libffi assumes that you have a pointer to the function you wish
to call and that you know the number and types of arguments to pass
it, as well as the return type of the function.
The first thing you must do is create an ffi_cif object that
matches the signature of the function you wish to call. This is a
separate step because it is common to make multiple calls using a
single ffi_cif. The cif in ffi_cif stands for
Call InterFace. To prepare a call interface object, use the function
ffi_prep_cif.
This initializes cif according to the given parameters.
abi is the ABI to use; normally FFI_DEFAULT_ABI is what
you want. Multiple ABIs for more information.
nargs is the number of arguments that this function accepts.
rtype is a pointer to an ffi_type structure that
describes the return type of the function. See Types.
argtypes is a vector of ffi_type pointers.
argtypes must have nargs elements. If nargs is 0,
this argument is ignored.
ffi_prep_cif returns a libffi status code, of type
ffi_status. This will be either FFI_OK if everything
worked properly; FFI_BAD_TYPEDEF if one of the ffi_type
objects is incorrect; or FFI_BAD_ABI if the abi parameter
is invalid.
If the function being called is variadic (varargs) then
ffi_prep_cif_var must be used instead of ffi_prep_cif.
This initializes cif according to the given parameters for
a call to a variadic function. In general its operation is the
same as for ffi_prep_cif except that:
nfixedargs is the number of fixed arguments, prior to any variadic arguments. It must be greater than zero.
ntotalargs the total number of arguments, including variadic and fixed arguments. argtypes must have this many elements.
ffi_prep_cif_var will return FFI_BAD_ARGTYPE if any of
the variable argument types are ffi_type_float (promote to
ffi_type_double first), or any integer type small than an int
(promote to an int-sized type first).
Note that, different cif’s must be prepped for calls to the same function when different numbers of arguments are passed.
Also note that a call to ffi_prep_cif_var with
nfixedargs=nototalargs is NOT equivalent to a call to
ffi_prep_cif.
Note that the resulting ffi_cif holds pointers to all the
ffi_type objects that were used during initialization. You
must ensure that these type objects have a lifetime at least as long
as that of the ffi_cif.
To call a function using an initialized ffi_cif, use the
ffi_call function:
This calls the function fn according to the description given in
cif. cif must have already been prepared using
ffi_prep_cif.
rvalue is a pointer to a chunk of memory that will hold the
result of the function call. This must be large enough to hold the
result, no smaller than the system register size (generally 32 or 64
bits), and must be suitably aligned; it is the caller’s responsibility
to ensure this. If cif declares that the function returns
void (using ffi_type_void), then rvalue is
ignored.
In most situations, libffi will handle promotion according to
the ABI. However, for historical reasons, there is a special case
with return values that must be handled by your code. In particular,
for integral (not struct) types that are narrower than the
system register size, the return value will be widened by
libffi. libffi provides a type, ffi_arg, that
can be used as the return type. For example, if the CIF was defined
with a return type of char, libffi will try to store a
full ffi_arg into the return value.
avalues is a vector of void * pointers that point to the
memory locations holding the argument values for a call. If cif
declares that the function has no arguments (i.e., nargs was 0),
then avalues is ignored.
Note that while the return value must be register-sized, arguments
should exactly match their declared type. For example, if an argument
is a short, then the entry in avalues should point to an
object declared as short; but if the return type is
short, then rvalue should point to an object declared as
a larger type – usually ffi_arg.