# Programming-Idioms

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# Idiom #190 Call an external C function

Declare an external C function with the prototype

void foo(double *a, int n);

and call it, passing an array (or a list) of size 10 to a and 10 to n.

Use only standard features of your language.

``````void foo(double *a, int n);
double a[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
foo (a, sizeof(a)/sizeof(*a));``````
``with Interfaces.C;``
``````procedure C_Foo (A : access Interfaces.C.double;
N : Interfaces.C.int) with
Import        => True,
Convention    => C,
External_Name => "foo";

procedure Test_Foo is
type Double_Array is array (Positive range <>) of aliased Interfaces.C.double;
A : Double_Array (1 .. 10) := (1.0, 2.0, 3.0, 4.0, 5.0,
6.0, 7.0, 8.0, 9.0, 10.0);
begin
C_Foo (A (A'First)'Access, A'Length);
end Test_Foo;``````
``extern "C" void foo(double *a, int n);``
``````module x
use iso_c_binding
interface
subroutine foo (a, n) bind(c)
import
real(kind=c_double), dimension(*) :: a
integer (kind=c_int), value :: n
end subroutine foo
end interface
contains
subroutine call_foo
real(kind=c_double) :: a(10)
call foo(a,10)
end subroutine call_foo
end module x``````
``````// void foo(double *a, int n);
// double a[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
import "C"

C.foo(C.a, 10)``````
``procedure foo(a: pdouble; n: integer); external 'some_c_dll' name 'foo'; cdecl;``
``use FFI::Platypus qw();``
``````FFI::Platypus
->new(lib => '/tmp/mylib/libmylib.so')
->attach(foo => ['double[]', 'int']);
foo([1..10], 10);
``````
``from ctypes import *``
``````a = (c_int * 10)(*range(10))
n = 10
libc.foo(c_void_p(a), c_int(n))
``````
``````extern "C" {
/// # Safety
///
/// `a` must point to an array of at least size 10
fn foo(a: *mut libc::c_double, n: libc::c_int);
}

let mut a = [0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0];
let n = 10;
unsafe {
foo(a.as_mut_ptr(), n);
}``````

tkoenig