Pointers and static variables in C

Question

I'm having some problems with pointers in a school task, since I haven't programmed in C in a long time. I'm trying to store a value called rssi on a device running Contiki-NG which is like an OS for IOT devices.

I initialized the variable rssi in the beginning of the code:

static int8_t *rssi;

Then, in a function called on_reception, I'm updating the variable:

void on_reception(const void *data,
    uint16_t len,
    const linkaddr_t *src,
    const linkaddr_t *dest)
{
   ....
   // Update variable
   rssi = &packetbuf_attr(PACKETBUF_ATTR_RSSI);
}

And then in the main loop I'm trying to store this value using a provided cfs_write function:

PROCESS_THREAD(receiver_process, ev, data)
{
   PROCESS_BEGIN();
   ....

   while(1) {
    PROCESS_WAIT_EVENT_UNTIL(etimer_expired(&periodic_timer));
    etimer_reset(&periodic_timer);
    entry = akes_nbr_head();
    if(entry && entry->permanent) {
      akes_trickle_stop();
      break;
    }
    
    // Store rssi value
    r = cfs_write(fd, &rssi, sizeof(rssi));
  }
}

However I'm getting problems like:

"lvalue required as unary '&' operand"

I've tried using '*' instead of '&' and what not but I can't seem to get it to work. Could anyone point me in the right direction?

EDIT: PACKETBUF_ATTR is a struct that contains a value according to the header file:

typedef uint16_t packetbuf_attr_t;

struct packetbuf_attr {
  packetbuf_attr_t val;
};

They gave me an example of printing the values:

  memcpy(&counter, data, sizeof(counter));
  printf("B,PING,%lu,%u,%lu,%i,0\n",
      clock_time(),
      (uint8_t)packetbuf_attr(PACKETBUF_ATTR_CHANNEL),
      counter,
      (int8_t)packetbuf_attr(PACKETBUF_ATTR_RSSI));

@SupportUkraine Yeah, that's true. I tried using calloc() instead, but then it just complains that the "initializer element is not constant": static int8_t *rssi = calloc(1, sizeof(int8_t)); — yerboi
– yerboi, Commented May 2, 2022 at 10:02
"I'm not sure what PACKETBUF_ATTR is returning": Well, since your source uses this symbol, you should be able to track it into some header file to reveal its nature. -- What is packetbuf_attr? -- "I updated the answer." Do you mean the question? — the busybee
– the busybee, Commented May 2, 2022 at 10:15
I initialized the variable rssi No, the variable is not initialized besides automatic initialization with 0. — Gerhardh
– Gerhardh, Commented May 2, 2022 at 10:19
@SupportUkraine Yeah, that was kind of what the initial problem was. I'm not sure if it should be a pointer or not, and I've tried not using pointers but haven't gotten it to work. — yerboi
– yerboi, Commented May 2, 2022 at 10:40

kfx · Accepted Answer · 2022-05-03 08:52:11Z

1

The function packetbuf_attr() returns value of type packetbuf_attr_t, which is just another name for uint16_t (unsigned 16 bit integer):

typedef uint16_t packetbuf_attr_t;

You don't need to use pointers. The important issue is that RSSI is a signed integer, usually below zero, and always fits in 8 bits. Taking this into account, you need to cast the return value of packetbuf_attr() to the correct type:

static int8_t rssi;
...
rssi = (int8_t)packetbuf_attr(PACKETBUF_ATTR_RSSI);

answered May 3, 2022 at 8:52

kfx

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