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

[Tony Rogvall]

> 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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