Attempting to run su using pipes, dup2, execl from a c++ app

OK here is what my app is actually doing.  I have a c++ application that basically receives input from a php page.  That part is coded perfectly fine,  I can send info from the page, and receive it from the app, as well as the app send info to the page.

Here is my problem.  I want the user to be able to type in their name and password, this name and password also is the same as the username and password they have on the linux box (RH9).  Originally I was going to try to compare the encrypted password in the shadow file to what the user types in, but I don't see that as possible after doing some research.

Is there an easy way to do this?  Check to see if a password matches that for a username?

My other solution was the following:
NOTE: the application will not be running as root

I could have a c++ app run "su username_entered_from_web_page_here"
then have the c++ app type in the corresponding password I received from the page.
The function below will return true if the login is executed correctly, and false otherwise.
If the user logs in correctly I would also have the ability to execute some more commands.

Assuming I have the username in a string username, and the password in a string password, how can I accomplish this?

Exact code would be GREAT, here is the code I have so far:

bool verifyUser(string user, string pass){
      cout << "Starting verification process\n";

      int filedes1[2], filedes2[2];
      int pid;
 
      /* Make our first pipe. */

      if (pipe(filedes1) == -1){
            perror ("pipe");
            return false;
      }

      /* Make our second pipe. */

      if (pipe(filedes2) == -1){
            perror ("pipe");
            return false;
      }

      /* Fork a child */

      if ((pid = fork()) == 0){
            dup2(filedes1[0], fileno(stdin)); /* Copy the reading end of the pipe. */
            dup2(filedes2[1], fileno(stdout)); /* Copy the writing end of the pipe */

      /* Uncomment this if you want stderr sent too.

      dup2(filedes2[1], fileno(stderr));

      */

      /* If execl() returns at all, there was an error. */

            if (execl("/bin/su","su",user.c_str(),NULL)){
                  perror("execl");
                  return false;
            }
      }
      else if (pid == -1){
            perror("fork");
            return false;
      }
      else{
            int output;
            char c;

            write(filedes1[1], pass.c_str(), strlen(pass.c_str())); /* Write the string */

            int outputLength;
            char buffer[2048];
            outputLength = read(filedes2[0], buffer, 2048);
            string s(buffer,outputLength);

            //cout << s << "\n";
            //cout << "Done reading\n";
            return false;
      }
}

and of course this does not work, please help
Pretty much anyway I've tried it I tend to get this error:
"standard in must be a tty"