CODEWITHSUNDEEP
clibcurl
PublicEnemy
PublicEnemy

Reputation: 23

Retrieving data in C with libcurl

I'm using C and libcurl to log in to a website and retrieve a value from the form(ie put the string "username=random" into a char array). This is what I have so far:

curl = curl_easy_init();
if(curl) {
    curl_easy_setopt(curl, CURLOPT_USERAGENT, "Mozilla/4.0");
    curl_easy_setopt(curl, CURLOPT_AUTOREFERER, 1 );
    curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1 );
    curl_easy_setopt(curl, CURLOPT_COOKIEFILE, " "); 
    curl_easy_setopt(curl, CURLOPT_URL, "http://www.website.com/login");
    curl_easy_perform(curl);
    curl_easy_setopt(curl, CURLOPT_REFERER, "http://www.website.com/login");
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS,fields );
    curl_easy_perform(curl);
    curl_easy_setopt(curl, CURLOPT_URL, "http://www.website.com/form-with-data");

    res = curl_easy_perform(curl);
    curl_easy_cleanup(curl);

    }

but I'm not sure where to go from there. I've tried writing the whole page into a file then manually searching for the string, but that did not work.

I'm sure there is a simple answer for this, I'm just new to both C and libcurl :)

Upvotes: 2

Views: 1554

Answers (2)

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

Are you familiar with HTTP and HTML? You should at least know what GET, HEAD, and POST requests are, and what exactly happens at the protocol level when a form is submitted by a user thru his browser. You may practice by using telnet and typing manually the HTTP requests.

Then, you want to make a POST request programmatically. The libcurl has several examples, you should look inside postit2.c

You could be concerned with HTTP cookies, and look into cookie_interface.c example. Your CURLOPT_COOKIEFILE is wrong (you are giving a file named by a single space).

Don't forget to enable all warnings and debugging info when compiling. If on Linux, compile with gcc -Wall -g yourexample.c -lcurl -o yourprog and improve your code till no warnings are given. Then learn to use gdb for debugging.

Upvotes: 1

user529758
user529758

Reputation:

Your current code will do one thing: it will write the data to the standard output. To accumulate the data, you have to do something like this:

size_t write_clbk(void *data, size_t blksz, size_t nblk, void *ctx)
{
    static size_t sz = 0;
    size_t currsz = blksz * nblk;

    size_t prevsz = sz;
    sz += currsz;
    void *tmp = realloc(*(char **)ctx, sz);
    if (tmp == NULL) {
        // handle error
        free(*(char **)ctx);
        *(char **)ctx = NULL;
        return 0;
    }
    *(char **)ctx = tmp;

    memcpy(*(char **)ctx + prevsz, data, currsz);
    return currsz;
}

hndl = curl_easy_init();
// Set up the easy handle, i. e. specify URL, user agent, etc.
// Do the ENTIRE setup BEFORE calling `curl_easy_perform()'.
// Afterwards the calls to `curl_easy_setopt()' won't be effective anymore

char *buf = NULL;
curl_easy_setopt(hndl, CURLOPT_WRITEFUNCTION, write_clbk);
curl_easy_setopt(hndl, CURLOPT_WRITEDATA, &buf);
curl_easy_perform(hndl);
curl_easy_cleanup(hndl);

// here `buf' will contain the data
// after use, don't forget:
free(buf);

Upvotes: 2

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