Correctly using gumbo-parser to iterate and find the things I need?

Question

Correctly using gumbo-parser to iterate and find the things I need?

2.6k Views Asked by Jackson Ming Hu At 12 April 2025 at 18:23

I'm working on a pure C99 practising project which can do a login simulation for our school's CAS login system.

Now I'm trying to use Gumbo HTML parser to parse our school's login page. Here is the form section and I need to get the login ticket from it before I run the POST request to submit the form, which is the "hidden" type input element with name "lt". (i.e. line with <input type="hidden" name="lt" value="LT-000000-b4LktCXyzXyzXyzXyzXyzXyz" />, and I need to parse the "value").

I've wrote some code, but it seems cannot find out this input element. Here is my C program's function:

const char * parse_login_ticket_old(char * raw_html)
{
    // Parse HTML into Gumbo memory structure
    GumboOutput * gumbo_output = gumbo_parse(raw_html);

    // Prepare the node
    GumboNode * gumbo_root = gumbo_output->root;

    assert(gumbo_root->type == GUMBO_NODE_ELEMENT);
    assert(gumbo_root->v.element.children.length >= 2);

    const GumboVector* root_children = &gumbo_root->v.element.children;

    GumboNode* page_body = NULL;


    for (int i = 0; i < root_children->length; ++i)
    {
        GumboNode* child = root_children->data[i];
        if (child->type == GUMBO_NODE_ELEMENT && child->v.element.tag == GUMBO_TAG_BODY)
        {
            page_body = child;
            break;
        }
    }

    assert(page_body != NULL);

    GumboVector* page_body_children = &page_body->v.element.children;

    for (int i = 0; i < page_body_children->length; ++i)
    {
        GumboNode* child = page_body_children->data[i];

        GumboAttribute * input_name_attr = gumbo_get_attribute(&child->v.element.attributes, "name");

        if (child->type == GUMBO_NODE_ELEMENT && child->v.element.tag == GUMBO_TAG_INPUT && strcmp(input_name_attr->value, "lt") == 0)
        {
            GumboAttribute * input_value_attr = gumbo_get_attribute(&child->v.element.attributes, "value");
            return input_name_attr->value;
        }
    }

    return NULL;
}

In case someone need for debugging, here is a example of our schools page. Possible sensitive data has been removed.

<body>
    <div id="wrapper">
        <div id="contentArea" role="main">
            <div class="form login" role="form">
                <h2 class="hidden">Login</h2>
                <form id="fm1" class="fm-v clearfix" action="/schoolcas/login?jsessionid=1234567890" method="post"><div class="formRow">
                        <label for="username" class="label">Student ID</label>
                        <div class="textBox">
                            <input id="username" name="username" class="schoolcas text" aria-required="true" type="text" value="" size="25" maxlength="25"/></div>
                    </div>
                    <div class="formRow">
                        <label for="password" class="label">Password</label>
                        <div class="textBox">
                            <input id="password" name="password" class="schoolcas text" aria-required="true" type="password" value="" size="25" autocomplete="off"/></div>
                    </div>

                    <div class="formRow">
                        <input type="hidden" name="lt" value="LT-000000-b4LktCXyzXyzXyzXyzXyzXyz" />
                        <input type="hidden" name="execution" value="e2s1" />
                        <input type="hidden" name="_eventId" value="submit" />
                        <input class="button grey submit" name="submit" value="Login" type="submit" />
                    </div>

                </form>
            </div>
        </div>
    </div>
</body>

Anyway, my program seems just stop at the top of the body element and it returns NULL later on.

So I would like to know how to do a correct search, and find out the input element I need?

Original Q&A

There are 1 best solutions below

**Jackson Ming Hu** · Answer 1

I've figured it out by myself from the Google sample code (https://github.com/google/gumbo-parser/blob/master/examples/find_links.cc).

Here's the code. It's crappy but it works anyway.

const char * find_attribute(GumboNode * current_node, GumboTag element_tag_type,
                            char * element_term_key, char * element_term_value, char * desired_result_key)
{
    const char * lt_token = NULL;

    // Return NULL if it is in WHITESPACE
    if (current_node->type != GUMBO_NODE_ELEMENT)
    {
        return NULL;
    }

    // Set the element's term key, 
    // e.g. if we need to find something like <input name="foobar"> then element search term key is "name",
    //      and element search value is "foobar"
    GumboAttribute* lt_attr = gumbo_get_attribute(&current_node->v.element.attributes, element_term_key);

    if (lt_attr != NULL && current_node->v.element.tag == element_tag_type && (strcmp(lt_attr->value, element_term_value) == 0))
    {
        lt_token = gumbo_get_attribute(&current_node->v.element.attributes, desired_result_key)->value;
        return lt_token;
    }

    GumboVector* children = &current_node->v.element.children;

    for (unsigned int i = 0; i < children->length; ++i)
    {
        lt_token = find_attribute(children->data[i], element_tag_type,
                                  element_term_key, element_term_value, desired_result_key);

        // Force stop and return if it gets a non-null result.
        if(lt_token != NULL)
        {
            return lt_token;
        }
    }
}

Correctly using gumbo-parser to iterate and find the things I need?

There are 1 best solutions below

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