One of the most important issues for language extensions is
accepting and dealing with data passed via arguments. Most
extensions are built to deal with specific input data (or require
parameters to perform their specific actions), and function
arguments are the only real way to exchange data between the PHP
level and the C level. Of course, there's also the possibility of
exchanging data using predefined global values (which is also
discussed later), but this should be avoided by all means, as it's
extremely bad practice.
PHP doesn't make use of any formal function declarations; this is
why call syntax is always completely dynamic and never checked for
errors. Checking for correct call syntax is left to the user code.
For example, it's possible to call a function using only one
argument at one time and four arguments the next time - both
invocations are syntactically absolutely correct.
Since PHP doesn't have formal function definitions with support
for call syntax checking, and since PHP features variable
arguments, sometimes you need to find out with how many arguments
your function has been called. You can use the
ZEND_NUM_ARGS macro in this case. In previous
versions of PHP, this macro retrieved the number of arguments with
which the function has been called based on the function's hash
table entry, ht, which is passed in the
INTERNAL_FUNCTION_PARAMETERS list. As
ht itself now contains the number of arguments that
have been passed to the function, ZEND_NUM_ARGS
has been stripped down to a dummy macro (see its definition in
zend_API.h). But it's still good practice to
use it, to remain compatible with future changes in the call
interface. Note: The old PHP equivalent of
this macro is ARG_COUNT.
The following code checks for the correct number of arguments:
if(ZEND_NUM_ARGS() != 2) WRONG_PARAM_COUNT; |
If the function is not called with two
arguments, it exits with an error message. The code snippet above
makes use of the tool macro
WRONG_PARAM_COUNT,
which can be used to generate a standard error message (see
Rysunek 52-1).
This macro prints a default error message and then returns to the caller.
Its definition can also be found in zend_API.h and looks
like this:
ZEND_API void wrong_param_count(void);
#define WRONG_PARAM_COUNT { wrong_param_count(); return; } |
As you can see, it calls an internal function
named
wrong_param_count() that's responsible for printing
the warning. For details on generating customized error
messages, see the later section "Printing Information."
