6.5. Constructing Function Calls
Using the built-in functions described below, you can record
the arguments a function received, and call another function
with the same arguments, without knowing the number or types
of the arguments.
You can also record the return value of that function call,
and later return that value, without knowing what data type
the function tried to return (as long as your caller expects
that data type).
However, these built-in functions may interact badly with some
sophisticated features or other extensions of the language. It
is, therefore, not recommended to use them outside very simple
functions acting as mere forwarders for their arguments.
void *__builtin_apply_args ()
This built-in function returns a pointer to data
describing how to perform a call with the same arguments as were passed
to the current function.
The function saves the arg pointer register, structure value address,
and all registers that might be used to pass arguments to a function
into a block of memory allocated on the stack. Then it returns the
address of that block.
void *__builtin_apply (void (*function)(), void *arguments, size_t size)
This built-in function invokes function
with a copy of the parameters described by arguments
The value of arguments should be the value returned by
__builtin_apply_args. The argument size specifies the size
of the stack argument data, in bytes.
This function returns a pointer to data describing
how to return whatever value was returned by function. The data
is saved in a block of memory allocated on the stack.
It is not always simple to compute the proper value for size. The
value is used by __builtin_apply to compute the amount of data
that should be pushed on the stack and copied from the incoming argument
void__builtin_return (void *result)
This built-in function returns the value described by result from
the containing function. You should specify, for result, a value
returned by __builtin_apply.