Function returning uint16

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Function returning uint16

Stéphane Glondu
Hello,

I am trying to debug

   https://github.com/ocamllabs/ocaml-ctypes/issues/404

and I realized that the following code (on amd64):

#include <stdio.h>
#include <stdint.h>
#include <ffi.h>

uint16_t retrieve() {
   return 0x4242;
}

int main() {
   uint16_t r[2] = { 0xdead, 0xbeef };
   ffi_cif cif;
   printf("r = {%x, %x}\n", r[0], r[1]);
   ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 0, &ffi_type_uint16, NULL);
   ffi_call(&cif, FFI_FN(retrieve), &r[0], NULL);
   printf("r = {%x, %x}\n", r[0], r[1]);
   return 0;
}

returns:

r = {dead, beef}
r = {4242, 0}

Is that expected? I don't expect r[1] to be overwritten...


Cheers,

--
Stéphane
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Re: Function returning uint16

Richard Henderson-2
On 06/16/2016 08:03 AM, Stéphane Glondu wrote:

> Hello,
>
> I am trying to debug
>
>   https://github.com/ocamllabs/ocaml-ctypes/issues/404
>
> and I realized that the following code (on amd64):
>
> #include <stdio.h>
> #include <stdint.h>
> #include <ffi.h>
>
> uint16_t retrieve() {
>   return 0x4242;
> }
>
> int main() {
>   uint16_t r[2] = { 0xdead, 0xbeef };
>   ffi_cif cif;
>   printf("r = {%x, %x}\n", r[0], r[1]);
>   ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 0, &ffi_type_uint16, NULL);
>   ffi_call(&cif, FFI_FN(retrieve), &r[0], NULL);
>   printf("r = {%x, %x}\n", r[0], r[1]);
>   return 0;
> }
>
> returns:
>
> r = {dead, beef}
> r = {4242, 0}
>
> Is that expected? I don't expect r[1] to be overwritten...

It is expected.  In the documentation,

In most situations, @samp{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 @code{struct}) types that are narrower than the
system register size, the return value will be widened by
@samp{libffi}.  @samp{libffi} provides a type, @code{ffi_arg}, that
can be used as the return type.  For example, if the CIF was defined
with a return type of @code{char}, @samp{libffi} will try to store a
full @code{ffi_arg} into the return value.


r~