Question

I have several questions so I’ll put numbers in comments so that the questioned line is easier to find.

[1] How can char *p be assigned to token variable which in fact doesn’t exist?

[2] Why we don’t put ‘\0’ here, what is done in every other if condition?

[3] Why we copy () into token string only? And we don’t do that in the case of [] and alphanumeric chars ?

[4] Those return commands are weird IMO -> first: why does it not look just like this return PARENS, and second: when it returns tokentype = ‘(‘ it is a char so why the function gettoken is declared as returning integers ?

[5] SUPPOSING part: let input be ( a b c ) then: ( causes function to return tokentype ‘(‘ a b c enter if condition (isalpha(C)) and the last ) exits that condition causing ungetch. Does it go along main else condition then? Is my walkthrough correct?

#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  ALSO [4] !!! */
char token[MAXTOKEN]; /*last token string */
char name[MAXTOKEN]; /*identifier name */
char datatype[MAXTOKEN]; /*data type = char, int, etc. */
char out[1000];

main() /* 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;
}

int gettoken(void) /* return next token */
{
    int c, getch(void);
    void ungetch(int);
    char *p = token;        /* [1] */
    while ((c = getch()) == ' ' || c == '\t')
        ;
    if (c == '(') {
        if ((c = getch()) == ')') {
            strcpy(token, "()");            /* [2][3] */
            return tokentype = PARENS;           /* [4] */
        } 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()); ) /* SUPPOSING [5] */
            *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();
        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");
        }
}

THANKS IN ADVANCE!

Answer 1

1) token does exist, but it’s a global variable defined as char token[MAXTOKEN];

2) strcpy() copies the terminating 0 byte from the source, so we don’t need to do it manually

3) that appears to be special case handling for the literal string () – some parentheses with nothing in between, as opposed to handling the case where we have ( some stuff )

4) in accordance with (3), PARENS looks like the token type for an empty set of parentheses, while returning ( and ) separately as specific tokentypes is the case for when we have something between them

5) not sure I follow what you’re asking, but, since there doesn’t appear to be a special case for the closing parenthesis, it appears that it takes the final else branch, returning tokentype = c