Arduino: cannot pass union struct as pointer ac I can with gcc compiler

Question

I'm trying to use structs with Arduino but cannot pass a struct pointer with a function call. Everything works fine when adding a main function and compiling for my computer with gcc but with the Arduino IDE I get errors.

The code I tried was:

typedef union
{
    struct
    {
        unsigned unit   :2;
        unsigned channel:2;
        unsigned status :1;
        unsigned group  :1;
        unsigned remote :26;
    };
    unsigned long data;
} Signal;

Signal signal;

void testPassingStruct(Signal *variable)
{
    variable->status = 1;
}

void setup()
{
    signal.status = 1;
    testPassingStruct(&signal);
}

void loop()
{
}

And the errors was:

structtest:2: error: variable or field ‘testPassingStruct’ declared void
structtest:2: error: ‘Signal’ was not declared in this scope
structtest:2: error: ‘variable’ was not declared in this scope

artless-noise-bye-due2AI · Accepted Answer · 2014-01-10 23:18:48Z

3

You have several unnamed items. This happens with typedefs. You could try,

union sig_union
{
    struct
    {
        unsigned unit   :2;
        unsigned channel:2;
        unsigned status :1;
        unsigned group  :1;
        unsigned remote :26;
    } d;                       /* Note the name here. */
    unsigned long data;
} signal;
typedef union sig_union Signal;

Then in your code you need,

void testPassingStruct(Signal *variable)
{
    variable->d.status = 1;
}

You can also use defines for masks and bit fields. Often it makes control of what bit is where clearer and the compiler will generate similar code in both cases. For instance,

#define SIG_UNIT(d)    (d&0x3)
#define SIG_CHANNEL(d) (d&0xc>>2)
#define SIG_STATUS(d)  (d&0x10)
/* etc. */

Especially with system programming where the bits matter, this is better as I don't believe that the 'C' bit fields specify how they are placed. Most of all, I am sure it will compile.

answered Jan 10, 2014 at 23:18

artless-noise-bye-due2AI

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3 Comments

artless-noise-bye-due2AI Over a year ago

The ATMega/AVR may have troubles with your nested struct/union. It has limited 8 bit registers. I think the defines will definitely work although there is a little bit more boiler-plate code.

Ignacio Vazquez-Abrams Over a year ago

It's 4 bytes, which is 2 registers. Hardly a challenge for anything more powerful than a ATtiny10.

artless-noise-bye-due2AI Over a year ago

@IgnacioVazquez-Abrams Yes, but for most CPUs it would only be a single register; I am just saying that it is not unfathomable that the compiler may have issue with this; I think it is the other issues I noted. Especially, the error message would have been different if it was an register allocation.

cup · Accepted Answer · 2014-01-10 23:45:25Z

3

It is basically the union and the lack of a the name for the structure member (I've called it parts in the code below). Try this

union Signal
{
    struct
    {
        unsigned unit   :2;
        unsigned channel:2;
        unsigned status :1;
        unsigned group  :1;
        unsigned remote :26;
    } parts;
    unsigned long data;
};

union Signal signal;

void testPassingStruct(union Signal *variable)
{
    variable->parts.status = 1;
}

void setup()
{
    signal.parts.status = 1;
    testPassingStruct(&signal);
}

void loop()
{
}

answered Jan 10, 2014 at 23:45

cup

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2 Comments

tirithen Over a year ago

Will I then be able to use signal.data to iterate over all 5 parts that makes up the 32 bits? I need that to send the signal via a 433Mhz transmitter.

cup Over a year ago

Yes - in fact trying it would have been faster than asking.

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