odes it. If <i>idx</i> is NULL then only one occurrence of an extension is permissible, otherwise the first extension after index <i>*idx</i> is returned and <i>*idx</i> updated to the location of the extension. If <i>crit</i> is not NULL then <i>*crit</i> is set to a status value: -2 if the extension occurs multiple times (this is only returned if <i>idx</i> is NULL), -1 if the extension could not be found, 0 if the extension is found and is not critical and 1 if critical. A pointer to an extension specifi