colon in the structure

what is the use of colon in the structure.

Code:

---

int main ()
{
        struct {
                int p:4;
                int k:4;
        }l;

        l.p=10;
        l.k=1000;

        printf("%d\n",l.p);
        printf("%d\n",l.k);
        return 0;
}

output:
-6
-8

---

Had to do some testing, but it appears to limit the number of bits used to repsent the number.

As a test try this code out (adapted from the code you gave)

Code:

---

int main ()
{
struct {
int p:3;
int k:5;
int a:2;
}l;
int c;
for (c=0; c<20; ++c)
{
  l.a=c;
  l.p=c;
  l.k=c;
  printf("%d\t%d\t%d\t%d\n",c,l.p,l.k,l.a);
}
return 0;
}

---

Did some more tests and here is what I've found. The struct will be allocated 4 bytes (32 bits) blocks for all the variables. So you could have 4 variables inside the struct that each only takes up 8 bits each and only use the orginal 4 byte block. However if you try to use 33 bits for all your variables then you will get another 4 byte block.

So the following struct only takes 4 bytes

Code:

---

  struct {
    int a:4;
    int b:4;
    int c:8;
    int d:8;
    char ch;
    }

---

but the following takes 8 bytes

Code:

---

  struct {
    int a:5;
    int b:4;
    int c:8;
    int d:8;
    char ch;
    }

---

Just look up bit fields in any good C book

good detective work thantos

Think I should get a new book? Its only copyrighted 1987 :D