[PD] Getting 32 bit floating point number from 4 sequential bytes

[Christof Ressi]

> Using a union would be a bit more efficient as it doesn't copy any memory.

an optimizing compiler will very likely generate identical assembly. look here, how the call to memcpy is reduced to a single 'movss' instruction: https://godbolt.org/z/n5ueWD

@Arda: shouldn't 'wrd' and 'o' be local variables instead of struct members? or do you need somehow to cache the value for later?

Christof


Gesendet: Samstag, 06. April 2019 um 17:27 Uhr
Von: "Martin Peach" <chakekatzil at gmail.com>
An: Pd-List <pd-list at lists.iem.at>
Betreff: Re: [PD] Getting 32 bit floating point number from 4 sequential bytes

On Sat, Apr 6, 2019 at 10:54 AM Arda Eden <ardaeden at gmail.com[mailto:ardaeden at gmail.com]> wrote:

With this method, I got the compiler warning about breaking the strict aliasing rule:
uint8_t bytes[4] = {12, 34, 56, 78};
float f = *(float*)bytes;
 
Yes, modern c doesn't like type punning.
 

 
 
This is my code, and for now, it is working properly. But I am not sure if this is an efficient way or not.
 
typedef struct _xsensparse
{
    t_object x_obj;
    uint8_t wrd[4];
    t_float o;
    t_outlet *f1_out, *f2_out;
} t_xsensparse;
 
static void xsensparse_list(t_xsensparse *x, t_symbol *s, int argc, t_atom *argv)
{
    for(int i=0; i<argc; i++) {
    x->wrd[i]=(uint8_t)atom_getfloat(argv+3-i);
    memcpy(&x->o, &x->wrd, 4);
    }
    post("%f",  x->o);
    outlet_float(x->f1_out, x->o);
}
 
I think the memcpy statement should be outside the for loop. As it is, it operates the first three times uselessly.
Using a union would be a bit more efficient as it doesn't copy any memory.
 
Martin
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