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 Constructing function calls  

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Using the built-in functions described in the following discussion, 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).

__builtin_apply_args ()
This built-in function returns a pointer of type void * 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.
__builtin_apply (function, arguments, size)
This built-in function invokes function  (type void (*)()) with a copy of the parameters described by arguments (type void *) and size (type int).
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 of type void * 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 area.
__builtin_return (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.