcat(out, " of");
        }
}

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

    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;
}

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