Re: Function pointers?

[The Devils Jester <thedevilsjester at gmail dot com>]

I am not sure how that makes a difference.  With the above change, the
value stored in values[0] and the actual ptr printed in test_b are
still different.  Am I missing something obvious here?

On Tue, Mar 5, 2013 at 1:46 AM, Nathan Rossi <nathan.rossi@xilinx.com> wrote:
>> -----Original Message-----
>> From: libffi-discuss-owner@sourceware.org [mailto:libffi-discuss-
>> owner@sourceware.org] On Behalf Of The Devils Jester
>> Sent: Tuesday, March 05, 2013 4:06 PM
>> To: libffi-discuss@sourceware.org
>> Subject: Function pointers?
>>
>> I can pass pointers as arguments to a function without issue unless it
>> is a function pointer.  When I try and pass a function pointer, it
>> gets jumbled during the pass.  What am I doing wrong?
>>
>> Here is an example code that prints the value of the pointer before
>> its passed, and in the function its passed to.  Both values are
>> different, does anyone know why?
>>
>> (I am running OS X 10.8.2)
>>
>> #include <stdio.h>
>> #include <ffi.h>
>>
>> int test_a(void*ptr)
>> { return 0; }
>>
>> int test_b(void*ptr)
>> {
>>     printf("Address of passed ptr: %p\n", ptr );
>>     return 0;
>> }
>>
>> int main()
>> {
>>     ffi_cif cif;
>>     ffi_type *args[1];
>>     void *values[1];
>>     int rc;
>>
>>     args[0] = &ffi_type_pointer;
>>     values[0] = &test_a;
>>
>>     if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1, &ffi_type_uint, args) == FFI_OK)
>>      {
>>        printf("Address of ptr to pass: %p\n", values[0] );
>>        ffi_call(&cif, test_b, &rc, values);
>>      }
>>      return 0;
>> }
>
> Hi,
>
> The 'values' array you are passing to ffi_call needs to contain pointers to the associated data (in this case the data is the function pointer). Because of the C syntax "test_a == &test_a", and the & operator is ignored, and thus you are passing the address of the function as the pointer to the data.
>
> You will need to store the function pointer on the stack/heap before passing it to the ffi_call. E.g.
> ..
> void* arg_values[1];
> arg_values[0] = (void*)test_a;
> values[0] = &(arg_values[0]);
> ...
>
> Regards,
> Nathan
>
>