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| <h5 class="subsubsection">A.2.2.3 How Many Arguments Were Supplied</h5> |
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| <p><a name="index-number-of-arguments-passed-3718"></a><a name="index-how-many-arguments-3719"></a><a name="index-arguments_002c-how-many-3720"></a> |
| There is no general way for a function to determine the number and type |
| of the optional arguments it was called with. So whoever designs the |
| function typically designs a convention for the caller to specify the number |
| and type of arguments. It is up to you to define an appropriate calling |
| convention for each variadic function, and write all calls accordingly. |
| |
| <p>One kind of calling convention is to pass the number of optional |
| arguments as one of the fixed arguments. This convention works provided |
| all of the optional arguments are of the same type. |
| |
| <p>A similar alternative is to have one of the required arguments be a bit |
| mask, with a bit for each possible purpose for which an optional |
| argument might be supplied. You would test the bits in a predefined |
| sequence; if the bit is set, fetch the value of the next argument, |
| otherwise use a default value. |
| |
| <p>A required argument can be used as a pattern to specify both the number |
| and types of the optional arguments. The format string argument to |
| <code>printf</code> is one example of this (see <a href="Formatted-Output-Functions.html#Formatted-Output-Functions">Formatted Output Functions</a>). |
| |
| <p>Another possibility is to pass an “end marker” value as the last |
| optional argument. For example, for a function that manipulates an |
| arbitrary number of pointer arguments, a null pointer might indicate the |
| end of the argument list. (This assumes that a null pointer isn't |
| otherwise meaningful to the function.) The <code>execl</code> function works |
| in just this way; see <a href="Executing-a-File.html#Executing-a-File">Executing a File</a>. |
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