#multiple property for select not working even though documentation says it is supported

Why'd you close this?

Didn't #multiple adjust the #name attribute so that when the field is processed, each value is captured, instead of just one?

<select name="just_choose_one">
<option value="cookie">Cookie</option>
<option value="donut">Donut</option>
<option value="ice_cream">Ice Cream</option>
<option value="soda">Soda</option>
</select>

<select multiple="multiple" name="choose_many[]">
<option value="cookie">Cookie</option>
<option value="donut">Donut</option>
<option value="ice_cream">Ice Cream</option>
<option value="soda">Soda</option>
</select>

@_randy:
multiple is added to the #attributes array by form_process_select()

A select element has form_process_select() as a #process function, this is called by form_builder() as the form is built, see modules/system/system.module:424 for the form element declaration.

I'm puzzled then. That's what its supposed to do, but it presently doesn't or something is overriding it.

Here's what's at line 424 of modules/system/system.module. The #multiple default is still present in 7.

 ...
  $types['select'] = array(
    '#input' => TRUE,
    '#multiple' => FALSE,
    '#process' => array('form_process_select', 'ajax_process_form'),
    '#theme' => 'select',
    '#theme_wrappers' => array('form_element'),
  );
... 

And here's what's in includes/form.inc around form_process_select();
It looks as if $element['#multiple'] is interpreted as expected ...

 ...
function form_process_select($element) {
  // #multiple select fields need a special #name.
  if ($element['#multiple']) {
    $element['#attributes']['multiple'] = 'multiple';
    $element['#attributes']['name'] = $element['#name'] . '[]';
  }
  // A non-#multiple select needs special handling to prevent user agents from
  // preselecting the first option without intention. #multiple select lists do
  // not get an empty option, as it would not make sense, user interface-wise.
  else {
    $required = $element['#required'];
    // If the element is required and there is no #default_value, then add an
    // empty option that will fail validation, so that the user is required to
    // make a choice. Also, if there's a value for #empty_value or
    // #empty_option, then add an option that represents emptiness.
    if (($required && !isset($element['#default_value'])) || isset($element['#empty_value']) || isset($element['#empty_option'])) {
      $element += array(
        '#empty_value' => '',
        '#empty_option' => $required ? t('- Select -') : t('- None -'),
      );
      // The empty option is prepended to #options and purposively not merged
      // to prevent another option in #options mistakenly using the same value
      // as #empty_value.
      $empty_option = array($element['#empty_value'] => $element['#empty_option']);
      $element['#options'] = $empty_option + $element['#options'];
    }
  }
  return $element;
}
... 

As far as I can see, your patch would have been correct...and should be applied. Therefore, I'm re-activating this issue.

I guess I misunderstood the function call surrounding your change. I had mistakenly assumed it was a filter,
With that said, it looks like there was a significant change (or just missed) code in that function.

Here's what's in D6.

function theme_select($variables) {
$select = '';
  $size = $element['#size'] ? ' size="'. $element['#size'] .'"' : '';
  _form_set_class($element, array('form-select'));
  $multiple = $element['#multiple'];
  return theme('form_element', $element, '<select name="'. $element['#name'] .''. ($multiple ? '[]' : '') .'"'. ($multiple ? ' multiple="multiple" ' : '') . drupal_attributes($element['#attributes']) .' id="'. $element['#id'] .'" '. $size .'>'. form_select_options($element) .'</select>');
}

I'd generate a patch, but I don't know D7's internals well enough to say that simply porting D6's code would be accurate.

This seems to accomplish the trick!

Lets try this again, apparently my line endings were still in Windows format.

Here's exactly how I fixed up theme_select(). If the attached patch doesn't work this time around, could someone please generate one that works correctly?

function theme_select($variables) {
  $element = $variables['element'];
  _form_set_class($element, array('form-select'));
  if (isset($element['#multiple']) && $element['#multiple']==TRUE) {
    $element['#attributes'] += array('multiple'=>'multiple');
    $element['#name'] .= '[]';
  }
  element_set_attributes($element, array('id', 'name', 'size'));
  return '<select' . drupal_attributes($element['#attributes']) . '>' . form_select_options($element) . '</select>';
}

Huh, latest Core -dev seems to be ok now. theme_select hasn't changed any. I'm not sure whether to change the status of or close this thread.
Seems to have risen again in 7.2.

Is this still an issue?

This issue still seems to be relevant in Drupal 8. The #multiple default is still present, and form_process_select is relatively unchanged. With that said, I rerolled the latest patch for D8.

@_randy, Thank you for reporting this problem. We rely on issue reports like this to improve Drupal core.

I tested this on Drupal 9.4.x, standard install. I used the form_api_example of the Examples module which includes a demo of #select.

    // Multiple values option elements.
    $form['select_multiple'] = [
      '#type' => 'select',
      '#title' => 'Select (multiple)',
      '#multiple' => TRUE,
      '#options' => [
        'sat' => 'SAT',
        'act' => 'ACT',
        'none' => 'N/A',
      ],
      '#default_value' => ['sat'],
      '#description' => 'Select Multiple',
    ];

And it work as expected.

Therefore, closing as cannot reproduce. If you are experiencing this problem on a supported version of Drupal reopen the issue, by setting the status to 'Active', and provide complete steps to reproduce the issue (starting from "Install Drupal core").

Thanks!