[erlang-questions] Accessing A Binary as an Array of Floats/Doubles Within a NIF

[Sverker Eriksson]

Well, reference counted binaries have 8-byte aligned data.
Heap binaries only half of the time on 32-bit :-)

Both driver_alloc_binary() and enif_alloc_binary() guarantee
8-byte alignment of the returned payload data.

/Sverker


On 06/20/2016 10:07 PM, Tony Rogvall wrote:
> I just found the answer in binary.c.
> The answer is that all binary data is always 8 byte aligned.
> And that is nice :-)
>
> /Tony
>
>> On 20 jun 2016, at 21:59, Tony Rogvall <tony@REDACTED> wrote:
>>
>>> On 20 jun 2016, at 21:50, Sverker Eriksson <sverker.eriksson@REDACTED> wrote:
>>>
>>>
>>> If you use <<1.0/float-native>>
>>> then it's just type cast to a 'double' pointer:
>>>
>>> double* my_array;
>>>
>>> my_array = (double*) Bin.data;
>>>
>>> printf("%lf", my_array[0]);
>>>
>>>
>>> Default byte order is 'float-big', which will not work on little endian machines (x86).
>>>
>>> Depending on architecture you also might have to ensure 8 byte alignment.
>>> x86 will accept unaligned floats but the performance may suffer.
>>>
>> Is there an easy way to ensure 8 byte alignment in Erlang, when constructing binaries?
>> On a 64-bit machine/arch my guess is that Binary data is 8 byte aligned but
>> what about a on 32-bit machine/arch?
>>
>> /Tony
>>
>>> /Sverker, Erlang/OTP
>>>
>>>
>>> On 06/20/2016 09:04 PM, John Duffy wrote:
>>>> Hi
>>>> I'm using FreeBSD 10.3 & Erlang 18.3 and I'm trying to get to grips with passing a binary of floats to a NIF for further processing as as array of C doubles.
>>>> My simple NIF module compiles and I can access my "my_nif_1" function from the erlang shell. I can access the 'size' structure member but I can't figure out how to access the 'data' structure member as a C array. I understand that 'data' is a block of memory of UINT bytes but I can't figure out how to "cast" this to a C array within the NIF.
>>>> Any help would be mud appreciated.
>>>> Kind regards
>>>> John
>>>> #include "erl_nif.h"
>>>> static ERL_NIF_TERM my_nif_1(ErlNifEnv * env, int argc, const ERL_NIF_TERM argv[])
>>>> {
>>>>     ErlNifBinary Bin;
>>>>     if (!enif_inspect_binary(env, argv[0], &Bin)) {
>>>>         return enif_make_badarg(env);
>>>>     }
>>>>     // So far, so good...
>>>>     // From the erl shell...
>>>>     // my_nifs:my_nif_1(<<1.0/float>>). returns a Bin.size of 8
>>>>     // my_nifs:my_nif_1(<<1.0/float,2.0/float>>. returns a Bin.size of 16
>>>>     //
>>>>     // How do I access Bin.data as an array of doubles?
>>>>     //
>>>>     return enif_make_int(env, Bin.size); // For testing... this works.
>>>> }
>>>> static ErlNifFunc nif_funcs[] = {
>>>>     {"my_nif_1", 1, my_nif_1}
>>>> };
>>>> ERL_NIF_INIT(my_nifs, nif_funcs, NULL, NULL, NULL, NULL);
>>>>
>>>>
>>>>
>>>>
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