Exercise 1.24: Rudimentary Syntax Checker

Code

#include <u.h>
#include <libc.h>
#include <bio.h>

#define STDIN   0

/* rudimentary syntax error checker: unbalanced parentheses (, brackets [, and braces { */
void
main()
{
    Biobuf *bstdin;
    bstdin = Bfdopen(STDIN, OREAD);
    int inquote, nest, oparens, obrackets, obraces, cparens, cbrackets, cbraces, error;
    char c, prev, pprev;

    inquote = nest = oparens = obrackets = obraces = cparens = cbrackets = cbraces = error = 0;
    prev = Bgetc(bstdin);
    pprev = '\0';
    while((c = Bgetc(bstdin)) >= 0){
        /* set states */
        if(prev=='/' && c=='*'){
            nest++;
        }
        else if(prev=='*' && c=='/'){
            c = Bgetc(bstdin);
            nest--;
        }
        else if((c=='\'' || c=='\"') && (prev!='\\' || (prev=='\\' && pprev=='\\'))){
            if(inquote == 0){
                nest++;
                inquote = 1;
            }
            else{
                nest--;
                inquote = 0;
            }
        }               

        /* process state */
        if(nest > 0)
            ;
        else{
            if(c=='(' && prev!='\\'){
                oparens++;  
            }
            if(c=='[' && prev!='\\'){
                obrackets++;
            }
            if(c=='{' && prev!='\\'){
                obraces++;
            }
            if(c==')' && prev!='\\'){
                cparens++;
            }
            if(c==']' && prev!='\\'){
                cbrackets++;
            }
            if(c=='}' && prev!='\\'){
                cbraces++;
            }
        }
        pprev = prev;
        prev = c;
    }

    if(oparens != cparens){
        print("SYNTAX ERROR: Mismatched parentheses: %d open, %d closed.\n", oparens, cparens);
        error = 1;
    }
    if(obrackets != cbrackets){
        print("SYNTAX ERROR: Mismatched brackets: %d open, %d closed.\n", obrackets, cbrackets);
        error = 1;
    }
    if(obraces != cbraces){
        print("SYNTAX ERROR: Mismatched braces: %d open, %d closed.\n", obraces, cbraces);
        error = 1;
    }
    if(error == 0)
        print("No sytanx errors found.\n");

exits(0);
}

/* some tests; the compiler recognizes // comments and this program does not */
// }
// )
// ]
// ")" <- this one should not register
// ']' <- neither should this one

Output

$ 9c syntax.c; 9l syntax.o -o syntax
$ ./syntax < syntax.c
SYNTAX ERROR: Mismatched parentheses: 30 open, 31 closed.
SYNTAX ERROR: Mismatched brackets: 0 open, 1 closed.
SYNTAX ERROR: Mismatched braces: 17 open, 18 closed.