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| <h5 class="subsubsection">A.2.2.2 Receiving the Argument Values</h5> |
| |
| <p><a name="index-variadic-function-argument-access-3715"></a><a name="index-arguments-_0028variadic-functions_0029-3716"></a> |
| Ordinary fixed arguments have individual names, and you can use these |
| names to access their values. But optional arguments have no |
| names—nothing but ‘<samp><span class="samp">...</span></samp>’. How can you access them? |
| |
| <p><a name="index-stdarg_002eh-3717"></a>The only way to access them is sequentially, in the order they were |
| written, and you must use special macros from <samp><span class="file">stdarg.h</span></samp> in the |
| following three step process: |
| |
| <ol type=1 start=1> |
| <li>You initialize an argument pointer variable of type <code>va_list</code> using |
| <code>va_start</code>. The argument pointer when initialized points to the |
| first optional argument. |
| |
| <li>You access the optional arguments by successive calls to <code>va_arg</code>. |
| The first call to <code>va_arg</code> gives you the first optional argument, |
| the next call gives you the second, and so on. |
| |
| <p>You can stop at any time if you wish to ignore any remaining optional |
| arguments. It is perfectly all right for a function to access fewer |
| arguments than were supplied in the call, but you will get garbage |
| values if you try to access too many arguments. |
| |
| <li>You indicate that you are finished with the argument pointer variable by |
| calling <code>va_end</code>. |
| |
| <p>(In practice, with most C compilers, calling <code>va_end</code> does nothing. |
| This is always true in the GNU C compiler. But you might as well call |
| <code>va_end</code> just in case your program is someday compiled with a peculiar |
| compiler.) |
| </ol> |
| |
| <p>See <a href="Argument-Macros.html#Argument-Macros">Argument Macros</a>, for the full definitions of <code>va_start</code>, |
| <code>va_arg</code> and <code>va_end</code>. |
| |
| <p>Steps 1 and 3 must be performed in the function that accepts the |
| optional arguments. However, you can pass the <code>va_list</code> variable |
| as an argument to another function and perform all or part of step 2 |
| there. |
| |
| <p>You can perform the entire sequence of three steps multiple times |
| within a single function invocation. If you want to ignore the optional |
| arguments, you can do these steps zero times. |
| |
| <p>You can have more than one argument pointer variable if you like. You |
| can initialize each variable with <code>va_start</code> when you wish, and |
| then you can fetch arguments with each argument pointer as you wish. |
| Each argument pointer variable will sequence through the same set of |
| argument values, but at its own pace. |
| |
| <p><strong>Portability note:</strong> With some compilers, once you pass an |
| argument pointer value to a subroutine, you must not keep using the same |
| argument pointer value after that subroutine returns. For full |
| portability, you should just pass it to <code>va_end</code>. This is actually |
| an ISO C<!-- /@w --> requirement, but most ANSI C compilers work happily |
| regardless. |
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