指向结构的指针成员

Question

I am getting an error from my compiler as follow:

C51 COMPILER V9.01 - SN: C1ADC-HAI60D COPYRIGHT KEIL ELEKTRONIK GmbH 1987 - 2009 * WARNING C260 IN LINE 300 OF SEQUENCE.C: '=': pointer truncation * ERROR C190 IN LINE 301 OF SEQUENCE.C: '&': not an lvalue

The following is my code:

   struct myCond{
    unsigned char currStatus;
    unsigned char prevStatus;
    unsigned int *timer;
    unsigned char *flag;
   }
  struct myCond StatCond;

  unsigned int data timerdata;
  bit bdata timeflag;

  void someSubroutine (void)
  {
    struct myCond *tempCond;

tempCond = &StatCond;
tempCond->timer = &((unsigned int)timerdata);
tempCond->flag = &((unsigned char)timeflag);
  }

Answer 1

Are we supposed to guess which line is 301?

The problems, as I understand are here:

tempCond->timer = &((unsigned int)timerdata);
tempCond->flag = &((unsigned char)timeflag);

(unsigned int)timerdata and (unsigned char)timeflag are values, r-values to be precise. They cannot be modified or assigned unlike l-values , which plain timerdata and timeflag are. And so you can't take addresses of r-values with & . It would be the same as, say, writing &1 . 1 on its own does not exist as an object in data memory.

You should write instead:

tempCond->timer = &timerdata;
tempCond->flag = (unsigned char*)&timeflag;

And I'm not quite sure that it is legal to take an address of a bit variable. The last line may fail to compile.

Perhaps redefining the structure would help:

struct myCond{
...
    bit bdata *flag; // or maybe without bdata
   }

and then you'd write tempCond->flag = &timeflag; .

Answer 2

unsigned int data timerdata;  // what is 'data', is it defined?
bit bdata timerflag;  // what are 'bit' and 'bdata', are they defined?

Check your code with regard to my questions above. Compiler errors are often reported multiple lines after the real offense.