Question

In a header file, I declare a struct with an array and array length like so:

typedef struct {
    unsigned char *frame;
    int length;
} ResponseFrame ;

extern ResponseFrame rf;

In my main file, I have the following:

ResponseFrame rf;

int main(void)
{
   while(1) {
      if (receive() == 0x01) {
         uint8_t val;
         rf.length = 6;
         for(int i = 0; i < 6; i++){
            val = receive();
            rf.frame[i] = val;
            transmit(val);            // LINE 1
         }


         for (uint8_t i=0; i<rf.length; i++){
            transmit(rf.frame[i]);    // LINE 2
         }
      }
   }
}

The array I’m receiving is

{ 00 00 FF 00 FF 00 }

The initial transmit responds with this received data [see LINE 1] .
However, when I try to transmit using the global variable rf.frame [see LINE 2], the first value is different like this —-

{ 13 00 FF 00 FF 00 }

Why is that first initial value different like this??

Answer 1

You never allocate any memory for ResponseFrame.frame, so you’re running into undefined behaviour.

You’re assuming that by doing rf.length = 6;, the size of unsigned char *frame; somehow magically increases, but it doesn’t.

If you’re certain of the size, you need:

rf.length = 6;
rf.frame = malloc(sizeof(unsigned char) * 6);   //for C

or use a std::vector in C++.