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

[Tony Rogvall]

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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