Question

I am working on a simple mathematical parser. Something that just reads number = 1 + 2;

I have a vector containing these tokens. They store a type and string value of the character. I am trying to step through the vector to build an AST of these tokens, and I keep getting segmentation faults, even when I am under the impression my code should prevent this from happening.

Here is the bit of code that builds the AST:

struct ASTGen
{
    const vector<Token>            &Tokens;
    unsigned int                   size,
                                   pointer;

    ASTGen(const vector<Token> &t) : Tokens(t), pointer(0) 
    {
        size = Tokens.size() - 1;
    }

    unsigned int next()
    {
        return pointer + 1;
    }

    Node* Statement()
    {
        if(next() <= size)
        {
            switch(Tokens[next()].type)
            {
                case EQUALS
                :
                    Node* n = Assignment_Expr();
                    return n;
            }
        }

        advance();
    }

    void advance()
    {
        if(next() <= size) ++pointer;
    }

    Node* Assignment_Expr()
    {
        Node* lnode = new Node(Tokens[pointer], NULL, NULL);
        advance();
        Node* n = new Node(Tokens[pointer], lnode, Expression());
        return n;
    }

    Node* Expression()
    {
        if(next() <= size)
        {                        
            advance();
            if(Tokens[next()].type == SEMICOLON)
            {
                Node* n = new Node(Tokens[pointer], NULL, NULL);
                return n;
            }

            if(Tokens[next()].type == PLUS)
            {
                Node* lnode = new Node(Tokens[pointer], NULL, NULL);
                advance();
                Node* n = new Node(Tokens[pointer], lnode, Expression());
                return n;
            }
        }
    }
};

...

ASTGen AST(Tokens);
Node* Tree = AST.Statement();
cout << Tree->Right->Data.svalue << endl;

I can access Tree->Data.svalue and get the = Node’s token info, so I know that node is getting spawned, and I can also get Tree->Left->Data.svalue and get the variable to the left of the =

I have re-written it many times trying out different methods for stepping through the vector, but I always get a segmentation fault when I try to access the = right node (which should be the + node)

Any help would be greatly appreciated.

Answer 1

There’s plenty more code that we haven’t seen, so I can’t tell you precisely what’s going on, but I see a few things that are reasons for concern. One is that the Statement() method doesn’t always return a value. If the first if test doesn’t pass, then we call advance() and fall off the bottom of the routine without an explicit return. The caller will try to get the return value of the function but it’ll get garbage. This could lead to all sorts of problems, including things like double free() calls, etc, which can easily cause segfaults.

Expression() has the same problem.