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>Dealing with Arguments Passed by Reference</A
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><P
>&#13;    If your function accepts arguments passed by reference that you
    intend to modify, you need to take some precautions.
   </P
><P
>&#13;    What we didn't say yet is that under the circumstances presented so
    far, you don't have write access to any <VAR
CLASS="envar"
>zval</VAR
> containers
    designating function parameters that have been passed to you. Of course, you
    can change any <VAR
CLASS="envar"
>zval</VAR
> containers that you created within
    your function, but you mustn't change any <VAR
CLASS="envar"
>zval</VAR
>s that refer to
    Zend-internal data!
   </P
><P
>&#13;    We've only discussed the so-called <B
CLASS="function"
>*_ex()</B
> API
    so far. You may have noticed that the API functions we've used are
    called <B
CLASS="function"
>zend_get_parameters_ex()</B
> instead of
    <B
CLASS="function"
>zend_get_parameters()</B
>,
    <B
CLASS="function"
>convert_to_long_ex()</B
> instead of
    <B
CLASS="function"
>convert_to_long()</B
>, etc. The
    <B
CLASS="function"
>*_ex()</B
> functions form the so-called new
    "extended" Zend API. They give a minor speed increase over the old
    API, but as a tradeoff are only meant for providing read-only
    access.
   </P
><P
>&#13;    Because Zend works internally with references, different variables
    may reference the same value. Write access to a
    <VAR
CLASS="envar"
>zval</VAR
> container requires this container to contain
    an isolated value, meaning a value that's not referenced by any
    other containers. If a <VAR
CLASS="envar"
>zval</VAR
> container were
    referenced by other containers and you changed the referenced
    <VAR
CLASS="envar"
>zval</VAR
>, you would automatically change the contents
    of the other containers referencing this <VAR
CLASS="envar"
>zval</VAR
>
    (because they'd simply point to the changed value and thus change
    their own value as well).
   </P
><P
>&#13;    <B
CLASS="function"
>zend_get_parameters_ex()</B
> doesn't care about
    this situation, but simply returns a pointer to the desired
    <VAR
CLASS="envar"
>zval</VAR
> containers, whether they consist of references
    or not. Its corresponding function in the traditional API,
    <B
CLASS="function"
>zend_get_parameters()</B
>, immediately checks for
    referenced values. If it finds a reference, it creates a new,
    isolated <VAR
CLASS="envar"
>zval</VAR
> container; copies the referenced data
    into this newly allocated space; and then returns a pointer to the
    new, isolated value.
   </P
><P
>&#13;    This action is called <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>zval separation</I
></SPAN
>
    (or pval separation). Because the <B
CLASS="function"
>*_ex()</B
> API
    doesn't perform zval separation, it's considerably faster, while
    at the same time disabling write access.
   </P
><P
>&#13;    To change parameters, however, write access is required. Zend deals
    with this situation in a special way: Whenever a parameter to a function is
    passed by reference, it performs automatic zval separation. This means that
    whenever you're calling a function like 
    this in PHP, Zend will automatically ensure
    that <VAR
CLASS="envar"
>$parameter</VAR
> is being passed as an isolated value, rendering it
    to a write-safe state:
    <TABLE
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><TD
><PRE
CLASS="programlisting"
>my_function(&#38;$parameter);</PRE
></TD
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>
   </P
><P
>&#13;    But this <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>is not</I
></SPAN
> the case with regular parameters!
    All other parameters that are not passed by reference are in a read-only
    state.
   </P
><P
>&#13;    This requires you to make sure that you're really working with a
    reference - otherwise you might produce unwanted results. To check for a
    parameter being passed by reference, you can use the macro
    <VAR
CLASS="literal"
>PZVAL_IS_REF</VAR
>. This macro accepts a <VAR
CLASS="literal"
>zval*</VAR
>
    to check if it is a reference or not. Examples are given in
    in <A
HREF="zend.arguments.by-reference.html#example.pass-by-ref"
>Exemplu 34-3</A
>.
   </P
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><A
NAME="example.pass-by-ref"
></A
><P
><B
>Exemplu 34-3. Testing for referenced parameter passing.</B
></P
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><PRE
CLASS="programlisting"
>zval *parameter;

if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "z", &#38;parameter) == FAILURE)
    return;

/* check for parameter being passed by reference */
if (!PZVAL_IS_REF(*parameter)) {
{
    zend_error(E_WARNING, "Parameter wasn't passed by reference");
    RETURN_NULL();
}

/* make changes to the parameter */
ZVAL_LONG(*parameter, 10);</PRE
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